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2016-11-08 | Geometry of Adaptive Martensite in Ni-Mn-based Heusler alloys | Modulated martensites play an important role in magnetic shape memory alloys,
because all functional properties are closely connected to the twin
microstructure and the phase boundary. The nature of the modulated martensites
is still unclear. One approach is the concept of adaptive martensite, which
regards all modulated phases as nanotwinned microstructures. In this article,
we use the Ni-Mn-based shape memory alloys as an example to show the geometric
rationale behind this concept using analytic equations based on the
phenomenological theory of martensite. This could enhance discussions about the
implications of the adaptive martensite by showing the exact relations between
the various unit cells used to describe the structure. We use the concept to
discuss the compatibility at the habit plane, the nature of high-order twin
boundaries and the dependence of the lattice constants on the different types
of modulation. | 1611.02535v2 |
2020-10-06 | Tailoring magnetocaloric effect in all-d-metal Ni-Co-Mn-Ti Heusler alloys: a combined experimental and theoretical study | Novel Ni-Co-Mn-Ti all-d-metal Heusler alloys are exciting due to large
multicaloric effects combined with enhanced mechanical properties. An optimized
heat treatment for a series of these compounds leads to very sharp phase
transitions in bulk alloys with isothermal entropy changes of up to 38 J
kg$^{-1}$ K$^{-1}$ for a magnetic field change of 2 T. The differences of
as-cast and annealed samples are analyzed by investigating microstructure and
phase transitions in detail by optical microscopy. We identify different grain
structures as well as stoichiometric (in)homogenieties as reasons for
differently sharp martensitic transitions after ideal and non-ideal annealing.
We develop alloy design rules for tuning the magnetostructural phase transition
and evaluate specifically the sensitivity of the transition temperature towards
the externally applied magnetic fields ($\frac{dT_t}{\mu_0dH}$) by analyzing
the different stoichiometries. We then set up a phase diagram including
martensitic transition temperatures and austenite Curie temperatures depending
on the e/a ratio for varying Co and Ti content. The evolution of the Curie
temperature with changing stoichiometry is compared to other Heusler systems.
Density Functional Theory calculations reveal a correlation of T$_C$ with the
stoichiometry as well as with the order state of the austenite. This combined
approach of experiment and theory allows for an efficient development of new
systems towards promising magnetocaloric properties. Direct adiabatic
temperature change measurements show here the largest change of -4 K in a
magnetic field change of 1.93 T for Ni$_{35}$Co$_{15}$Mn$_{37}$Ti$_{13}$. | 2010.02620v2 |
2015-06-29 | Giant magnetocaloric effect from reverse martensitic transformation in Ni-Mn-Ga-Cu ferromagnetic shape memory alloys | In an effort to produce Giant Magnetocaloric effect (GMCE) near room
temperature, in a first ever such study, the austenite transformation
temperature (As) was fine tuned to ferromagnetic Curie temperature (TC) in
Ferromagnetic Shape Memory Alloys (FSMA) and a large GMCE of delta-SM = - 81.75
J/Kg-K was achieved in Ni50Mn18.5Cu6.5Ga25 alloy during reverse martensitic
transformation (heating cycle) for a magnetic field change of 9 T at 302.5 K.
Fine tuning of As with TC was achieved by Cu substitution in Ni50Mn25-xCuxGa25
(0< x < 7.0)-based FSMAs. Characterizations of these alloys were carried out
using Optical and Scanning Electron Microscopy, X-ray Diffraction (XRD),
Differential Scanning Calorimetry (DSC) and DC magnetization measurements.
Addition of Cu to stoichiometric Heusler type Ni2MnGa increases the martensitic
transformation temperatures and decreases TC. Concurrently, DSM increases with
Cu addition and peaks at 6.5 at. % Cu for which there is a virtual overlap
between TC and As. Maximum Refrigerant Capacity (RCP) of 327.01 J/Kg was also
achieved in the heating cycle for 9 T field change at 302.5 K. Corresponding
values for the cooling cycle measurements (measured during forward
transformation) were 30.4 J/Kg-K and 123.52 J/Kg respectively for the same 6.5
at. % Cu sample and same thermo-magnetic conditions. | 1506.08709v2 |
2006-01-30 | Electronic structure and spectroscopy of the quaternary Heusler alloy Co$_2$Cr$_{1-x}$Fe$_{x}$Al | Quaternary Heusler alloys Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe
ratio $x$ were investigated experimentally and theoretically. The electronic
structure and spectroscopic properties were calculated using the full
relativistic Korringa-Kohn-Rostocker method with coherent potential
approximation to account for the random distribution of Cr and Fe atoms as well
as random disorder. Magnetic effects are included by the use of spin dependent
potentials in the local spin density approximation.
Magnetic circular dichroism in X-ray absorption 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. Resonant
(560eV - 800eV) soft X-ray as well as high resolution - high energy ($\geq
3.5$keV) hard X-ray photo emission was used to probe the density of the
occupied states in Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al. | 0601670v1 |
2008-02-18 | Ab-initio determined electronic and magnetic properties of half-metallic NiCrSi and NiMnSi Heusler alloys; the role of interfaces and defects | Using state-of-the-art first-principles calculations we study the properties
of the ferromagnetic Heusler compounds NiYSi where Y stands for V, Cr or Mn.
NiCrSi and NiMnSi contrary to NiVSi are half-metallic at their equilibrium
lattice constant exhibiting integer values of the total spin magnetic moment
and thus we concentrate on these two alloys. The minority-spin gap has the same
characteristics as for the well-known NiMnSb alloy being around $\sim$1 eV.
Upon tetragonalization the gap is present in the density of states even for
expansion or contraction of the out-of-plane lattice parameter by 5%. The Cr-Cr
and Mn-Mn interactions make ferromagnetism extremely stable and the Curie
temperature exceeds 1000 K for NiMnSi. Surface and interfaces with GaP, ZnS and
Si semiconductors are not half-metallic but in the case of NiCrSi the Ni-based
contacts present spin-polarization at the Fermi level over 90%. Finally, we
show that there are two cases of defects and atomic-swaps. The first-ones which
involve the Cr(Mn) and Si atoms induce states at the edges of the gap which
persists for a moderate-concentration of defects. Defects involving Ni atoms
induce states localized within the gap completely destroying the
half-metallicity. Based on single-impurity calculations we associate these
states to the symmetry of the crystal. | 0802.2531v1 |
2016-10-19 | Bismuth based Half Heusler Alloys with giant thermoelectric figure of merit | Half Heusler (HH) thermoelectric alloys provide a wide platform to choose
materials with non-toxic and earth abundant elements. This article presents an
ab-initio theoretical evaluation of electrical and thermal transport properties
of three Bismuth-based most promising thermoelectric alloys, selected out of 54
stable HH compounds. These are brand new compounds which are recently proposed
to be stable (Nature Chem. 7, 308 (2015)) and may have interesting properties.
