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2016-03-03 | Predicted superconductivity of Ni2VAl and pressure dependence of superconductivity in Ni2NbX (X = Al, Ga and Sn) and Ni2VAl | A first-principles study of the electronic and superconducting properties of
the Ni$_2$VAl Heusler compound is presented. The electron-phonon coupling
constant of $\lambda_{ep}$ = 0.68 is obtained, which leads to a superconducting
transition temperature of T$_c$ = $\sim$4 $K$ (assuming a Coulomb
pseudopotential $\mu^*$ = 0.13), which is a relatively high transition
temperature for Ni based Heusler alloys. The electronic density of states
reveals a significant hybridization between Ni-$eg$ and V-$t_{2g}$ states
around the Fermi level. The Fermi surface, consisting of two electron pockets
around the X-points of the Brillouin zone, exhibits nesting and leads to a Kohn
anomaly of the phonon dispersion relation for the transverse acoustic mode TA2
along the (1,1,0) direction, which is furthermore found to soften with
pressure. As a consequence, T$_c$ and $\lambda_{ep}$ vary non-monotonically
under pressure. The calculations are compared to similar calculations performed
for the Ni$_2$NbX (X = Al, Ga and Sn) Heusler alloys, which experimentally have
been identified as superconductors. The experimental trend in T$_c$ is well
reproduced, and reasonable quantitative agreement is obtained. The calculated
T$_c$ of Ni$_2$VAl is larger than either calculated and observed T$_c$s of any
of the Nb compounds. The Fermi surfaces of Ni$_2$NbAl and Ni$_2$NbGa consist of
only a single electron pocket around the X point, however under compression
second electron pocket similar to that of Ni2VAl emerges only in Ni2NbAl and
the Tc increases non monotonically in all the compounds. Fermi surface nesting
and associated Kohn anomalies are a common feature of all four compounds,
albeit weakest in Ni$_2$VAl. | 1603.01104v1 |
2021-08-26 | Magnetoelastic anisotropy in Heusler-type Mn$_{2-δ}$CoGa$_{1+δ}$ films | Perpendicular magnetization is essential for high-density memory application
using magnetic materials. High-spin polarization of conduction electrons is
also required for realizing large electric signals from spin-dependent
transport phenomena. Heusler alloy is a well-known material class showing the
half-metallic electronic structure. However, its cubic lattice nature favors
in-plane magnetization and thus minimizes the perpendicular magnetic anisotropy
(PMA), in general. This study focuses on an inverse-type Heusler alloy,
Mn$_{2-\delta}$CoGa$_{1+\delta}$ (MCG) with a small off-stoichiometry
($\delta$) , which is expected to be a half-metallic material. We observed
relatively large uniaxial magnetocrystalline anisotropy constant
($K_\mathrm{u}$) of the order of 10$^5$ J/m$^3$ at room temperature in MCG
films with a small tetragonal distortion of a few percent. A positive
correlation was confirmed between the $c/a$ ratio of lattice constants and
$K_\mathrm{u}$. Imaging of magnetic domains using Kerr microscopy clearly
demonstrated a change in the domain patterns along with $K_\mathrm{u}$. X-ray
magnetic circular dichroism (XMCD) was employed using synchrotron radiation
soft x-ray beam to get insight into the origin for PMA. Negligible angular
variation of orbital magnetic moment ($\Delta m_\mathrm{orb}$) evaluated using
the XMCD spectra suggested a minor role of the so-called Bruno's term to
$K_\mathrm{u}$. Our first principles calculation reasonably explained the small
$\Delta m_\mathrm{orb}$ and the positive correlation between the $c/a$ ratio
and $K_\mathrm{u}$. The origin of the magnetocrystalline anisotropy was
discussed based on the second-order perturbation theory in terms of the
spin--orbit coupling, claiming that the mixing of the occupied $\uparrow$- and
the unoccupied $\downarrow$-spin states is responsible for the PMA of the MCG
films. | 2108.11547v2 |
2018-11-10 | Magnetic Order and Lattice Instabilities in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$ Heusler based Magnetic Shape-Memory Alloys | The magnetic correlations in the austenite phase and the consequent
martensitic transition in inverse magnetocaloric alloys,
Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$, have been a matter of debate for decades. We
conclusively establish using {\it ab initio} phonon calculations that the spin
alignment of excess Mn at the Sn site (Mn$_{Sn}$) with the existing Mn in the
unit cell in the high temperature cubic phase of Ni-Mn-Sn alloy is
ferromagnetic (FM), and not ferrimagnetic (FI), resolving a long lasting
controversy. Using first principles density functional perturbation theory
(DFPT), we observe an instability of the TA$_{2}$ mode along the $\Gamma$-M
direction in the FM phase, very similar to that observed in the prototypical
ferromagnetic shape memory alloy (FSMA) Ni$_{2}$MnGa. This specific instability
is not observed in the FI phase. Further finite temperature first principles
lattice dynamics calculations reveal that at 300 K the FM phase becomes
mechanically stable, while the FI phase continue to remain unstable providing
credence to the fact that the high-temperature phase has FM order. These
results will be primordial to understand the magneto-structural properties of
this class of compounds. | 1811.04221v1 |
2020-06-09 | Effect of partial substitution of iso-valent Mo at Cr-site on electronic structure and physical properties of Fe2CrAl | Heusler alloy Fe2CrAl exhibits a ferromagnetic behaviour below Curie
temperature (TC) ~ 202 K along with presence of cluster glass (CG) phase near
freezing temperature (Tf) ~ 3.9 K and Griffiths phase (GP) above 300 K. The
physical properties of this alloy are very sensitive to substitutions and
anti-site disorder. Here, we investigate the effect of partial substitution of
Mo at Cr-site on physical properties of Fe2CrAl. Structural and morphological
analysis confirms the single cubic structure of the substituted alloys.
Increment in Mo concentration shifts the TC towards lower temperature, which is
ascribed to the effect of increased hybridization strength between 3d-4d states
of Fe/Cr/Mo. Additionally, systematic analysis of AC susceptibility, magnetic
memory effect and time dependent magnetization studies confirm the presence of
CG-like phase near (Tf) ~ 3.5 K in Fe2Cr0.95Mo0.05Al. Such feature gets
suppressed towards lower temperature with an increase of Mo concentration, i.e.
below 1.8 K in Fe2Cr0.85Mo0.15Al. The origin of the glassy signature is
ascribed to the decrement in magnetic anisotropy with Mo concentration. A
partial increment in magnetic entropy change is also noted near TC with the
increase in Mo substitution. Interestingly, at high temperatures (above 350 K),
GP phase persists in both the alloys due to the presence of anti-site disorder. | 2006.05111v2 |
2002-07-10 | Martensitic transition and magnetoresistance in a Cu-Al-Mn shape memory alloy. Influence of aging | We have studied the effect of ageing within the miscibility gap on the
electric, magnetic and thermodynamic properties of a non-stoichiometric Heusler
Cu-Al-Mn shape-memory alloy, which undergoes a martensitic transition from a
$bcc$-based ($\beta$-phase) towards a close-packed structure ($M$-phase).
Negative magnetoresistance which shows an almost linear dependence on the
square of magnetization with different slopes in the $M$- and $\beta$-phases,
was observed. This magnetoresistive effect has been associated with the
existence of Mn-rich clusters with the Cu$_2$AlMn-structure. The effect of an
applied magnetic field on the martensitic transition has also been studied. The
entropy change between the $\beta$- and $M$-phases shows negligible dependence
on the magnetic field but it decreases significantly with annealing time within
the miscibility gap. Such a decrease is due to the increasing amount of
Cu$_2$MnAl-rich domains that do not transform martensitically. | 0207254v1 |
2007-07-06 | Effect of surfaces and interfaces on the electronic, magnetic and gap-related properties of the half-metal Co$_2$MnSn | We present state-of-the-art electronic structure calculations for the
Co$_2$MnSn full-Heusler alloy. We show that in its bulk form it is a
half-metallic ferromagnet with the Fermi level being located within a tiny gap
of the minority-spin density of states. Moreover the alloy shows the
Slater-Pauling behavior with a total spin magnetic moment in the unit cell of 5
$\mu_B$. In the case of the (001) surfaces, the broken bonds at the surface
form a minority band pinned exactly at the Fermi level destroying the
half-metallicity. Our calculations reveal that both the interfaces with the
non-magnetic metal V and the semiconductor InAs are no more half-metallic due
to the different environment of the atoms of the half-metal at the interface.
These interface states although localized only at the first few interface
layers can become conducting when coupled to defect states and kill the
spin-polarization of the current injected from the half-metal into the
semiconductor or the non-magnetic metallic spacer. | 0707.0941v1 |
2011-10-25 | Surface spin polarization of the non-stoichiometric Heusler compound Co2Mn(alpha)Si | Using a combined approach of spin-resolved photoemission spectroscopy, band
structure and photoemission calculations we investigate the influence of bulk
defects and surface states on the spin polarization of Co2Mn(alpha)Si thin
films with bulk L21 order. We find that for Mn-poor alloys the spin
polarization at EF is negative due to the presence of Co_Mn antisite and
minority surface state contributions. In Mn-rich alloys, the suppression of
Co(Mn) antisites leads to a positive spin polarization at the Fermi energy, and
the influence of minority surface states on the photoelectron spin polarization
is reduced. | 1110.5451v2 |
2014-07-11 | A 4-fold-symmetry hexagonal ruthenium for magnetic heterostructures exhibiting enhanced perpendicular magnetic anisotropy and tunnel magnetoresistance | An unusual crystallographic orientation of hexagonal Ru with a 4-fold
symmetry emerging in epitaxial MgO/Ru/Co2FeAl/MgO heterostructures is reported,
in which an approximately Ru(02-23) growth attributes to the lattice matching
among MgO, Ru, and Co2FeAl. Perpendicular magnetic anisotropy of the
Co2FeAl/MgO interface is substantially enhanced as compared with those with a
Cr(001) layer. The MTJs incorporating this structure gave rise to the largest
tunnel magnetoresistance for perpendicular MTJs using low damping Heusler
alloys. The 4-fold-symmetry hexagonal Ru arises from an epitaxial growth with
an unprecedentedly high crystal index, opening a unique pathway for the
development of perpendicular anisotropy films of cubic and tetragonal
ferromagnetic alloys. | 1407.3160v2 |
2015-06-28 | Calorimetric and magnetic study for Ni$_{50}$Mn$_{36}$In$_{14}$ and relative cooling power in paramagnetic inverse magnetocaloric systems | The non-stoichiometric Heusler alloy Ni$_{50}$Mn$_{36}$In$_{14}$ undergoes a
martensitic phase transformation in the vicinity of 345 K, with the high
temperature austenite phase exhibiting paramagnetic rather than ferromagnetic
behavior, as shown in similar alloys with lower-temperature transformations.
Suitably prepared samples are shown to exhibit a sharp transformation, a
relatively small thermal hysteresis, and a large field-induced entropy change.
We analyzed the magnetocaloric behavior both through magnetization and direct
field-dependent calorimetry measurements. For measurements passing through the
first-order transformation, an improved method for heat-pulse relaxation
calorimetry was designed. The results provide a firm basis for the analytic
evaluation of field-induced entropy changes in related materials. An analysis
of the relative cooling power (RCP), based on the integrated field-induced
entropy change and magnetizing behavior of the Mn spin system with
ferromagnetic correlations, shows that a significant RCP may be obtained in
these materials by tuning the magnetic and structural transformation
temperatures through minor compositional changes or local order changes. | 1506.08351v1 |
2016-10-17 | Designing compensated magnetic states in tetragonal Mn3Ge-based alloys | Magnetic compensated state attracted much interests due to the observed large
exchange bias and large coercivity, and its potential applications in the
antiferromagnetic spintronics with merit of no stray field. In this work, by ab
initio calculations with KKR-CPA for the treatment of random substitution, we
obtain the complete compensated states in the Ni (Pd, Pt) doped Mn3Ge-based
D022-type tetragonal Heusler alloys. We find the total moment change is
asymmetric across the compensation point (at ~ x = 0.3) in Mn3-xYxGe (Y = Ni,
Pd, Pt), which is highly conforming to that experimentally observed in Mn3Ga.
In addition, an uncommon discontinuous jump is observed across the critical
zero-moment point, indicating that some non-trivial properties can emerge at
this point. Further electronic analysis for the three compensation compositions
reveals large spin polarizations, together with the high Curie temperature of
the host Mn3Ge, making them promising candidates for spin transfer torque
applications. | 1610.04971v1 |
2019-04-04 | First-principles investigations of orthorhombic-cubic phase transition and its effect on thermoelectric properties in cobalt-based ternary alloys | We screened six cobalt-based 18-VEC systems CoVSi, CoNbSi, CoTaSi (Si-group)
and CoVGe, CoNbGe, CoTaGe (Ge-group) by the first-principles approach, with the
motivation of stabilizing these orthorhombic phases into the cubic symmetry --
favorable for thermoelectrics. Remarkably, it was found that the Ge-group is
energetically more favorable in the cubic symmetry than the hitherto
orthorhombic phase. We account the cubic ground state of the Si-group to the
interplay of internal pressure and covalent interactions. The principle of
covalent interactions will provide an insight and could be vital in speeding
the search of missing cubic half-Heusler alloys. Meanwhile, the calculated
transport properties of all the systems on \textit{p}-type doping, except
CoVSi, are more promising than the well-known CoTiSb. We also provide
conservative estimates of the figure of merit, exceeding the CoTiSb. Based on
our findings, we suggest possible new phases of ternary compounds for
thermoelectric applications. | 1904.02485v1 |
2020-04-01 | Importance of site occupancy and absence of strain glassy phase in Ni$_{2-x}$Fe$_{x}$Mn$_{1.5}$In$_{0.5}$ | Martensitic transition temperature steadily decreases in
Ni$_{2-x}$Fe$_{x}$Mn$_{1.5}$In$_{0.5}$ and is completely suppressed at $x$ =
0.2. Despite suppression of martensitic transition,
Ni$_{1.8}$Fe$_{0.2}$Mn$_{1.5}$In$_{0.5}$ does not display the expected strain
glassy phase. Instead, a ground state with dominant ferromagnetic interactions
is observed. A study of structural and magnetic properties of $x$ = 0.2 reveal
that the alloy consists of a major Fe rich cubic phase and a minor Fe deficient
monoclinic phase favoring a ferromagnetic ground state. This is exactly
opposite of that observed in Ni$_2$Mn$_{1-y}$Fe$_{y}$In$_{0.5}$ wherein a
strain glassy phase is observed for $y$ = 0.1. The change in site symmetry of
Fe when doped for Ni in contrast to Mn in the Heusler composition seems to
support the growth of the ferromagnetic phase. | 2004.00256v1 |
2019-09-28 | Size-Dependent Structural and Magnetic Properties of Disordered Co2FeAl Heusler Alloy Nanoparticles | Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by
chemical route. The effect of the size of NPs upon the structure and
magnetization compared to its bulk counterpart was investigated. The structure
and composition were determined from X-ray diffraction (XRD) and electron
microscopy. XRD analysis shows that the samples are having single (A2-type)
disordered phase. Magnetization measurements suggest that the samples are soft
ferromagnetic in nature with very low coercivity. Enhanced magnetic properties
like saturation magnetization, coercive force, retentivity, and
Curie-temperature are observed with a decrease in particle size. The effect of
particle size on hysteresis losses is also discussed. The smallest particles of
size 16 nm exhibited the highest saturation magnetization and transition
temperature of 180.73 emu/g and 1261 K, respectively. The origin of enhancement
in the magnetization of Co2FeAl nano-alloy is attributed to the strong Co-Co
exchange interaction due to disorder present in the systems. | 1909.13088v1 |
2020-03-04 | Theoretical study of phase stability, electronic and magnetic properties of Rh$_2$CrGe$_{1-x}$Al$_x$ ($x = 0$, $0.25$, $0.50$, $0.75$ and $1$) Heusler alloys by FP-LAPW method | First-principle calculations were performed within the framework of the
density functional theory (DFT) using FP-LAPW method as implemented in WIEN2k
code to determine the structural stability, electronic and magnetic properties
of Rh$_2$CrGe$_{1-x}$Al$_x $($x = 0$, $0.25$, $0.50$, $0.75$ and $1$). The
results showed that for Rh$_2$CrAl and Rh$_2$CrGe, the Cu$_2$MnAl-type
structure is energetically more stable than Hg$_2$CuTi-type structure at the
equilibrium volume. The calculated densities of states for Rh$_2$CrAl and
Rh$_2$CrGe show half-metallic and nearly half-metallic behavior, respectively.
