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2017-06-06 | Coupling between a charge density wave and magnetism in an Heusler material | The Prototypical magnetic memory shape alloy Ni$_2$MnGa undergoes various
phase transitions as a function of temperature, pressure, and doping. In the
low-temperature phases below 260 K, an incommensurate structural modulation
occurs along the [110] direction which is thought to arise from softening of a
phonon mode. It is not at present clear how this phenomenon is related, if at
all, to the magnetic memory effect. Here we report time-resolved measurements
which track both the structural and magnetic components of the phase transition
from the modulated cubic phase as it is brought into the high-symmetry phase.
The results suggest that the photoinduced demagnetization modifies the Fermi
surface in regions that couple strongly to the periodicity of the structural
modulation through the nesting vector. The amplitude of the periodic lattice
distortion, however, appears to be less affected by the demagnetizaton. | 1706.01685v1 |
2019-09-12 | Single pulse all-optical toggle switching of magnetization without Gd: The example of Mn2RuxGa | Energy-efficient control of magnetization without the help of a magnetic
field is a key goal of spintronics. Purely heat-induced single-pulse
all-optical toggle switching has been demonstrated, but so far only in Gd based
amorphous ferrimagnet films. In this work, we demonstrate toggle switching in
the half-metallic compensated ferrimagnetic Heusler alloys Mn2RuxGa, which have
two crystallographically-inequivalent Mn sublattices. Moreover, we observe the
switching at room temperature in samples that are immune to external magnetic
fields in excess of 1 T, provided they exhibit compensation above room
temperature. Observations of the effect in compensated ferrimagnets without Gd
challenges our understanding of all-optical switching. The dynamic behavior
indicates that Mn2RuxGa switches in 2 ps or less. Our findings widen the basis
for fast optical switching of magnetization and break new ground for engineered
materials that can be used for nonvolatile ultrafast switches using ultrashort
pulses of light. | 1909.05809v2 |
2020-02-06 | Thermal dependence of the mechanical properties of NiTiSn using first-principles calculations and high-pressure X-ray diffraction | In this work we aim to study the effect of temperature on the mechanical
properties of a solid. For this, we have introduced a new first-principles
based methodology to obtain the thermal variation of the elastic constants of
NiTiSn, a multifunctional Heusler compound. In parallel using X-ray
diffraction, we have measured the isothermal bulk modulus at 300~K. The
agreement between the calculations and the experiments is within the
experimental error bars showing the accuracy of the calculations. Using two
other numerical methods, which give all coherent results, we have shown that
NiTiSn conserves its very good mechanical properties up to 1500~K. In
particular at 700~K (the best working temperature for thermoelectric
applications), NiTiSn remains a ductile and robust material making it a
compound of choice for applications in which large temperature fluctuations are
present. | 2002.02161v1 |
2021-02-22 | Suppression of spinodal instability by disorder in an athermal system | We observed asymmetric critical slowing down and asymmetric dynamical scaling
exponent in the superheating and supercooling kinetic processes during the
thermally-induced metal-insulator transition of MnNiSn based heusler alloy.
During the transition to the insulator phase, the critical-like features get
enhanced compared to the transition back to the metal phase. These experimental
findings suggest that the metastable phase in the cooling branch of hysteresis
has approached close to the spinodal instability. On the other hand, the
extended disorder, generated over and above the intrinsic crystal defects
during heating, triggers the excess heterogeneous nucleation before reaching
the spinodal point. Zero temperature random field Ising model (ZTRFIM)
simulation, inscribed for the athermal martensitic transitions, support the
argument that the disorder smears the spinodal instabilities as the correlation
length is bounded by the average distance between the disorder points. | 2102.11347v2 |
2021-08-19 | IrCrMnZ (Z=Al, Ga, Si, Ge) Heusler alloys as electrode materials for MgO-based magnetic tunneling junctions: A first-principles study | We study IrCrMnZ (Z=Al, Ga, Si, Ge) systems using first-principles
calculations from the perspective of their application as the electrode
materials of MgO-based MTJs. These materials have highly spin-polarized
conduction electrons with partially occupied $\Delta_1$ band, which is
important for coherent tunneling in parallel magnetization configuration. The
Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K) as
predicted from mean-field-approximation. The stability of ordered phase against
various antisite disorders has been investigated. We discuss here the effect of
"spin-orbit-coupling" on the electronic structure around Fermi level. Further,
we investigate the electronic structure of IrCrMnZ/MgO heterojunction along
(001) direction. IrCrMnAl/MgO and IrCrMnGa/MgO maintain half-metallicity even
at the MgO interface, with no interfacial states at/around Fermi level in the
minority-spin channel. Large majority-spin conductance of IrCrMnAl/MgO/IrCrMnAl
and IrCrMnGa/MgO/IrCrMnGa is reported from the calculation of ballistic
spin-transport property for parallel magnetization configuration. We propose
IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa as promising MTJs with a weaker
temperature dependence of tunneling magnetoresistance ratio, owing to their
very high Curie temperatures. | 2108.08501v1 |
2021-10-26 | Giant quadratic magneto-optical response of thin YIG films for sensitive magnetometric experiments | We report on observation of a magneto-optical effect quadratic in
magnetization (Cotton-Mouton effect) in 50 nm thick layer of Yttrium-Iron
Garnet (YIG). By a combined theoretical and experimental approach, we managed
to quantify both linear and quadratic magneto-optical effects. We show that the
quadratic magneto-optical signal in the thin YIG film can exceed the linear
magneto-optical response, reaching values of 450 urad that are comparable with
Heusler alloys or ferromagnetic semiconductors. Furthermore, we demonstrate
that a proper choice of experimental conditions, particularly with respect to
the wavelength, is crucial for optimization of the quadratic magneto-optical
effect for magnetometry measurement. | 2110.13679v1 |
2022-04-04 | Phase-field modeling of paramagnetic austenite-ferromagnetic martensite transformation coupled with mechanics and micromagnetics | A three-dimensional phase-field model is proposed for simulating the magnetic
martensitic phase transformation. The model considers a paramagnetic cubic
austenite to ferromagnetic tetragonal martensite transition, as it occurs in
magnetic Heusler alloys like Ni2 MnGa, and is based on a Landau 2-3-4
polynomial with temperature dependent coefficients. The
paramagnetic-ferromagnetic transition is recaptured by interpolating the
micromagnetic energy as a function of the order parameter for the ferroelastic
domains. The model is numerically implemented in real space by finite element
(FE) method. FE simulations in the martensitic state show that the model is
capable to correctly recapture the ferroelastic and -magnetic microstructures,
as well as the influence of external stimuli. Simulation results indicate that
the paramagnetic austenite to ferromagnetic martensite transition shifts
towards higher temperatures when a magnetic field or compressive stress is
applied. The dependence of the phase transition temperature shift on the
strength of the external stimulus is uncovered as well. Simulation of the phase
transition in magnetocaloric materials is of high interest for the development
of energy-efficient magnetocaloric cooling devices. | 2204.01308v1 |
2022-10-10 | Numerical stability and efficiency of response property calculations in density functional theory | Response calculations in density functional theory aim at computing the
change in ground-state density induced by an external perturbation. At finite
temperature these are usually performed by computing variations of orbitals,
which involve the iterative solution of potentially badly-conditioned linear
systems, the Sternheimer equations. Since many sets of variations of orbitals
yield the same variation of density matrix this involves a choice of gauge.
Taking a numerical analysis point of view we present the various gauge choices
proposed in the literature in a common framework and study their stability.
Beyond existing methods we propose a new approach, based on a Schur complement
using extra orbitals from the self-consistent-field calculations, to improve
the stability and efficiency of the iterative solution of Sternheimer
equations. We show the success of this strategy on nontrivial examples of
practical interest, such as Heusler transition metal alloy compounds, where
savings of around 40% in the number of required cost-determining Hamiltonian
applications have been achieved. | 2210.04512v2 |
2022-12-15 | Resonant and off-resonant magnetoacoustic waves in epitaxial Fe$_3$Si/GaAs hybrid structures | Surface acoustic waves (SAWs) provide an efficient dynamical coupling between
strain and magnetization in micro/nano-metric devices. Using a hybrid device
composed of a piezoelectric, GaAs, and a ferromagnetic Heusler alloy thin film,
Fe$_3$Si, we are able to quantify the amplitude of magnetoacoustic waves
generated with SAWs via magnetic imaging in an X-ray photoelectron microscope.
The cubic anisotropy of the sample together with a low damping coefficient
allows for the observation of resonant and non-resonant magnetoelastic
coupling. Additionally, via micromagnetic simulation, we verify the
experimental behavior and quantify the magnetoelastic shear strain component in
Fe$_3$Si that appears to be very large ($b_2=14\times 10^6$ J/m$^3$), much
larger than the one found in Nickel. | 2212.07994v1 |
2022-12-21 | Electronic and phonon contributions to the Thermoelectric properties of newly discovered half-Heusler alloys XHfPb (X= Ni, Pd, and Pt) | In this work we calculate the thermoelectric figure of merit of XHfPb (X= Ni,
Pd, and Pt) by computing the both the power factor and the lattice thermal
conductivity by first principles. We make reasonable approximations: we use the
Constant Relaxation Time Approximation (CRTA) to compute the electron transport
contribution and the modified Debye-Callaway model to calculate the thermal
lattice conductivity. We also report the dielectric properties of these
semiconductors and the mode Gr\"uneisen parameters. Not surprisingly we find
that the average Gr\"uneisen coefficient correlates with the tehrmal
conductivity. Next, we consider a realistic relaxation time $\tau$ and carrier
concentration $n$ from experimental data on ZrHfPb and obtain the figure of
merit $ZT$ as a function of temperature. Our main finding is that despite the
Pt is isoelectronic with Ni and Pd, the $ZT$ of PtHfPb is larger and behaves
differently from the other two materials, suggesting that PtHfPb is better
suited for high temperature thermoelectric generators. | 2212.10848v1 |
2023-01-24 | Ru$_{2-x}$Mn$_{1+x}$Al thin films | The cubic Heusler alloy Ru$_{2-x}$Mn$_{1+x}$Al is grown in thin film form on
MgO and MgAl$_2$O$_4$ substrates. It is a highly spin-polarised ferrimagnetic
metal, with weak magnetocrystalline anisotropy. Although structurally and
chemically similar to $\text{Mn}_2\text{Ru}_x\text{Ga}$, it does not exhibit
ferrimagnetic compensation, or large magneto galvanic effects. The differences
are attributed to a combination of atomic order and the hybridisation with the
group 13 element Al or Ga. The spin polarisation is around 50 to 60 %. There is
a gap in the density of states just above the Fermi level in fully ordered
compounds. | 2301.10148v2 |
2023-03-21 | Unveiling the magnetic structure and phase transition of Cr$_2$CoAl using neutron diffraction | We report the detailed analysis of temperature dependent neutron diffraction
pattern of the Cr$_2$CoAl inverse Heusler alloy and unveil the magnetic
structure up to the phase transition as well as its fully compensated
ferrimagnetic nature. The Rietveld refinement of the diffraction pattern using
the space group I$\bar4${\it m}2 confirm the inverse tetragonal structure over
the large temperature range from 100~K to 900~K. The refinement of the magnetic
phase considering the wave vector $k=$ (0, 0, 0) reveals the ferrimagnetic
nature of the sample below 730$\pm$5~K. This transition temperature is obtained
from empirical power law fitting of the variation in the ordered net magnetic
moment and intensity of (110) peak as a function of temperature. The spin
configuration of the microscopic magnetic structure suggests the nearly fully
compensated ferrimagnetic behavior where the magnetic moments of Cr2 are
antiparallel with respect to the Cr1, and Co moments. Moreover, the observed
anomaly in the thermal expansion and lattice parameters at 730$\pm$5~K suggest
that the distortion in crystal structure may play an important role in the
magnetic phase transition. | 2303.11869v1 |
2023-05-15 | Magnetic order and electronic transport properties in the Mn$_3$Al compound: the role of the structural state | Electronic transport and magnetic properties of bulk and rapid melt quenched
samples of the Mn$_3$Al Heusler alloy were studied. A correlation between the
magnetic and structural states was established. For a cast sample, there is no
ferromagnetic moment, and the behavior of the magnetic susceptibility (break at
low temperatures and the Curie-Weiss law with high values of the paramagnetic
Curie temperature) indicates a frustrated antiferromagnetic state. At the same
time, for a rapid melt quenched sample, a ferrimagnetic state is observed with
a moment close to compensation. The results of measurements of the electrical
resistivity and the Hall effect evidence as well in favor of the implementation
of these magnetic states. | 2305.08646v1 |
2024-03-20 | Record-high Anomalous Ettingshausen effect in a micron-sized magnetic Weyl semimetal on-chip cooler | Solid-state cooling devices offer compact, quiet, reliable and
environmentally friendly solutions that currently rely primarily on the
thermoelectric (TE) effect. Despite more than two centuries of research,
classical thermoelectric coolers suffer from low efficiency which hampers wider
application. In this study, the less researched Anomalous Ettingshausen effect
(AEE), a transverse thermoelectric phenomenon, is presented as a new approach
for on-chip cooling. This effect can be boosted in materials with non-trivial
band topologies as demonstrated in the Heusler alloy $\text{Co}_2\text{MnGa}$.
Enabled by the high quality of our material, in situ scanning thermal
microscopy experiments reveal a record-breaking anomalous Ettingshausen
coefficient of $-2.1$~mV in $\mu$m-sized on-chip cooling devices at room
temperature. A significant 44\% of the effect is contributed by the intrinsic
topological properties, in particular the Berry curvature of
$\text{Co}_2\text{MnGa}$, emphasising the unique potential of magnetic Weyl
semimetals for high-performance spot cooling in nanostructures. | 2403.13598v1 |
2021-11-04 | Influence of microstructure on the application of Ni-Mn-In Heusler compounds for multicaloric cooling using magnetic field and uniaxial stress | Novel multicaloric cooling utilizing the giant caloric response of
Ni-Mn-based metamagnetic shape-memory alloys to different external stimuli such
as magnetic field, uniaxial stress and hydrostatic pressure is a promising
candidate for energy-efficient and environmentally-friendly refrigeration.