The calculated band structure of the three compounds, namely HfRhBi, ZrIrBi and
ZrRhBi, served as a hint for their promising thermoelectric properties. To gain
confidence on the theoretical predictions of these unreported systems, we first
checked our calculated results for a well studied similar compound, ZrNiSn, and
showed reasonable agreement with the measured ones. HfRhBi and ZrIrBi turn out
to be narrow band gap while ZrRhBi is a moderate band gap semiconductor. A
detailed study of the carrier concentration and temperature dependance of the
Seebeck coefficient (S), Power factor (S$^2 \sigma$), lattice ($\kappa_L$) and
electronic ($\kappa_e$) thermal conductivity and hence the figure of merit (ZT)
is carried out. In contrast to most promising known thermoelectric materials,
we found high power factor for these materials (highest S$^2 \sigma\sim$17.36
mWm$^{-1}$K$^{-2}$ for p-type ZrIrBi). All the three systems (specially p-type)
show high figure of merit, with ZT value as high as 0.45 for ideal crystal.
Maximum ZT and the corresponding optimal n- and p-type doping concentrations
($n_c$) are calculated for all the three compounds, which shall certainly pave
guidance to future experimental work. | 1610.06038v2 |
2017-04-01 | Native defects in the Co$_2$Ti$Z$ ($Z=$ Si, Ge, Sn) full Heusler alloys: formation and influence on the thermoelectric properties | We have performed first-principles investigations on the native defects in
the full Heusler alloys Co$_2$Ti$Z$ ($Z$ one of the group IV elements Si, Ge,
Sn), determining their formation energies and how they influence the transport
properties. We find that Co vacancies (Vc) in all compounds and the
Ti$_\text{Sn}$ anti-site exhibit negative formation energies. The smallest
positive values occur for Co in excess on anti-sites (Co$_Z$ or Co$_\text{Ti}$)
and for Ti$_Z$. The most abundant native defects were modeled as dilute alloys,
treated with the coherent potential approximation in combination with the
multiple-scattering theory Green function approach. The self-consistent
potentials determined this way were used to calculate the residual resistivity
via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck
coefficient of the systems. The latter is shown to depend significantly on the
type of defect, leading to variations that are related to subtle, spin-orbit
coupling induced, changes in the electronic structure above the half-metallic
gap. Two of the systems, Vc$_\text{Co}$ and Co$_Z$, are found to exhibit a
negative Seebeck coefficient. This observation, together with their low
formation energy, offers an explanation for the experimentally observed
negative Seebeck coefficient of the Co$_2$Ti$Z$ compounds as being due to
unintentionally created native defects. | 1704.00149v2 |
2017-10-06 | Relative Cooling Power Enhancement by Tuning Magneto-structural Stability in Ni-Mn-In Heusler Alloys | Off-stoichiometric Ni$_2$MnIn Heusler alloys have drawn recent attention due
to their large magnetocaloric entropy change associated with the first-order
magneto-structural transition. Here we present crystal structural, calorimetric
and magnetic studies of three compositions. Temperature-dependent X-ray
diffraction shows clear structural transition from a 6M modulated monoclinic to
a L2$_1$ cubic. A significant enhancement of relative cooling power (RCP) was
achieved by tuning the magnetic and structural stability through minor
compositional changes, with the measured results quantitatively close to the
prediction as a function of the ratio between the martensitic transition
($T_m$) temperature and austenite Curie temperature ($T_C$) although the
maximal magnetic induced entropy change ($\Delta S_{max}$) reduction is
observed in the same time. The results provided an evaluation guideline of RCPs
as well as magnetic-induced entropy change in designing practical active
materials. | 1710.02522v6 |
2020-10-22 | Multicaloric effects in Metamagnetic Heusler Ni-Mn-In under uniaxial stress and magnetic field | The world's growing hunger for artificial cold on the one hand, and the ever
more stringent climate targets on the other, pose an enormous challenge to
mankind. Novel, efficient and environmentally friendly refrigeration
technologies based on solid-state refrigerants can offer a way out of the
problems arising from climate-damaging substances used in conventional
vapor-compressors. Multicaloric materials stand out because of their large
temperature changes which can be induced by the application of different
external stimuli such as a magnetic, electric, or a mechanical field. Despite
the high potential for applications and the interesting physics of this group
of materials, only few studies focus on their investigation by direct methods.
In this paper, we report on the advanced characterization of all relevant
physical quantities that determine the multicaloric effect of a Ni-Mn-In
Heusler compound. We have used a purpose-designed calorimeter to determine the
isothermal entropy and adiabatic temperature changes resulting from the
combined action of magnetic field and uniaxial stress on this metamagnetic
shape-memory alloy. From these results, we can conclude that the multicaloric
response of this alloy by appropriate changes of uniaxial stress and magnetic
field largely outperforms the caloric response of the alloy when subjected to
only a single stimulus. We anticipate that our findings can be applied to other
multicaloric materials, thus inspiring the development of refrigeration devices
based on the multicaloric effect. | 2010.11511v1 |
2020-11-03 | Fully-Compensated Ferrimagnetic Spin Filter Materials within the Cr$\textit{M}\textit{N}$Al Equiatomic Quaternary Heusler Alloys | XX'YZ equiatomic quaternary Heusler alloys (EQHA's) containing Cr, Al, and
select Group IVB elements ($\textit{M}$ = Ti, Zr, Hf) and Group VB elements
($\textit{N}$ = V, Nb, Ta) were studied using state-of-the-art density
functional theory to determine their effectiveness in spintronic applications.
Each alloy is classified based on their spin-dependent electronic structure as
a half-metal, a spin gapless semiconductor, or a spin filter material. We
predict several new fully-compensated ferrimagnetic spin filter materials with
small electronic gaps and large exchange splitting allowing for robust spin
polarization with small resistance. CrVZrAl, CrVHfAl, CrTiNbAl, and CrTiTaAl
are identified as particularly robust spin filter candidates with an exchange
splitting of $\sim 0.20$ eV. In particular, CrTiNbAl and CrTiTaAl have
exceptionally small band gaps of $\sim 0.10$ eV. Moreover, in these compounds,
a spin asymmetric electronic band gap is maintained in 2 of 3 possible atomic
arrangements they can take, making the electronic properties less susceptible
to random site disorder. In addition, hydrostatic stress is applied to a subset
of the studied compounds in order to determine the stability and tunability of
the various electronic phases. Specifically, we find the CrAlV$\textit{M}$
subfamily of compounds to be exceptionally sensitive to hydrostatic stress,
yielding transitions between all spin-dependent electronic phases. | 2011.01389v1 |
2023-07-31 | Ballistic spin-transport properties of magnetic tunnel junctions with MnCr-based ferrimagnetic quaternary Heusler alloys | We investigate the suitability of nearly half-metallic ferrimagnetic
quaternary Heusler alloys, CoCrMnZ (Z=Al, Ga, Si, Ge) to assess the feasibility
as electrode materials of MgO-based magnetic tunnel junctions (MTJ). Low
magnetic moments of these alloys originated from the anti-ferromagnetic
coupling between Mn and Cr spins ensure a negligible stray field in spintronics
devices as well as a lower switching current required to flip their spin
direction. We confirmed mechanical stability of these materials from the
evaluated values of elastic constants, and the absence of any imaginary
frequency in their phonon dispersion curves. The influence of swapping
disorders on the electronic structures and their relative stability are also
discussed. A high spin polarization of the conduction electrons are observed in
case of CoCrMnZ/MgO hetrojunctions, independent of terminations at the
interface. Based on our ballistic transport calculations, a large coherent
tunnelling of the majority-spin $s$-like $\Delta_1$ states can be expected
through MgO-barrier. The calculated tunnelling magnetoresistance (TMR) ratios
are in the order of 1000\%. A very high Curie temperatures specifically for
CoCrMnAl and CoCrMnGa, which are comparable to $bcc$ Co, could also yield a
weaker temperature dependece of TMR ratios for CoCrMnAl/MgO/CoCrMnAl (001) and
CoCrMnGa/MgO/CoCrMnGa (001) MTJ. | 2307.16442v1 |
2024-01-22 | Off-stoichiometric effect on magnetic and electron transport properties of Fe$_2$VAl$_{1.35}$ in respect to Ni$_2$VAl; Comparative study | Density functional theory (DFT) calculations confirm that the structurally
ordered Fe$_2$VAl Heusler alloy is nonmagnetic narrow-gap semiconductor. This
compound is apt to form various disordered modifications with high
concentration of antisite defects. We study the effect of structural disorder
on the electronic structure, magnetic, and electronic transport properties of
the full Heusler alloy Fe$_2$VAl and its off-stoichiometric equivalent
Fe$_2$VAl$_{1.35}$. Data analysis in relation to {\it ab initio} calculations
indicates an appearance of antisite disorder mainly due to Fe--V and Fe--Al
stoichiometric variations. The data for weakly magnetic Fe$_2$VAl$_{1.35}$ are
discussed in respect to Ni$_2$VAl. Fe$_2$VAl$_{1.35}$ can be classified as a
nearly ferromagnetic metal with a pronounced spin glassy contribution, which,
however, does not give a predominant effect on its thermoelectric properties.