Rh$_2$CrGe$_{1-x}$Al$_x$ ($x = 0.25$, $0.50$, $0.75$) these alloys show a
half-metallic character, and these compounds are predicted to be good
candidates for spintronic applications. | 2003.02067v1 |
2020-03-25 | Crystal and magnetic structure of antiferromagnetic Mn$_{2}$PtPd | We have investigated the crystal and magnetic structure of Mn${}_{2}$PtPd
alloy using powder x-ray and neutron diffraction experiments. This compound is
believed to belong to the Heusler family having crystal symmetry
$\mathit{I}$4/$\mathit{mmm}$ (TiAl${}_{3}$-type). However, in this work we
found that the Pd and Pt atoms are disordered and thus Mn${}_{2}$PtPd
crystallizes in the $\mathit{L}$1${}_{0}$ structure having
$\mathit{P}$4/$\mathit{mmm}$ symmetry (CuAu-I type) like MnPt and MnPd binary
alloys. The lattice constants are $\mathit{a}$ = 2.86 \r{A} and $\mathit{c}$ =
3.62 \r{A} at room temperature. Mn${}_{2}$PtPd has a collinear
antiferromagnetic spin structure below the N\'{e}el temperature
$\mathit{T}$${}_{N}$ = 866 K, where Mn moments of $\mathrm{\sim}$4
$\mu$${}_{B}$ lie in the $\mathit{ab}$-plane. We observed a strong change in
the lattice parameters near $\mathit{T}$${}_{N}$. The sample exhibits metallic
behaviour, where electrical resistivity and carrier concentration are of the
order of 10${}^{-5}$ $\Omega$ cm and 10${}^{21}$ cm${}^{-3}$, respectively. | 2003.11569v1 |
2020-10-14 | Ab-initio study of electronic and magnetic properties of Mn$_2$RuZ/MgO (001) heterojunctions (Z= Al, Ga, Si, Ge) | Using first-principles calculations, we studied Mn$_2$RuZ (Z=Al, Ga, Si, Ge)
and their heterojunctions with MgO along (001) direction. All these alloys
possess Hg$_2$CuTi-type inverse Heusler alloy structure and ferrimagnetic
ground state. Our study reveals the half-metallic electronic structure with
highly spin-polarized $\Delta_1$ band, which is robust against atomic disorder.
Next we studied the electronic structure of Mn$_2$RuAl/MgO and Mn$_2$RuGe/MgO
heterojunctions. We found that the MnAl- or MnGe-terminated interface is
energetically more favorable compared to the MnRu-terminated interface.
Interfacial states appear at the Fermi level in the minority-spin gap for the
Mn$_2$RuGe/MgO junction. We discuss the origin of these interfacial states in
terms of local environment around each constituent atom. On the other hand, in
the Mn$_2$RuAl/MgO junction, high spin polarization of bulk Mn$_2$RuAl is
preserved independent of its termination. | 2010.06761v1 |
2018-11-30 | Spin Gapless Semiconducting Nature in Co-rich Co1+xFe1-xCrGa: Insight and Advancements | In this report, we present structural, electronic, magnetic and transport
properties of Co-rich spin gapless semiconductor CoFeCrGa using both
theoretical and experimental techniques. The key advantage of Co-rich samples
$\mathrm{Co_{1+x}Fe_{1-x}CrGa}$ is the high Curie temperature (T$\mathrm{_C}$)
and magnetization, without compromising the SGS nature (up to x = 0.4), and
hence our choice. The quaternary Heusler alloys $\mathrm{Co_{1+x}Fe_{1-x}CrGa}$
(x = 0.1 to 0.5) are found to crystallize in LiMgPdSn-type structure having
space group $F\bar{4}3m$ (\# 216). The measured Curie temperature increases
from 690 K (x = 0) to 870 K (x = 0.5). Observed magnetization values follow the
Slater-Pauling rule. Measured electrical resistivity, in the temperature range
of 5-350 K, suggests that the alloys retain the SGS behavior up to x = 0.4,
beyond which it reflects metallic character. Unlike conventional
semiconductors, the conductivity value ($\mathrm{\sigma_{xx}}$) at 300 K lies
in the range of 2289 S $\mathrm{cm^{-1}}$ to 3294 S $\mathrm{cm^{-1}}$, which
is close to that of other reported SGS materials. The anomalous Hall effect is
comparatively low. The intrinsic contribution to the anomalous Hall
conductivity increase with x, which can be correlated with the enhancement in
chemical order. The anomalous Hall coefficient is found to increase from 38
S/cm for x = 0.1 to 43 S/cm for 0.3. Seebeck coefficients turn out to be
vanishingly small below 300 K, another signature for being SGS. All the alloys
(for different x) are found to be both chemically and thermally stable.
Simulated magnetization agrees fairly with the experiment. As such Co-rich
CoFeCrGa is a promising candidate for room temperature spintronic applications,
with enhanced T$\mathrm{_C}$, magnetic properties and SGS nature. | 1811.12684v1 |
2012-09-22 | Structural and electronic properties of superconducting Heusler alloy Ni$_{2}$Nb$_{1+x}$Sn$_{1-x}$: \textit{Ab initio} approach | Using \textit{ab initio} calculation, we investigate systematically the
structural and electronic properties of Ni$_{2}$Nb$_{1+x}$Sn$_{1-x}$ ($x$ = 0,
0.25, 0.50). Here, projector augmented wave approach (PAW) implemented in the
Vienna \textit{ab initio} simulation package (VASP) within generalized gradient
approximation (GGA) for the exchange-correlation functional has been used. In
this article, it is reported that though Ni$_{2}$NbSn and
Ni$_{2}$Nb$_{1.25}$Sn$_{0.75}$ have no structural transformation,
Ni$_{2}$Nb$_{1.5}$Sn$_{0.5}$ can transform to tetragonal structure from cubic
L2$_{1}$ phase. The cubic lattice parameter decreases with Nb doping at Sn
sites in off-stoichiometric alloys. The alloys are in paramagnetic phase in all
the structures. The hybridization between Ni and Nb 3d states triggers the
tetragonal distortion. Due to Nb doping in cubic L2$_{1}$ phase, there is a
significant change in total density of states (DOSs) at Fermi energy (E$_{F}$)
(N(E$_{F}$)). N(E$_{F}$) increases with increasing Nb doping. But, N(E$_{F}$)
decreases during structural transformation of Ni$_{2}$Nb$_{1.5}$Sn$_{0.5}$. The
superconducting critical temperature (T$_{C}$) also changes with Nb doping in
cubic phase and tetragonal distortion because T$_{C}$ very much depends on
N(E$_{F}$). | 1209.4964v3 |
2022-08-03 | Machine Learning-Based Classification, Interpretation, and Prediction of High-Entropy-Alloy Intermetallic Phases | The design of high-entropy alloys (HEA) with desired properties is
challenging due to their large compositional space. While various machine
learning (ML) models can predict specific HEA solid-solution phases (SS),
predicting high-entropy intermetallic phases (IM) is underdeveloped due to
limited datasets and inadequate ML features. This paper introduces feature
engineering-assisted ML models that achieve detailed phase classification and
high accuracy. By combining phase-diagram-based and physics-based features, it
is found that the ML models trained on the Random Forest (RF) and Support
Vector Machine (SVM) regressors, are able to classify individual SS and common
IM (Sigma, Laves, Heusler, and refractory B2 phases) with accuracies ranging
from 80 - 94%. The machine-learned features also enable the interpretation of
IM formation. Furthermore, the efficacies of the RF, SVM, and neural network
(NN) models are critically evaluated. The phase classification accuracies are
found to decrease upon utilizing the NN model to train the datasets. The
accuracy of the model prediction is validated by synthesizing 86 new alloys.
This approach provides a practical and robust framework for guiding HEA phase
design, particularly for technologically significant IM phases. | 2208.02141v3 |
2024-02-29 | Enhanced metamagnetic shape memory effect in Heusler-type Ni37Co11Mn43Sn9 polycrystalline ferromagnetic shape memory alloy | Polycrystalline Ni-Co-Mn-Sn based ferromagnetic shape memory alloys (FSMAs)
show promise as actuator materials, but their practical application involving
magnetic field induced strain (MFIS) is often limited by three factors: the
requirement for high magnetic fields (> 5 T), martensitic transition
temperature away from room temperature, and limited recovery of pre-strain
applied to the martensite phase. Current work investigates the martensitic
transition (MT) and shape memory effect under the application of magnetic field
for bulk polycrystalline Ni37Co11Mn43Sn9 alloy. The outcome of the study
reveals a metamagnetic transition from the martensitic phase to the austenitic
phase at a low field of 2.8 T at 300 K which results 0.25% spontaneous MFIS.
Interestingly, 1.3% pre-strained specimen registers a 100% recovery with the
application of magnetic field of 4.5 T. Furthermore, the pre-strained specimen
exhibited a two-way shape memory effect between a strain value of 1.0% to 1.55%
during the field loading and unloading sequences. Notably, this study also
demonstrates, to the best of our knowledge , for the first time, that the
spontaneous strain and pre-strain add together. This finding paves the way for
achieving a giant MFIS by pre-straining a Ni-Mn-Sn/In class of FSMAs which
shows large spontaneous MFIS. | 2402.18992v1 |
2005-10-08 | Design of magnetic materials: Co$_2$Cr$_{1-x}$Fe$_{x}$Al | Doped Heusler compounds Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe
ratio $x$ were investigated experimentally and theoretically. The electronic
structure of the ordered, doped Heusler compound Co$_2$Cr$_{1-x}$Fe$_{x}$Al
($x=n/4, n=0,1,2,3,4)$ was calculated using different types of band structure
calculations. The ordered compounds turned out to be ferromagnetic with small
Al magnetic moment being aligned anti-parallel to the 3d transition metal
moments. All compounds show a gap around the Fermi-energy in the minority
bands. The pure compounds exhibit an indirect minority gap, whereas the
ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism
(MCD) in X-ray absorption spectra was measured at the $L_{2,3}$ edges of Co,
Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine
element specific magnetic moments. Calculations and measurements show an
increase of the magnetic moments with increasing iron content. The
experimentally observed reduction of the magnetic moment of Cr can be explained
by Co-Cr site-disorder. The presence of the gap in the minority bands of
Co$_2$CrAl can be attributed to the occurrence of pure Co$_2$ and mixed CrAl
(001)-planes in the $L2_1$ structure. It is retained in structures with
different order of the CrAl planes but vanishes in the $X$-structure with
alternating CoCr and CoAl planes. | 0510203v1 |
2012-09-29 | Hard X-ray photoelectron spectroscopy on buried, off-stoichiometric CoxMnyGez (x : z = 2 : 0.38) Heusler thin films | Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric
Co2-based Heusler alloy shows a intense dependency of the tunnel
magnetoresistance (TMR) on the Mn composition, demonstrating giant TMR ratios
of up to 1995% at 4.2 K for 1. This work reports on the electronic structure of
non-stoichiometric CoxMnyGez thin films with a fixed Co/Ge ratio of x : z = 2 :
0.38. The electronic structure was investigated by high energy, hard X-ray
photoelectron spectroscopy combined with first-principles calculations. The
high-resolution measurements of the valence band of the non-stoichiometric
CoxMnyGez films close to the Fermi energy indicate a shift of the spectral
weight compared to bulk Co2MnGe. This is in agreement with the changes in the
density of states predicted by the calculations. Furthermore it is shown that
the co-sputtering of Co2MnGe together with additional Mn is an appropriate
technique to adjust the stoichiometry of the CoxMnyGez film composition. The
resulting changes of the electronic structure within the valence band will
allow to tune the magnetoresistive characteristics of CoxMnyGez based tunnel
junctions as verified by the calculations and photoemission experiments. | 1210.0146v1 |
2014-08-07 | Weak Antilocalization Effect and Noncentrosymmetric Superconductivity in a Topologically Nontrivial Semimetal LuPdBi | A large number of half-Heusler compounds have been recently proposed as
three-dimensional (3D) topological insulators (TIs) with tunable physical
properties.However, no transport measurements associated with the topological
surface states have been observed in these half-Heusler candidates due to the
dominating contribution from bulk electrical conductance. Here we show that, by
reducing the mobility of bulk carriers, a two-dimensional (2D) weak
antilocalization (WAL) effect, one of the hallmarks of topological surface
states, was experimentally revealed from the tilted magnetic field dependence
of magnetoconductance in a topologically nontrivial semimetal LuPdBi. Besides
the observation of a 2D WAL effect, a superconducting transition was revealed
at Tc~1.7 K in the same bulk LuPdBi. Quantitative analysis within the framework
of a generalized BCS theory leads to the conclusion that the noncentrosymmetric
superconductivity of LuPdBi is fully gapped with a possibly unconventional
pairing character. The co-existence of superconductivity and the transport
signature of topological surface states in the same bulk alloy suggests that
LuPdBi represents a very promising candidate as a topological superconductor. | 1408.1543v1 |
2014-11-21 | A new spin gapless semiconductor: quaternary Heusler CoFeCrGa alloy | Despite a plethora of materials suggested for spintronic applications, a new
class of materials has emerged, namely spin gapless semiconductors (SGS), that
offers potentially more advantageous properties than existing ones. These
magnetic semiconductors exhibit a finite band gap for one spin channel and a
closed gap for the other. Here, supported by the first-principles,
electronic-structure calculations, we report the first experimental evidence of
SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (L21)
structure but exhibiting chemical disorder (DO3 structure). CoFeCrGa is found
to transform from SGS to half-metallic phase under pressure, which is
attributed to unique electronic-structure features. The saturation
magnetization (MS) obtained at 8 K agrees with the Slater-Pauling rule and the
Curie temperature (TC) is found to exceed 400 K. Carrier concentration (up to
250 K) and electrical conductivity are observed to be nearly temperature
independent, prerequisites for SGS. The anomalous Hall coefficient is estimated
to be 185 S/cm at 5 K. Considering the SGS properties and high TC, this
material appears to be promising for spintronic applications. | 1411.5772v2 |
2015-03-05 | Monitoring surface resonances on Co2MnSi(100) by spin-resolved photoelectron spectroscopy | The magnitude of the spin polarization at the Fermi level of ferromagnetic
materials at room temperature is a key property for spintronics. Investigating
the Heusler compound Co$_2$MnSi a value of 93$\%$ for the spin polarization has
been observed at room temperature, where the high spin polarization is related
to a stable surface resonance in the majority band extending deep into the
bulk. In particular, we identified in our spectroscopical analysis that this
surface resonance is embedded in the bulk continuum with a strong coupling to
the majority bulk states. The resonance behaves very bulk-like, as it extends
over the first six atomic layers of the corresponding (001)-surface. Our study
includes experimental investigations, where the bulk electronic structure as
well as surface-related features have been investigated using spin-resolved
photoelectron spectroscopy (SR-UPS) and for a larger probing depth
spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The
results are interpreted in comparison with first-principles band structure and
photoemission calculations which consider all relativistic, surface and
high-energy effects properly. | 1503.01573v1 |
2017-07-13 | The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X = Hf, Zr, Ti) half-Heusler alloys | Thermoelectric application of half-Heusler compounds suffers from their
fairly high thermal conductivities. Insight into how effective various
scattering mechanisms are in reducing the thermal conductivity of fabricated
XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial.