However, the role of microstructure when several external fields are applied
simultaneously or sequentially has been scarcely discussed in literature. Here,
we synthesized ternary Ni-Mn-In alloys by suction casting and arc melting and
analyzed the microstructural influence on the response to magnetic fields and
uniaxial stress. By combining SEM-EBSD and stress-strain data, a significant
effect of texture on the stress-induced martensitic transformation is revealed.
It is shown that a <001> texture can strongly reduce the critical
transformation stresses. The effect of grain size on the material failure is
demonstrated and its influence on the magnetic-field-induced transformation
dynamics is investigated. Temperature-stress and temperature-magnetic field
phase diagrams are established and single caloric performances are
characterized in terms of ${\Delta}{s_T}$ and ${\Delta}{T_{ad}}$. The cyclic
${\Delta}{T_{ad}}$ values are compared to the ones achieved in the multicaloric
exploiting-hysteresis cycle. It turns out that a suction-cast microstructure
and the combination of both stimuli enables outstanding caloric effects in
moderate external fields which can significantly exceed the single caloric
performances. In particular for Ni-Mn-In, the maximum cyclic effect in magnetic
fields of 1.9 T is increased by more than 200 % to -4.1 K when a moderate
sequential stress of 55 MPa is applied. Our results illustrate the crucial role
of microstructure for multicaloric cooling using Ni-Mn-based metamagnetic
shape-memory alloys. | 2111.03092v2 |
2022-12-26 | In plane reorientation induced single laser pulse magnetization reversal in rare-earth based multilayer | Single Pulse All Optical Helicity Independent Switching (AO-HIS) represents
the ability to reverse the magnetic moment of a nanostructure using a
femtosecond single laser pulse. It is an ultrafast method to manipulate
magnetization without the use of any applied field. Since the first switching
experiments carried on GdFeCo ferrimagnetic systems, single pulse AO-HIS has
been restricted for a while to Gd-based alloys or Gd/FM bilayers where FM is a
ferromagnetic layer. Only recently has AO-HIS been extended to a few other
materials: MnRuGa ferrimagnetic Heusler alloys and Tb/Co multilayers with a
very specific range of thickness and composition. Here, we demonstrate that
single pulse AO-HIS observed in Tb/Co results from a different mechanism than
the one for Gd based samples and that it can be obtained for a large range of
rare earth-transition metal (RE-TM) multilayers, making this phenomenon much
more general. Surprisingly, in this large family of (RE-TM) multilayer systems,
the threshold fluence for switching is observed to be independent of the pulse
duration, up to at least 12 ps. Moreover, at high laser intensities, concentric
ring domain structures are induced, unveiling multiple fluence thresholds.
These striking switching features, which are in contrast to those of AO-HIS in
GdFeCo alloys, concomitant with the demonstration of an in-plane reorientation
of the magnetization, point towards an intrinsic precessional reversal
mechanism. Our results allow expanding the variety of materials with tunable
magnetic properties that can be integrated in complex heterostructures and
provide a pathway to engineer materials for future applications based on
all-optical control of magnetic order. | 2212.13279v1 |
2005-04-20 | First-principles study of thin magnetic transition-metal silicide films on Si(001) | In order to combine silicon technology with the functionality of magnetic
systems, a number of ferromagnetic (FM) materials have been suggested for the
fabrication of metal/semiconductor heterojunctions. In this work, we present a
systematic study of several candidate materials in contact with the Si surface.
We employ density-functional theory calculations to address the thermodynamic
stability and magnetism of both pseudomorphic CsCl-like $M$Si ($M$=Mn, Fe, Co,
Ni) thin films and Heusler alloy $M_2$MnSi ($M$=Fe, Co, Ni) films on Si(001).
Our calculations show that Si-termination of the $M$Si films is energetically
preferable during epitaxy since it minimizes the energetic cost of broken bonds
at the surface. Moreover, we can explain the calculated trends in thermodynamic
stability of the $M$Si thin films in terms of the $M$-Si bond-strength and the
$M$ 3d orbital occupation. From our calculations, we predict that ultrathin
MnSi films are FM with sizable spin magnetic moments at the Mn atoms, while
FeSi and NiSi films are nonmagnetic. However, CoSi films display itinerant
ferromagnetism. For the $M_2$MnSi films with Heusler-type structure, the MnSi
termination is found to have the highest thermodynamic stability. In the FM
ground state, the calculated strength of the effective coupling between the
magnetic moments of Mn atoms within the same layer approximately scales with
the measured Curie temperatures of the bulk $M_2$MnSi compounds. In particular,
the Co$_2$MnSi/Si(001) thin film has a robust FM ground state as in the bulk,
and is found to be stable against a phase separation into CoSi/Si(001) and
MnSi/Si(001) films. Hence this material is of possible use in FM-Si
heterojunctions and deserves further experimental investigations. | 0504515v2 |
2017-08-25 | Topological superconductivity of spin-3/2 carriers in a three-dimensional doped Luttinger semimetal | We investigate topological Cooper pairing, including gapless Weyl and fully
gapped class DIII superconductivity, in a three-dimensional doped Luttinger
semimetal. The latter describes effective spin-3/2 carriers near a quadratic
band touching and captures the normal-state properties of the 227 pyrochlore
iridates and half-Heusler alloys. Electron-electron interactions may favor
non-$s$-wave pairing in such systems, including even-parity $d$-wave pairing.
We argue that the lowest energy $d$-wave pairings are always of complex (e.g.,
$d + i d$) type, with nodal Weyl quasiparticles. This implies $\varrho(E) \sim
|E|^2$ scaling of the density of states (DoS) at low energies in the clean
limit, or $\varrho(E) \sim |E|$ over a wide critical region in the presence of
disorder. The latter is consistent with the $T$-dependence of the penetration
depth in the half-Heusler compound YPtBi. We enumerate routes for experimental
verification, including specific heat, thermal conductivity, NMR relaxation
time, and topological Fermi arcs. Nucleation of any $d$-wave pairing also
causes a small lattice distortion and induces an $s$-wave component; this gives
a route to strain-engineer exotic $s+d$ pairings. We also consider odd-parity,
fully gapped $p$-wave superconductivity. For hole doping, a gapless Majorana
fluid with cubic dispersion appears at the surface. We invent a generalized
surface model with $\nu$-fold dispersion to simulate a bulk with winding number
$\nu$. Using exact diagonalization, we show that disorder drives the surface
into a critically delocalized phase, with universal DoS and multifractal
scaling consistent with the conformal field theory (CFT) SO($n$)${}_\nu$, where
$n \rightarrow 0$ counts replicas. This is contrary to the naive expectation of
a surface thermal metal, and implies that the topology tunes the surface
renormalization group to the CFT in the presence of disorder. | 1708.07825v2 |
2016-12-18 | Heusler 4.0: Tunable Materials | Heusler compounds are a large family of binary, ternary and quaternary
compounds that exhibit a wide range of properties of both fundamental and
potential technological interest. The extensive tunability of the Heusler
compounds through chemical substitutions and structural motifs makes the family
especially interesting. In this article we highlight recent major developments
in the field of Heusler compounds and put these in the historical context. The
evolution of the Heusler compounds can be described by four major periods of
research. In the latest period, Heusler 4.0 has led to the observation of a
variety of properties derived from topology that includes: topological metals
with Weyl and Dirac points; a variety of non-collinear spin textures including
the very recent observation of skyrmions at room temperature; and giant
anomalous Hall effects in antiferromagnetic Heuslers with triangular magnetic
structures. Here we give a comprehensive overview of these major achievements
and set research into Heusler materials within the context of recent emerging
trends in condensed matter physics. | 1612.05947v2 |
2004-06-05 | Role of shuffles and atomic disorder in Ni-Mn-Ga | We report results of \textit{ab-initio} calculations of the ferromagnetic
Heusler alloy Ni-Mn-Ga. Particular emphasis is placed on the stability of the
low temperature tetragonal structure with $c/a = 0.94$. This structure cannot
be derived from the parent L2$_1$ structure by a simple homogeneous strain
associated with the soft elastic constant $C'$. In order to stabilise the
tetragonal phase, one has to take into account shuffles of atoms, which form a
wave-like pattern of atomic displacements with a well defined period
(modulation). While the modulation is related to the soft acoustic [110]-TA$_2$
phonon mode observed in Ni$_2$MnGa, we obtain additional atomic shuffles, which
are related to acoustic-optical coupling of the phonons in Ni$_2$MnGa. In
addition, we have simulated an off-stoichiometric systems, in which 25 % of Mn
atoms are replaced by Ni. The energy of this structure also exhibits a local
minimum at $c/a = 0.94$. This allows us to conclude that both shuffles and
atomic disorder stabilize the $c/a = 0.94$ structure. In both cases the
stability seems to be associated with a dip in the minority-spin density of
states (DOS) at the Fermi level, being related to the formation of hybrid
states of Ni-\textit{d} and Ga-\textit{p} minority-spin orbitals. | 0406139v1 |
2011-02-10 | An effective quantum parameter for strongly correlated metallic ferromagnets | The correlated motion of electrons in multi-orbital metallic ferromagnets is
investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital
degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J
using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum
parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained
which determines, in analogy with 1/S for quantum spin systems and 1/N for the
N-orbital Hubbard model, the strength of correlation-induced quantum
corrections to magnetic excitations. The rapid suppression of this quantum
parameter with Hund's coupling J, especially for large {\cal N}, provides
fundamental insight into the phenomenon of strong stabilization of metallic
ferromagnetism by orbital degeneracy and Hund's coupling. This approach is
illustrated for the case of ferromagnetic iron and the half metallic Heusler
alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness
and Curie temperature values obtained are in quantitative agreement with
measurements. Significantly, the contribution of long wavelength modes is shown
to yield a nearly ~25% reduction in the calculated Curie temperature. Finally,
an outline is presented for extending the approach to generic multi-band
metallic ferromagnets including realistic band-structure features of
non-degenerate orbitals and inter-orbital hopping as obtained from LDA
calculations. | 1102.2115v1 |
2011-10-10 | Ab-initio calculation of effective exchange interactions, spin waves, and Curie temperature in L2_1- and L1_2-type local moment ferromagnets | Employing first-principles electronic structure calculations in conjunction
with the frozen-magnon method we study the effective exchange interactions and
spin waves in local moment ferromagnets. As prototypes we have chosen three
L2$_1$-type full Heusler alloys Cu$_2$MnAl, Ni$_2$MnSn and Pd$_2$MnSn, and the
L1$_2$-type XPt$_3$ compounds with X= V, Cr and Mn. We have also included
CoPt$_3$ which is a usual ferromagnet. In all compounds due to the large
spatial separation ($\sim 4$ \AA) of the magnetic transition metal atoms, the
3\textit{d} states belonging to different atoms overlap weakly and as a
consequence the exchange coupling is indirect, mediated by the \textit{sp}
electrons. Calculated effective exchange parameters are long range and show
RKKY-type oscillations. The spin-wave dispersion curves are in reasonable
agreement with available experimental data. Using the calculated exchange
parameters we have estimated the Curie temperatures within both the mean-field
and the random-phase approximations. In local moment ferromagents deviations of
the estimated Curie temperature with respect to the available experimental data
occur when the ground-state electronic structure calculations overestimate the
values of the spin magnetic moments as in VPt$_3$. | 1110.2156v1 |
2013-10-23 | Magnetoelastic coupling induced magnetic anisotropy in Co$_2$(Fe/Mn)Si thin films | The influence of epitaxial strain on uniaxial magnetic anisotropy of
Co$_{2}$FeSi (CFS) and Co$_{2}$MnSi (CMS) Heusler alloy thin films grown on
(001) SrTiO$_3$ (STO) and MgO is reported. The in-plane biaxial strain is
susceptible to tune by varying the thickness of the films on STO, while on MgO
the films show in-plane easy axis for magnetization (\overrightarrow{M})
irrespective of their thickness. A variational analysis of magnetic free energy
functional within the Stoner-Wohlfarth coherent rotation model with
out-of-plane uniaxial anisotropy for the films on STO showed the presence of
magnetoelastic anisotropy with magnetostriction constant $\approx$
(12.22$\pm$0.07)$\times 10^{-6}$ and (2.02$\pm$0.06)$\times 10^{-6}$, in
addition to intrinsic magnetocrystalline anisotropy $\approx$ -1.72$\times
10^{6}$ erg/cm$^{3}$ and -3.94$\times 10^{6}$ erg/cm$^{3}$ for CFS and CMS,
respectively. The single-domain phase diagram reveals a gradual transition from
in-plane to out-of-plane orientation of magnetization with the decreasing film
thickness. A maximum canting angle of 41.5$^{\circ}$ with respect to film plane
is predicted for the magnetization of the thinnest (12 nm) CFS film on STO. The
distinct behaviour of \overrightarrow{M} in the films with lower thickness on
STO is attributed to strain-induced tetragonal distortion. | 1310.6204v1 |
2014-12-02 | Structural, electrical and magnetic properties of nanostructured Mn2Ni1.6Sn0.4 melt spun ribbons | Nanocrystalline ribbons of inverse Heusler alloy Mn2Ni1.6Sn0.4 have been
synthesised by melt spinning of the arc melted bulk precursor. The single phase
ribbons crystallize into a cubic structure and exhibit very fine crystallite
size of < 2 nm. Temperature dependent magnetization (M-T) measurements reveal
that austenite (A)-martensite (M) phase transition begins at T~248 K and
finishes at T~238 K during cooling cycle and these values increase to T~267 K
and T~259 K while warming. In cooling cycle, the A-phase shows ferromagnetic
(FM) ordering with a Curie temperature T~267 K, while both the
FM-antiferromagnetic (AFM) and M-transitions occur at T~242 K. The M-phase
undergoes FM transition at T~145 K. These transitions are also confirmed by
temperature dependent resistivity measurements. The observed hysteretic
behaviour of magnetization and resistivity in the temperature regime spanned by
the A-M transition is a manifestation of the first order phase transition.