The figure of merit $ZT$ is at 300 K about 0.05 for the Fe sample and 0.02 for
Ni one, respectively. However, it is documented that the narrow $d$ band
resulting from Fe/V site exchange can be responsible for the unusual
temperature dependencies of the physical properties of the Fe2TiAl$_{1.35}$
alloy, characteristic of strongly correlated electron systems. As an example,
the magnetic susceptibility of Fe$_2$VAl$_{1.35}$ exhibits singularity
characteristic of a Griffiths phase, appearing as an inhomogeneous electronic
state below $T_G\sim 200$ K. We also performed numerical analysis which
supports the Griffiths phase scenario. | 2401.12218v1 |
2016-11-21 | Adaptive modulation in Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy | The origin of incommensurate structural modulation in Ni-Mn based Heusler
type magnetic shape memory alloys (MSMAs) is still an unresolved issue inspite
of intense focus on this due to its role in the magnetic field induced
ultra-high strains. In the archetypal MSMA Ni2MnGa, the observation of
non-uniform displacement of atoms from their mean positions in the modulated
martensite phase, premartensite phase and charge density wave as well as the
presence of phason broadening of satellite peaks have been taken in support of
the electronic instability model linked with a soft acoustic phonon. We present
here results of a combined high resolution synchrotron x-ray powder diffraction
(SXRPD) and neutron powder diffraction (NPD) study on Ni2Mn1.4In0.6 using
(3+1)D superspace group approach, which reveal not only uniform atomic
displacements in the modulated structure of the martensite phase with
physically acceptable ordered magnetic moments in the antiferromagnetic phase
at low temperatures but also the absence of any premartensite phase and phason
broadening of the satellite peaks. Our HRTEM studies and first principles
calculations of the ground state also support uniform atomic displacements
predicted by powder diffraction studies. All these observations suggest that
the structural modulation in the martensite phase of Ni2Mn1.4In0.6 MSMA can be
explained in terms of the adaptive phase model. The present study underlines
the importance of superspace group analysis using complimentary SXRPD and NPD
in understanding the physics of the origin of modulation as well as the
magnetic and the modulated ground states of the Heusler type MSMAs. Our work
also highlights the fact that the mechanism responsible for the origin of
modulated structure in different Ni-Mn based MSMAs may not be universal and it
must be investigated thoroughly in different alloy compositions. | 1611.06688v2 |
2021-05-14 | Understanding Magnetic Phase Coexistence in Ru$_2$Mn$_{1-x}$Fe$_x$Sn Heusler Alloys: A Neutron Scattering, Thermodynamic, and Phenomenological Analysis | The random substitutional solid solution between the antiferromagnetic (AFM)
full-Heusler alloy Ru$_2$MnSn and the ferromagnetic (FM) full-Heusler alloy
Ru$_2$FeSn provides a rare opportunity to study FM-AFM phase competition in a
near-lattice-matched, cubic system, with full solubility. At intermediate $x$
in Ru$_2$Mn$_{1-x}$Fe$_x$Sn this system displays suppressed magnetic ordering
temperatures, spatially coexisting FM and AFM order, and strong coercivity
enhancement, despite rigorous chemical homogeneity. Here, we construct the most
detailed temperature- and $x$-dependent understanding of the magnetic phase
competition and coexistence in this system to date, combining
wide-temperature-range neutron diffraction and small-angle neutron scattering
with magnetometry and specific heat measurements on thoroughly characterized
polycrystals. A complete magnetic phase diagram is generated, showing FM-AFM
coexistence between $x \approx 0.30$ and $x \approx 0.70$. Important new
insight is gained from the extracted length scales for magnetic phase
coexistence (25-100 nm), the relative magnetic volume fractions and ordering
temperatures, in addition to remarkable $x$-dependent trends in magnetic and
electronic contributions to specific heat. An unusual feature in the magnetic
phase diagram (an intermediate FM phase) is also shown to arise from an
extrinsic effect related to a minor Ru-rich secondary phase. The established
magnetic phase diagram is then discussed with the aid of phenomenological
modeling, clarifying the nature of the mesoscale phase coexistence with respect
to the understanding of disordered Heisenberg models. | 2105.06629v1 |
2023-08-18 | Large thermo-spin effects in Heusler alloy based spin-gapless semiconductor thin films | Recently, Heusler alloys-based spin gapless semiconductors (SGSs) with high
Curie temperature (TC) and sizeable spin polarization have emerged as potential
candidates for tunable spintronic applications. We report comprehensive
investigation of the temperature dependent ANE and intrinsic longitudinal spin
Seebeck effect (LSSE) in CoFeCrGa thin films grown on MgO substrates. Our
findings show the anomalous Nernst coefficient for the MgO/CoFeCrGa (95 nm)
film is $\cong 1.86$ micro V/K at room temperature which is nearly two orders
of magnitude higher than that of the bulk polycrystalline sample of CoFeCrGa (=
0.018 micro V/K) but comparable to that of the magnetic Weyl semimetal Co2MnGa
thin film (2-3 micro V/K). Furthermore, the LSSE coefficient for our
MgO/CoFeCrGa(95nm)/Pt(5nm) heterostructure is $\cong 20.5$ $\mu$V/K/$\Omega$ at
room temperature which is twice larger than that of the half-metallic
ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_3$ thin films ($\cong$ 20.5
$\mu$V/K/$\Omega$). We show that both ANE and LSSE coefficients follow
identical temperature dependences and exhibit a maximum at $\cong$ 225 K which
is understood as the combined effects of inelastic magnon scatterings and
reduced magnon population at low temperatures. Our analyses not only indicate
that the extrinsic skew scattering is the dominating mechanism for ANE in these
films but also provide critical insights into the functional form of the
observed temperature dependent LSSE at low temperatures. Furthermore, by
employing radio frequency transverse susceptibility and broadband ferromagnetic
resonance in combination with the LSSE measurements, we establish a correlation
among the observed LSSE signal, magnetic anisotropy and Gilbert damping of the
CoFeCrGa thin films, which will be beneficial for fabricating tunable and
highly efficient Heusler alloys based spincaloritronic nanodevices. | 2308.09843v1 |
2016-01-22 | Phase Separation and Superparamagnetism in the Martensitic Phase of $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ | $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ shape memory alloys in the approximate range
$5 \le x \le 10$ display desirable properties for applications as well as
intriguing magnetism. These off-stoichiometric Heusler alloys undergo a
martensitic phase transformation at a temperature $T_{M}$ of 300 - 400 K, from
ferromagnetic (F) to nonferromagnetic, with unusually low thermal hysteresis
and a large change in magnetization. The low temperature magnetic structures in
the martensitic phase of such alloys, which are distinctly inhomogeneous, are
of great interest but are not well understood. Our present use of spin echo
NMR, in the large hyperfine fields at $^{55}Mn$ sites, provides compelling
evidence that nanoscale magnetic phase separation into F and antiferromagnetic
(AF) regions occurs below $T_{M}$ in alloys with x in the range 0 to 7. At
finite Co substitution the F regions are found to be of two distinct types,
corresponding to high and low local concentrations of Co on Ni sites. Estimates
of the size distributions of both the F and AF nanoregions have been made. At x
= 7 the AF component is not long-range ordered, even below 4 K, and is quite
different to the AF component found at x = 0; by x = 14 the F phase is
completely dominant. Of particular interest, we find, for x = 7, that field
cooling leads to dramatic changes in the AF regions. These findings provide
insight into the origins of magnetic phase separation and superparamagnetism in
these complex alloys, particularly their intrinsic exchange bias, which is of
considerable current interest. | 1601.05993v1 |
2018-04-11 | NMR and the antiferromagnetic crystal phase regions in rapidly quenched ribbons and in alloys of the type $Cu-Mn-Al$ | It was shown that anomalous resistivity behavior of the $Cu-Mn-Al$ ribbons is
explained by the s-d interaction between conduction electrons and the clustered
Mn atoms. While nuclear magnetic resonance measurements show the
antiferromagnetic and ferromagnetic clusters of Mn atom coexisting without
long-range order, it is an interesting problem to study magnetic resonance
properties also for the antiferromagnetic crystal phase regions (which have
long-range order for larger regions) and which may also occur in these ribbons.