Here, we show that such insight can be obtained through a concerted
theory-experiment comparison of how the lattice thermal conductivity kLat(T)
depends on temperature and crystallite size. Comparing theory and experiment
for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and
in the present paper revealed that grain boundary scattering plays the most
important role in bringing down kLat, in particular so for unmixed compounds.
Our concerted analysis approach was corroborated by a good qualitative
agreement between the measured and calculated kLat of polycrystalline samples,
where the experimental average crystallite size was used as an input parameter
for the calculations. The calculations were based on the Boltzmann transport
equation and ab initio density functional theory. Our analysis explains the
significant variation of reported kLat of nominally identical XNiSn samples and
is expected to provide valuable insights into the dominant scattering
mechanisms even for other materials. | 1707.04302v2 |
2017-11-29 | Methods to induce perpendicular magnetic anisotropy in full-Heusler Co2FeSi thin layers in a magnetic tunnel junction structure | In this study, to obtain perpendicular magnetic tunnel junctions (p-MTJs)
using half-metallic ferromagnets (HMFs), several methods were developed to
induce perpendicular magnetic anisotropy (PMA) in full-Heusler Co2FeSi (CFS)
alloy thin layers in an MTJ multilayer composed of a layered CFS/MgO/CFS
structure. Oxygen exposure at 2.0 Pa for 10 min after deposition of the bottom
CFS layer was effective for obtaining PMA in the CFS layer. One of the reasons
for the PMA is the formation of nearly ideal CFS/MgO interfaces due to oxygen
exposure before the deposition of the MgO layer. The annealing process was
effective for obtaining PMA in the top CFS layer capped with a Pd layer. PMA
was clearly observed in the top CFS layer of a Cr(40 nm)/Pd(50 nm)/bottom
CFS(0.6 nm)/MgO(2.0 nm)/top CFS(0.6 nm)/Pd(10 nm) multilayer, where the top CFS
and Pd thin films were deposited at RT and subsequently annealed at 300{\deg}C.
In addition to the continuous layer growth of the films, the crystalline
orientation alignment at the top CFS/Pd interface probably attributes to the
origin of PMA at the top CFS layer. | 1711.10722v3 |
2017-04-26 | Ordering tendencies and electronic properties in quaternary Heusler derivatives | The phase stabilities and ordering tendencies in the quaternary full-Heusler
alloys NiCoMnAl and NiCoMnGa have been investigated by in-situ neutron
diffraction, calorimetry and magnetization measurements. NiCoMnGa was found to
adopt the L2$_1$ structure, with distinct Mn and Ga sublattices but a common
Ni-Co sublattice. A second-order phase transition to the B2 phase with disorder
also between Mn and Ga was observed at 1160 K. In contrast, in NiCoMnAl slow
cooling or low-temperature annealing treatments are required to induce
incipient L2$_1$ ordering, otherwise the system displays only B2 order. Linked
to this L2$_1$ ordering, a drastic increase in the magnetic transition
temperature was observed in NiCoMnAl, while annealing affected the magnetic
behavior of NiCoMnGa only weakly due to the low degree of quenched-in disorder.
First principles calculations were employed to study the thermodynamics as well
as order-dependent electronic properties of both compounds. It was found that a
near half-metallic pseudo-gap emerges in the minority spin channel only for the
completely ordered Y structure, which however is energetically unstable
compared to the predicted ground state of a tetragonal structure with
alternating layers of Ni and Co. The experimental inaccessibility of the
totally ordered structures is explained by kinetic limitations due to the low
ordering energies. | 1704.08100v1 |
2019-10-07 | Accurate high-throughput screening of I-II-V 8-electron Half-Heusler compounds for renewable-energy applications | Renewable energy resources have emerged as the best alternatives to fossil
fuel energy which are rapidly declining with time. Here, eight valence-electron
count Half-Heusler(HH) alloys have been studied using reliable first principles
calculations in the search of potential candidates for renewable energy
applications like thermoelectric (TE), solar harvesting, topological insulator
(TI) and transparent conductor (TC) applications. The initial screening
parameters used for our study are chemical and thermal stability, band gap,
nature of bandgap and band inversion strength. We have performed quasistatic
G0W0 calculation starting from HSE groundstate wavefunction to predict the most
accurate estimation of bandgap for these class of compounds. A total of 960
compounds were simulated. 121 out of 960 compounds were found to be thermally
and chemically stable. 31 compounds with bandgap less than 1.5 eV were studied
for thermoelectric application out of which 13 compounds were found to show
thermoelectric figure of merit ZT > 0.7 for both p-type and n-type conduction.
30 compounds with band gap 1-1.8 eV were studied for optoelectronic application
out of which 13 compounds were found to show Spectroscopic Limited Maximum
Efficiency (SLME) more than 20%, comparable to existing state of the art
materials. 21 compounds were found to show band inversion at ambient conditions
which is a necessary condition for topological insulators. The surface band
structure calculations for one of the promising candidate was done to check
robustness of the topological behaviour. 29 compounds were found to have
bandgap more than 2 eV which are promoted for transparent conductor
applications with further band engineering. We strongly believe that our
calculations will give useful insights to experimentalists for synthesizing and
investigating proposed compounds for different energy applications. | 1910.02984v1 |
2019-10-15 | Impact of the scattering physics on the power factor of complex thermoelectric materials | We assess the impact of the scattering physics assumptions on the
thermoelectric properties of five Co-based p-type half-Heusler alloys by
considering full energy-dependent scattering times, versus the commonly
employed constant scattering time. For this, we employ DFT bandstructures and a
full numerical scheme that uses Fermi's Golden Rule to extract the momentum
relaxation times of each state at every energy, momentum, and band. We consider
electron-phonon scattering (acoustic and optical), as well as ionized impurity
scattering, and evaluate the qualitative and quantitative differences in the
power factors of the materials compared to the case where the constant
scattering time is employed. We show that the thermoelectric power factors
extracted from the two different methods differ in terms of i) their ranking
between materials, ii) the carrier density where the peak power factor appears,
and iii) their trends with temperature. We further show that the constant
relaxation time approximation smoothens out the richness in the bandstructure
features, thus limiting the possibilities of exploring this richness for
material design and optimization. These details are more properly captured
under full energy/momentum-dependent scattering time considerations. Finally,
by mapping the conductivities extracted within the two schemes, we provide
appropriate density-dependent constant relaxation times that could be employed
as a fast first order approximation for extracting charge transport properties
in the half-Heuslers we consider. | 1910.06628v3 |
2018-12-12 | First-principles prediction of half-Heusler half-metals above room temperature | Half-metallicity (HM) offers great potential for engineering spintronic
applications, yet only few magnetic materials present metallicity in just one
spin channel. In addition, most HM systems become magnetically disordered at
temperatures well below ambient conditions, which further hinders the
development of spin-based electronic devices. Here, we use first-principles
methods based on density functional theory (DFT) to investigate the electronic,
magnetic, structural, mixing, and vibrational properties of $90$ $XYZ$
half-Heusler (HH) alloys ($X =$ Li, Na, K, Rb, Cs; $Y =$ V,Nb, Ta; $Z =$ Si,
Ge, Sn, S, Se, Te). We disclose a total of $28$ new HH compounds that are
ferromagnetic, vibrationally stable, and HM, with semiconductor band gaps in
the range of $1$-$4$ eV and HM band gaps of $0.2$-$0.8$ eV. By performing Monte
Carlo simulations of a spin Heisenberg model fitted to DFT energies, we
estimate the Curie temperature, $T_{\rm C}$, of each HM compound. We find that
$17$ HH HM remain magnetically ordered at and above room temperature, namely,
$300 \le T_{\rm C} \le 450$ K, with total magnetic moments of $2$ and $4$
$\mu_{\rm B}$. A further materials sieve based on zero-temperature mixing
energies let us to conclude $5$ overall promising ferromagnetic HH HM at and
above room temperature: NaVSi, RbVTe, CsVS, CsVSe, and RbNbTe. We also predict
$2$ ferromagnetic materials that are semiconductor and magnetically ordered at
ambient conditions: LiVSi and LiVGe. | 1812.04813v1 |
2020-08-09 | Ab initio study of the half-metallic full-Heusler compounds Co$_2$ZAl [Z = Sc, Ti, V, Cr, Mn, Fe]; the role of electronic correlations | We study the structural, electronic, and magnetic properties of Co$_2$ZAl
compounds employing a pseudopotential electronic bandstructure method. The
stability of the compounds is established through the formation and cohesive
energy calculations. The effect of the lattice parameter variation on the
electronic and magnetic properties of the compounds is investigated and
meticulous explanation is provided for the observed behavior. The variation of
the individual spin magnetic moments and the stability of the total spin
magnetic moment during the expansion and contraction of the lattice parameter
is observed and an attempt is made to understand the obtained behavior.
Finally, we implement DFT+U to examine its consequences on the electronic and
magnetic properties of the Co$_2$ZAl compounds. We find that the use of DFT+U
is not justified for these compounds and in some cases like Co$_2$MnAl it
produces unrealistic properties. The exception is Co2FeAl where the desired
half-metallicity is restored after the inclusion of on-site correlations. We
explain why the on-site correlations might be important for Co$_2$FeAl by
comparing it with other Heusler alloys where the correlation was found to be
meaningful to explain the observed magnetic moments. | 2008.03732v1 |
2020-08-28 | Effect of Bi-substitution on Structural Stability and Improved Thermoelectric Performance of p-type Half-Heusler TaSbRu: A First-principles Study | Recently, Fang et al. have predicted a high ZT of 1.54 in TaSbRu alloys at
1200 K from first-principles without considering spin-orbit interaction,
accurate electronic structure, details of phonon scattering, and
energy-dependent holes relaxation time. Here, we report the details of
structural stability and thermoelectric performance of Bi-Substituted p-type
TaSbRu from first-principles calculations considering theses important
parameters. This indirect bandgap semiconductor (Eg=0.8 eV by TB-mBJ+SOC) has
highly dispersive and degenerate valence bands, which lead to a maximum power
factor, 3.8 mWm-1K-2 at 300K. As Sb-5p has a small contribution to the bandgap
formation, the substitution of Bi on the Sb site does not cause significant
change to the electronic structure. Although the Seebeck coefficient increases
by Bi due to slight changes in the bandgap, electrical conductivity, and hence,
the power factor reduces to ~3 mW m-1K-2 at 300K (50% Bi). On the other side,
lattice thermal conductivity drops effectively to 5 from 20 W/m K as Bi
introduces a significant contribution in the acoustic phonon region and
intensify phonon scattering. Thus, ZT value is improved through
Bi-substitution, reaching 1.1 (50% Bi) at 1200 K from 0.45 (pure TaSbRu) only.
Therefore, the present study suggests how to improve the TE performance of
Sb-based half-Heusler compounds and TaSbRu (with 50% Bi) is a promising
material for high-temperature applications. | 2008.12564v1 |
2019-12-21 | Non-collinear antiferromagnetic states in Ru-based Heusler compounds induced by biquadratic coupling | We investigate the magnetic properties of Ru$_{2}$Mn$Z$ ($Z$ = Sn, Sb, Ge,
Si) chemically ordered full Heusler compounds for zero as well as finite
temperatures. Based on first principles calculations we derive the interatomic
isotropic bilinear and biquadratic couplings between Mn atoms from the
paramagnetic state. We find frustrated isotropic couplings for all compounds
and in case of $Z$ = Si and Sb a nearest-neighbor biquadratic coupling that
favors perpendicular alignment between the Mn spins. By using an extended
classical Heisenberg model in combination with spin dynamics simulations we
obtain the magnetic equilibrium states. From these simulations we conclude that
the biquadratic coupling, in combination with the frustrated isotropic
interactions, leads to non-collinear magnetic ground states in the Ru$_{2}$MnSi
and Ru$_{2}$MnSb compounds. In particular, for these alloys we find two
distinct, non-collinear ground states which are energetically equivalent and
can be identified as $3-q$ and $4-q$ states on a frustrated fcc lattice.