Magnetization and susceptibility data also provide unambiguous evidence in
favour of spin glass . The scaling of the glass freezing temperature (Tf) with
frequency, extracted from the frequency dependent AC susceptibility
measurements, confirms the existence of canonical spin glass at T<145 K. The
occurrence of canonical spin glass has been explained in terms of the
nanostructuring modified interactions between the FM correlations in the
martensitic phase and the coexisting AFM. | 1412.0859v1 |
2015-01-16 | Direct measurement of the magnetic anisotropy field in Mn--Ga and Mn--Co--Ga Heusler films | The static and dynamic magnetic properties of tetragonally distorted Mn--Ga
based alloys were investigated. Static properties are determined in magnetic
fields up to 6.5~T using SQUID magnetometry. For the pure Mn$_{1.6}$Ga film,
the saturation magnetisation is 0.36~MA/m and the coercivity is 0.29~T. Partial
substitution of Mn by Co results in Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$. The
saturation magnetisation of those films drops to 0.2~MA/m and the coercivity is
increased to 1~T.
Time-resolved magneto-optical Kerr effect (TR-MOKE) is used to probe the
high-frequency dynamics of Mn--Ga. The ferromagnetic resonance frequency
extrapolated to zero-field is found to be 125~GHz with a Gilbert damping,
$\alpha$, of 0.019. The anisotropy field is determined from both SQUID and
TR-MOKE to be 4.5~T, corresponding to an effective anisotropy density of
0.81~MJ/m$^3$.
Given the large anisotropy field of the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film,
pulsed magnetic fields up to 60~T are used to determine the field strength
required to saturate the film in the plane. For this, the extraordinary Hall
effect was employed as a probe of the local magnetisation. By integrating the
reconstructed in--plane magnetisation curve, the effective anisotropy energy
density for Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ is determined to be 1.23~MJ/m$^3$. | 1501.03973v1 |
2015-11-25 | Accelerated screening of thermoelectric materials by first-principles computations of electron-phonon scattering | Recent discovery of new materials for thermoelectric energy conversion is
enabled by efficient prediction of materials' performance from
first-principles, without empirically fitted parameters. The novel simplified
approach for computing electronic transport properties is described, which
achieves good accuracy and transferability while greatly reducing complexity
and computation cost compared to the existing methods. The first-principles
calculations of the electron-phonon coupling demonstrate that the energy
dependence of the electron relaxation time varies significantly with chemical
composition and carrier concentration, suggesting that it is necessary to go
beyond the commonly used approximations to screen and optimize materials'
composition, carrier concentration, and microstructure. The new method is
verified using high accuracy computations and validated with experimental data
before applying it to screen and discover promising compositions in the space
of half-Heusler alloys. By analyzing data trends the effective electron mass is
identified as the single best general descriptor determining material's
performance. The Lorenz number is computed from first principles and the
universality of the Wiedemann-Franz law in thermoelectrics is discussed. | 1511.08115v7 |
2017-08-03 | Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co$_2$FeAl/MgO interface upon annealing | We investigate thickness series of films of the Heusler alloy Co$_2$FeAl in
order to study the effect of annealing on the interface with a MgO layer and on
the bulk magnetic properties. Our results reveal that while the perpendicular
interface anisotropy constant $K^{\perp}_{\rm S}$ is zero for the as-deposited
samples, its value increases with annealing up to a value of $1.14\, \pm
\,0.07$~mJ/m$^2$ for the series annealed at 320$^{\rm o}$C and of $2.07\, \pm
\,0.7$~mJ/m$^2$ for the 450$^{\rm o}$C annealed series owing to a strong
modification of the interface during the thermal treatment. This large value
ensures a stabilization of a perpendicular magnetization orientation for a
thickness below 1.7~nm. The data additionally shows that the in-plane biaxial
anisotropy constant has a different evolution with thickness in as-deposited
and annealed systems. The Gilbert damping parameter $\alpha$ shows minima for
all series for a thickness of 40~nm and an absolute minimum value of
$2.8\pm0.1\cdot10^{-3}$. The thickness dependence is explained in terms of an
inhomogenous magnetization state generated by the interplay between the
different anisotropies of the system and by crystalline disorder. | 1708.01126v2 |
2018-10-23 | Uncovering the puzzle of complex magnetism in Fe16N2: a first-principles based study | The electronic structure and magnetic exchange interactions in pure and
V-doped Fe16N2 are studied within the framework of density functional theory.
The Curie temperatures were obtained with both mean field approximation (MFA)
as well as Monte Carlo (MC) calculations. The Curie temperature (TC) for pure
Fe16N2 obtained within MFA are significantly larger than the experimental
value, suggesting the importance of thermal fluctuations in these systems, and
has a resemblance of a lower dimensional spin system. We also briefly discuss
about the various possible factors which may lead to a large magnetic moment in
this material. The calculated magnetic susceptibility at zero field shows sharp
peak at T=TC which resemble a local moment system. From the nature of exchange
interactions we try to figure out the nature of the Fesites which might contain
localized d-states. Finally, we point out that Fe16N2 can also act as a good
spin injector for the III-V semiconductors in addition to its well promised
application as permanent magnet since it has a very high spin polarization
(larger compared to elemental ferromagnets) as well as quite smaller lattice
mismatch (compared to half-metallic Heusler alloys) with the conventional III-V
semiconductors such as GaAs or InGaAs. We further demonstrate this through our
calculations for Fe16N2(001)/InGaAs(001) heterostructures which shows the
non-negligible spin polarization in the semiconductor (InGaAs) region implying
a long spin diffusion length. | 1810.09818v1 |
2019-07-24 | Nonlinear anomalous Hall effect for Néel vector detection | Antiferromagnetic (AFM) spintronics exploits the N\'eel vector as a state
variable for novel spintronic devices. Recent studies have shown that the
field-like and antidamping spin-orbit torques (SOT) can be used to switch the
N\'eel vector in antiferromagnets with proper symmetries. However, the precise
detection of the N\'eel vector remains a challenging problem. In this letter,
we predict that the nonlinear anomalous Hall effect (AHE) can be used to detect
the N\'eel vector in most compensated antiferromagnets supporting the
antidamping SOT. We show that the magnetic crystal group symmetry of these
antiferromagnets combined with spin-orbit coupling produce a sizable Berry
curvature dipole and hence the nonlinear AHE. As a specific example, we
consider half-Heusler alloy CuMnSb, which N\'eel vector can be switched by the
antidamping SOT. Based on density functional theory calculations, we show that
the nonlinear AHE in CuMnSb results in a measurable Hall voltage under
conventional experimental conditions. The strong dependence of the Berry
curvature dipole on the N\'eel vector orientation provides a new detection
scheme of the N\'eel vector based on the nonlinear AHE. Our predictions enrich
the material platform for studying non-trivial phenomena associated with the
Berry curvature and broaden the range of materials useful for AFM spintronics. | 1907.10696v3 |
2020-05-10 | Phase stability and the effect of lattice distortions on electronic properties and half-metallic ferromagnetism of Co2FeAl Heusler alloy: An ab initio study | Density functional theory calculations within the generalized gradient
approximation are employed to study the ground state of Co2FeAl. Various
magnetic configurations are considered to find out its most stable phase. The
ferromagnetic ground state of the Co2FeAl is energetically observed with an
optimized lattice constant of 5.70 {\AA}. Thereafter, the system was subjected
under uniform and non-uniform strains to see their effects on spin polarization
(P) and half-metallicity. The effect of spin orbit coupling is considered in
the present study. Half-metallicity (and 100 % P) is only retained under
uniform strains started from 0 to +4%, and dropped rapidly from 90% to 16% for
the negative strains started from -1% to -6%. We find that the present system
is much sensitive under tetragonal distortions as half-metallicity (and 100% P)
is preserved only for the cubic case. The main reason for the loss of
half-metallicity is due to the shift of the bands with respect to the Fermi
level. We also discuss the influence of these results on spintronics devices. | 2005.04634v2 |
2020-06-04 | Material Descriptors for the Discovery of Efficient Thermoelectrics | The predictive performance screening of novel compounds can significantly
promote the discovery of efficient, cheap, and non-toxic thermoelectric
materials. Large efforts to implement machine-learning techniques coupled to
materials databases are currently being undertaken, but the adopted
computational methods can dramatically affect the outcome. With regards to
electronic transport and power factor calculations, the most widely adopted and
computationally efficient method, is the constant relaxation time approximation
(CRT). This work goes beyond the CRT and adopts the proper, full energy and
momentum dependencies of electron-phonon and ionized impurity scattering, to
compute the electronic transport and perform power factor optimization for a
group of half-Heusler alloys. Then the material parameters that determine the
optimal power factor based on this more advanced treatment are identified. This
enables the development of a set of significantly improved descriptors that can
be used in materials screening studies, and which offer deeper insights into
the underlying nature of high performance thermoelectric materials. We have
identified $n_v$$\epsilon_r$ / $D_o^2m_{cond}$ as the most useful and generic
descriptor, a combination of the number of valleys, the dielectric constant,
the conductivity effective mass, and the deformation potential for the dominant
electron-phonon process. The proposed descriptors can accelerate the discovery
of new efficient and environment friendly thermoelectric materials in a much
more accurate and reliable manner, and some predictions for very high
performance materials are presented. | 2006.02789v2 |
2022-06-06 | Quantitative theory of magnetic interactions in solids | In this report we review the method of explicit calculations of interatomic
exchange interactions of magnetic materials. This involves exchange mechanisms
normally referred to as Heisenberg exchange, Dzyaloshinskii-Moriya interaction
and anisotropic symmetric exchange. The connection between microscopic theories
of the electronic structure, such as density functional theory or dynamical
mean field theory, and interatomic exchange, is given in detail. The different
aspects of extracting information for an effective spin Hamiltonian that
involves thousands of atoms, from electronic structure calculations considering
significantly fewer atoms (1-50) is highlighted. Examples of exchange
interactions of a large group of materials is presented, which involves heavy
elements of the 3d period, alloys between transition metals, Heusler compounds,
multilayer systems as well as overlayers and adatoms on a substrate, transition
metal oxides, 4f elements, magnetic materials in two dimensions and molecular
magnets. Where possible, a comparison to experimental data is made, that
naturally becomes focused on the magnon dispersion. The influence of relativity
is reviewed for a few cases, as is the importance of dynamical correlations.
Development to theories that handle out of equilibrium conditions is also
described here. The review ends with a short description of extensions of the
theories behind explicit calculations of interatomic exchange, to non-magnetic
situations, e.g. that describe chemical (charge) order and superconductivity. | 2206.02415v2 |
2022-08-09 | Structural, magnetic and transport properties of Co$_2$CrAl epitaxial thin films | We report the physical properties of Co$_2$CrAl Heusler alloy epitaxial thin
films grown on single crystalline MgO(001) substrate using pulsed laser
deposition technique. The x-ray diffraction pattern in $\theta$-2$\theta$ mode
showed the film growth in single phase B2-type ordered cubic structure with the
presence of (002) and (004) peaks, and the film oriented along the MgO(001)
direction. The $\phi$~scan along the (220) plane confirms the four-fold
symmetry and the epitaxial growth relation found to be
Co$_2$CrAl(001)[100]$\vert$$\vert$MgO(001)[110]. The thickness of about 12~nm
is extracted through the analysis of x-ray reflectivity data. The isothermal
magnetization (M--H) curves confirm the ferromagnetic (FM) nature of the thin
film having significant hysteresis at 5 and 300~K. From the in-plane M--H
curves, the saturation magnetization values are determined to be
2.1~$\mu$$_{\rm B}$/f.u.~at 5~K and 1.6~$\mu$$_{\rm B}$/f.u. at 300~K, which
suggests the soft FM behavior in the film having the coercive field $\approx$
522~Oe at 5~K. The thermo-magnetization measurements at 500~Oe magnetic field
show the bifurcation between field-cooled and zero-field-cooled curves below
about 100~K. The normalized field-cooled magnetization curve follows the T$^2$
dependency, and the analysis reveal the Curie temperature around 335$\pm$11~K.
Moreover, the low-temperature resistivity indicates semiconducting behavior
with the temperature, and we find a negative temperature coefficient of
resistivity (5.2 $\times$ 10$^{-4}$ /K). | 2208.04687v1 |
2023-01-13 | Spin and current transport in the robust half-metallic magnet $c$-CoFeGe | Spintronics is an emerging form of electronics based on the electrons' spin
degree of freedom for which materials with robust half-metallic ferromagnet
(HMF) character are very attractive. Here we determine the structural
stability, electronic, magnetic, and mechanical properties of the half-Heusler
(hH) compound CoFeGe, in particular also in its cubic form. The
first-principles calculations suggest that the electronic structure is robust
with 100 \% spin polarization at the Fermi level under hydrostatic pressure and
uni-axial strain. Both the longitudinal and Hall current polarization are
calculated and the longitudinal current polarization ($P_{L}$) is found to be
$>99\%$ and extremely robust under uniform pressure and uni-axial strain. The
anomalous Hall conductivity (AHC) and Spin Hall conductivity (SHC) of hH cubic
CoFeGe (\textit{c}-CoFeGe) are found to be $\sim -100$ S/cm and $\sim
39~\hbar/e$ S/cm, respectively. Moreover, the Curie temperature of the alloy is
calculated to be $\sim$524 K with a 3 $\mu_{B}$ magnetic moment. Lastly, the
calculated mechanical properties indicate that \textit{c}-CoFeGe is ductile and
mechanically stable with a bulk modulus of $\approx$ 154 GPa. Overall, this
analysis reveals that cubic CoFeGe is a robust half-metallic ferromagnet and an
interesting material for spintronic applications. | 2301.05493v1 |
2023-10-05 | Exotic rare earth-based materials for emerging spintronic technology | The progress in materials science has always been associated with the
development of functional materials systems, which enables us to design
proof-of-concept devices. To advance further, theoretical predictions of new
novel materials and their experimental realization is very important. This
chapter reviews the intriguing properties of rare earth-based materials and
their applications in spintronics. Spintronics is an emerging technology, which
exploits spin degree of freedom of an electron along with its charge property.
Discovery of various physical phenomena and their industrial applications in
the field of magnetic sensors, magnetic recording and non-volatile memories
such as magnetic random access memory (MRAM) and spin-transfer torque (STT)
MRAM opens several new directions in this field. Materials with large spin
polarization, strong spin-orbit coupling, and tunable electronic and magnetic
properties offer an excellent platform for the spintronics technology.
Combination of rare earths with other elements such as transition metals show
broad range of structural, electronic, and magnetic properties which make them
excellent candidates for various spintronic applications. This chapter
discusses many such materials ranging from Heusler alloys, topological
insulators to two-dimensional ferromagnets and their potential applications.