The Heusler Type $Cu-Mn-Al$ Alloy has a composition half way between
$Cu_{2}MnAl$ and $Cu_{3}Al$. Electron microscopy of the premartensitic $\beta
Cu-Zn-Al$ alloy has shown that the $\beta Cu-Zn-Al$ alloy quenched from high
temperature has the electron diffraction patterns of this alloy well explained
by the model with the existence of small particles with an orthorhombic
structure. It was noted that an important aspect of improvement in the material
properties is to create a nanostructured state in matrix, which has significant
advantages in magnetic and mechanical characteristics in contrast to the bulk
materials in crystalline or amorphous state. It is an interesting problem to
study magnetic resonance properties not only for the Mn atoms and clusters
without long-range order but also for the antiferromagnetic crystal phase
regions (which have long-range order for larger regions) which may also occur
in ribbons. This is the aim of our paper. | 1804.04196v1 |
2023-11-30 | Vanishing of the anomalous Hall effect and enhanced carrier mobility in the spin-gapless ferromagnetic Mn2CoGa1-xAlx alloys | Spin gapless semiconductor (SGS) has attracted long attention since its
theoretical prediction, while concrete experimental hints are still lack in the
relevant Heusler alloys. Here in this work, by preparing the series alloys of
Mn2CoGa1-xAlx (x=0, 0.25, 0.5, 0.75 and 1), we identified the vanishing of
anomalous Hall effect in the ferromagnetic Mn2CoGa (or x=0.25) alloy in a wide
temperature interval, accompanying with growing contribution from the ordinary
Hall effect. As a result, comparatively low carrier density (1020 cm-3) and
high carrier mobility (150 cm2/Vs) are obtained in Mn2CoGa (or x=0.25) alloy in
the temperature range of 10-200K. These also lead to a large dip in the related
magnetoresistance at low fields. While in high Al content, despite the
magnetization behavior is not altered significantly, the Hall resistivity is
instead dominated by the anomalous one, just analogous to that widely reported
in Mn2CoAl. The distinct electrical transport behavior of x=0 and x=0.75 (or 1)
is presently understood by their possible different scattering mechanism of the
anomalous Hall effect due to the differences in atomic order and conductivity.
Our work can expand the existing understanding of the SGS properties and offer
a better SGS candidate with higher carrier mobility that can facilitate the
application in the spin-injected related devices. | 2311.18335v1 |
2003-11-19 | Shape memory ferromagnets | In ferromagnetic alloys with shape memory large reversible strains can be
obtained by rearranging the martensitic domain structure by a magnetic field.
Magnetization through displacement of domain walls is possible in the presence
of high magnetocrystalline anisotropy, when martensitic structure rearrangement
is energetically favorable compared to the reorientation of magnetic moments.
In ferromagnetic Heusler alloys Ni$_{2+x}$Mn$_{1-x}$Ga the Curie temperature
exceeds the martensitic transformation temperature. The fact that these two
temperatures are close to room temperature offers the possibility of
magnetically controlling the shape and size of ferromagnets in the martensitic
state. In Ni$_{2+x}$Mn$_{1-x}$Ga single crystals, a reversible strain of $\sim
6$% is obtained in fields of $\sim 1$ T. | 0311433v1 |
2003-12-04 | Magnetocaloric effect and magnetization in a Ni-Mn-Ga Heusler alloy in the vicinity of magnetostructural transition | The magnetic and thermodynamic properties of a Ni2.19Mn0.81Ga alloy with
coupled magnetic and structural (martensitic) phase transitions were studied
experimentally and theoretically. The magnetocaloric effect was measured by a
direct method in magnetic fields 0-26 kOe at temperatures close to the
magnetostructural transition temperature. For theoretical description of the
alloy properties near the magnetostructural transition a statistical model is
suggested, that takes into account the coexistence of martensite and austenite
domains in the vicinity of martensite transformation point. | 0312110v1 |
2004-05-07 | Magnetic properties and magnetostructural phase transitions in Ni2+xMn1-xGa shape memory alloys | A systematic study of magnetic properties of Ni2+xMn1-xGa (0 \le x \le 0.19)
Heusler alloys undergoing structural martensite-austenite transformations while
in ferromagnetic state has been performed. From measurements of spontaneous
magnetization, Ms(T), jumps \Delta M at structural phase transitions were
determined. Virtual Curie temperatures of the martensite were estimated from
the comparison of magnetization in martensitic and austenitic phases. Both
saturation magnetic moments in ferromagnetic state and effective magnetic
moments in paramagnetic state of Mn and Ni atoms were estimated and the
influence of delocalization effects on magnetism in these alloys was discussed.