Investigating the thermal stability of the non-collinear phase we find that in
case of Ru$_{2}$MnSi the multiple$-q$ phase undergoes a transition to the
single$-q$ phase, while in case of Ru$_{2}$MnSb the corresponding transition is
not obtained due to the larger magnitude of the nearest-neighbor biquadratic
coupling. | 1912.10299v1 |
2020-10-16 | THz range Faraday rotation in the Weyl Semimetal Candidate $\mathrm{Co_2TiGe}$ | The $\mathrm{Co_2}$ family of ferromagnetic Heusler alloys have attracted
interest due to their fully spin-polarized nature, making them ideal for
applications in spintronic devices. More recently, the existence of room
temperature time-reversal-breaking Weyl nodes near the Fermi level was
predicted and confirmed in these systems. As a result of the presence of these
Weyl nodes, these systems possess a non-zero momentum space Berry curvature
that can dramatically influence transport properties such as the anomalous Hall
effect. One of these candidate compounds is $\mathrm{Co_2 Ti Ge}$. Recently,
high quality molecular beam epitaxy-grown thin films of $\mathrm{Co_2 Ti Ge}$
have become available. In this work, we present THz-range measurement of
MBE-grown $\mathrm{Co_2 Ti Ge}$ films. We measure the THz-range Faraday
rotation, which can be understood as a measure of the anomalous Hall effect. We
supplement this work with electronic band structure calculations showing that
the principal contribution to the anomalous Hall effect in the this material
stems from the Berry curvature of the material. Our work shows that this class
of Heusler materials shows promise for Weyl semimetal based spintronics. | 2010.08589v2 |
2020-12-03 | When Band Convergence is Not Beneficial for Thermoelectrics | Band convergence is considered a clear benefit to thermoelectric performance
because it increases the charge carrier concentration for a given Fermi level,
which typically enhances charge conductivity while preserving the Seebeck
coefficient. However, this advantage hinges on the assumption that interband
scattering of carriers is weak or insignificant. With first-principles
treatment of electron-phonon scattering in
CaMg$_{2}$Sb$_{2}$-CaZn$_{2}$Sb$_{2}$ Zintl system and full Heusler
Sr$_{2}$SbAu, we demonstrate that the benefit of band convergence can be
intrinsically negated by interband scattering depending on the manner in which
bands converge. In the Zintl alloy, band convergence does not improve weighted
mobility or the density-of-states effective mass. We trace the underlying
reason to the fact that the bands converge at one k-point, which induces strong
interband scattering of both the deformation-potential and the polar-optical
kinds. The case contrasts with band convergence at distant k-points (as in the
full Heusler), which better preserves the single-band scattering behavior
thereby successfully leading to improved performance. Therefore, we suggest
that band convergence as thermoelectric design principle is best suited to
cases in which it occurs at distant k-points. | 2012.02272v2 |
2023-05-25 | Optically controlling the competition between spin flips and intersite spin transfer in a Heusler half-metal on sub-100 fs timescales | The direct manipulation of spins via light may provide a path toward
ultrafast energy-efficient devices. However, distinguishing the microscopic
processes that can occur during ultrafast laser excitation in magnetic alloys
is challenging. Here, we study the Heusler compound Co2MnGa, a material that
exhibits very strong light-induced spin transfers across the entire M-edge. By
combining the element-specificity of extreme ultraviolet high harmonic probes
with time-dependent density functional theory, we disentangle the competition
between three ultrafast light-induced processes that occur in Co2MnGa:
same-site Co-Co spin transfer, intersite Co-Mn spin transfer, and ultrafast
spin-flips mediated by spin-orbit coupling. By measuring the dynamic magnetic
asymmetry across the entire M-edges of the two magnetic sublattices involved,
we uncover the relative dominance of these processes at different probe energy
regions and times during the laser pulse. Our combined approach enables a
comprehensive microscopic interpretation of laser-induced magnetization
dynamics on timescales shorter than 100 fs. | 2305.16455v2 |
2023-09-09 | Accelerating Discovery of Vacancy Ordered 18-Valence Electron Half-Heusler Compounds: A Synergistic Approach of Machine Learning and Density Functional Theory | In this study, we attempted to model vacancy ordered half Heusler compounds
with 18 valence electron count (VHH) derived from 19 VEC compounds such as
TiNiSb such that the compositions will be Ti0.75NiSb, Zr0.75NiSb and Hf0.75NiSb
with semiconducting behavior. The main motivation is that such a
vacancy-ordered phase not only introduces semi conductivity but also it
disrupts the phonon conducting path in HH alloys and thus reduces the thermal
conductivity and as a consequence enhances the thermoelectric figure of merit.
In order to predict the formation energy ({\Delta}Hf) from composition and
crystal structure we have used 4684 compounds for their {\Delta}Hf values are
available in the material project database and trained a machine learning model
with R2 value of 0.943. Using this trained model, we have predicted the
{\Delta}Hf of a list of VHH. From the predicted database of VHH we have
selected Zr0.75NiSb and Hf0.75NiSb to validate the machine learning prediction
using accurate DFT calculation. The calculated {\Delta}Hf for these two
compounds from DFT calculation are found to be comparable with our ML
prediction. The calculated electronic and lattice dynamics properties show that
these materials are narrow band gap semiconductors and are dynamically stable
as their all-phonon dispersion curves are having positive frequencies. The
calculated Seebeck coefficient, electrical conductivity as well as thermal
conductivity, power factor and thermoelectric figure of merit are analyzed. | 2309.04692v1 |
2017-11-14 | Growth, electrical, structural, and magnetic properties of half-Heusler CoTi$_{1-x}$Fe$_x$Sb | Epitaxial thin films of the substitutionally alloyed half-Heusler series
CoTi$_{1-x}$Fe$_x$Sb were grown by molecular beam epitaxy on InAlAs/InP(001)
substrates for concentrations 0.0$\leq$x$\leq$1.0. The influence of Fe on the
structural, electronic, and magnetic properties was studied and compared to
that expected from density functional theory. The films are epitaxial and
single crystalline, as measured by reflection high-energy electron diffraction
and X-ray diffraction. Using in-situ X-ray photoelectron spectroscopy, only
small changes in the valence band are detected for x$\leq$0.5. For films with
x$\geq$0.05, ferromagnetism is observed in SQUID magnetometry with a saturation
magnetization that scales linearly with Fe content. A dramatic decrease in the
magnetic moment per formula unit occurs when the Fe is substitutionally alloyed
on the Co site indicating a strong dependence on the magnetic moment with site
occupancy. A crossover from both in-plane and out-of-plane magnetic moments to
only in-plane moment occurs for higher concentrations of Fe. Ferromagnetic
resonance indicates a transition from weak to strong interaction with a
reduction in inhomogeneous broadening as Fe content is increased.
Temperature-dependent transport reveals a semiconductor to metal transition
with thermally activated behavior for x$\leq$0.5. Anomalous Hall effect and
large negative magnetoresistance (up to -18.5% at 100 kOe for x=0.3) are
observed for higher Fe content films. Evidence of superparamagnetism for x=0.3
and x=0.2 suggests for moderate levels of Fe, demixing of the
CoTi$_{1-x}$Fe$_x$Sb films into Fe rich and Fe deficient regions may be
present. Atom probe tomography is used to examine the Fe distribution in a
x=0.3 film. Statistical analysis reveals a nonhomogeneous distribution of Fe
atoms throughout the film, which is used to explain the observed magnetic and
electrical behavior. | 1711.05320v1 |
2019-03-07 | Uniaxial anisotropy, intrinsic and extrinsic damping in Co$_{2}$FeSi Heusler alloy thin films | Ferromagnetic resonance (FMR) technique has been used to study the
magnetization relaxation processes and magnetic anisotropy in two different
series of the Co2FeSi (CFS) Heusler alloy thin films, deposited on the Si(111)
substrate by UHV sputtering. While the CFS films of fixed (50 nm) thickness,
deposited at different substrate temperatures (TS) ranging from room
temperature (RT) to 600^C, constitute the series-I, the CFS films with
thickness t varying from 12 nm to 100 nm and deposited at 550^C make up the
series-II. In series-I, the CFS films deposited at TS = RT and 200^C are
completely amorphous, the one at TS = 300^C is partially crystalline, and those
at TS equal 450^C, 550^C and 600^C are completely crystalline with B2 order. By
contrast, all the CFS films in series-II are in the fully-developed B2
crystalline state. Irrespective of the strength of disorder and film thickness,
angular variation of the resonance field in the film plane unambiguously
establishes the presence of global in-plane uniaxial anisotropy. Angular
variation of the linewidth in the film plane reveals that, in the CFS thin
films of varying thickness, a crossover from the in-plane local four-fold
symmetry (cubic anisotropy) to local two-fold symmetry (uniaxial anisotropy)
occurs as t exceeds 50 nm. Gilbert damping parameter {\alpha} decreases
monotonously from 0.047 to 0.0078 with decreasing disorder strength (increasing
TS) and jumps from 0.008 for the CFS film with t = 50 nm to 0.024 for the film
with t equal 75 nm. Such variations of {\alpha} with TS and t are understood in
terms of the changes in the total (spin-up and spin-down) density of states at
the Fermi level caused by the disorder and film thickness. | 1903.02976v1 |
2019-09-18 | Neutron diffraction and magnetic properties of Co$_2$Cr$_{1-x}$Ti$_x$Al Heusler alloys | We report the structural, magnetic, and magnetocaloric properties of
Co$_2$Cr$_{1-x}$Ti$_x$Al ($x=$ 0--0.5) Heusler alloys for spintronic and
magnetic refrigerator applications. Room temperature X-ray diffraction and
neutron diffraction patterns along with Rietveld refinements confirm that the
samples are of single phase and possess a cubic structure. Interestingly,
magnetic susceptibly measurements indicate a second order phase transition from
paramagnetic to ferromagnetic where the Curie temperature (T$_{\rm C}$) of
Co$_2$CrAl increases from 330~K to 445~K with Ti substitution. Neutron powder
diffraction data of the $x=$ 0 sample across the magnetic phase transition
taken in a large temperature range confirm the structural stability and exclude
the possibility of antiferromagnetic ordering. The saturation magnetization of
the $x=$ 0 sample is found to be 8000~emu/mol (1.45~$\mu_{\rm B}$/{\it f.u.})
at 5~K, which is in good agreement with the value (1.35$\pm$0.05~$\mu_{\rm
B}$/{\it f.u.}) obtained from the Rietveld analysis of the neutron powder
diffraction pattern measured at temperature of 4~K. By analysing the
temperature dependence of the neutron data of the $x=$ 0 sample, we find that
the change in the intensity of the most intense Bragg peak (220) is consistent
with the magnetization behavior with temperature. Furthermore, an enhancement
of change in the magnetic entropy and relative cooling power values has been
observed for the $x=$ 0.25 sample. Interestingly, the critical behavior
analysis across the second order magnetic phase transition and extracted
exponents ($\beta\approx$ 0.496, $\gamma\approx$ 1.348, and $\delta\approx$
3.71 for the $x=$ 0.25 sample) suggest the presence of long-range ordering,
which deviates towards 3D Heisenberg type interactions above T$_{\rm C}$,
consistent with the interaction range value $\sigma$. | 1909.08292v1 |
2021-11-02 | Microstructure engineering of metamagnetic Ni-Mn-based Heusler compounds by Fe-doping: A roadmap towards excellent cyclic stability combined with large elastocaloric and magnetocaloric effects | Ni-Mn-based metamagnetic shape-memory alloys exhibit a giant thermal response
to magnetic fields and uniaxial stress which can be utilized in single caloric
or multicaloric cooling concepts for energy-efficient and sustainable
refrigeration. However, during cyclic operation these alloys suffer from
structural and functional fatigue as a result of their high intrinsic
brittleness. Here, we present based on Fe-doping of Ni-Mn-In a microstructure
design strategy which simultaneously improves cyclic stability and maintains
the excellent magnetocaloric and elastocaloric properties. Our results reveal
that precipitation of a strongly Fe-enriched and In-depleted coherent secondary
gamma-phase at grain boundaries can ensure excellent mechanical stability by
hindering intergranular fracture during cyclic loading. In this way, a large
elastocaloric effect of -4.5 K was achieved for more than 16000 cycles without
structural or functional degradation, which corresponds to an increase of the
cyclic stability by more than three orders of magnitude as compared to
single-phase Ni-Mn-In-(Fe). In addition, we demonstrate that the large
magnetocaloric effect of single-phase Ni-Mn-In-(Fe) can be preserved in the
dual-phase material when the secondary gamma-phase is exclusively formed at
grain boundaries as the martensitic transformation within the Heusler matrix is
barely affected. This way, an adiabatic temperature change of -3 K and an
isothermal entropy change of 15 $Jkg^{-1}K^{-1}$ was obtained in 2 T for
dual-phase Ni-Mn-In-Fe. We expect that this concept can be applied to other
single caloric and mutlicaloric materials, therewith paving the way for
solid-state caloric cooling applications. | 2111.01621v2 |
2023-01-10 | Dissipation losses limiting first-order phase transition materials in cryogenic caloric cooling: A case study on all-d-metal Ni(-Co)-Mn-Ti Heusler alloys | Ni-Mn-based Heusler alloys, in particular all-d-metal Ni(-Co)-Mn-Ti, are
highly promising materials for energy-efficient solid-state refrigeration as
large multicaloric effects can be achieved across their magnetostructural
martensitic transformation. However, no comprehensive study on the crucially
important transition entropy change $\Delta s_t$ exists so far for
Ni(-Co)-Mn-Ti. Here, we present a systematic study analyzing the composition
and temperature dependence of $\Delta s_t$. Our results reveal a substantial
structural entropy change contribution of approximately 65 J(kgK)$^{-1}$, which
is compensated at lower temperatures by an increasingly negative entropy change
associated with the magnetic subsystem. This leads to compensation temperatures
$T_{comp}$ of 75 K and 300 K in Ni$_{35}$Co$_{15}$Mn$_{50-y}$Ti$_{y}$ and
Ni$_{33}$Co$_{17}$Mn$_{50-y}$Ti$_{y}$, respectively, below which the
martensitic transformations are arrested. In addition, we simultaneously
measured the responses of the magnetic, structural and electronic subsystems to
the temperature- and field-induced martensitic transformation near $T_{comp}$,
showing an abnormal increase of hysteresis and consequently dissipation energy
at cryogenic temperatures. Simultaneous measurements of magnetization and
adiabatic temperature change $\Delta T_{ad}$ in pulsed magnetic fields reveal a
change in sign of $\Delta T_{ad}$ and a substantial positive and irreversible
$\Delta T_{ad}$ up to 15 K at 15 K as a consequence of increased dissipation
losses and decreased heat capacity. Most importantly, this phenomenon is
universal, it applies to any first-order material with non-negligible
hysteresis and any stimulus, effectively limiting the utilization of their
caloric effects for gas liquefaction at cryogenic temperatures. | 2301.03934v1 |
2023-03-15 | High spin-polarization in a disordered novel quaternary Heusler alloy FeMnVGa | In this work, we report the successful synthesis of a Fe-based novel
half-metallic quaternary Heusler alloy FeMnVGa and its structural, magnetic and
transport properties probed through different experimental methods and
theoretical technique. Density functional theory (DFT) calculations performed
on different types of structure reveal that Type-2 ordered structure (space
group: F-43m, Ga at 4a, V at 4b, Mn at 4c and Fe at 4d) possess minimum energy
among all the ordered variants. Ab-initio simulations in Type 2 ordered
structure further reveal that the compound is half-metallic ferromagnet (HMF)
having a large spin-polarization (89.9 %). Neutron diffraction reveal that the
compound crystalizes in disordered Type-2 structure (space group: Fm-3m) in
which Ga occupy at 4a, V at 4b and Fe/Mn occupy 4c/4d sites with 50:50
proportions. The structural disorder is further confirmed by X-ray diffraction
(XRD), extended X-ray absorption fine structure (EXAFS),57Fe Mossbauer
spectrometry results and DFT calculations. Magnetisation studies suggest that
the compound orders ferromagnetically below TC ~ 293 K and the saturation
magnetization follows Slater-Pauling rule. Mossbauer spectrometry, along with
neutron diffraction suggest that Mn is the major contributor to the total
magnetism in the compound consistent with the theoretical calculations. First
principle calculations indicate that spin-polarization remain high (81.3 %)
even in the presence of such large atomic disorder. The robustness of the HMF
property in presence of disorder is a quite unique characteristic over other
reported HMF in literature and make this compound quiet promising for
spintronics applications. | 2303.08579v1 |
2015-09-30 | Roadmap for Emerging Materials for Spintronic Device Applications | The Technical Committee of the IEEE Magnetics Society has selected 7 research
topics to develop their roadmaps, where major developments should be listed
alongside expected timelines; (i) hard disk drives, (ii) magnetic random access
memories, (iii) domain-wall devices, (iv) permanent magnets, (v) sensors and
actuators, (vi) magnetic materials and (vii) organic devices. Among them,
magnetic materials for spintronic devices have been surveyed as the first
exercise. In this roadmap exercise, we have targeted magnetic tunnel and
spin-valve junctions as spintronic devices. These can be used for example as a
cell for a magnetic random access memory and spin-torque oscillator in their
vertical form as well as a spin transistor and a spin Hall device in their
lateral form. In these devices, the critical role of magnetic materials is to
inject spin-polarised electrons efficiently into a non-magnet. We have
accordingly identified 2 key properties to be achieved by developing new
magnetic materials for future spintronic devices: (1) Half-metallicity at room
temperature (RT); (2) Perpendicular anisotropy in nano-scale devices at RT. For
the first property, 5 major magnetic materials are selected for their
evaluation for future magnetic/spintronic device applications: Heusler alloys,
ferrites, rutiles, perovskites and dilute magnetic semiconductors. These alloys
have been reported or predicted to be half-metallic ferromagnets at RT. They
possess a bandgap at the Fermi level EF only for its minority spins, achieving
100% spin polarisation at EF. We have also evaluated L10-alloys and
D022-Mn-alloys for the development of a perpendicularly anisotropic ferromagnet
with large spin polarisation. We have listed several key milestones for each
material on their functionality improvements, property achievements, device
implementations and interdisciplinary applications within 35 years time scale. | 1509.08997v1 |
2017-07-16 | Competing magnetic and spin gap-less semiconducting behaviour in fully compensated ferrimagnet CrVTiAl: Theory and Experiment | We report the structural, magnetic and transport properties of
polycrystalline CrVTiAl alloy along with first principles calculations. It
crystallizes in the LiMgPdSn type structure with lattice parameter 6.14 \AA\ at
room temperature. Absence of (111) peak along with the presence of a weak (200)
peak indicates the antisite disorder of Al with Cr and V atoms. The
magnetization measurements reveal a ferrimagnetic transition near 710 K and a
coercive field of 100 Oe at 3 K. Very low moment and coercive field indicate
fully compensated ferrimagnetism in the alloy. Temperature coefficient of
resistivity is found to be negative, indicating a characteristic of
semiconducting nature. Absence of exponential dependence of resistivity on
temperature indicates a gapless/spin-gapless semiconducting behaviour.