The review gives an insight of how rare-earth materials can play a key role in
emerging future technology and have great potential in many new spintronic
related applications. | 2310.03541v1 |
2023-11-07 | The role of electronic bandstructure shape in improving the thermoelectric power factor of complex materials | The large variety of complex electronic structure materials and their alloys,
offer highly promising directions for improvements in thermoelectric (TE) power
factors (PF). Their electronic structure contains rich features, referred to as
'surface complexity', one of them being the highly anisotropic warped energy
surface shapes with elongated features and threads in some cases. In this work
we use Boltzmann transport simulations to quantify the influence that the shape
of the electronic structure energy surfaces has on the PF. Using both
analytical ellipsoidal bands, as well as realistic bands from the group of
half-Heuslers, we show that band shape complexity alone can offer an advantage
to the PF of ~3x in realistic cases. The presence of anisotropic scattering
mechanisms such as ionized impurity or polar optical phonon scattering,
however, can reduce these improvements by up to ~50%. We show that expressions
based on the simple ratio of the density-of-states to the conductivity
effective masses, mDOS/mC, together with the number of valleys, can capture the
anisotropy shape with a moderate to high degree of correlation. For this, we
use a convenient way to extract these masses by mapping the complex
bandstructures of materials to parabolic electronic structures, without the
need for Boltzmann transport codes. Despite the fact that the PF depends on
many parameters, information about the benefits of the band shape alone, would
be very useful for identifying and understanding the performance of novel
thermoelectric materials. | 2311.03935v1 |
2014-09-23 | Guidelines for understanding cubic manganese-rich Heusler compounds | Manganese-rich Heusler compounds are attracting much interest in the context
of spin transfer torque and rare-earth free hard magnets. Here we give a
comprehensive overview of the magnetic properties of non-centrosymmetric cubic
Mn$_2$-based Heusler materials, which are characterized by an antiparallel
coupling of magnetic moments on Mn atoms. Such a ferrimagnetic order leads to
the emergence of new properties that are absent in ferromagnetic
centrosymmetric Heusler structures. In terms of the band structure
calculations, we explain the formation of this magnetic order and the Curie
temperatures. This overview is intended to establish guidelines for a basic
understanding of magnetism in Mn2 -based Heusler compounds. | 1409.6532v1 |
2012-02-17 | Insights into ultrafast demagnetization in pseudo-gap half metals | Interest in femtosecond demagnetization experiments was sparked by Bigot's
discovery in 1995. These experiments unveil the elementary mechanisms coupling
the electrons' temperature to their spin order. Even though first quantitative
models describing ultrafast demagnetization have just been published within the
past year, new calculations also suggest alternative mechanisms.
Simultaneously, the application of fast demagnetization experiments has been
demonstrated to provide key insight into technologically important systems such
as high spin polarization metals, and consequently there is broad interest in
further understanding the physics of these phenomena. To gain new and relevant
insights, we perform ultrafast optical pump-probe experiments to characterize
the demagnetization processes of highly spin-polarized magnetic thin films on a
femtosecond time scale. Previous studies have suggested shifting the Fermi
energy into the center of the gap by tuning the number of electrons and thereby
to study its influence on spin-flip processes. Here we show that choosing
isoelectronic Heusler compounds (Co2MnSi, Co2MnGe and Co2FeAl) allows us to
vary the degree of spin polarization between 60% and 86%. We explain this
behavior by considering the robustness of the gap against structural disorder.
Moreover, we observe that Co-Fe-based pseudo gap materials, such as partially
ordered Co-Fe-Ge alloys and also the well-known Co-Fe-B alloys, can reach
similar values of the spin polarization. By using the unique features of these
metals we vary the number of possible spin-flip channels, which allows us to
pinpoint and control the half metals electronic structure and its influence
onto the elementary mechanisms of ultrafast demagnetization. | 1202.3874v1 |
2018-01-06 | Development of half metallicity within mixed magnetic phase of Cu$_{1-x}$Co$_x$MnSb alloy | Cubic Half-Heusler Cu$_{1-x}$Co$_x$MnSb (0 $\leq$ $x$ $\leq$ 0.1) compounds
have been investigated both experimentally and theoretically for their
magnetic, transport and electronic properties in search of possible half
metallic antiferromagnetism. The systems (Cu,Co)MnSb are of particular interest
as the end member alloys CuMnSb and CoMnSb are semi metallic (SM)
antiferromagnetic (AFM) and half metallic (HM) ferromagnetic (FM),
respectively. Clearly, Co-doping at the Cu-site of CuMnSb introduces changes in
the carrier concentration at the Fermi level that may lead to half-metallic
ground state but there remains a persistent controversy whether the AFM to FM
transition occurs simultaneously. Our experimental results reveal that the AFM
to FM magnetic transition occurs through a percolation mechanism where
Co-substitution gradually suppresses the AFM phase and forces FM polarization
around every dopant cobalt. As a result a mixed magnetic phase is realized
within this composition range while a nearly HM band structure is developed
already at the 10% Co-doping. Absence of T$^2$ dependence in the resistivity
variation at low T-region serves as an indirect proof of opening up an energy
gap at the Fermi surface in one of the spin channels. This is further
corroborated by the ab-initio electronic structure calculations that suggests a
nearly ferromagnetic half-metallic ground state is stabilized by Sb-p holes
produced upon Co doping. | 1801.02035v1 |
2013-01-12 | Effect of Co-Fe substitutions on the room-temperature spin polarization in Co_3-xFe_xSi Heusler-compound films | Using low-temperature molecular beam epitaxy, we study substitutions of Fe
atoms for Co ones in Co_3-xFe_xSi Heusler-compound films grown on Si and Ge.
Even for the low-temperature grown Heusler-compound films, the Co-Fe atomic
substitution at A and C sites can be confirmed by the conversion electron
M"ossbauer spectroscopy measurements. As a result, the magnetic moment and
room-temperature spin polarization estimated by nonlocal spin-valve
measurements are systematically changed with the Co-Fe substitutions. This
study experimentally verified that the Co-Fe substitution in Co_3-xFe_xSi
Heusler compounds can directly affect the room-temperature spin polarization. | 1301.2645v1 |
2013-01-09 | New iron-based Heusler compounds Fe2YZ: Comparison with theoretical predictions of the crystal structure and magnetic properties | The present work reports on the new soft ferromagnetic Heusler phases
Fe2NiGe, Fe2CuGa, and Fe2CuAl, which in previous theoretical studies have been
predicted to exist in a tetragonal regular Heusler structure. Together with the
known phases Fe2CoGe and Fe2NiGa these materials have been synthesized and
characterized by powder XRD, 57 Fe M\"ossbauer spectroscopy, SQUID and EDX
measurements. In particular M\"ossbauer spectroscopy was used to monitor the
degree of local atomic order/disorder and to estimate magnetic moments at the
Fe sites from the hyperfine fields. It is shown that in contrast to the
previous predictions all the materials except Fe2NiGa basically adopt the
inverse cubic Heusler (X-) structure with differing degrees of disorder. The
disorder is more enhanced in case of Fe2NiGa, which was predicted as an inverse
Heusler phase. The experimental data are compared with results from ab-inito
electronic structure calculations on LDA level incorporating the effects of
atomic disorder by using the coherent potential approximation (CPA). A good
agreement between calculated and experimental magnetic moments is found for the
cubic inverse Heusler phases. Model calculations on various atomic
configurations demonstrate that antisite disorder tends to enhance the
stability of the X-structure. Given the fundamental scientific and
technological importance of tetragonal Heusler phases the present results call
for further investigations to unravel the factors stabilizing tetragonal
Heusler materials. | 1301.1988v1 |
2019-01-28 | An Enormous Class of Double Half-Heusler Compounds with Low Thermal Conductivity | Since their discovery around a century ago, the structure and chemistry of
the multi-functional half-Heusler semiconductors have been studied extensively
as three component systems. The elemental groups constituting these ternary
compounds with the nominal formula XYZ are well established. From the very same
set of well-known elements we explore a phase space of quaternary double
($X'X''Y_2Z_2$, $X_2Y'Y''Z_2$, and $X_2Y_2Z'Z''$), triple ($X_2'X''Y_3Z_3$) and
quadruple ($X_3'X''Y_4Z_4$) half-Heusler compositions which 10 times larger in
size. Using a reliable, first-principles thermodynamics methodology on a
selection of 347 novel compositions, we predict 127 new stable quaternary
compounds, already more than the 89 reported almost exhaustively for ternary
systems. Thermoelectric performance of the state-of-the-art ternary
half-Heusler compounds are limited by their intrinsically high lattice thermal
conductivity ($\kappa_{L}$). In comparison to ternary half-Heuslers, thermal
transport in double half-Heuslers is dominated by low frequency phonon modes
with smaller group velocities and limited by disorder scattering. The double
half-Heusler composition Ti$_2$FeNiSb$_2$ was synthesized and confirmed to have
a significantly lower lattice thermal conductivity (factor of 3 at room
temperature) than TiCoSb, thereby providing a better starting point for
thermoelectric efficiency optimization. We demonstrate a dependable strategy to
assist the search for low thermal conductivity half-Heuslers and point towards
a huge composition space for implementing it. Our findings can be extended for
systematic discovery of other large families of multi-component intermetallic
semiconductors. | 1901.09800v1 |
2013-01-25 | All-Heusler giant-magnetoresistance junctions with matched energy bands and Fermi surfaces | We present an all-Heusler architecture which could be used as a rational
design scheme for achieving high spin-filtering efficiency in the
current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices. A
Co2MnSi/Ni2NiSi/Co2MnSi trilayer stack is chosen as the prototype of such an
architecture, of which the electronic structure and magnetotransport properties
are systematically investigated by first principles approaches. Almost
perfectly matched energy bands and Fermi surfaces between the all-Heusler
electrode-spacer pair are found, indicating large interfacial spin-asymmetry,
high spin-injection efficiency, and consequently high GMR ratio. Transport
calculations further confirms the superiority of the all-Heusler architecture
over the conventional Heusler/transition-metal(TM) structure by comparing their
transmission coefficients and interfacial resistances of parallel conduction
electrons, as well as the macroscopic current-voltage (I-V) characteristics. We
suggest future theoretical and experimental efforts in developing novel
all-Heusler GMR junctions for the read heads of the next generation
high-density hard disk drives (HDDs). | 1301.6106v1 |
2018-02-13 | Designing and discovering a new family of semiconducting quaternary Heusler compounds based on the 18-electron rule | Intermetallic compounds with sizable band gaps are attractive for their
unusual properties but rare. Here, we present a new family of stable
semiconducting quaternary Heusler compounds, designed and discovered by means
of high-throughput \textit{ab initio} calculations based on the 18-electron
rule. The 99 new semiconductors reported here adopt the ordered quaternary
Heusler structure with the prototype of LiMgSnPd (F$\bar{\mathbf{4}}$3m,
No.\,216) and contain 18 valence electrons per formula unit. They are realized
by filling the void in the half Heusler structure with a small and
electropositive atom, i.e., lithium. These new stable quaternary Heusler
semiconductors possess a range of band gaps from 0.3 to 2.5\,eV, and exhibit
some unusual properties different from conventional semiconductors, such as
strong optical absorption, giant dielectric screening, and high Seebeck
coefficient, which suggest these semiconductors have potential applications as
photovoltaic and thermoelectric materials. While this study opens up avenues
for further exploration of this novel class of semiconducting quaternary
Heuslers, the design strategy used herein is broadly applicable across a
potentially wide array of chemistries to discover new stable materials. | 1802.04875v1 |
2019-05-09 | Half-Heusler Compounds: Promising Materials For Mid-To-High Temperature Thermoelectric Conversion | Half-Heusler compounds (space group Fm3m) has garnered increasing attention
in recent years in the thermoelectric community. Three decades ago, refractory
RNiSn half-Heusler compounds (R represents refractory metals such as Hf, Zr,
Ti) were found to be narrow-gap semiconductors with large Seebeck coefficients
in 100s of micro-volt per Kelvin. Today, half-Heusler (HH) compounds have
emerged as promising thermoelectric materials in the intermediate temperature
range (400-800oC). HH materials are endowed with good thermal stability and
scalability. Thermoelectric n-p modules based on HH materials demonstrate
conversion efficiency near 10% and power density output near 9 W/cm2. The
objective of this article is to present a historical account of the research
and development of thermoelectric half-Heusler compounds. Particularly, there
have been notable achievements since 2012 thanks to the emergence of new
approaches. As a result, ZT has risen from ~1 to 1.5. The various advances made
since the early 1990s to the present are recounted by categorizing half-Heusler
materials into three generations (Gen): Gen-1 Gen-2, and Gen-3 HH materials. | 1905.03845v1 |
2013-11-21 | Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloy as magnetic electrodes | HRTEM, nano-beam electronic diffraction, energy dispersive X-rays scanning
spectroscopy, Vibrating Sample Magnetometry (VSM) and FerroMagnetic Resonance
(FMR) techniques are used in view of comparing (static and dynamic) magnetic
and structural properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel
magnetic junctions (TMJ), deposited on various single crystalline substrates
(a-plane sapphire, MgO(100) and Si(111)). They allow for providing a
correlation between these magnetic properties and the fine structure
investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and
contains a large concentration of Co atoms, which, however, is significantly
reduced when using a sapphire substrate. The Co layer is polycrystalline and
shows larger grains for films grown on a sapphire substrate. The VSM
investigation reveals in-plane anisotropy only for samples grown on a sapphire
substrate. The FMR spectra of the TMJs are compared to the obtained ones with a
single Co and Co2MnGe films of identical thickness deposited on a sapphire
substrate. As expected, two distinct modes are detected in the TMJs while only
one mode is observed in each single film. For the TMJ grown on a sapphire
substrate the FMR behavior does not significantly differ from the superposition
of the individual spectra of the single films, allowing for concluding that the
exchange coupling between the two magnetic layers is too small to give rise to
observable shifts. For TMJs grown on a Si or on a MgO substrate the resonance
spectra reveal one mode which is nearly identical to the obtained one in the
single Co film, while the other observed resonance shows a considerably smaller
intensity and cannot be described using the magnetic parameters appropriate to
the single Co2MnGe film. | 1311.5589v1 |
2019-07-25 | Superconducting switching due to triplet component in the Pb/Cu/Ni/Cu/Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ spin-valve structure | We report the superconducting properties of the
Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$/Cu/Ni/Cu/Pb spin-valve structure which magnetic
part comprises the Heusler alloy layer HA = Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ with a
high degree of spin polarization (DSP) of the conduction band and the Ni layer
of variable thickness. We obtained that the separation between the
superconducting transition curves measured for the parallel ($\alpha =
0^\circ$) and perpendicular ($\alpha = 90^\circ$) orientation of the
magnetizations of the HA and Ni layers reaches up to 0.5 K ($\alpha$ is an
angle between the magnetization of two ferromagnetic layers). For all studied
samples the dependence of the superconducting transition temperature $T_c$ on
$\alpha$ demonstrates a deep minimum in the vicinity of the perpendicular
configuration of magnetizations. This suggests that the observed minimum and
the corresponding full switching effect of the spin valve is caused by the
long-range triplet component of the superconducting condensate in the
multilayer. Such a large effect can be attributed to a half-metallic nature of
the HA layer which in the orthogonal configuration efficiently draws off the
spin-polarized Cooper pairs from the space between the HA and Ni layers. Our
results indicate a significant potential of the concept of the superconducting
spin-valve multilayer comprising a half-metallic ferromagnet recently proposed
by A. Singh et al., Phys. Rev. X 5, 021019 (2015) in achieving large values of
the switching effect. | 1907.11176v2 |
2021-01-25 | Computing and Memory Technologies based on Magnetic Skyrmions | Solitonic magnetic excitations such as domain walls and, specifically,
skyrmionics enable the possibility of compact, high density,
ultrafast,all-electronic, low-energy devices, which is the basis for the
emerging area of skyrmionics. The topological winding of skyrmion spins affects
their overall lifetime, energetics and dynamical behavior. In this review, we
discuss skyrmionics in the context of the present day solid state memory
landscape, and show how their size, stability and mobility can be controlled by
material engineering, as well as how they can be nucleated and detected.