The experimental results obtained show that the shift of martensitic transition
temperature depends weakly on composition. The values of this shift are in good
correspondence with Clapeyron-Clausius formalism taking into account the
experimental data on latent heat at martensite-austenite transformations. | 0405134v1 |
2007-09-01 | Local atomic arrangement and martensitic transformation in Ni$_{50}$Mn$_{35}$In$_{15}$: An EXAFS Study | Heusler alloys that undergo martensitic transformation in ferromagnetic state
are of increasing scientific and technological interest. These alloys show
large magnetic field induced strains upon martensitic phase change thus making
it a potential candidate for magneto-mechanical actuation. The crystal
structure of martensite is an important factor that affects both the magnetic
anisotropy and mechanical properties of such materials. Moreover, the local
chemical arrangement of constituent atoms is vital in determining the overall
physical properties. Ni$_{50}$Mn$_{35}$In$_{15}$ is one such ferromagnetic
shape memory alloy that displays exotic properties like large magnetoresistance
at moderate field values. In this work, we present the extended x-ray
absorption fine-structure measurements (EXAFS) on the bulk
Ni$_{50}$Mn$_{35}$In$_{15}$ which reveal the local structural change that
occurs upon phase transformation. The change in the bond lengths between
different atomic species helps in understanding the type of hybridization which
is an important factor in driving such Ni-Mn based systems towards martensitic
transformation. | 0709.0030v1 |
2009-06-19 | Lattice dynamics in magnetic superelastic Ni-Mn-In alloys. Neutron scattering and ultrasonic experiments | Neutron scattering and ultrasonic methods have been used to study the lattice
dynamics of two single crystals of Ni-Mn-In Heusler alloys close to
Ni$_{50}$Mn$_{34}$In$_{16}$ magnetic superelastic composition. The paper
reports the experimental determination of the low-lying phonon dispersion
curves and the elastic constants for this alloy system. We found that the
frequencies of the TA$_{2}$ branch are relatively low and it exhibits a small
dip anomaly at a wave number $\xi_{0} \approx 1/3$, which softens with
decreasing temperature. Associated with the softening of this phonon, we also
observed the softening of the shear elastic constant $C'=(C_{11}-C_{12})/2$.
Both temperature softenings are typical for bcc based solids which undergo
martensitic transformations and reflect the dynamical instability of the cubic
lattice against shearing of $\{110\}$ planes along $<1\bar{1}0>$ directions.
Additionally, we measured low-lying phonon dispersion branches and elastic
constants in applied magnetic fields aimed to characterize the magnetoelastic
coupling. | 0906.3622v1 |
2010-05-26 | Correlation between Local Structure Distortions and Martensitic Transformation in Ni-Mn-In alloys | The local structural distortions arising as a consequence of increasing Mn
content in Ni_2Mn_1+xIn_1-x (x=0, 0.3, 0.4, 0.5 and 0.6) and its effect on
martensitic transformation have been studied using Extended X-ray Absorption
Fine Structure (EXAFS) spectroscopy. Using the room temperature EXAFS at the Ni
and Mn K-edges in the above compositions, the changes associated with respect
to the local structure of these absorbing atoms are compared. It is seen that
in the alloys exhibiting martensitic transformation ($x \ge 0.4$) there is a
significant difference between the Ni-In and Ni-Mn bond lengths even in the
austenitic phase indicating atomic volume to be the main factor in inducing
martensitic transformation in Ni-Mn-In Heusler alloys. | 1005.4873v1 |
2012-12-03 | Role of covalent hybridization in martensitic structure and magnetic properties of shape memory alloys: the case of Ni50Mn5+xGa35-xCu10 | We have investigated the impact of covalent hybridization on martensitic
structure and magnetic properties of Ni50Mn5+xGa35-xCu10 shape memory alloys.
We found that the lattice distortion ((c-a)/a) of L10 martensite monotonously
changes with the substitution of Mn for Ga atoms and shows a kink behavior at
Ga(at.%)= 25 due to the weakened covalent effect between main-group and
transition-metal atoms. Moreover, owing to the competition between covalence
hybridization and magnetic ordering of introduced Mn atoms, the molecular
magnetic moment and Curie temperature coincidently show maximums at Ga(at.%)=25
as well. These behaviors are closely associated with corresponding changes of
the strength of covalent hybridization. The results therefore suggest that
careful control of the concentration of main-group atoms in Heusler alloys can
serve as an additional general tuning parameter for searching new
multifunctional materials. | 1212.0302v1 |
2014-03-28 | First-principle prediction of Martensitic transformation and magnetic properties of Heusler-type Pt2-xMn1+xGa alloys | The electronic structure, magnetism and phase stability of Pt2-xMn1+xGa(x=0,
0.25, 0.5, 0.75, 1) alloys are studied by first-principle calculations. The
calculations reveal that a potential magnetic martensitic transformation can be
expected in all the series. In addition, a large magnetic-field-induced strain
is likely to appear in Pt2-xMn1+xGa(x=0, 0.25, 0.75, 1) alloys. The electronic
structure calculations indicate that the tetragonal phase is stabilized upon
the distortion because of the pseudogap formation at the Fermi Level. The
magnetic structure is also investigated and the total magnetic moment of the
tetragonal phase is a little larger than that of the cubic austenite phase in
all the series. | 1403.7318v3 |
2014-04-27 | Magnetic shape memory microactuator | Bimetallic composite nanotweezers based on Ti2NiCu alloy with shape memory
effect (SME) have recently demonstrated the ability to manipulate real
nano-objects, such as nanotubes, and bionanoparticles when heated to 40-60 C by
laser radiation. The possibility of developing nanotweezers operating at
constant temperature is of particular importance mainly for the manipulation of
biological objects. In this work, a microactuator was produced using a
composite bilayer made of a layer of rapidly quenched Ni53Mn24Ga23
ferromagnetic shape memory Heusler alloy and an elastic layer of Pt. The size
of the microactuator is 25x2.3x1.7 micro-meters3. A controlled bending
deformation of the actuator of 1.2 %, with a deflection of the end of the
actuator higher than 2 micro-meter was obtained by applying a magnetic field of
8 T at T = 62 C. The possibility of the development of new technologies for
magnetic-field-controlled nanotools operating at a constant temperature using
the new multifunction magnetic shape memory alloys will be discussed. | 1404.6762v1 |
2017-12-13 | Co$_{2}$PtGa: A promising magnetic shape memory alloy with high martensite transition temperature | In the present work, a combined theoretical and experimental study on
Co$_{2}$PtGa Heusler alloy shows that it exhibits a martensite transition
around 1320 K with a small thermal hysteresis (10 K). Dynamical stability of
Co$_2$PtGa in the tetragonal phase has been established by the theoretically
calculated phonon dispersion curves. Magnetization measurements suggest that
this alloy is a ferromagnetic material with a saturation magnetic moment of
2.83 $\mu_B$/f.u. at 2 K, which is in excellent agreement with the value
obtained from \textit{ab-initio} calculations (2.87 $\mu_B$/f.u.). Our present
study demonstrates that Co$_{2}$PtGa is a promising material for high
temperature magnetic shape memory application. | 1712.04765v4 |
2020-08-27 | Lattice strain accommodation and absence of pre-transition phases in Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ | The stoichiometric Ni$_{50}$Mn$_{25}$In$_{25}$ Heusler alloy transforms from
a stable ferromagnetic austenitic ground state to an incommensurate modulated
martensitic ground state with a progressive replacement of In with Mn without
any pre-transition phases. The absence of pre-transition phases like strain
glass in Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ alloys is explained to be the ability
of the ferromagnetic cubic structure to accommodate the lattice strain caused
by atomic size differences of In and Mn atoms. Beyond the critical value of $x$
= 8.75, the alloys undergo martensitic transformation despite the formation of
ferromagnetic and antiferromagnetic clusters and the appearance of a super spin
glass state. | 2008.11982v1 |
2019-12-08 | Investigation of Thermoelectric properties of Magnetic Insulator FeRuTiSi Using First Principle Calculation | In this work, we have investigated the electronic structure and
thermoelectric properties of quaternary heusler alloy, FeRuTiSi, using first
principle DFT tools implemented in WIEN2k and BoltzTraP code. Electronic
structure calculations using TB-mBJ potential shows appearance of flat band at
the conduction band edge, thus electron in conduction band have the large
effective mass (me*), and therefore mainly contribute for negatively large
value of Seebeck coefficient (S). This alloy has indirect band gap of 0.59 eV,
and shows the n-type transport behavior. Under the constant relaxation time
approximation (tau = 10 -14 s), temperature dependent Seebeck coefficient,
electrical conductivity (sigma), and electronic thermal conductivity (ke) were
also estimated. The maximum figure-of-merit (ZT), for the FeRuTiSi compound is
found to be ~0.86 at 840 K, with n-type doping, which suggests that this
quaternary alloy can be a good candidate among the n-type material for
thermoelectric applications in high-temperature reg | 1912.03708v1 |
2022-01-31 | Disorder-mediated quenching of magnetization in NbVTiAl: Theory and Experiment | In this paper, we present the structural, electronic, magnetic and transport
properties of a equiatomic quaternary alloy NbVTiAl. The absence of (111) and
(200) peaks in X-ray diffraction (XRD) data confirms the A2-type structure.