Electronic and magnetic properties of CrVTiAl for three possible
crystallograpic configurations are studied theoretically. All the three
configurations are found to be different forms of semiconductors. Ground state
configuration is a fully compensated ferrimagnet with band gaps 0.58 eV and
0.30 eV for up and down spin bands respectively. The next higher energy
configuration is also ferrimagnetic, but has spin-gapless semiconducting
nature. The highest energy configuration corresponds to a non-magnetic gapless
semiconductor. The energy differences among these configurations are quite
small ($<$ 1 $\mathrm{mRy/atom}$) which hints that at finite temperatures, the
alloy exists in a disordered phase, which is a mixture of the three
configurations. By taking into account the theoretical and the experimental
findings, we conclude that CrVTiAl is a fully compensated ferrimagnet with
predominantly spin gap-less semiconductor nature. | 1707.04854v1 |
2010-11-01 | Modeling materials with optimized transport properties | Following demands for materials with peculiar transport properties, e.g. in
magnetoelectronics or thermoelectrics, there is a need for materials modeling
at the quantum-mechanical level. We combine density-functional with various
scale-bridging tools to establish correlations between the macroscopic
properties and the atomic structure of materials. For examples, magnetic memory
devices exploiting the tunneling magneto-resistance (TMR) effect depend
crucially on the spin polarization of the electrodes. Heusler alloys, e.g.
Co2MnSi, if perfectly ordered, are ferromagnetic half-metals with (ideally)
100% spin polarization. Their performance as electrodes in TMR devices is
limited by atomic disorder and deviations from perfect stoichiometry, but also
by interface states at the tunneling barrier. We use ab initio thermodynamics
in conjunction with the cluster expansion technique to show that excess
manganese in the alloy and at the interface helps to preserve the desired
half-metallic property. As another example, nanostructured materials with a
reduced thermal conductivity but good electrical conductivity are sought for
applications in thermoelectrics. Semiconductor heterostructures with a regular
arrangement of nanoscale inclusions ('quantum dot superlattices') hold the
promise of a high thermoelectric figure of merit. Our theoretical analysis
reveals that an increased figure of merit is to be expected if the quantum dot
size, the superlattice period and the doping level are all suitably fine-tuned.
Such a superlattice thus constitutes a material whose transport properties are
controlled by geometrical features at the nanoscale. | 1011.0324v1 |
2012-10-19 | Ferromagnetic structures in Mn2CoGa and Mn2CoAl doped by Co, Cu, V, and Ti | The structure and magnetic properties in doped Heusler alloys of Mn2CoGa and
Mn2CoAl have been investigated by experiments and calculations. The main group
elements of Ga and Al are substituted by the magnetic or non-magnetic
transition metals, Co, Cu, V, and Ti in the alloy systems. Three kinds of local
ferromagnetic structures, Co-Mn-Co, Mn-Co-Mn and Mn-Co-V, have been found. They
embed in the native ferrimagnetic matrix and increase the magnetization with
different increments. The Co-Mn-Co ferromagnetic structure shows the largest
increment of 6.18{\mu}B /atom. In addition, interesting results for
non-magnetic Cu increasing the magnetization and the V atom having a large
ferromagnetic moment of about 1.0{\mu}B have been obtained. The exchange
interaction energy can be increased by the newly added Co and depleted by
supporting a ferromagnetic coupling in other substitution cases, and showing
the variation of the TC. Our calculation of electronic structure verifies the
strong d-d hybridization when the three ferromagnetic structures are achieved.
It has also been found that the covalent effect from the Ga and Al determines
the generation of the local ferromagnetic structure and the tolerance for
dopant content. | 1210.5357v1 |
2020-11-01 | Bipolar Magnetic Semiconducting Behavior in VNbRuAl: A New Spintronic Material for Spin Filters | We report the theoretical prediction of a new class of spintronic materials,
namely bipolar magnetic semiconductor (BMS), which is also supported by our
experimental data. BMS acquires a unique band structure with unequal band gaps
for spin up and down channels, and thus are useful for tunable spin transport
based applications such as spin filters. The valence band (VB) and conduction
band (CB) in BMS approach the Fermi level through opposite spin channels, and
hence facilitate to achieve reversible spin polarization which are controllable
via applied gate voltage. We report the quaternary Heusler alloy VNbRuAl to
exactly possess the band structure of BMS. The alloy is found to crystallize in
LiMgPdSn prototype structure (space group $F\bar{4}3m$) with B$2$ disorder and
lattice parameter 6.15 \AA . The resistivity and Hall measurements show a two
channel semiconducting behavior and a quasi linear dependence of negative
magneto resistance (MR) indicating the possible semiconducting nature.
Interestingly, VNbRuAl also shows a fully compensated ferrimagnetic (FCF)
behavior with vanishing net magnetization (m$_s$$\sim$ $10^{-3}$ $\mu_B/f.u.$)
and significantly high ordering temperature ($> 900$ K). Unlike conventional
FCF, vanishing moment in this case appears to be the result of a combination of
long range antiferromagnetic (AFM) ordering and the inherent B2 disorder of the
crystal. This study opens up the possibility of finding a class of materials
for AFM spintronics, with great significance both from fundamental and applied
fronts. | 2011.00533v1 |
2022-05-16 | First principle studies on electronic and thermoelectric properties of Fe$_{2}$TiSn based multinary Heusler alloys | The alloys with 8/18/24 valence electron count (VEC) are promising candidates
for efficient energy conversion and refrigeration applications at low as well
as high temperatures. The full potential linearized augmented plane wave method
as implemented in WIEN2k code was used to investigate electronic structure and
TE transport properties with the PBE$-$GGA and TB$-$mBJ exchange potentials and
Boltzmann transport theory. The calculated single crystal elastic constants,
phonon dispersion and phonon density of states confirm that these systems are
mechanically and dynamically stable. The TE transport properties is calculated
by including the lattice part of thermal conductivity ($\kappa_{L}$) obtained
from two methods one from the calculated elastic properties calculation
($\kappa^{elastic}_{L}$) and the other from phonon dispersion curve
($\kappa^{phonon}_{L}$). The strong phonon$-$phonon scattering by large mass
difference/strain fluctuation of isovalent/aliovalent substitution at Ti/Sn
sites of Fe$_{2}$TiSn reduces the lattice thermal conductivity which results in
high \textit{ZT} value of 0.81 at 900\,K for
Fe$_{2}$Sc$_{0.25}$Ti$_{0.5}$Ta$_{0.25}$Al$_{0.5}$Bi$_{0.5}$. The comparative
analysis of TE transport properties using the band structures calculated with
the PBE$-$GGA and TB$-$mBJ functional shows that the \textit{ZT} value obtained
from TB$-$mBJ scheme is found to be significantly higher than that based on
PBE$-$GGA. The calculated relatively low lattice thermal conductivity and high
\textit{ZT} values suggest that isovalent/aliovalent substituted Fe$_{2}$TiSn
are promising candidates for medium to high temperature waste heat recovery. | 2205.07688v1 |
2022-12-05 | Quantifying nonadiabaticity in major families of superconductors | The classical Bardeen-Cooper-Schrieffer and Eliashberg theories of the
electron-phonon-mediated superconductivity are based on the Migdal theorem,
which is an assumption that the energy of charge carriers, $k{_B}T{_F}$,
significantly exceeds the phononic energy, $\hbar{\omega{_D}} $, of the
crystalline lattice. This assumption, which is also known as adiabatic
approximation, implies that the superconductor exhibits fast charge carriers
and slow phonons. This picture is valid for pure metals and metallic alloys
because these superconductors exhibit $\hbar{\omega{_D}}$/$k{_B}T{_F}<0.01$.
However, n-type doped semiconducting $SrTiO_3$ was the first superconductor
which beyond this adiabatic approximation, because this material exhibits
$\hbar{\omega{_D}}$/$k{_B}T{_F} $~$ 50$. There is growing number of newly
discovered superconductors which also beyond the adiabatic approximation. Here,
leaving apart pure theoretical aspects of nonadiabatic superconductors, we
classified major classes of superconductors (including, elements, A-15 and
Heusler alloys, Laves phases, intermetallics, noncentrosymmetric compounds,
cuprates, pnictides, highly-compressed hydrides and oxygen, and magic-angle
twisted bilayer graphene) by the strength of nonadiabaticity (for which the
ratio of the Debye temperature to the Fermi temperature, $T{_\theta}/T{_F}$, is
used as a criterion for the nonadiabaticity) versus the superconducting
transition temperature, $T{_c}$. The discussion of this classification scheme
and its relation to other known classification counterparts is given. | 2212.02396v3 |
2023-12-29 | Effect of Point Defects and Lattice Distortions on the Structural, Electronic, and Magnetic properties of Co$_2$MnAl Heusler alloy | The effects of various point defects and lattice distortions on the
structural, electronic, and magnetic properties of Co$_2$MnAl alloy are
investigated using density functional theory calculations. For the point
defects, six types of binary antisites, three types of ternary antisites, and
three kinds of vacancies have been simulated with different disorder degrees,
up to a maximum of 12.50%. For the lattice distortions, cubic strain within
-10% $\leq$$\Delta{V/V_0}$$\leq$ 10% (corresponding to 5.50\r{A} $\leq$ a
$\leq$5.88\r{A}) and tetragonal distortions with
0.5$\leq$$\textit{c/a}$$\leq$1.5 at three different unit-cell volumes -
$\textit{V}_0$ and ($\textit{V}_0$$\pm5$%$\textit{V}_0$) have been considered.