Ferrimagnetsnear their compensation points are important candidates for this
application, leading to detailed exploration of amorphous CoGd as well as the
study of emergent materials such as Mn$_4$N and Inverse Heusler alloys. Along
with material properties, geometrical parameters such as film thickness, defect
density and notches can be used to tune skyrmion properties, such as their size
and stability. Topology, however, can be a double-edged sword, especially for
isolated metastable skyrmions, as it brings stability at the cost of additional
damping and deflective Magnus forces compared to domain walls. Skyrmion
deformation in response to forces also makes them intrinsically slower than
domain walls. We explore potential analog applications of skyrmions, including
temporal memory at low density, and decorrelator for stochastic computing at a
higher density that capitalizes on their interactions. We summarize the main
challenges to achieve a skyrmionics technology, including maintaining
positional stability with very high accuracy, electrical readout, especially
for small ferrimagnetic skyrmions, deterministic nucleation and annihilation,
and overall integration with digital circuits with the associated circuit
overhead. | 2101.09947v4 |
2021-02-26 | Role of chemical disorder in tuning the Weyl points in vanadium doped Co$_2$TiSn | The lack of time-reversal symmetry and Weyl fermions give exotic transport
properties to Co-based Heusler alloys. In the present study, we have
investigated the role of chemical disorder on the variation of Weyl points in
Co\textsubscript{2}Ti\textsubscript{1-x}V\textsubscript{x}Sn magnetic Weyl
semimetal candidate. We employ the first principle approach to track the
evolution of the nodal lines responsible for the appearance of Weyl node in
Co$_2$TiSn as a function of V substitution in place of Ti. By increasing the V
concentration in place of Ti, the nodal line moves toward Fermi level and
remains at Fermi level around the middle composition. Further increase of the V
content, leads shifting of nodal line away from Fermi level. Density of state
calculation shows half-metallic behavior for the entire range of composition.
The magnetic moment on each Co atom as a function of V concentration increases
linearly up to x=0.4, and after that, it starts decreasing. We also
investigated the evolution of the Weyl nodes and Fermi arcs with chemical
doping. The first-principles calculations reveal that via replacing almost half
of the Ti with V, the intrinsic anomalous Hall conductivity increased twice as
compared to the undoped composition. Our results indicate that the composition
close to the 50\% V doped Co$_2$TiSn, will be an ideal composition for the
experimental investigation of Weyl physics. | 2102.13389v2 |
2022-09-30 | Coexisting structural disorder and robust spin-polarization in half-metallic FeMnVAl | Half-metallic ferromagnets (HMF) are on one of the most promising materials
in the field of spintronics due to their unique band structure consisting of
one spin sub-band having metallic characteristics along with another sub-band
with semiconductor-like behavior. In this work, we report the synthesis of a
novel quaternary Heusler alloy FeMnVAl and have studied the structural,
magnetic, transport, and electronic properties complemented with
first-principles calculations. Among different possible structurally ordered
arrangements, the optimal structure is identified by theoretical energy
minimization. The corresponding spin-polarized band structure calculations
indicates the presence of a half-metallic ferromagnetic ground state. A
detailed and careful investigation of the x-ray diffraction data, M\"{o}ssbauer
and nuclear magnetic resonance spectra suggest the presence of site-disorder
between the Fe and Mn atoms in the stable ordered structure of the system. The
magnetic susceptibility measurement clearly establishes a ferromagnetic-like
transition below $\sim$213 K. The ${^{57}}$Fe M\"{o}ssbauer spectrometry
measurements suggest only the Mn-spins could be responsible for the magnetic
order, which is consistent with our theoretical calculation. Surprisingly, the
density-functional-theory calculations reveal that the spin-polarization value
is almost immunized (92.4\% ${\rightarrow}$ 90.4\%) from the Mn-Fe structural
disorder, even when nonmagnetic Fe and moment carrying Mn sites are entangled
inseparably. Robustness of spin polarization and half metallicity in the
studied FeMnVAl compound comprising structural disorder is thus quite
interesting and could provide a new direction to investigate and understand the
exact role of disorders on spin polarization in these class of materials, over
the available knowledge. | 2209.15243v1 |
2022-11-25 | Two-band conduction as a pathway to non-linear Hall effect and unsaturated negative magnetoresistance in the martensitic compound GdPd2Bi | The present work aims to address the electronic and magnetic properties of
the intermetallic compound GdPd$_2$Bi through a comprehensive study of the
structural, magnetic, electrical and thermal transport on a polycrystalline
sample, followed by theoretical calculations. Our findings indicate that the
magnetic ground state is antiferromagnetic in nature. Magnetotransport data
present prominent hysteresis loop hinting a structural transition with further
support from specific heat and thermopower measurements, but no such signature
is observed in the magnetization study. Temperature dependent powder x-ray
diffraction measurements confirm martensitic transition from the
high-temperature (HT) cubic Heusler $L2_1$ structure to the low-temperature
(LT) orthorhombic $Pmma$ structure similar to many previously reported shape
memory alloys. The HT to LT phase transition is characterized by a sharp
increase in resistivity associated with prominent thermal hysteresis. Further,
we observe robust Bain distortion between cubic and orthorhombic lattice
parameters related by $a_{orth} = \sqrt{2}a_{cub}$, $b_{orth} = a_{cub}$ and
$c_{orth} = a_{cub}/\sqrt{2}$, that occurs by contraction along $c$-axis and
elongation along $a$-axis respectively. The sample shows an unusual
`non-saturating' $H^2$-dependent negative magnetoresistance for magnetic field
as high as 150 kOe. In addition, non-linear field dependence of Hall
resistivity is observed below about 30 K, which coincides with the sign change
of the Seebeck coefficient. The electronic structure calculations confirm
robust metallic states both in the LT and HT phases. It indicates complex
nature of the Fermi surface along with the existence of both electron and hole
charge carriers. The anomalous transport behaviors can be related to the
presence of both electron and hole pockets. | 2211.13982v1 |
2023-01-14 | CoRuVSi: A potential candidate for spin semimetal with promising spintronic and thermoelectric properties | Based on our experimental and theoretical studies, we report the
identification of the quaternary Heusler alloy, CoRuVSi as a new member of the
recently discovered spin semimetals class. Spin polarised semimetals possess a
unique band structure in which one of the spin bands shows semimetallic nature,
while the other shows semiconducting/insulating nature. Our findings show that
CoRuVSi possesses interesting spintronic and thermoelectric properties.
Magnetization data reveal a weak ferri-/antiferro magnetic ordering at low
temperatures, with only a very small moment $\sim$ 0.13 $\mu_B$/f.u.,
attributed to the disorder. Transport results provide strong evidence of
semimetallicity dominated by two-band conduction, while magnetoresistance data
show a non-saturating, linear, positive, magnetoresistance. Spin polarization
measurements using point-contact Andreev reflection spectra reveal a reasonably
high spin polarization of $\sim$ 50\%, which matches fairly well with the
simulated result. Furthermore, CoRuVSi shows a high thermopower value of $0.7$
$m Watt/ m-K^{2}$ at room temperature with the dominant contribution from the
semimetallic bands, rendering it as a promising thermoelectric material as
well. Our ab-initio simulation not only confirms a unique semimetallic feature,
but also reveals that the band structure hosts a linear band crossing at $\sim$
-0.4 eV below the Fermi level incorporated by a band-inversion. In addition,
the observed topological non-trivial features of the band structure is
corroborated with the simulated Berry curvature, intrinsic anomalous Hall
conductivity and the Fermi surface. The coexistence of many interesting
properties relevant for spintronic, topological and thermoelectric applications
in a single material is extremely rare and hence this study could promote a
similar strategy to identify other potential materials belonging to same class. | 2301.05854v1 |
2018-02-11 | Heusler, Weyl, and Berry | Heusler materials, initially discovered by Fritz Heusler more than a century
ago, have grown into a family of more than 1000 compounds, synthesized from
combinations of more than 40 elements. These materials show a wide range of
properties, but new properties are constantly being found. Most recently, by
incorporating heavy elements that can give rise to strong spin-orbit coupling
(SOC), non-trivial topological phases of matter, such as topological insulators
(TIs), have been discovered in Heusler materials. Moreover, the interplay of
symmetry, SOC and magnetic structure allows for the realization of a wide
variety of topological phases through Berry curvature design. Weyl points and
nodal lines can be manipulated by various external perturbations, which results
in exotic properties such as the chiral anomaly, and large anomalous spin and
topological Hall effects. The combination of a non-collinear magnetic structure
and Berry curvature gives rise a non-zero anomalous Hall effect, which was
first observed in the antiferromagnets Mn3Sn and Mn3Ge. Besides this k-space
Berry curvature, Heusler compounds with non-collinear magnetic structures also
possess real-space topological states in the form of magnetic antiskyrmions,
which have not yet been observed in other materials. The possibility of
directly manipulating the Berry curvature shows the importance of understanding
both the electronic and magnetic structures of Heusler compounds. Together,
with the new topological viewpoint and the high tunability, novel physical
properties and phenomena await discovery in Heusler compounds. | 1802.03771v1 |
2009-01-12 | Screening and Fabrication of Half-Heusler phases for thermoelectric applications | Half-Heusler phases have gained recently much interest as thermoelectric
materials. Screening of possible systems was performed by ab-initio simulation
using VASP-software. The energy-versus-Volume (E(V)) curves were calculated and
calibrated. For TiCoSb, NbNiSn, FeMoSb the stability of Half-Heusler phase
against concurrent crystal structures like TiNiSi, ZrCoAl, ZrBeSi, FeSiV, ZrNiP
and Full Heusler was confirmed. However, the thermo-dynamical driving force as
calculated from the difference in lattice energies is less than 0.1eV/atom.
Hence, the fabrication of Half Heusler phases is a challenge and requires three
steps, surface activation of the raw material by ball milling, arc-melting of
pressed pellets and finally long-term annealing treatment in a vacuum furnace.
On doped TiCoSb specimens, Seebeck coefficients up to 0.1 mV/K, on NiNbSn 0.16
mV/K were measured, although the microstructure was not yet optimized. | 0901.1491v3 |
2013-04-01 | First-principles investigation of half-metallic ferromagnetism of half-Heusler compounds XYZ | We investigate the electronic structure and magnetism of half-Heusler
compounds XYZ (X, Y=V, Cr, Mn, Fe, Co and Ni; Z=Al, Ga, In, Si, Ge, Sn, P, As,
and Sb) using the ab initio density functional theory calculations. Nine
half-metals with half-Heusler structure have been predicted with the
half-metallic gap of 0.07-0.67 eV. The calculations show that the formation
energies for these nine half-Heusler compounds range from -1.32 to -0.12
eV/f.u., and for CoCrSi, CoCrGe, CoFeGe, CoMnSi, CoMnGe, FeMnGe and FeMnAs, the
total energy differences between the half-Heusler structure and the
corresponding ground-state structure are small (0.07-0.76 eV/f.u.), thus it is
expected that they would be realized in the form of thin films under metastable
conditions for spintronic applications. The stability of the half-metallicity
of CoCrGe and FeMnAs to the lattice distortion is also investigated in detail. | 1304.0344v2 |
2013-09-27 | A first-principles investigation of the thermodynamic and mechanical properties of Ni-Ti-Sn Heusler and half-Heusler materials | First principles calculations of the vibrational, thermodynamic and
mechanical properties of the Ni-Ti-Sn Heusler and half-Heusler compounds have
been performed. First, we have calculated the Raman and infrared spectra of
NiTiSn, providing benchmark theoretical data directly useful for the
assignments of its experimental spectra and clarifying the debate reported in
the literature on the assignment of its modes. Then, we have discussed the
significant vibrational density-of-states of Ni2TiSn at low-frequencies. These
states are at the origin of (i) its smaller free energy, (ii) its higher
entropy, and (iii) its lower Debye temperature, with respect to NiTiSn.