Magnetization measurements indicate a high Curie temperature and a negligibly
small magnetic moment ($\sim 10^{-3} \mu_B/f.u.$) These observations are
indicative of fully compensated ferrimagnetism in the alloy.
Temperature-dependent resistivity indicates metallic nature. Ab-initio
calculation of fully ordered NbVTiAl structure confirms a nearly half metallic
behavior with a high spin polarization ($\sim$ 90 \%) and a net magnetic moment
of 0.8 $\mu_B/f.u.$ (in complete contrast to the experimental observation). One
of the main objective of the present paper is to resolve and explain the
long-standing discrepancy between theoretical prediction and experimental
observation of magnetization for V-based quaternary Heusler alloys, in general.
To gain an in-depth understanding, we modelled various disordered states and
its subsequent effect on the magnetic and electronic properties. The
discrepancy is attributed to the A2 disorder present in the system, as
confirmed by our XRD data. The presence of disorder also causes the emergence
of finite states at the Fermi level, which impacts the spin polarization of the
system. | 2201.13037v1 |
2015-12-24 | Importance of spin-orbit coupling in power factor calculations for half-Heusler ANiB (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi) | We investigate the spin-orbit coupling (SOC) effects on the electronic
structures and semi-classic transport coefficients of half-Heusler
$\mathrm{ANiB}$ (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi) by using generalized gradient
approximation (GGA). Calculated results show that SOC splits the valence bands
at high symmetry $\Gamma$ point, and modifies the outline of $\Gamma$-centered
valence bands, which has remarkable effects on the electron transport
properties. Thermoelectric properties are performed through solving Boltzmann
transport equations within the constant scattering time approximation. It is
found that the compounds containing Sn atom have larger power factor in p-type
doping than ones in n-type doping, and it is just the opposite for compounds
containing Sb and Bi elements. The SOC has obvious detrimental influence on
power factor in p-type doping, while has a negligible effect in n-type doping.
These can be understood by considering the effects of SOC on the valence bands
and conduction bands. The maximum power factors (MPF) are extracted in n-type
and p-type doping with GGA and GGA+SOC, and the MPF at 300 K with SOC is
predicted to be about 4.25\%$\sim$44.13\% smaller than that without SOC in the
case of p-type doping for $\mathrm{ANiB}$ (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi).
Therefore, it is crucial to consider SOC effects for theoretical analysis in
the case of p-type doping in half-Heusler compounds composed of heavy elements. | 1512.07710v1 |
2017-08-03 | A comparative study of different exchange-correlation functionals in understanding structural, electronic and thermoelectric properties of Fe$_{2}$VAl and Fe$_{2}$TiSn compounds | Fe$_{2}$VAl and Fe$_{2}$TiSn are full Heusler compounds with non-magnetic
ground state. The two compouds are good thermoelectric materials. PBE and
LDA(PW92) are the two most commonly used density functionals to study the
Heusler compounds. Along with these two well studied exchange-correlation
functionals, recently developed PBEsol, mBJ and SCAN functionals are employed
to study the two compounds. Using the five functionals equilibrium lattice
parameter and bulk modulus are calculated. Obtained values are compared with
experimental reports wherever available. Electronic structure properties are
studied by calculating dispersion curves, total and partial density of states.
For Fe$_{2}$VAl, band gap of 0.22 eV is obtained from the mBJ potential which
is in reasonable agreement with experimental value while, for Fe$_{2}$TiSn band
gap of 0.68 eV is obtained. Fe$_{2}$VAl is predicted to be semimetallic with
different values of negative gaps from LDA,PBEsol,PBE and SCAN functionals.
Whereas, Fe$_{2}$TiSn is found to be semimetallic(semiconducting) from
LDA,PBEsol(PBE,SCAN) functionals employed calculations. From the dispersion
curve effective mass values are also computed to see the contribution to the
Seebeck coefficient. In Fe$_{2}$TiSn, a flat band is present along the
$\Gamma$-X direction with calculated value of effective mass $\sim$36 more than
the mass of electron. The improvements or inadequacies among the functionals in
explaining the properties of full Heusler alloys for thermoelectric application
are thus observed through this study. | 1708.01180v1 |
2020-08-31 | Robust topological Hall effect driven by tunable noncoplanar magnetic state in Mn-Pt-In inverse tetragonal Heusler alloys | Manipulation of magnetic ground states by effective control of competing
magnetic interactions has led to the finding of many exotic magnetic states. In
this direction, the tetragonal Heusler compounds consisting of multiple
magnetic sublattices and crystal symmetry favoring chiral Dzyaloshinskii-Moriya
interaction (DMI) provide an ideal base to realize non-trivial magnetic
structures. Here, we present the observation of a large robust topological Hall
effect (THE) in the multi-sublattice Mn$_{2-x}$PtIn Heusler magnets. The
topological Hall resistivity, which originates from the non-vanishing real
space Berry curvature in the presence of non-zero scalar spin chirality,
systematically decreases with decreasing the magnitude of the canting angle of
the magnetic moments at different sublattices. With help of first principle
calculations, magnetic and neutron diffraction measurements, we establish that
the presence of a tunable non-coplanar magnetic structure arising from the
competing Heisenberg exchanges and chiral DMI from the D$_{2d}$ symmetry
structure is responsible for the observed THE. The robustness of the THE with
respect to the degree of non-collinearity adds up a new degree of freedom for
designing THE based spintronic devices. | 2008.13505v1 |
2017-10-05 | First-principles investigation of competing magnetic interactions in (Mn,Fe)Ru$_2$Sn Heusler solid solutions | Many Heusler compounds possess magnetic properties well-suited for
applications as spintronic materials. The pseudo-binary
Mn$_{0.5}$Fe$_{0.5}$Ru$_2$Sn, formed as a solid solution of two full Heuslers,
has recently been shown to exhibit exchange hardening suggestive of two
magnetic phases, despite existing as a \textit{single} chemical phase. We have
performed a first-principles study of the chemical and magnetic degrees of
freedom in the Mn$_{1-x}$Fe$_{x}$Ru$_2$Sn pseudo-binary to determine the origin
of the unique magnetic behavior responsible for exchange hardening within a
single phase. We find a transition from antiferromagnetic (AFM) to
ferromagnetic (FM) behavior upon replacement of Mn with Fe, consistent with
experimental results. The lowest energy orderings in Mn$_{1-x}$Fe$_{x}$Ru$_2$Sn
consist of chemically- and magnetically-uniform (111) planes, with Fe-rich
regions preferring FM ordering and Mn-rich regions preferring AFM ordering,
independent of the overall composition. Analysis of the electronic structure
suggests that the magnetic behavior of this alloy arises from a competition
between AFM-favoring Sn-mediated superexchange and FM-favoring RKKY exchange