The Co$_{Al}$ and Mn$_{Al}$ binary antisite disordered structures (namely,
Co$_{2.0625}$MnAl$_{0.9375}$, Co$_{2.125}$MnAl$_{0.875}$,
Co$_2$Mn$_{1.0625}$Al$_{0.9375}$ and Co$_2$Mn$_{1.125}$Al$_{0.875}$) and
(Co$_{Al}$+Mn$_{Al}$) ternary antisite disordered structure
(Co$_{2.0625}$Mn$_{1.0625}$Al$_{0.875}$) exhibit perfect half-metallicity. The
rest of the antisite disorders have a marginal effect on the half-metallic
properties of Co$_2$MnAl, along with high spin polarization ($\textit{P}$
$\geq$ 70%) and nearly same magnetization ($\textit{M$_s$}$) as that for ideal
structure. Conversely, the vacancy defects significantly affect the electronic
and magnetic properties. The cubic strained structures exhibit high
$\textit{P}$ and constant $\textit{M$_s$}$. Under negative strain within -10%
$\leq$$\Delta{V/V_0}$$\leq$ -7% (for 5.50\r{A} $\leq$ a $\leq$ 5.58\r{A}), the
strained structures have perfect half-metallicity. On the other hand,
tetragonal distortions lead to significant degradation in half-metallic
behavior, except for small distortion values $\Delta{c/a}$, irrespective of
their volume. | 2312.17545v1 |
2002-10-22 | First-principles calculations of spin spirals in Ni2MnGa and Ni2MnAl | We report here non-collinear magnetic configurations in the Heusler alloys
Ni2MnGa and Ni2MnAl which are interesting in the context of the magnetic shape
memory effect. The total energies for different spin spirals are calculated and
the ground state magnetic structures are identified. The calculated dispersion
curves are used to estimate the Curie temperature which is found to be in good
agreement with experiments. In addition, the variation of the magnetic moment
as a function of the spiral structure is studied. Most of the variation is
associated with Ni, and symmetry constraints relevant for the magnetization are
identified. Based on the calculated results, the effect of the constituent
atoms in determining the Curie temperature is discussed. | 0210482v1 |
2003-07-04 | Detection of weak-order phase transitions in ferromagnets by ac resistometry | It is shown that ac resistometry can serve as an effective tool for the
detection of phase transitions, such as spin reorientation or premartensitic
phase transitions, which generally are not disclosed by dc resistivity
measurement. Measurement of temperature dependence of impedance, $Z(T)$, allows
one to unmask the anomaly, corresponding to a weak-order phase transition. The
appearance of such an anomaly is accounted for by a change in the effective
permeability $\mu$ of a sample upon the phase transition. Moreover, frequency
dependence of $\mu$ makes it possible to use the frequency of the applied ac
current as an adjusting parameter in order to make this anomaly more
pronounced. The applicability of this method is tested for the rare earth Gd
and Heusler alloy Ni$_2$MnGa. | 0307096v1 |
2003-11-13 | Structural, magnetic and transport properties of thin films of the Heusler alloy Co2MnSi | Thin films of Co2MnSi have been grown on a-plane sapphire substrates from
three elemental targets by dc magnetron co-sputtering. These films are single
phase, have a strong (110) texture and a saturation magnetization of 4.95
uB/formula unit at 10 K. Films grown at the highest substrate temperature of
715 K showed the lowest resistivity (47 uOhm cm at 4.2 K) and the lowest
coercivity (18 Oe). The spin polarization of the transport current was found to
be of the order of 54% as determined by point contact Andreev reflection
spectroscopy. A decrease in saturation magnetization with decreasing film
thickness and different transport behaviour in thinner films indicate a graded
disorder in these films grown on non-lattice matched substrates. | 0311316v1 |
2004-03-09 | Resistivity and Thermopower of Ni2.19Mn0.81Ga | In this paper, we report results of the first studies on the thermoelectric
power (TEP) of the magnetic heusler alloy Ni$_{2.19}$Mn$_{0.81}$Ga. We explain
the observed temperature dependence of the TEP in terms of the crystal field
(CF) splitting and compare the observed behavior to that of the stoichiometric
system Ni$_2$MnGa. The resistivity as a function of temperature of the two
systems serves to define the structural transition temperature, T$_M$, which is
the transition from the high temperature austenitic phase to low temperatures
the martensitic phase. Occurrence of magnetic (Curie-Weiss) and the martensitic
transition at almost the same temperature in Ni$_{2.19}$Mn$_{0.81}$Ga has been
explained from TEP to be due to changes in the density of states (DOS) at the
Fermi level. | 0403232v2 |
2004-08-10 | Towards half-metallic interfaces: the Co$_2$CrAl/InP contacts | Although the interest on half-metallic Heusler alloys, susceptible to be used
in spintronic applications, has considerably grown, their interfaces with
semiconductors show very low spin-polarization. I identify mechanisms which can
keep the high spin-polarization at the interface (more than 80% of the
electrons at the Fermi level are of majority spin) although the
half-metallicity is lost. The large enhancement of the Cr moment at the
interface between a CrAl terminated Co$_2$CrAl(001) spacer and the InP(001)
semiconductor weakens the effect of the interface states resulting in this high
spin-polarization. On the other hand the Co$_2$CrAl/InP interfaces made up by a
Co layer and either an In or a P one show a severe decrease of the Co spin
moment but Cr in the subinterface layer is bulklike and the resulting
spin-polarization is similar to the CrAl-based interfaces. | 0408204v3 |
2004-09-13 | Interface properties of the NiMnSb/InP and NiMnSb/GaAs contacts | We study the electronic and magnetic properties of the interfaces between the
half-metallic Heusler alloy NiMnSb and the binary semiconductors InP and GaAs
using two different state-of-the-art full-potential \textit{ab-initio}
electronic structure methods. Although in the case of most NiMnSb/InP(001)
contacts the half-metallicity is lost, it is possible to keep a high degree of
spin-polarization when the interface is made up by Ni and P layers. In the case
of the GaAs semiconductor the larger hybridization between the Ni-$d$ and
As-$p$ orbitals with respect to the hybridization between the Ni-$d$ and P-$p$
orbitals destroys this polarization. The (111) interfaces present strong
interface states but also in this case there are few interfaces presenting a
high spin-polarization at the Fermi level which can reach values up to 74%. | 0409333v1 |
2004-12-22 | Investigation of the growth and magnetic properties of highly oriented films of the Heusler alloy Co2MnSi on GaAs(001) | Highly (001) oriented thin films of Co2MnSi have been grown on lattice
matched GaAs(001) without a buffer layer. Stoichiometric films exhibited a
saturation magnetization slightly reduced from the bulk value and films grown
at the highest substrate temperature of 689 K showed the lowest resistivity (33
micro.ohm.cm at 4.2 K) and the lowest coercivity (14 Oe). The spin polarization
of the transport current was found to be of the order of 55% as determined by
point contact Andreev reflection spectroscopy. The reduced magnetization
obtained was attributed to the antiferromagnetic Mn2As phase. Twofold in-plane
magnetic anisotropy was observed due to the inequivalence of the <110>
directions, and this was attributed to the nature of the bonding at the
reconstructed GaAs surface. | 0412636v1 |
2005-02-24 | Structural and magnetic properties of the (001) and (111) surfaces of the half-metal NiMnSb | Using the full potential linearised augmented planewave method we study the
electronic and magnetic properties of the (001) and (111) surfaces of the
half-metallic Heusler alloy NiMnSb from first-principles. We take into account
all possible surface terminations including relaxations of these surfaces.
Special attention is paid to the spin-polarization at the Fermi level which
governs the spin-injection from such a metal into a semiconductor. In general,
these surfaces lose the half-metallic character of the bulk NiMnSb, but for the
(111) surfaces this loss is more pronounced. Although structural optimization
does not change these features qualitatively, specifically for the (111)
surfaces relaxations can compensate much of the spin-polarization at the Fermi
surface that has been lost upon formation of the surface. | 0502597v1 |
2006-05-15 | Vortex fluctuations and freezing of dipolar-coupled granular moments in thin ferromagnetic films | Below the Curie temperature T_c of a Heusler-alloy film, consisting of
densely packed, but exchange-decoupled nanograins, the spontaneous
magnetization M_s(T) and static in-plane susceptibility \chi_{||}(T) increase
very slowly signalizing a suppression of magnetization fluctuations. The
unpredicted variation \chi_{||}(T) ~ G_d^2(T), where G_d ~ M_s^2 is the
intergranular dipolar coupling, and also the magnetic freezing observed in the
dynamic susceptibility at lower temperatures is quantitatively reproduced by
Monte Carlo (MC) simulations with 10^4 dipolar-coupled moments on a disordered
triangular lattice. At high temperatures, the MC spin configurations clearly
reveal a dense gas of local vortex structures, which at low temperatures
condense in regions with stronger disorder. This vortex depletion upon
decreasing temperature seems to be responsible for the observed
\textit{increase} of the magnetic relaxation time. For weak disorder, the
temperature dependence of the MC vorticity and a singularity of the specific
heat at T_v=1/2 G_d(T_v)/k_B indicate a thermal transition from a vortex gas to
a state with a single vortex center plus linear vortex structures. | 0605377v1 |
2007-11-06 | Half-metallic ferromagnets: From band structure to many-body effects | A review of new developments in theoretical and experimental electronic
structure investigations of half-metallic ferromagnets (HMF) is presented.
Being semiconductors for one spin projection and metals for another ones, these
substances are promising magnetic materials for applications in spintronics
(i.e., spin-dependent electronics). Classification of HMF by the peculiarities
of their electronic structure and chemical bonding is discussed. Effects of
electron-magnon interaction in HMF and their manifestations in magnetic,
spectral, thermodynamic, and transport properties are considered. Especial
attention is paid to appearance of non-quasiparticle states in the energy gap,
which provide an instructive example of essentially many-body features in the
electronic structure. State-of-art electronic calculations for correlated
$d$-systems is discussed, and results for specific HMF (Heusler alloys,
zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed. | 0711.0872v1 |
2008-10-10 | Anomalous Magnetic Properties in Ni50Mn35In15 | We present here a comprehensive investigation of the magnetic ordering in
Ni50Mn35In15 composition. A concomitant first order martensitic transition and
the magnetic ordering occurring in this off-stoichiometric Heusler compound at
room temperature signifies the multifunctional character of this magnetic shape
memory alloy. Unusual features are observed in the dependence of the
magnetization on temperature that can be ascribed to a frustrated magnetic
order. It is compelling to ascribe these features to the cluster type
description that may arise due to inhomogeneity in the distribution of magnetic
atoms. However, evidences are presented from our ac susceptibility, electrical
resistivity and dc magnetization studies that there exists a competing
ferromagnetic and antiferromagnetic order within crystal structure of this
system. We show that excess Mn atoms that substitute the In atoms have a
crucial bearing on the magnetic order of this compound. These excess Mn atoms
are antiferromagnetically aligned to the other Mn, which explains the peculiar
dependence of magnetization on temperature. | 0810.1850v1 |
2008-11-14 | Effect of chemical disorder on NiMnSb investigated by Appearance Potential Spectroscopy: a theoretical study | The half-Heusler alloy NiMnSb is one of the local-moment ferromagnets with
unique properties for future applications. Band structure calculations predict
exclusively majority bands at the Fermi level, thus indicating {100%} spin
polarization there. As one thinks about applications and the design of
functional materials, the influence of chemical disorder in these materials
must be considered. The magnetization, spin polarization, and electronic
structure are expected to be sensitive to structural and stoichiometric
changes. In this contribution, we report on an investigation of the
spin-dependent electronic structure of NiMnSb. We studied the influence of
chemical disorder on the unoccupied electronic density of states by use of the
ab-initio Coherent Potential Approximation method. The theoretical analysis is
discussed along with corresponding spin-resolved Appearance Potential
Spectroscopy measurements. Our theoretical approach describes the spectra as
the fully-relativistic self-convolution of the matrix-element weighted,
orbitally resolved density of states. | 0811.2300v1 |
2009-01-21 | Ruderman-Kittel-Kasuya-Yosida spin density oscillations: impact of the finite radius of the exchange interaction | A non-interacting electron gas on a one-dimensional ring is considered at
finite temperatures. The localized spin is embedded at some point on the ring
and it is assumed that the interaction between this spin and the electrons is
the exchange interaction being the basis of the Ruderman-Kittel-Kasuya-Yosida
indirect exchange effect. When the number of electrons is large enough, it
turns out that any small but finite interaction radius value can always produce
an essential change of the spin density oscillations in comparison with the
zero interaction radius traditionally used to model the
Ruderman-Kittel-Kasuya-Yosida effect. | 0901.3339v2 |
2009-12-04 | A new spin-functional MOSFET based on magnetic tunnel junction technology: pseudo-spin-MOSFET | We fabricated and characterized a new spin-functional MOSFET referred to as a
pseudo-spin-MOSFET (PS-MOSFET). The PS-MOSFET is a circuit using an ordinary
MOSFET and magnetic tunnel junction (MTJ) for reproducing functions of
spin-transistors. Device integration techniques for a bottom gate MOSFET using
a silicon-on-insulator (SOI) substrate and for an MTJ with a full-Heusler alloy
electrode and MgO tunnel barrier were developed. The fabricated PS-MOSFET
exhibited high and low transconductance controlled by the magnetization
configurations of the MTJ at room temperature. This is the first observation of
spin-transistor operations for spin-functional MOSFETs. | 0912.0835v1 |
2010-02-19 | Interplay of magnetic order and Jahn-Teller distortion in a model with strongly correlated electron system | The Hubbard model has been employed successfully to understand many aspects
of correlation driven physical properties, in particular, the magnetic order in
itenerant electron systems. In some systems such as Heusler alloys, manganites
etc., it is known that, in addition to magnetic order, distortion induced by
Jahn-Teller(J-T) effect also exists. In this paper, based on two-fold
degenerate Hubbard model, the influence of magnetic order on J-T distortion is
investigated. The electron correlation is treated using a spectral density
approach and J-T interaction is added to the model. We find that magnetic order
and structural distortion coexist at low temperature $T$ for a certain range of
electron correlation strength $U$, J-T coupling strength $G$ and band
occupation $n$. At T=0, for a given $n$ and $U$, magnetic order is present but
distortion appears only for a $G$ larger than a critical value. We also studied
the temperature dependence of lattice strain and magnetization choosing a $G$
close to the critical value. | 1002.3718v2 |
2010-02-28 | A Frequency-Controlled Magnetic Vortex Memory | Using the ultra low damping NiMnSb half-Heusler alloy patterned into
vortex-state magnetic nano-dots, we demonstrate a new concept of non-volatile
memory controlled by the frequency. A perpendicular bias magnetic field is used
to split the frequency of the vortex core gyrotropic rotation into two distinct
frequencies, depending on the sign of the vortex core polarity $p=\pm1$ inside
the dot. A magnetic resonance force microscope and microwave pulses applied at
one of these two resonant frequencies allow for local and deterministic
addressing of binary information (core polarity). | 1003.0158v1 |
2010-03-26 | Tunnel effect in ferromagnetic half-metal Co$_2$CrAl-superconductor heterostructures | Ferromagnetic half-metal Co$_2$CrAl films and tunnel contacts Co$_2$CrAl -
insulator (I) - Pb are fabricated and investigated. It is found that the
normalized differential conductivity $\sigma ^{\rm FS} $ of such tunnel
junctions with low resistance is larger than the normalized differential
conductivity $\sigma ^{\rm NS} $ of known normal metal - I - superconductor
type tunnel junctions. It is shown that the observed increase in $\sigma ^{\rm
FS} $ is caused by the accumulation of spin polarized electrons in a
superconductor and can be used for estimating the spin polarization degree $P$
in ferromagnets. This method shows that $P$ of L2$_1$-type ordered Co$_2$CrAl
Heusler alloy films at $T = 4.2{\rm K}$ is close to 1. | 1003.5093v1 |
2012-04-03 | Effect of interfacial strain on spin injection and spin polarization of Co2CrAl/NaNbO3/Co2CrAl magnetic tunneling junction | First-principles calculations were carried out to investigate interfacial
strain effects on spin injection and spin polarization of a magnetic tunnel
junction consisting of half-metallic full-Heusler alloy Co2CrAl and
ferroelectric perovskite NaNbO3. Spin-dependent coherent tunneling was
calculated within the framework of non-equilibrium Green's function technique.