Finally, we have reported the mechanical properties of the two compounds. In
particular, we have found that the half-Heusler compound has the largest
stiffness. Paradoxically, its bulk modulus is also the smallest. This unusual
behavior has been related to the Ni-vacancies that weaken the structure under
isostatic compression. Both compounds show a ductile behavior. | 1309.7195v1 |
2014-10-26 | Half-Heusler topological insulators | Ternary semiconducting or metallic half-Heusler compounds with an atomic
composition 1:1:1 are widely studied for their flexible electronic properties
and functionalities. Recently, a new material property of half-Heusler
compounds was predicted based on electronic structure calculations: the
topological insulator. In topological insulators, the metallic surface states
are protected from impurity backscattering due to spin-momentum locking. This
opens up new perspectives in engineering multifunctional materials. In this
article, we introduce half Heusler materials from the crystallographic and
electronic structure point of view. We present an effective model Hamiltonian
from which the topological state can be derived, notably from a non-trivial
inverted band structure. We discuss general implications of the inverted band
structure with a focus on the detection of the topological surface states in
experiments by reviewing several exemplary materials. Special attention is
given to superconducting half-Heusler materials, which have attracted ample
attention as a platform for non-centrosymmetric and topological
superconductivity. | 1410.7011v1 |
2018-12-04 | Recent Advances in Thermoelectric Performance of Half-Heusler Compounds | Half-Heusler phases (space group F43m, C1b) have recently captured much
attention as promising thermoelectric materials for heat-to-electric power
conversion in the mid-to-high temperature range. The most studied ones are the
RNiSn-type half-Heusler compounds, where R represents refractory metals Hf, Zr,
and Ti. These compounds have shown a high-power factor and high-power density,
as well as good material stability and scalability. Due to their high thermal
conductivity, however, the dimensionless figure of merit (zT) of these
materials has stagnated near 1 for a long time. Since 2013, the verifiable ZT
of half-Heusler compounds has risen from 1 to near 1.5 for both n- and p-type
compounds in the temperature range of 500 to 900 degrees C. In this brief
review, we summarize recent advances as well as approaches in achieving the
high ZT reported. In particular, we discuss the less-exploited strain-relief
effect and dopant resonant state effect studied by the author and his
collaborators in more detail. Finally, we point out directions for further
development.
Keywords: half-Heusler compounds; figure of merit; power density; lattice
disorder; dopant resonant states | 1812.01709v1 |
2019-07-31 | Perspective: Heusler interfaces -- opportunities beyond spintronics? | Heusler compounds, in both cubic and hexagonal polymorphs, exhibit a
remarkable range of electronic, magnetic, elastic, and topological properties,
rivaling that of the transition metal oxides. To date, research on these
quantum materials has focused primarily on bulk magnetic and thermoelectric
properties or on applications in spintronics. More broadly, however, Heuslers
provide a platform for discovery and manipulation of emergent properties at
well-defined crystalline interfaces. Here, motivated by advances in the
epitaxial growth of layered Heusler heterostructures, I present a vision for
Heusler interfaces, focusing on the frontiers and challenges that lie beyond
spintronics. The ability to grow these materials epitaxially on technologically
important semiconductor substrates, such as GaAs, Ge, and Si, provides a direct
path for their integration with modern electronics. Further advances will
require new methods to control the stoichiometry and defects to "electronic
grade" quality, and to control the interface abruptness and ordering at the
atomic scale. | 1908.00101v1 |
2021-09-03 | Magnetic Properties of the Heusler Ru$_2$Mn$_X$ ($X$ = Nb, Ta or V) Compounds: Monte Carlo Simulations | In this paper, we have focused on a comparison of the different magnetic
properties of the three nano-Heusler Ru$_2$Mn$_X$ (X = Nb, Ta or V) compounds
using the Blume-Capel Ising model. The Heusler structures are composed by
different mixed spins. In fact, the Ru and Mn atoms are modeled by spin-5/2 and
spin-1/2, respectively. While, the X atoms ($X$ = Nb, Ta and V) are represented
by the spin-7/2, spin-3/2 and spin-5/2, respectively. This study is carried out
by using the Monte Carlo simulations under the Metropolis algorithm. The
magnetic behaviors of the three nano-Heusler compounds have been studied and
discussed. It is found that Ferrimagnetic to superparamagnetic transitions were
observed corresponding to different blocking temperatures. Besides, the effect
of the crystal field, the exchange coupling interactions and the external
magnetic field have been inspected on the magnetization of each nano-Heusler
compound Ru$_2$Mn$_X$ ($X$ = Nb, Ta or V). | 2109.01708v1 |
2005-10-08 | Slater-Pauling Rule and Curie-Temperature of Co$_2$-based Heusler compounds | A concept is presented serving to guide in the search for new materials with
high spin polarization. It is shown that the magnetic moment of half-metallic
ferromagnets can be calculated from the generalized Slater-Pauling rule.
Further, it was found empirically that the Curie temperature of Co$_2$ based
Heusler compounds can be estimated from a seemingly linear dependence on the
magnetic moment. As a successful application of these simple rules, it was
found that Co$_2$FeSi is, actually, the half-metallic ferromagnet exhibiting
the highest magnetic moment and the highest Curie temperature measured for a
Heusler compound. | 0510210v1 |
2005-11-18 | Investigation of Co$_2$FeSi: The Heusler compound with Highest Curie Temperature and Magnetic Moment | This work reports on structural and magnetic investigations of the Heusler
compound Co$_2$FeSi. X-Ray diffraction and M\"o\ss bauer spectrometry indicate
an ordered $L2_1$ structure. Magnetic measurements by means of X-ray magnetic
circular dichroism and magnetometry revealed that this compound is, currently,
the material with the highest magnetic moment ($6 \mu_B$) and Curie-temperature
(1100K) in the classes of Heusler compounds as well as half-metallic
ferromagnets. | 0511462v1 |
2010-10-11 | Half-Heusler Topological Insulators: A First-Principle Study with the Tran-Blaha Modified Becke-Johnson Density Functional | We systematically investigate the topological band structures of half-Heusler
compounds using first-principles calculations. The modified Becke-Johnson
exchange potential together with local density approximation for the
correlation potential (MBJLDA) has been used here to obtain accurate band
inversion strength and band order. Our results show that a large number of
half-Heusler compounds are candidates for three-dimensional topological
insulators. The difference between band structures obtained using the local
density approximation (LDA) and MBJLDA potential is also discussed. | 1010.2179v1 |
2011-03-30 | The efficient spin injector scheme based on Heusler materials | We present the rational design scheme intended to provide the stable high
spin-polarization at the interfaces of the magneto-resistive junctions by
fulfilling the criteria of structural and chemical compatibilities at the
interface. This can be realized by joining the semiconducting and half-metallic
Heusler materials with similar structures. The present first-principal
calculations verify that interface remains half-metallic if the nearest
interface layers effectively form a stable Heusler material with the properties
intermediate between the surrounding bulk parts. This leads to a simple rule
for selecting the proper combinations. | 1103.5928v1 |
2011-09-26 | Anomalous Hall effect in the Co-based Heusler compounds Co$_{2}$FeSi and Co$_{2}$FeAl | The anomalous Hall effect (AHE) in the Heusler compounds Co$_{2}$FeSi and
Co$_{2}$FeAl is studied in dependence of the annealing temperature to achieve a
general comprehension of its origin. We have demonstrated that the crystal
quality affected by annealing processes is a significant control parameter to
tune the electrical resistivity $\rho_{xx}$ as well as the anomalous Hall
resistivity $\rho_{ahe}$. Analyzing the scaling behavior of $\rho_{ahe}$ in
terms of $\rho_{xx}$ points to a temperature-dependent skew scattering as the
dominant mechanism in both Heusler compounds. | 1109.5498v1 |
2012-05-02 | Superconductivity in the Heusler Family of Intermetallics | Several physical properties of the superconducting Heusler compounds,
focusing on two systems (Y, Lu, Sc)Pd2Sn and APd2M, where A=Hf, Zr and M=Al,
In, are summarized and compared. The analysis of the data shows the importance
of the electron-phonon coupling for superconductivity in this family. We report
the superconducting parameters of YPd2Sn, which has the highest Tc among all
known Heusler superconductors. | 1205.0433v1 |
2013-12-10 | Electronic structure of Zr-Ni-Sn systems: role of clustering and nanostructures in Half-Heusler and Heusler limits | Half-Heusler and Heusler compounds have been of great interest for several
decades for thermoelectric, magnetic, half-metallic and many other interesting
properties. Among these systems, Zr-Ni-Sn compounds are interesting
thermoelectrics which can go from semiconducting half-Heusler (HH) limit,
ZrNiSn, to metallic Heusler (FH) limit, ZrNi$_2$Sn. Recently Makogo et al. [J.
Am. Chem. Soc. 133, 18843 (2011)] found that dramatic improvement in the
thermoelectric power factor of HH can be achieved by putting excess Ni into the
system. This was attributed to an energy filtering mechanism due to the
formation of FH nanostructures in the HH matrix. Using density functional
theory we have investigated clustering and nanostructure formation in
HH$_{1-x}$FH$_x$ systems near the HH and FH ends and found that excess Ni atoms
in HH tend to stay close to each other and form nanoclusters of FH. On the
other hand, there is competing interaction between Ni-vacancies in FH which
prevent them from forming HH nano clusters. Effects of nano inclusions on the
electronic structure at both HH and FH ends will be discussed. | 1312.2985v2 |
2020-10-23 | Machine-Learning-based Prediction of Lattice Thermal Conductivity for Half-Heusler Compounds using Atomic Information | The half-Heusler compound has drawn attention in a variety of fields as a
candidate material for thermoelectric energy conversion and spintronics
technology. This is because it has various electronic structures, such as
semi-metals, semiconductors, and a topological insulator. When the half-Heusler
compound is incorporated into the device, the control of high lattice thermal
conductivity owing to high crystal symmetry is a challenge for the thermal
manager of the device. The calculation for the prediction of lattice thermal
conductivity, which is an important physical parameter for controlling the
thermal management of the device, requires a calculation cost of several 100
times as much as the usual density functional theory calculation. Therefore, we
examined whether lattice thermal conductivity prediction by machine learning
was possible on the basis of only the atomic information of constituent
elements for thermal conductivity calculated by the density functional theory
calculation in various half-Heusler compounds. Consequently, we constructed a
machine learning model, which can predict the lattice thermal conductivity with
high accuracy from the information of only atomic radius and atomic mass of
each site in the half-Heusler type crystal structure. Applying our results, the
lattice thermal conductivity for an unknown half-Heusler compound can be
immediately predicted. In the future, low-cost and short-time development of
new functional materials can be realized, leading to breakthroughs in the
search of novel functional materials. | 2010.12467v1 |
2022-05-05 | Electronic band structure screening for Dirac points in Heuslers | The Heusler compounds have provided a playground of material candidates for
various technological applications based on their highly diverse and tunable
properties, controlled by chemical composition and crystal structure. However,
physical exploration of the Heusler chemical space en masse is impossible in
practice, hindering the exploration of the chemical composition vs. proprieties
relationship. Many of these applications are related to the Heuslers electron
transport characteristics, which are embedded in their electronic band
structure (EBS). Here we we created a Heuslers dataset using the Materials
Project (MP) database -- retrieving both chemical composition and their EBSs.
We then used machine learning to develop a model correlating the composition
vs. number of Dirac points in the EBS for Heuslers and also other Cubic
compounds by identifying said Dirac points using an automated algorithm as well
as generating chemical composition and global crystal structure features. Our
ML model captures the overall trend, as well as identifies significant
electronic and global crystal structure features, however, the ML model
suffered from significant variance due to the lack of site specific features.
Future work on a methodology for handling atomic site specific features will
allow ML models to better match the underlying quantum mechanics governing the
properties (also based on site specific properties) and capture the electronic
properties in a more generalized approach. | 2205.02547v1 |
1996-07-01 | On the Uniqueness of the Papapetrou--Majumdar Metric | We establish the equality of the ADM mass and the total electric charge for
asymptotically flat, static electrovac black hole spacetimes with completely
degenerate, not necessarily connected horizon. | 9607001v1 |
2013-01-03 | Enhanced thermoelectric performance in TiNiSn-based half-Heuslers | Thermoelectric figures of merit, ZT > 0.5, have been obtained in arc-melted
TiNiSn-based ingots. This promising conversion efficiency is due to a low
lattice thermal conductivity, which is attributed to excess nickel in the
half-Heusler structure. | 1301.0419v1 |
2023-12-10 | Microstructure Thermal Stability and Superplastic Behavior of Al-6%Mg-0.12%Sc-0.10%Zr-0.10%(Yb, Er, Hf) Ultrafine-Grained Alloys | Superplastic behavior of ultrafine-grained (UFG) Al-6Mg-0.12Sc-0.10Zr-0.1X
alloys, where X = Yb (Alloy #1-Yb), Er (Alloy #2-Er), and Hf (Alloy #3-Hf), has
been studied. The total content of Sc, Zr, Yb, Er, Hf in the alloys was 0.32
wt.% (0.117-0.118 at.%). The alloys used for benchmarking were
Al-6Mg-0.12Sc-0.20Zr (Alloy #4-Zr) and Al-6Mg-0.22Sc-0.10Zr (Alloy #5-Sc).
Their UFG microstructure was formed with ECAP. Two different types of
deformation behavior during superplasticity were demonstrated. A simultaneous
increase in yield stress and elongation to failure during superplastic
deformation was discovered. High deformation temperatures were shown to cause a
competition between dynamic (strain-induced) grain growth and dynamic
recrystallization, leading to a finer grain microstructure. The values of
strain hardening factor (n), strain rate sensitivity factor (m), and
superplastic deformation threshold stress (Sp) were determined. The impact of
the type and concentration of alloying elements on the deformation behavior and
dynamic grain growth of Al-6%Mg alloys was investigated. It was established
that the maximum elongation to failure in Alloy #1-Yb and Alloy #2-Er is
observed at lower deformation temperatures than in Alloy #4-Zr and Alloy #5-Sc.