mediated by spin-polarized conduction electrons. Changes in valency upon
replacement of Mn with Fe shifts the balance from superexchange-dominated
interactions to RKKY-dominated interactions. | 1710.02089v1 |
2006-11-17 | Ab initio prediction of half-metallic properties for the ferromagnetic Heusler alloys Co$_2$MSi (M=Ti, V, Cr) | By means of density functional calculations the magnetic and electronic
properties and phase stabilities of the Heusler compounds Co$_2$MSi (with M=Ti,
V, Cr, Mn, Fe, Co, Ni) were investigated. Based on the calculated results we
predict the ferromagnetic phases of the compounds Co$_2$TiSi, Co$_2$VSi and
Co$_2$CrSi to be half-metals. Of particular interest is Co$_2$CrSi because of
its high density of majority spin states at Fermi energy in combination with a
reasonably high estimated Curie temperature of 747K. The compounds Co$_2$TiSi
and Co$_2$VSi are thermodynamically stable, whereas Co$_2$CrSi is a metastable
phase which might be stabilized by suitable experimental techniques. | 0611466v1 |
2006-12-10 | Substituting the main group element in cobalt - iron based Heusler alloys: Co$_2$FeAl$_{1-x}$Si$_x$ | This work reports about electronic structure calculations for the Heusler
compound Co$_2$FeAl$_{1-x}$Si$_x$. Particular emphasis was put on the role of
the main group element in this compound. The substitution of Al by Si leads to
an increase of the number of valence electrons with increasing Si content and
may be seen as electron-doping. Self-consistent electronic structure
calculations were performed to investigate the consequences of the electron
doping for the magnetic properties. The series Co$_2$FeAl$_{1-x}$Si$_x$ is
found to exhibit half-metallic ferromagnetism and the magnetic moment follows
the Slater-Pauling rule. It is shown that the electron-doping stabilises the
gap in the minority states for $x=0.5$. | 0612241v1 |
2007-06-25 | Optical and magneto-optical properties of ferromagnetic full-Heusler films: experiments and first-principles calculations | We report a joint theoretical and experimental study focused on understanding
the optical and magneto-optical properties of Co-based full-Heusler compounds.
We show that magneto-optical spectra calculated within ab-initio density
functional theory are able to uniquely identify the features of the
experimental spectra in terms of spin resolved electronic transitions. As
expected for 3d-based magnets, we find that the largest Kerr rotation for these
alloys is of the order of 0.3o in polar geometry. In addition, we demonstrate
that (i) multilayered structures have to be carefully handled in the
theoretical calculations in order to improve the agreement with experiments,
and (ii) combined theoretical and experimental investigations constitute a
powerful approach to designing new materials for magneto-optical and
spin-related applications | 0706.3613v1 |
2010-02-28 | A new platform for topological quantum phenomena : Topological Insulator states in thermoelectric Heusler-related ternary compounds | Topological insulators (TI) realize a novel state of quantum matter that are
distinguished by topological invariants of bulk band structure rather than
spontaneously broken symmetries. A number of exotic quantum phenomena have been
predicted to exist in multiply-connected geometries which require an enormous
amount of materials flexibility. We have extended our previous search for TI
materials from binary (Bi2X3 series) to the thermoelectric ternary compounds.
We discover that the distorted LuPtSb is the first ternary compound harboring a
3D topological insulator state. We also show that the half-Heusler LuPtSb-type
series is a natural platform that hosts a range of candidate compounds, alloys
and artificial heterostructures (quantum-wells). We also discovered several
different paradigms of trivial and non-trivial topological ordering in this
class, including a metallic nontrivial topological state in YAuPb. Some of
these materials are grown (results will be reported separately). | 1003.0155v1 |
2011-08-19 | Magnetic and structural anisotropies of Co2FeAl Heusler alloy epitaxial thin films | This paper shows the correlation between chemical order, lattice strains and
magnetic properties of Heusler Co2FeAl films epitaxially grown on MgO(001). A
detailed magnetic characterization has been performed using vector field
magnetometery combined with numerical Stoner-Wohlfarth analysis. We demonstrate
the presence of three types of in-plane anisotropies: one biaxial, as expected
for the cubic symmetry, and other two uniaxial ones. The three anisotropies
show different behavior with the annealing temperature. The biaxial anisotropy
shows a monotonous increase. The uniaxial anisotropy, parallel with the hard
biaxial axes, related to the chemical homogeneity, decreases, while the other,
supposed to have magnetostatic origin, remains constant. | 1108.4043v2 |
2013-06-26 | Effective Scattering Cross-section in Lattice Thermal Conductivity Calculation with Differential Effective Medium Method | To further reduce the lattice thermal conductivity of thermoelectric
materials, the technique of embedding nano-inclusions into bulk matrix
materials, in addition to point defect scattering via alloying, was widely
applied. Differential Effective Medium (DEM) method was employed to calculate
two-phase heterogeneous systems. However, in most effective medium treatment,
the interface scattering of matrix phonons by embedded nanoparticle was
underestimated by adopting particle's projected area as scattering
cross-section. Herein, modified cross-section calculations, as well as grain
sizes dispersions, are applied in DEM, with the calculations then validated by
comparing with Monte-Carlo simulations and existing experimental data.
Predictions of lattice thermal conductivity reduction on in-situ formed Full
Heusler(FH)/Half Heusler(HH) nano/matrix system are discussed. | 1306.6274v1 |
2014-04-22 | First principles investigation of magnetocrystalline anisotropy at the L2$_1$ Full Heusler|MgO interfaces and tunnel junctions | Magnetocrystalline anisotropy at Heusler alloy$|$MgO interfaces have been
studied using first principles calculations. It is found that Co terminated
Co$_{2}$FeAl$|$MgO interfaces show perpendicular magnetic anisotropy up to 1.31
mJ/m$^2$, while those with FeAl termination exhibit in-plane magnetic
anisotropy. Atomic layer resolved analysis indicates that the origin of
perpendicular magnetic anisotropy in Co$_{2}$FeAl$|$MgO interfaces can be
attributed to the out-of-plane orbital contributions of interfacial Co atoms.
At the same time, Co$_{2}$MnGe and Co$_{2}$MnSi interfaced with MgO tend to
favor in-plane magnetic anisotropy for all terminations. | 1404.5646v2 |
2015-10-07 | Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films | The half-metallic half-Heusler alloy NiMnSb is a promising candidate for
applications in spintronic devices due to its low magnetic damping and its rich
anisotropies. Here we use ferromagnetic resonance (FMR) measurements and
calculations from first principles to investigate how the composition of the
epitaxially grown NiMnSb influences the magnetodynamic properties of saturation
magnetization $M_S$, Gilbert damping $\alpha$, and exchange stiffness $A$.