Both spin polarization and tunnel magnetoresistance (TMR) are affected by the
interfacial strain but their responses to compressive and tensile strains are
different. Spin polarization across the interface is fully preserved under a
compressive strain due to stronger coupling between interfacial atoms, whereas
a tensile strain significantly enhances interface states and lead to
substantial drops in spin polarization and TMR. | 1204.0611v1 |
2012-06-11 | Residual disorder and diffusion in thin Heusler alloy films | Co2FeSi/GaAs(110) and Co2FeSi/GaAs(111)B hybrid structures were grown by
molecular-beam epitaxy and characterized by transmission electron microscopy
(TEM) and X-ray diffraction. The films contained inhomogeneous distributions of
ordered L2_1 and B2 phases. The average stoichiometry was controlled by lattice
parameter measurements, however diffusion processes lead to inhomogeneities of
the atomic concentrations and the degradation of the interface, influencing
long-range order. An average long-range order of 30-60% was measured by
grazing-incidence X-ray diffraction, i.e. the as-grown Co2FeSi films were
highly but not fully ordered. Lateral inhomogeneities of the spatial
distribution of long-range order in Co2FeSi were found using dark-field TEM
images taken with superlattice reflections. | 1206.2242v1 |
2012-08-15 | Two-dimensional electron-gas-like charge transport at magnetic Heusler alloy-SrTiO$_3$ interface | We report remarkably low residual resistivity, giant residual resistivity
ratio, free-electron-like Hall resistivity and high mobility ($\approx$ 10$^4$
cm$^2$V$^{-1}$s$^{-1}$) charge transport in epitaxial films of Co$_2$MnSi and
Co$_2$FeSi grown on (001) SrTiO$_3$. This unusual behavior is not observed in
films deposited on other cubic oxide substrates of comparable lattice
parameters. The scaling of the resistivity with thickness of the films allow
extraction of interface conductance, which can be attributed to a layer of
oxygen vacancies confined within 1.9 nm of the interface as revealed by
atomically resolved electron microscopy and spectroscopy. The high mobility
transport observed here at the interface of a fully spin polarized metal is
potentially important for spintronics applications. | 1208.3099v2 |
2013-03-29 | Formation of double ring patterns on Co2MnSi Heusler alloy thin film by anodic oxidation under scanning probe microscope | Double ring formation on Co2MnSi (CMS) films is observed at electrical
breakdown voltage during local anodic oxidation (LAO) using atomic force
microscope (AFM). Corona effect and segregation of cobalt in the vicinity of
the rings is studied using magnetic force microscopy and energy dispersive
spectroscopy. Double ring forma-tion is attributed to the interaction of
ablated material with the induced magnetic field during LAO. Steepness of
forward bias transport characteristics from the unperturbed region of the CMS
film suggest a non equilibrium spin contribution. Such mesoscopic textures in
magnetic films by AFM tip can be potentially used for memory storage
applications. | 1303.7470v2 |
2013-05-30 | Evolution of ferromagnetic and spin-wave resonances with crystalline order in thin films of full-Heusler alloy Co2MnSi | We report the evolution of magnetic moment as well as magnetic anisotropy
with crystalline order in Co$_2$MnSi thin films grown on $(100)$ MgO by pulsed
laser deposition. The films become more ordered as the annealing temperature
($T_A$) increases from 400 to 600 $^0$C. The extent of \emph{L}$2_1$ ordering
in the films annealed at 600 $^0$C is $\approx 96%$. The static magnetization
measurements by vibrating sample magnetometry shows a maximum moment of 4.95
$\mu_B$ per formula unit with low coercivity ($H_C$ $\approx$ 65 Oe) in the
films annealed at 600 $^0$C. A rigorous analysis of the azimuthal and polar
angle dependent ferromagnetic resonance (FMR) measured at several temperatures
allows determination of various anisotropy fields relevant to our system as a
function of $T_A$. Finally, we have evaluated the exchange stiffness constant
down to 100 K using spin wave modes in FMR spectra. We have also estimated the
exchange energies as well as stiffness constant by varying the lattice
parameter \emph{ab-initio} using the Korringa-Kohn-Rostoker method. | 1305.7048v1 |
2013-07-14 | Effect of annealing on the magnetic, magnetocaloric and magnetoresistance properties of Ni-Co-Mn-Sb melt spun ribbons | The structural, magnetic, magnetocaloric and magnetotransport properties of
Ni46Co4Mn38Sb12 melt spun ribbons have been systematically investigated. The
partially ordered B2 phase of the as-spun ribbon transforms to fully ordered
L21 phase upon annealing, which signifies a considerable change of the atomic
ordering in the system. The presence of atomic disorder in the as-spun ribbon
gives rise to a higher martensitic transition temperature and a lower
magnetization as compared to the bulk sample. However, annealing the ribbons
helps in regaining the bulk properties to a large extent. Significant changes
in magnetocaloric effect, exchange bias and magnetoresistance have been
observed between the as-spun and the annealed ribbons, indicating the role of
atomic ordering on the functional as well as fundamental properties in the
Heusler system. Importantly, the study shows that one can reduce the hysteresis
loss by preparing melt spun alloys and subjecting them to appropriate annealing
conditions, which enable them to become practical magnetic refrigerants. | 1307.3776v1 |
2013-07-14 | In-plane and out of plane magnetic properties in Ni46Co4Mn38Sb12 Heusler alloys ribbons | Magnetic, magnetocaloric and exchange bias properties have been
systematically investigated in Ni46Co4Mn38Sb12 ribbon by applying magnetic
field along (IP) and perpendicular (OP) to the ribbon plane. From the
thermo-magnetization curves, the sharpness of the martensitic transition is
observed to be nearly the same for both IP and OP ribbons. The thermomagnetic
irreversibility region is found to be larger in the OP ribbon at 500 Oe,
indicating that the magnetic anisotropy is larger in this case. The OP ribbon
shows the Hopkinson maximum at 500 Oe, both for the FCC and ZFC modes. The
magnetization curve for IP ribbon shows a faster approach to saturation,
compared to the OP ribbon. Isothermal magnetic entropy change at 50 kOe has
been found to be nearly same for both the ribbons. At 5 K the coercivity and
exchange bias values are larger for the OP ribbon. Crystallographic texturing
of the ribbons and its effect in the easy magnetization direction are found to
be the reason behind the differences between the two ribbons. | 1307.3778v1 |
2014-01-31 | First-principles calculation of the instability leading to giant inverse magnetocaloric efects | The structural and magnetic properties of functional Ni-Mn-Z (Z = Ga, In, Sn)
Heusler alloys are studied by first-principles and Monte Carlo methods. The
\textit{ab initio} calculations give a basic understanding of the underlying
physics which is associated with the strong competition of ferro- and
antiferromagnetic interactions with increasing chemical disorder. The resulting
$d$-electron orbital dependent magnetic ordering is the driving mechanism of
magnetostructural instability which is accompanied by a drop of magnetization
governing the size of the magnetocaloric effect. The thermodynamic properties
are calculated by using the \textit{ab initio} magnetic exchange coupling
constants in finite-temperature Monte Carlo simulations, which are used to
accurately reproduce the experimental entropy and adiabatic temperature changes
across the magnetostructural transition. | 1401.8148v1 |
2014-02-28 | The fate of half-metallicity near interfaces: the case of NiMnSb/MgO and NiMnSi/MgO | The electronic and magnetic properties of the interfaces between the
half-metallic Heusler alloys NiMnSb, NiMnSi and MgO have been investigated
using first-principles density-functional calculations with projector augmented
wave potentials generated in the generalized gradient approximation. In the
case of the NiMnSb/MgO (100) interface the half-metallicity is lost, whereas
the MnSb/MgO contact in the NiMnSb/MgO (100) interface maintains a substantial
degree of spin polarization at the Fermi level ($\sim 60$%). Remarkably, the
NiMnSi/MgO (111) interface shows 100% spin polarization at the Fermi level,
despite considerable distortions at the interface, as well as rather short Si/O
bonds after full structural optimization. This behavior markedly distinguishes
NiMnSi/MgO (111) from the corresponding NiMnSb/CdS and NiMnSb/InP interfaces. | 1403.0889v1 |
2014-03-14 | Investigation on Mn$_{3-δ}$Ga/MgO interface for magnetic tunneling junctions | The Mn$_3$Ga Heusler compound and related alloys are the most promising
materials for the realization of spin-transfer-torque switching in magneto
tunneling junctions. Improved performance can be achieved by high quality
interfaces in these multilayered structured devices. In this context, the
interface between Mn$_{1.63}$Ga and MgO is of particular interest because of
its spin polarization properties in tunneling junctions. We performed a
chemical characterization of the MgO/Mn$_{1.63}$Ga junction by hard x-ray
photoelectron spectroscopy (HAXPES). The experiment indicated the formation of
Ga-O bonds at the interface and evidenced changes in the local environment of
Mn atoms in the proximity of the MgO film. In addition, we show that the
insertion of a metallic Mg-layer interfacing the MgO and Mn--Ga film strongly
suppresses the oxidation of gallium. | 1403.3556v4 |
2014-11-03 | Importance of two current model in understanding the electronic transport behavior of inverse Heusler alloy: Fe 2 CoSi | Here we explore the applicability of the two current model in understanding
the transport behavior of Fe 2 CoSi compound by using the first principles
calculations in combination with the Boltzmann transport theory. The
spin-unpolarized calculation shows large density of states (DOS) at Fermi level
(E F) and is unable to provide the correct temperature dependence of transport
coefficients. The spin-polarised calculation shows reduced DOS at the E F in
the spin-up channel, whereas spin-dn channel have almost zero DOS at the E F .
The absolute value of Seebeck coefficient in the spin-up channel shows linear
increment with the temperature and in the spin-dn channel it varies
non-linearly. The electrical conductivity also shows non-linear temperature
dependence in both the spin channels whereas, the electronic thermal
conductivity shows linear temperature dependence. The values of transport
coefficients and their temperature dependence obtained by using the two current
model are found to be in fairly good agreement with the experimental data.
Present work clearly suggests the importance of two current model in
understanding the transport properties of the compound. | 1411.0367v1 |
2015-05-29 | Strain behavior and lattice dynamics in Ni50Mn35In15 | The lattice dynamics in the polycrystalline shape-memory Heusler alloy
Ni$_{50}$Mn$_{35}$In$_{15}$ has been studied by means of resonant ultrasound
spectroscopy (RUS). RUS spectra were collected in a frequency range $100-1200$
kHz between 10 and 350 K. Ni$_{50}$Mn$_{35}$In$_{15}$ exhibits a ferromagnetic
transition at 313 K in the austenite and a martensitic transition at 248 K
accompanied by a change of the magnetic state. Furthermore it displays a
antiferromagnetic to ferromagnetic transition within the martensitic phase. We
determined the temperature dependence of the shear modulus and the acoustic
attenuation of Ni$_{50}$Mn$_{35}$In$_{15}$ and compared it with magnetization
data. Following the structural softening, which accompanies the martensitic
transition as a pretransitional phenomenon, a strong stiffening of the lattice
is observed at the martensitic magneto-structural transition. Only a weak
magnetoelastic coupling is evidenced at the Curie temperatures both in
austenite and martensite phase. The large acoustic damping in the martensitic
phase compared with the austenitic phase reflects the motion of the twin walls,
which freezes out in the low temperature region. | 1505.08090v1 |
2015-06-29 | Magnetization and magneto-transport studies on Fe$_2$VAl$_{1-x}$Si$_x$ | We report on magnetoresistance, Hall and magnetization measurements of
Fe2VAl1-xSix Heusler compounds for x= 0.005, 0.015, 0.02. There is a systematic
change in the temperature coefficient of resistance (TCR) from negative to
positive as the Si composition is increased. The Hall co-efficient shows that
the carriers are electron like and the carrier density increases with Si
concentration. Resistance measurements under magnetic field indicate a
decreasing behavior under the application of magnetic field at low temperature
region (T< 60 K), suggesting the suppression of scattering by magnetic field.