The superplastic properties of Alloy #3-Hf are superior to those of Alloy #4-Zr
and Alloy #5-Sc with high content of alloying elements (in at.%). Alloy #1-Yb
manifests good elongation to failure (910%) at low temperatures (400 oC). The
satisfiability of Hart's criterion for calculating uniform deformation value
under superplastic conditions was verified. It was demonstrated that cavitation
when pores are formed in large Al3X particles at high temperatures causes early
failure of aluminum alloys. | 2312.05813v1 |
2007-02-22 | Alloying induced degradation of the absorption edge of InAs_{x}Sb_{1-x} | InAs_{x}Sb_{1-x} alloys show a strong bowing in the energy gap, the energy
gap of the alloy can be less than the gap of the two parent compounds. We
demonstrate that a consequence of this alloying is a systematic degradation in
the sharpness of the absorption edge. The alloy disorder induced band-tail
(Urbach tail) characteristics are quantitatively studied for
InAs_{0.05}Sb_{0.95}. | 0702518v1 |
2021-06-16 | Development of competitive high-entropy alloys using commodity powders | One of the main drawbacks of the powder metallurgy route for High-Entropy
Alloys (HEAs) is the unavailability of fully pre-alloyed powders in the market.
Using commodity powders (commercial Ni, Fe and Co base fully pre-alloyed
powders, fully available in large quantities and at competitive prices) to
produce HEAs presents a completely new and competitive scenario for obtaining
viable alloys for high-performance applications. | 2106.08576v1 |
2015-03-09 | Magnetic Cluster Expansion model for random and ordered magnetic face-centered cubic Fe-Ni-Cr alloys | A Magnetic Cluster Expansion (MCE) model for ternary face-centered cubic
Fe-Ni-Cr alloys has been developed using DFT data spanning binary and ternary
alloy configurations. Using this MCE model Hamiltonian, we perform Monte Carlo
simulations and explore magnetic structures of alloys over the entire range of
alloy compositions, considering both random and ordered alloy structures. In
random alloys, the removal of magnetic collinearity constraint reduces the
total magnetic moment but does not affect the predicted range of compositions
where the alloys adopt low temperature ferromagnetic configurations. During
alloying of ordered fcc Fe-Ni compounds with Cr, chromium atoms tend to replace
nickel rather than iron atoms. Replacement of Ni by Cr in alloys with high iron
content increases the Curie temperature of the alloys. This can be explained by
strong antiferromagnetic Fe-Cr coupling, similar to that found in bcc Fe-Cr
solutions, where the Curie temperature increase, predicted by simulations as a
function of Cr concentration, is confirmed by experimental observations. | 1503.02481v1 |
2022-12-20 | In vitro evaluation of a novel Mg-Sn-Ge ternary alloy for orthopedic applications | Magnesium (Mg) and its alloys have attracted considerable attention owing to
their excellent biodegradable properties and biocompatibility. Novel Mg-Sn-Ge
ternary Mg alloys were developed as potential biodegradable materials for
orthopedic applications because of their alloying elements naturally present in
humans. The feasibility of these alloys was investigated in terms of mechanical
properties, degradation, cytocompatibility, and hemocompatibility. The hardness
and elastic modulus of Mg-2Sn-xGe alloys were improved significantly by
increasing the Ge content. Among all the alloys, the Mg-2Sn-3Ge alloy displays
outstanding biodegradable properties, as evidenced by the electrochemical tests
and hydrogen evolution. The degradation products detected on the corroded alloy
surfaces weaken at higher Ge levels. The in vitro cytotoxicity assay and
hemolysis test showed that the Mg-2Sn-xGe alloys exhibit favorable
biocompatibility and hemocompatibility, except for the Mg-2Sn-2Ge alloy. | 2212.10296v1 |
2005-05-25 | Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb | The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the
Mn-based class of Heuslers and half-Heuslers that contains several conventional
and half metallic ferromagnets, shows a peculiar stability of its magnetic
order in high magnetic fields. Density functional based studies reveal an
unusual nature of its unstable (and therefore unseen) paramagnetic state, which
for one electron less (CuMnSn, for example) would be a zero gap semiconductor
(accidentally so) between two sets of very narrow, topologically separate bands
of Mn 3d character. The extremely flat Mn 3d bands result from the environment:
Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below
the Fermi level, and the other four tetrahedrally coordinated sites are empty,
leaving chemically isolated Mn 3d states. The AFM phase can be pictured
heuristically as a self-doped Cu$^{1+}$Mn$^{2+}$Sb$^{3-}$ compensated semimetal
with heavy mass electrons and light mass holes, with magnetic coupling
proceeding through Kondo and/or antiKondo coupling separately through the two
carrier types. The ratio of the linear specific heat coefficient and the
calculated Fermi level density of states indicates a large mass enhancement
$m^*/m \sim 5$, or larger if a correlated band structure is taken as the
reference. | 0505624v1 |
2008-08-18 | A Ni-based Superconductor: the Heusler Compound ZrNi$_2$Ga | This work reports on the novel Heusler superconductor ZrNi2Ga. Compared to
other nickel-based superconductors with Heusler structure, ZrNi2Ga exhibits a
relatively high superconducting transition temperature of Tc=2.9 K and an upper
critical field of 1.5 T. Electronic structure calculations show that this
relatively high transition temperature is caused by a van Hove singularity,
which leads to an enhanced density of states at the Fermi energy. The van Hove
singularity originates from a higher order valence instability at the L-point
in the electronic structure. The enhanced density of states at the Fermi level
was confirmed by specific heat and susceptibility measurements. Although many
Heusler compounds are ferromagnetic, our measurements of ZrNi2Ga indicate a
paramagnetic state above Tc and could not reveal any traces of magnetic order
down to temperatures of at least 0.35 K. We investigated in detail the
superconducting state with specific heat, magnetization, and resistivity
measurements. The resulting data show the typical behavior of a conventional,
weakly coupled BCS (s-wave) superconductor. | 0808.2356v1 |
2010-03-10 | Evidence for triplet superconductivity in Josephson junctions with ferromagnetic Cu$_{2}$MnAl-Heusler barriers | We have studied Josephson junctions with barriers prepared from the Heusler
compound Cu$_2$MnAl. In the as-prepared state the Cu$_2$MnAl layers are non
ferromagnetic and the critical Josephson current density $j_{c}$ decreases
exponentially with the thickness of the Heusler layers $d_{F}$. On annealing
the junctions at 240\degree C the Heusler layers develop ferromagnetic order
and we observe a dependence $j_{c}(d_{F}$) with $j_{c}$ strongly enhanced and
weakly thickness dependent in the thickness range 7.0 nm < $d_{F}$ < 10.6 nm.
We attribute this feature to a triplet component in the superconducting pairing
function generated by the specific magnetization profile inside thin Cu$_2$MnAl
layers. | 1003.2082v1 |
2013-01-23 | Data Storage: Review of Heusler Compounds | In the recent decade, the family of Heusler compounds has attracted
tremendous scientific and technological interest in the field of spintronics.
This is essentially due to their exceptional magnetic properties, which qualify
them as promising functional materials in various data-storage devices, such as
giant-magnetoresistance spin valves, magnetic tunnel junctions, and
spin-transfer torque devices. In this article, we provide a comprehensive
review on the applications of the Heusler family in magnetic data storage. In
addition to their important roles in the performance improvement of these
devices, we also try to point out the challenges as well as possible solutions,
of the current Heusler-based devices. We hope that this review would spark
further investigation efforts into efficient incorporation of this eminent
family of materials into data storage applications by fully arousing their
intrinsic potential. | 1301.5455v2 |
2014-04-17 | Large Noncollinearity and Spin Reorientation in the Novel Mn2RhSn Heusler Magnet | Noncollinear magnets provide essential ingredients for the next generation
memory technology. It is a new prospect for the Heusler materials, already well
known due to the diverse range of other fundamental characteristics. Here, we
present a combined experimental and theoretical study of novel noncollinear
tetragonal Mn2RhSn Heusler material exhibiting unusually strong canting of its
magnetic sublattices. It undergoes a spin-reorientation transition, induced by
a temperature change and suppressed by an external magnetic field. Because of
the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn2RhSn
is suggested to be a promising candidate for realizing the Skyrmion state in
the Heusler family. | 1404.4581v2 |
2014-05-06 | Theoretical search for half-Heusler topological insulators | We have performed ab-initio band structure calculations on more than two
thousand half-Heusler compounds in order to search for new candidates for
topological insulators. Herein, LiAuS and NaAuS are found to be the strongest
topological insulators with the bulk band gap of 0.20 and 0.19 eV,
respectively, different from the zero band gap feature reported in other
Heusler topological insulators. Due to the inversion asymmetry of the Heusler
structure, their topological surface states on the top and bottom surfaces
exhibit p-type and n-type carriers, respectively. Thus, these materials may
serve as an ideal platform for the realization of topological magneto-electric
effects as polar topological insulators. Moreover, these topological surface
states exhibit the right-hand spin-texture in the upper Dirac cone, which
distinguish them from currently known topological insulator materials. Their
topological nontrivial character remains robust against in-plane strains, which
makes them suitable for epitaxial growth of films. | 1405.1305v1 |
2014-12-01 | Quality of Heusler Single Crystals Examined by Depth Dependent Positron Annihilation Techniques | Heusler compounds exhibit a wide range of different electronic ground states
and are hence expected to be applicable as functional materials in novel
electronic and spintronic devices. Since the growth of large and defect-free
Heusler crystals is still challenging, single crystals of Fe2TiSn and Cu2MnAl
were grown by the optical floating zone technique. Two positron annihilation
techniques -Angular Correlation of Annihilation Radiation (ACAR) and Doppler
Broadening Spectroscopy (DBS)- were applied in order to study both, the
electronic structure and lattice defects. Recently, we succeeded to observe
clearly the anisotropy of the Fermi surface of Cu2MnAl, whereas the spectra of
Fe2TiSn were disturbed by foreign phases. In order to estimate the defect
concentration in different samples of Heusler compounds the positron diffusion
length was determined by DBS using a monoenergetic positron beam. | 1412.0435v1 |
2015-02-11 | High thermoelectric figure of merit in p-type Half-Heuslers by intrinsic phase separation | Improvements in the thermoelectric properties of Half-Heusler materials have
been achieved by means of a micrometer-scale phase separation that increases
the phonon scattering and reduces the lattice thermal conductivity. A detailed
study of the p-type Half-Heusler compounds Ti(1-x)Hf(x)CoSb0.85Sn0.15 using
high-resolution synchrotron powder X-ray diffraction and element mapping
electron microscopy evidences the outstanding thermoelectric properties of this
system. A combination of intrinsic phase separation and adjustment of the
carrier concentration via Sn substitution is used to realize a record
thermoelectric figure of merit for p-type Half-Heusler compounds of ZT around
1.15 at 710C in Ti0.25Hf0.75CoSb0.85Sn0.15. The phase separation approach can
form a significant alternative to nanostructuring processing time, energy
consumption and increasing the thermoelectric efficiency. | 1502.03336v1 |
2015-06-04 | Low-moment ferrimagnetic phase of the Heusler compound Cr2CoAl | Synthesizing half-metallic fully-compensated ferrimagnets that form in the
inverse Heusler phase could lead to superior spintronic devices. These
materials would have high spin polarization at room temperature with very
little fringing magnetic fields. Previous theoretical studies indicated that
Cr2CoAl should form in a stable inverse Heusler lattice due to its low
activation energy. Here, stoichiometric Cr2CoAl samples were arc-melted and
annealed at varying temperatures, followed by studies of their structural and
magnetic properties. High-resolution synchrotron X-ray diffraction revealed a
chemically ordered Heusler phase in addition to CoAl and Cr phases. Soft X-ray
magnetic circular dichroism revealed that the Cr and Co magnetic moments are
antiferromagnetically oriented leading to the observed low magnetic moment in
Cr2CoAl. | 1506.01738v1 |
2016-02-18 | Observation of Unusual Topological Surface States in Half-Heusler Compounds LnPtBi (Ln=Lu, Y) | Topological quantum materials represent a new class of matter with both
exotic physical phenomena and novel application potentials. Many Heusler
compounds, which exhibit rich emergent properties such as unusual magnetism,
superconductivity and heavy fermion behaviour, have been predicted to host
non-trivial topological electronic structures. The coexistence of topological
order and other unusual properties makes Heusler materials ideal platform to
search for new topological quantum phases (such as quantum anomalous Hall
insulator and topological superconductor). By carrying out angle-resolved
photoemission spectroscopy (ARPES) and ab initio calculations on rare-earth
half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observed the unusual
topological surface states on these materials, establishing them as first
members with non-trivial topological electronic structure in this class of
materials. Moreover, as LnPtBi compounds are non-centrosymmetric
superconductors, our discovery further highlights them as promising candidates
of topological superconductors. | 1602.05633v2 |
2020-01-06 | Giant anomalous Hall and Nernst effect in magnetic cubic Heusler compounds | The interplay of magnetism and topology opens up the possibility for exotic
linear response effects, such as the anomalous Hall effect and the anomalous
Nernst effect, which can be strongly enhanced by designing a strong Berry
curvature in the electronic structure. It is even possible to utilize this to
create a quantum anomalous Hall state at high temperatures by reducing the
dimensionality. Magnetic Heusler compounds are a promising class of materials
for this purpose because they grow in thin films, have a high Curie
temperature, and their electronic structure hosts strong topological features.