Temperature and field dependent magnetization measurements did not show any
significant change apart from the fact that the presence of super paramagnetic
(SPM) cluster and its ordering at low temperatures. Arrott plot analysis of
magnetization versus field also indicates the magnetic ordering with applied
field below 60 K. | 1506.08516v1 |
2015-07-22 | An indirect magnetic approach for determining entropy change in first-order magnetocaloric materials | Taking into account the phase fraction during transition for the first-order
magnetocaloric materials, an improved isothermal entropy change determination
has been put forward based on the Clausius-Clapeyron (CC) equation. It was
found that the isothermal entropy change value evaluated by our method is in
excellent agreement with those determined from the Maxwell-relation (MR) for
Ni-Mn-Sn Heusler alloys, which usually presents a weak field-induced phase
transforming behavior. In comparison with MR, this method could give rise to a
favorable result derived from few thermomagnetic measurements. More
importantly, we can eliminate the isothermal entropy change overestimation
derived from MR, which always exists in the cases of Ni-Co-Mn-In and MnAs
systems with a prominent field-induced transition. These results confirmed that
such a CC-equation-based method is quite practical and superior to the MR-based
ones in eliminating the spurious spike and reducing measuring cost. | 1507.06133v2 |
2015-10-09 | Better Band Gaps with Asymptotically Corrected Local Exchange Potentials | We formulate a spin-polarized van Leeuwen and Baerends (vLB) correction to
the local density approximation (LDA) exchange potential [Phys. Rev. A 49, 2421
(1994)] that enforces the ionization potential (IP) theorem following Stein et
al. [Phys. Rev. Lett. 105, 266802 (2010)]. For electronic-structure problems,
the vLB-correction replicates the behavior of exact-exchange potentials, with
improved scaling and well-behaved asymptotics, but with the computational cost
of semi-local functionals. The vLB+IP corrections produces large improvement in
the eigenvalues over that from LDA due to correct asympotic behavior and atomic
shell structures, as shown on rare-gas, alkaline-earth, zinc-based oxides,
alkali-halides, sulphides, and nitrides. In half-Heusler alloys, this
asymptotically-corrected LDA reproduces the spin-polarized properties
correctly, including magnetism and half-metallicity. We also considered
finite-sized systems [e.g., ringed boron-nitirde (B$_{12}$N$_{12}$) and
graphene (C$_{24}$)] to emphasize the wide applicability of the method. | 1510.02688v3 |
2016-05-13 | Spin injection into multilayer graphene from highly spin-polarized Co2FeSi Heusler alloy | We demonstrate electrical spin injection into multilayer graphene (MLG) in a
lateral spin valve device from a highly spin-polarized Co2FeSi (CFS) Huesler
electrode. Exfoliated MLG was transferred onto pre-patterned epitaxial CFS
wires grown on an Si(111) substrate by a polymer-based transfer method. This
method enabled us to fabricate multiple single-crystal CFS electrodes in
contact with MLG. Electrical spin injection from CFS to MLG was detected
through non-local magnetoresistance (MR) measurement. A non-local spin signal
of 430 ohm was observed; this is the largest value among all reported non-local
MR values in graphene-based devices. | 1605.04213v1 |
2016-08-31 | Spin polarization ratios of resistivity and density of states estimated from anisotropic magnetoresistance ratio for nearly half-metallic ferromagnets | We derive a simple relational expression between the spin polarization ratio
of resistivity, $P_\rho$, and the anisotropic magnetoresistance ratio $\Delta
\rho/\rho$, and that between the spin polarization ratio of the density of
states at the Fermi energy, $P_{\rm DOS}$, and $\Delta \rho/\rho$ for nearly
half-metallic ferromagnets. We find that $P_\rho$ and $P_{\rm DOS}$ increase
with increasing $|\Delta \rho/\rho|$ from 0 to a maximum value. In addition, we
roughly estimate $P_\rho$ and $P_{\rm DOS}$ for a Co$_2$FeGa$_{0.5}$Ge$_{0.5}$
Heusler alloy by substituting its experimentally observed $\Delta \rho/\rho$
into the respective expressions. | 1608.08888v1 |
2016-10-12 | Spin injection and detection up to room temperature in Heusler~alloy/$n$-GaAs spin valves | We have measured the spin injection efficiency and spin lifetime in
Co$_2$FeSi/$n$-GaAs lateral nonlocal spin valves from 20 to 300 K. We observe
large ($\sim$40 $\mu$V) spin valve signals at room temperature and injector
currents of $10^3~$A/cm$^2$, facilitated by fabricating spin valve separations
smaller than the 1 $\mu$m spin diffusion length and applying a forward bias to
the detector contact. The spin transport parameters are measured by comparing
the injector-detector contact separation dependence of the spin valve signal
with a numerical model accounting for spin drift and diffusion. The apparent
suppression of the spin injection efficiency at the lowest temperatures
reflects a breakdown of the ordinary drift-diffusion model in the regime of
large spin accumulation. A theoretical calculation of the D'yakonov-Perel spin
lifetime agrees well with the measured $n$-GaAs spin lifetime over the entire
temperature range. | 1610.03797v1 |
2017-03-29 | Observation of Skyrmions at Room Temperature in Co2FeAl Heusler Alloy Ultrathin Films | Magnetic skyrmions are topological spin structures having immense potential
for energy efficient spintronic devices. However, observations of skyrmions at
room temperature are limited to patterned nanostructures. Here, we report the
observation of stable skyrmions in unpatterned Ta/Co2FeAl(CFA)/MgO thin film
heterostructures at room temperature and in zero external magnetic field
employing magnetic force microscopy. The skyrmions are observed in a trilayer
structure comprised of heavy metal (HM)/ferromagnet (FM)/Oxide interfaces which
result in strong interfacial Dzyaloshinskii-Moriya interaction (i-DMI) as
evidenced by Brillouin light scattering measurements, in agreement with the
results of micromagnetic simulations. We also emphasize on room temperature
observation of multiple skyrmions which can be stabilized for suitable choices
of CFA layer thickness, perpendicular magnetic anisotropy, and i-DMI. These
results open up a new paradigm for designing room temperature spintronic
devices based on skyrmions in FM continuous thin films. | 1703.10224v1 |
2017-07-27 | Reversible tuning of magnetocaloric Ni-Mn-Ga-Co films on ferroelectric PMN-PT substrates | Tuning functional properties of thin caloric films by mechanical stress is
currently of high interest. In particular, a controllable magnetisation or
transition temperature is desired for improved usability in magnetocaloric
devices. Here, we present results of epitaxial magnetocaloric Ni-Mn-Ga-Co thin
films on ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)$_{0.72}$Ti$_{0.28}$O$_3$
(PMN-PT) substrates. Utilizing X-ray diffraction measurements, we demonstrate
that the strain induced in the substrate by application of an electric field
can be transferred to the thin film, resulting in a change of the lattice
parameters. We examined the consequences of this strain on the magnetic
properties of the thin film by temperature and electric field dependent
measurements. We did not observe a change of martensitic transformation
temperature but a reversible change of magnetisation within the austenitic
state, which we attribute to the intrinsic magnetic instability of this
metamagnetic Heusler alloy. | 1707.08839v1 |
2017-08-03 | Room-temperature spin transport in n-Ge probed by four-terminal nonlocal measurements | We demonsrtate electrical spin injection and detection in $n$-type Ge
($n$-Ge) at room temperature using four-terminal nonlocal spin-valve and
Hanle-effect measurements in lateral spin-valve (LSV) devices with
Heusler-alloy Schottky tunnel contacts. The spin diffusion length
($\lambda$$_{\rm Ge}$) of the Ge layer used ($n \sim$ 1 $\times$ 10$^{19}$
cm$^{-3}$) at 296 K is estimated to be $\sim$ 0.44 $\pm$ 0.02 $\mu$m.
Room-temperature spin signals can be observed reproducibly at the low bias
voltage range ($\le$ 0.7 V) for LSVs with relatively low resistance-area
product ($RA$) values ($\le$ 1 k$\Omega$$\mu$m$^{2}$). This means that the
Schottky tunnel contacts used here are more suitable than ferromagnet/MgO
tunnel contacts ($RA \ge$ 100 k$\Omega$$\mu$m$^{2}$) for developing Ge
spintronic applications. | 1708.00962v1 |
2017-09-19 | Effect of Co substitution on Ni$_{2}$MnGe Heusler alloy: ab initio study | Ab initio calculations shown that the Co substitution instead of Ni in
Ni$_{2}$MnGe with the L2$_{1}$ crystallographic structure leads to a decrease
of the lattice constant and an increase of the total magnetic moment of the
Ni$_{2-x}$Co$_{x}$MnGe compounds. The Mn(B) has the largest local moment above
$3~\mu_{B}$ coupled parallel to moments on the Ni(A,C) and Co(A,C), which are
found in the ranges of $0.19\div0.26~\mu_{B}$ for Ni(A,C) and
$1.03\div0.97~\mu_{B}$ for Co(A,C) for studied range of $x$. Using the results
stemming from the total energy calculations, the values of bulk modulus and its
pressure derivatives are estimated according to the Murnaghan EOS. | 1709.06490v1 |
2017-12-06 | First-principles investigation of magnetocrystalline anisotropy oscillations in Co$_{2}$FeAl/Ta heterostructures | We report first-principles investigations of magnetocrystalline anisotropy
energy (MCAE) oscillations as a function of capping layer thickness in Heusler
alloy Co\textsubscript{2}FeAl/Ta heterostructures. Substantial oscillation is
observed in FeAl-interface structure. According to $k$-space and
band-decomposed charge density analyses, this oscillation is mainly attributed
to the Fermi-energy-vicinal quantum well states (QWS) which are confined
between Co\textsubscript{2}FeAl/Ta interface and Ta/vacuum surface. The smaller
oscillation magnitude in the Co-interface structure can be explained by the
smooth potential transition at the interface. These findings clarify that MCAE
in Co\textsubscript{2}FeAl/Ta is not a local property of the interface and that
the quantum well effect plays a dominant role in MCAE oscillations of the
heterostructures. This work presents the possibility of tuning MCAE by QWS in
capping layers, and paves the way for artificially controlling magnetic
anisotropy energy in magnetic tunnel junctions. | 1712.02139v2 |
2018-06-03 | Chiral Magnetic Skyrmions with Arbitrary Topological Charge ("skyrmionic sacks") | We show that continuous and spin-lattice models of chiral ferro- and
antiferromagnets provide the existence of an infinite number of stable soliton
solutions of any integer topological charge. A detailed description of the
morphology of new skyrmions and the corresponding energy dependencies are
provided. The considered model is general, and is expected to predict a
plethora of particle-like states which may occur in various chiral magnets
including atomic layers, e.g., PdFe/Ir(111), rhombohedral GaV$_4$S$_8$
semiconductor, B20-type alloys as Mn$_{1-x}$Fe$_x$Ge, Mn$_{1-x}$Fe$_x$Si,
Fe$_{1-x}$Co$_x$Si, Cu$_2$OSeO$_3$, acentric tetragonal Heusler compounds. | 1806.00782v2 |
2019-01-24 | Effect of secondary LuNiSn phase on thermoelectric properties of half-Heusler alloy LuNiSb | We report on the high-temperature (350 K <T< 1000 K) thermoelectric
properties of the ternary compounds LuNiSb (cubic) and LuNiSn (orthorhombic)
and a composite material (LuNiSb)$_{0.5}$(LuNiSn)$_{0.5}$. The electrical
conductivity in LuNiSn is metallic, while it is semiconducting-like in LuNiSb.
The Seebeck coefficient reaches -5.5 $\mu$V/K at 700 K for the former compound
and 66 $\mu$V/K at 607 K for the latter one. The composite sample
(LuNiSb)$_{0.5}$(LuNiSn)$_{0.5}$ constructed from orthorhombic matrix with
cubic inclusions combines the electrical conductivity of LuNiSn with the
thermoelectric properties of LuNiSb. Nonetheless, no enhancement of the
thermoelectric performance occurs for this material. | 1901.08399v1 |
2018-08-06 | Formation of Spatial Patterns by Spin-selective Excitations of Interacting Fermions | We describe the formation of charge- and spin-density patterns induced by
spin-selective photoexcitations of interacting fermionic systems in the
presence of a microstructure. As an example, we consider a one-dimensional
Hubbard-like system with a periodic magnetic microstructure, which has a
uniform charge distribution in its ground state, and in which a long-lived
charge-density pattern is induced by the spin-selective photoexcitation. Using
tensor-network methods, we study the full quantum dynamics in the presence of
electron-electron interactions and identify doublons as the main decay channel
for the induced charge pattern. Our setup is compared to the OISTR mechanism,
in which ultrafast optically induced spin transfer in Heusler and magnetic
compounds is associated to the difference of the local density of states of the
different elements in the alloys. We find that applying a spin-selective
excitation there induces spatially periodic patterns in local observables.
Implications for pump-probe experiments on correlated materials and experiments
with ultracold gases on optical lattices are discussed. | 1808.02138v3 |
2020-04-02 | $d$ band filling and magnetic phase separation in transition metal-doped Mn$_3$SnC | The structural and magnetic properties of transition metal-doped Mn$_3$SnC
are studied with an aim to understand the effect of transition metal atom on
magnetostructural properties of the antiperovskite compound. The doped
Mn$_{2.8}$T$_{0.2}$SnC (T = Cr, Fe, Co, Ni and Cu) compounds show a distinctly
different magnetic behavior which can be related to electronic filling of the
$d$ band of the transition metal atom. While Cr and Fe doped Mn$_3$SnC show
properties similar to that of Mn$_3$SnC, the Co, Ni and Cu doped compounds
exhibit nucleation of secondary phases which are devoid of carbon and having
Heusler and DO19 type hexagonal structure. A strong magnetic interaction is
observed between the impurity phases and the major antiperovskite phase leading
to a sharp decrease in magnetostructural transition temperature of the
antiperovskite phase and a cluster glassy ground state. | 2004.00804v1 |
2020-04-21 | Building hierarchical martensite | Martensitic materials show a complex, hierarchical microstructure containing
structural domains separated by various types of twin boundaries. Several
concepts exist to describe this microstructure on each length scale, however,
there is no comprehensive approach bridging the whole range from the nano- up
to the macroscopic scale. Here, we describe for a Ni-Mn-based Heusler alloy how
this hierarchical microstructure is built from scratch with just one key
parameter: the tetragonal distortion of the basic building block at the atomic
level. Based on this initial block, we introduce five successive levels of
nested building blocks. At each level, a larger building block is formed by
twinning the preceding one to minimise the relevant energy contributions
locally. This naturally explains the occurrence of different types of twin
boundaries. We compare this scale-bridging approach of nested building blocks
with experiments in real and reciprocal space. Our approach of nested building
blocks is versatile as it can be applied to the broad class of functional
materials exhibiting diffusionless transformations. | 2004.09768v1 |
2020-05-07 | Ultrafast All Optical Magnetization Control for Broadband Terahertz Spin Wave Generation | Terahertz spin waves could be generated on-demand via all-optical
manipulation of magnetization by femtosecond laser pulse. Here, we present an
energy balance model, which explains the energy transfer rates from laser pulse
to electron bath coupled with phonon, spin, and magnetization of five different
magnetic metallic thin films: Iron, Cobalt, Nickel, Gadolinium and Ni$_{2}$MnSn
Heusler alloy. Two types of transient magnetization dynamics emerge in metallic
magnetic thin films based on their Curie temperatures (T$_{C}$): type I (Fe,
Co, and Ni with T$_{C}$ > room temperature, RT) and type II films (Gd and
Ni$_{2}$MnSn with T$_{C}$ $\approx$ RT). We study the effect of laser fluence
and pulse width for single Gaussian laser pulses and the effect of metal film
thickness on magnetization dynamics. Spectral dynamics show that broadband spin
waves up to 24 THz could be generated by all-optical manipulation of
magnetization in these nanofilms. | 2005.03493v1 |
2016-03-30 | Magnetic Interactions and Electronic Structure of Pt$_{2}$Mn$_{1-x}$Y$_{x}$Ga (Y = Cr and Fe) system : An ab-initio calculation | First principles density functional theory based calculations have been
carried out to predict the effects of Mn replacement by Fe and Cr on electronic
as well as magnetic properties of Pt$_{2}$MnGa as well as Ni$_{2}$MnGa. All the
materials studied here are predicted to have conventional Heusler alloy
structure in their ground state and they are found to be electronically stable
on the basis of their respective formation energy. The replacement of Mn by Fe
leads to a ferromagnetic ground state whereas in case of Mn replacement by Cr
an {\it intra-sublattice} anti-ferromagnetic configuration has been observed to
have lower energy. We study the magnetic exchange interaction between the atoms
for the materials with ferromagnetic and anti-ferromagnetic configurations to
show the effects of Fe and Cr substitution at Mn site on the magnetic
interactions of these systems. Detailed analysis of electronic structure in
terms of density of states has been carried out to study the effect of
substitution. | 1603.09139v1 |
2016-12-22 | Type-II Dirac semimetals in the YPd$_2$Sn class | The Lorentz-invariance-violating Weyl and Dirac fermions have recently
attracted intensive interests as new types of particles beyond high-energy
physics, and they demonstrate novel physical phenomena such as angle-dependent
chiral anomaly and topological Lifshitz transition. Here we predict the
existence of Lorentz-invariance-violating Dirac fermions in the YPd$_2$Sn class
of Heusler alloys that emerge at the boundary between the electron-like and
hole-like pockets in the Brillouin zone, based on the first-principles
electronic structure calculations. In combination with the fact that this class
of materials was all reported to be superconductors, the YPd$_2$Sn class
provides an appropriate platform for studying exotic physical properties
distinguished from conventional Dirac fermions, especially for realizing
possible topological superconductivity. | 1612.07456v1 |
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