Here, we provide a comprehensive study of the intrinsic anomalous transport for
magnetic cubic full Heusler compounds and we illustrate that several Heusler
compounds outperform the best so far reported materials. The results reveal the
importance of symmetries, especially mirror planes, in combination with
magnetism for giant anomalous Hall and Nernst effects, which should be valid in
general for linear responses (spin Hall effect, spin orbital torque, etc.)
dominated by intrinsic contributions. | 2001.01698v3 |
2017-04-06 | A critical study of the elastic properties and stability of Heusler compounds: Phase change and tetragonal $X_{2}YZ$ compounds | In the present work, the elastic constants and derived properties of
tetragonal and cubic Heusler compounds were calculated using the high accuracy
of the full-potential linearized augmented plane wave (FPLAPW). To find the
criteria required for an accurate calculation, the consequences of increasing
the numbers of $k$-points and plane waves on the convergence of the calculated
elastic constants were explored. Once accurate elastic constants were
calculated, elastic anisotropies, sound velocities, Debye temperatures,
malleability, and other measurable physical properties were determined for the
studied systems. The elastic properties suggested metallic bonding with
intermediate malleability, between brittle and ductile, for the studied Heusler
compounds. To address the effect of off-stoichiometry on the mechanical
properties, the virtual crystal approximation (VCA) was used to calculate the
elastic constants. The results indicated that an extreme correlation exists
between the anisotropy ratio and the stoichiometry of the Heusler compounds,
especially in the case of Ni$_{2}$MnGa. | 1704.01741v1 |
2018-08-14 | Screening potential topological insulators in half-Heusler compounds via compressed-sensing | Ternary half-Heusler compounds with widely tunable electronic structures,
present a new platform to discover topological insulators. Due to
time-consuming computations and synthesis procedures, the identification of new
topological insulators is however a rough task. Here, we adopt a
compressed-sensing approach to rapidly screen potential topological insulators
in half-Heusler family, which is realized via a two-dimensional descriptor that
only depends on the fundamental properties of the constituent atoms. Beyond the
finite training data, the proposed descriptor is employed to screen many new
half-Heusler compounds, including those with integer and fractional
stoichiometry, and a larger number of possible topological insulators are
predicted. | 1808.04748v5 |
2018-12-14 | Formation of two-dimensional electron and hole gases at the interface of half-Heusler semiconductors | Heuslers are a prominent family of multi-functional materials that includes
semiconductors, half metals, topological semimetals, and magnetic
superconductors. Owing to their same crystalline structure, yet quite different
electronic properties and flexibility in chemical composition, Heusler-based
heterostructures can be designed to show intriguing properties at the
interface. Using electronic structure calculations, we show that two
dimensional electron or hole gases (2DEG or 2DHG) form at the interface of
half-Heusler (HH) semiconductors without any chemical doping. We use
CoTiSb/NiTiSn as an example, and show that the 2DEG at the TiSb/Ni(001)
termination and the 2DHG at the Co/TiSn(001) termination are intrinsic to the
interface, and hold rather high charge densities of 3x10^14 carriers/cm^2.
These excess charge carriers are tightly bound to the interface plane and are
fully accommodated in transition-metal d sub-bands. The formation of 2DEG and
2DHG are not specific to the CoTiSb/NiTiSn system; a list of combinations of HH
semiconductors that are predicted to form 2DEG or 2DHG is provided based on
band alignment, interface termination, and lattice mismatch. | 1812.05991v1 |
2017-10-03 | Search for Thermoelectrics with High Figure of Merit in half-Heusler compounds with multinary substitution | In order to improve the thermoelectric performance of TiCoSb we have
substituted 50% of Ti equally with Zr and Hf at Ti site and Sb with Sn and Se
equally at Sb site. The electronic structure of Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5
is investigated using the full potential linearized augmented plane wave method
and the thermoelectric transport properties are calculated on the basis of
semi-classical Boltzmann transport theory. Our band structure calculations show
that Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5 has semiconducting behavior with indirect
band gap value of 0.98 eV which follow the empirical rule of 18
valence-electron content to bring semiconductivity in half Heusler compounds,
indicating that one can have semiconducting behavior in multinary phase of half
Heusler compounds if they full fill the 18 VEC rule and this open-up the
possibility of designing thermoelectrics with high figure of merit in half
Heusler compounds. We show that at high temperature of around 700K
Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5 has high thermoelectric figure of merit of ZT =
1.05 which is higher than that of TiCoSb (~ 0.95) suggesting that by going from
ternary to multinary phase system one can enhance the thermoelectric figure of
merit at higher temperatures. | 1710.01012v1 |
2021-02-03 | First principles design of Ohmic spin diodes based on quaternary Heusler compounds | The Ohmic spin diode (OSD) is a recent concept in spintronics, which is based
on half-metallic magnets (HMMs) and spin-gapless semiconductors (SGSs).
Quaternary Heusler compounds offer a unique platform to realize the OSD for
room temperature applications as these materials possess very high Curie
temperatures as well as half-metallic and spin-gapless semiconducting behavior
within the same family. Using state-of-the-art first-principles calculations
combined with the non-equilibrium Green's function method we design four
different OSDs based on half-metallic and spin-gapless semiconducting
quaternary Heusler compounds. All four OSDs exhibit linear current-voltage
($I-V$) characteristics with zero threshold voltage $V_T$. We show that these
OSDs possess a small leakage current, which stems from the overlap of the
conduction and valence band edges of opposite spin channels around the Fermi
level in the SGS electrodes. The obtained on/off current ratios vary between
$30$ and $10^5$. Our results can pave the way for the experimental fabrication
of the OSDs within the family of ordered quaternary Heusler compounds. | 2102.01919v1 |
2021-06-02 | MgPd$_2$Sb -- the first Mg-based Heusler-type superconductor | We report the synthesis and physical properties of a full Heusler compound,
MgPd$_2$Sb, which we found to show superconductivity below $T_c$ = 2.2 K.
MgPd$_2$Sb was obtained by a two-step solid-state reaction method and its
purity and cubic crystal structure (Fm-3m, a=6.4523(1) \r{A}) were confirmed by
powder x-ray diffraction. Normal and superconducting states were studied by
electrical resistivity, magnetic susceptibility, and heat capacity
measurements. The results show that MgPd$_2$Sb is a type-II, weak coupling
superconductor ($\lambda_{e-p}$ = 0.53). The observed pressure dependence of
$T_c$ ($\Delta T_c / p \approx $ -0.23 K/GPa) is one of the strongest reported
for a superconducting Heusler compound. The electronic structure, phonons, and
electron-phonon coupling in MgPd$_2$Sb were theoretically investigated. The
obtained results are in agreement with the experiment, confirming the
electron-phonon coupling mechanism of superconductivity. We compare the
superconducting parameters tothose of all reported Heusler-type
superconductors. | 2106.01133v2 |
2021-10-20 | Anomalous Hall effect from gapped nodal line in Co2FeGe Heusler compound | Full Heusler compounds with Cobalt as a primary element show anomalous
transport properties owing to the Weyl fermions and broken time-reversal
symmetry. We present here the study of anomalous Hall effect (AHE) in Co2FeGe
Heusler compound. The experiment reveals anomalous Hall conductivity (AHC) 100
S/cm at room temperature with an intrinsic contribution of 78 S/cm . The
analysis of anomalous Hall resistivity suggests the scattering independent
intrinsic mechanism dominates the overall behaviour of anomalous Hall
resistivity. The first principles calculation reveals that the Berry curvature
originated by gapped nodal line near EF is the main source of AHE in Co2FeGe
Heusler compound. The theoretically calculated AHC is in agreement with the
experiment. | 2110.10677v1 |
2005-09-18 | Covalent bonding and the nature of band gaps in some half-Heusler compounds | Half-Heusler compounds \textit{XYZ}, also called semi-Heusler compounds,
crystallize in the MgAgAs structure, in the space group $F\bar43m$. We report a
systematic examination of band gaps and the nature (covalent or ionic) of
bonding in semiconducting 8- and 18- electron half-Heusler compounds through
first-principles density functional calculations. We find the most appropriate
description of these compounds from the viewpoint of electronic structures is
one of a \textit{YZ} zinc blende lattice stuffed by the \textit{X} ion. Simple
valence rules are obeyed for bonding in the 8-electron compound. For example,
LiMgN can be written Li$^+$ + (MgN)$^-$, and (MgN)$^-$, which is isoelectronic
with (SiSi), forms a zinc blende lattice. The 18-electron compounds can
similarly be considered as obeying valence rules. A semiconductor such as
TiCoSb can be written Ti$^{4+}$ + (CoSb)$^{4-}$; the latter unit is
isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and
18-electron compounds, when \textit{X} is fixed as some electropositive cation,
the computed band gap varies approximately as the difference in Pauling
electronegativities of \textit{Y} and \textit{Z}. What is particularly exciting
is that this simple idea of a covalently bonded \textit{YZ} lattice can also be
extended to the very important \textit{magnetic} half-Heusler phases; we
describe these as valence compounds \textit{ie.} possessing a band gap at the
Fermi energy albeit only in one spin direction. The \textit{local} moment in
these magnetic compounds resides on the \textit{X} site. | 0509472v1 |
2015-11-15 | Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films | The discovery of topological insulators (TIs), materials with bulk band gaps
and protected cross-gap surface states, in compounds such as Bi2Se3 has
generated much interest in identifying topological surface states (TSSs) in
other classes of materials. In particular, recent theory calculations suggest
that TSSs may be found in half-Heusler ternary compounds. If experimentally
realizable, this would provide a materials platform for entirely new
heterostructure spintronic devices that make use of the structurally-identical
but electronically-varied nature of Heusler compounds. Here, we show the
presence of a TSS in epitaxially grown thin films of the half-Heusler compound
PtLuSb. Spin and angle-resolved photoemission spectroscopy (ARPES),
complemented by theoretical calculations, reveals a surface state with linear
dispersion and a helical tangential spin texture consistent with previous
predictions. This experimental verification of TI behavior is a significant
step forward in establishing half-Heusler compounds as a viable material system
for future spintronics devices. | 1511.04778v3 |
2018-12-07 | Systematic understanding of half-metallicity of ternary compounds in Heusler and Inverse Heusler structures with 3$d$ and 4$d$ elements | Employing {\it ab initio} electronic structure calculations we extensively
study ternary Heusler compounds having the chemical formula X$_2$X$^\prime$Z,
where X = Mn, Fe or Co; Z = Al or Si; and X$^\prime$ changes along the row of
4$d$ transition metals. A comprehensive overview of these compounds, addressing
the trends in structural, electronic, magnetic properties and Curie temperature
is presented here along with the search for new materials for spintronics
applications. A simple picture of hybridization of the $d$ orbitals of the
neighboring atoms is used to explain the origin of the half-metallic gap in
these compounds. We show that arrangements of the magnetic atoms in different
Heusler lattices are largely responsible for the interatomic exchange
interactions that are correlated with the features in their electronic
structures as well as possibility of half-metallicity. We find seven
half-metallic magnets with 100\% spin polarization. We identify few other
compounds with high spin polarisation as "near half-metals" which could be of
potential use in applications as well. We find that the major features in the
electronic structures remain intact if a 3$d$ X$^{\prime}$ constituent is
replaced with an isoelectronic 4$d$, implying that the total number of valence
electrons can be used as a predictor of half-metallic nature in compounds from
Heusler family. | 1812.02856v1 |
2019-05-20 | Band alignment and scattering considerations for enhancing the thermoelectric power factor of complex materials: The case of Co-based half-Heuslers | Half-Heuslers, an emerging thermoelectric material group, has complex
bandstructures with multiple bands that can be aligned through band engineering
approaches, giving us an opportunity to improve their power factor. In this
work, going beyond the constant relaxation time approximation, we perform an
investigation of the benefits of band alignment in improving the thermoelectric
power factor under different density of states dependent scattering scenarios.
As a test case we consider the Co-based p-type half-Heuslers TiCoSb, NbCoSn and
ZrCoSb. First, using simplified effective mass models combined with Boltzmann
transport, we investigate the conditions of band alignment that are beneficial
to the thermoelectric power factor under three different carrier scattering
scenarios: i) the usual constant relaxation time approximation, ii) intra-band
scattering restricted to the current valley with the scattering rates
proportional to the density of states as dictated by Fermi's Golden Rule, and
iii) both intra- and inter-band scattering across all available valleys, with
the rates determined by the total density of states at the relevant energies.
We demonstrate that the band-alignment outcome differs significantly depending
on the scattering details. Next, using the density functional theory calculated
bandstructures of the half-Heuslers we study their power factor behavior under
strain induced band alignment. We show that strain can improve the power factor
of half-Heuslers, but the outcome heavily depends on the curvatures of the
bands involved, the specifics of the carrier scattering mechanisms, and the
initial band separation. Importantly, we also demonstrate that band alignment
is not always beneficial to the power factor. | 1905.07951v1 |
2020-03-25 | Tetragonal superstructure of the antiskyrmion hosting Heusler compound Mn1.4PtSn | Skyrmions in non-centrosymmetric magnets are vortex-like spin arrangements,
viewed as potential candidates for information storage devices. The crystal
structure and non-collinear magnetic structure together with magnetic and
spin-orbit interactions define the symmetry of the Skyrmion structure. We
outline the importance of these parameters in the Heusler compound Mn1.4PtSn
which hosts antiskyrmions, a vortex-like spin texture related to skyrmions.1 We
overcome the challenge of growing large micro-twin-free single crystals of
Mn1.4PtSn which has proved to be the bottleneck for realizing bulk
skyrmionic/antiskyrmionic states in a compound. The use of 5d-transition metal,
platinum, together with manganese as constituents in the Heusler compound such
as Mn1.4PtSn is a precondition for the non-collinear magnetic structure. Due to
the tetragonal inverse Heusler structure, Mn1.4PtSn exhibits large
magneto-crystalline anisotropy and D2d symmetry, which are necessary for
antiskyrmions. The superstructure in Mn1.4PtSn is induced by Mn-vacancies which
enables a ferromagnetic exchange interaction to occur. Mn1.4PtSn, the first
known tetragonal Heusler superstructure compound, opens up a new research
direction for properties related to the superstructure in a family containing
thousands of compounds. | 2003.11344v1 |
2021-01-26 | Co$_2$FeAl full Heusler compound based spintronic terahertz emitter | To achieve a large terahertz (THz) amplitude from a spintronic THz emitter
(STE), materials with 100\% spin polarisation such as Co-based Heusler
compounds as the ferromagnetic layer are required. However, these compounds are
known to loose their half-metallicity in the ultrathin film regime, as it is
difficult to achieve L2$_1$ ordering, which has become a bottleneck for the
film growth. Here, the successful deposition using room temperature DC
sputtering of the L2$_1$ and B2 ordered phases of the Co$_2$FeAl full Heusler
compound is reported. Co$_2$FeAl is used as ferromagnetic layer together with
highly orientated Pt as non-ferromagnetic layer in the Co$_2$FeAl/Pt STE, where
an MgO(10 nm) seed layer plays an important role to achieve the L2$_1$ and B2
ordering of Co$_2$FeAl. The generation of THz radiation in the CFA/Pt STE is
presented, which has a bandwidth in the range of 0.1-4 THz. The THz electric
field amplitude is optimized with respect to thickness, orientation, and growth
parameters using a thickness dependent model considering the optically induced
spin current, superdiffusive spin current, inverse spin Hall effect and the
attenuation of THz radiation in the layers. This study, based on the full
Heusler Co$_2$FeAl compound opens up a plethora possibilities in STE research
involving full Heusler compounds. | 2101.10911v1 |
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