$M_S$ and $A$ are shown to have a maximum for stoichiometric composition, while
the Gilbert damping is minimum. We find excellent quantitative agreement
between theory and experiment for $M_S$ and $\alpha$. The calculated $A$ shows
the same trend as the experimental data, but has a larger magnitude.
Additionally to the unique in-plane anisotropy of the material, these
tunabilities of the magnetodynamic properties can be taken advantage of when
employing NiMnSb films in magnonic devices. | 1510.01894v1 |
2016-07-19 | Lattice Thermal Conductivity of NiTiSn Half-Heusler Thermoelectric Materials from First-Principles Calculations | The microscopic physics behind the lattice thermal conductivity of NiTiSn is
investigated using first-principles-based anharmonic lattice dynamics. The
calcu lated lattice thermal conductivity of bulk materials (5.3 W/m.K) is in
good agreement with the experimental value at the optimal working temper ature
(700 K), but is overestimated below this temperature. The calculated values can
be strongly affected by the size of the crystalline grains. We show tha t the
lattice thermal conductivity is dominated by the acoustic (transverse and
mostly longitudinal) modes with no contribution from the optical modes. The a
coustic phonons are located below 150 cm-1 and involve mainly the tin atoms.
The calculated mean free path of the most heat carrying phonons is around f
ifty nanometers with a maximum life time of approx. 100 ps. These theoretical
results are a step forward in developing the experimental design of low thermal
conductivity NiTiSn Heusler based materials. | 1607.05558v1 |
2016-10-09 | Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure | The thermoelectric properties of n type semiconductor, TiNiSn is optimized by
partial substitution with metallic, MnNiSb in the half Heusler structure.
Herein, we study the transport properties and intrinsic phase separation in the
system. The Ti1-xMnxNiSn1-xSbx alloys were prepared by arc-melting and were
annealed at temperatures obtained from differential thermal analysis and
differential scanning calorimetry results. The phases were characterized using
powder X-ray diffraction patterns, energy dispersive X-ray spectroscopy, and
differential scanning calorimetry. After annealing the majority phase was
TiNiSn with some Ni rich sites and the minority phases was majorly Ti6Sn5, Sn,
and MnSn2. Ni rich sites were caused by Frenkel defects, this led to a
metal-like behavior of the semiconducting specimens at low temperature. For x
up to 0.05 the samples showed an activated conduction, whereas for x>0.05 they
showed metallic character. The figure of merit for x=0.05 was increased by 61%
(ZT=0.45) in comparison to the pure TiNiSn. | 1610.02657v1 |
2017-08-16 | Ab-initio design of new Heusler materials for thermoelectric applications | In search of new prospects for thermoelectric materials, using ab-initio
calculations and semi-classical Boltzmann theory, we have systematically
investigated the electronic structure and transport properties of 18-valence
electron count cobalt based half-Heusler alloys with prime focus on CoVSn,
CoNbSn, CoTaSn, CoMoIn, and CoWIn. The effect of doping on transport properties
has been studied under the rigid band approximation. The maximum power factor,
S$^2\sigma$, for all systems is obtained on hole doping and is comparable to
the existing thermoelectric material CoTiSb. The stability of all the systems
is verified by phonon calculations. Based on our calculations, we suggest that
CoVSn, CoNbSn, CoTaSn, CoMoIn and CoWIn could be potential candidates for high
temperature thermoelectric materials. | 1708.04768v1 |
2018-01-26 | Stoichiometric and off-stoichiometric full Heusler $\mathbf {Fe_2V_{1-x}W_xAl} $ thermoelectric systems | A series of full-Heusler alloys, $\rm Fe_2V_{1-x}W_xAl$, $0 \leq x \leq 0.2$,
was prepared, characterized and relevant physical properties to account for the
thermoelectric performance were studied in a wide temperature range.
Additionally, off-stoichiometric samples with similar compositions have been
included, and a 10~\% improvement of the thermoelectric figure of merit was
obtained. The V/W substitution causes i) a change of the main carrier type,
from holes to electrons as evidenced from Seebeck and Hall measurements and ii)
a substantial reduction of the lattice thermal conductivity due to a creation
of lattice disorder by means of a distinct different mass and metallic radius
upon the V/W substitution. Moreover $ZT$ values above 0.2 have been obtained. A
microscopic understanding of the experimental data observed is revealed from
ab-initio calculations of the electronic and phononic structure. | 1801.08966v2 |
2018-05-07 | High-temperature thermoelectric properties of half-Heusler phases Er$_{1-x}$Ho$_x$NiSb | Polycrystalline samples of Er$_{1-x}$Ho$_x$NiSb ($x$ = 0, 0.2, 0.3, 0.5, 0.7,
0.8, 1) were characterized by means of x-ray powder diffraction (XRD), scanning
electron microscopy (SEM), and optical metallography. The results proved the
formation of half-Heusler alloys in the entire composition range. Their
electrical transport properties (resistivity, thermoelectric power) were
studied in the temperature interval 350-1000 K. The measured electrical
resistivity spanned between 5 and 25 $\mu \Omega$m. The maximum thermopower of
50-65 $\mu$V/K was observed at temperatures 500-650 K. Replacing Ho for Er
resulted in a non-monotonous variation of the thermoelectric power factor ($PF
= S^2/\rho$). The largest $PF$ of 4.6 $\mu$WcmK$^{-2}$ was found at 660 K for
Er$_{0.5}$Ho$_{0.5}$NiSb. This value is distinctly larger than PF determined
for the terminal phases ErNiSb and HoNiSb. | 1805.02435v1 |
2018-08-08 | High throughput screening for spin-gapless semiconductors in quaternary Heusler compounds | Based on high throughput density functional theory calculations, we performed
systematic screening for spin-gapless semiconductors (SGSs) in quaternary
Heusler alloys XX 0 YZ (X, X 0 , and Y are transition metal elements without
Tc, and Z is one of B, Al, Ga, In, Si, Ge, Sn, Pb, P, As, Sb, and Bi).
Following the empirical rule, we focused on compounds with 21, 26, or 28
valence electrons, resulting in 12, 000 possible chemical compositions. After
systematically evaluating the thermodynamic, mechanical, and dynamical
stabilities, we successfully identified 70 stable SGSs, confirmed by explicit
electronic structure calculations with proper magnetic ground states. It is
demonstrated that all four types of SGSs can be realized, defined based on the
spin characters of the bands around the Fermi energy, and the type-II SGSs show
promising transport properties for spintronic applications. The effect of
spin-orbit coupling is investigated, resulting in large anisotropic
magnetoresistance and anomalous Nernst effects. | 1808.02684v1 |
2018-12-20 | Theoretical study of the structural stability, electronic and magnetic properties of XVSb (X $=$ Fe, Ni, and Co) half-Heusler compounds | The structural, electronic and magnetic properties of half-Heusler compounds
XVSb (X $=$ Fe, Co and Ni) are investigated by using the density functional
theory with generalized gradient approximation (GGA), and Tran-Blaha modified
Becke-Johnson (TB-mBJ) exchange potential approximation. It is found that the
half-metallic gaps are generally reasonably widened by mBJ as compared to the
GGA approximation. The magnetic proprieties of XVSb (X $=$ Fe, Co and Ni) are
well defined within mBJ with an exact integer value of magnetic moment. The
band gaps given by TB-mBJ are in good agreement with the available theoretical
data. The FeVSb exhibits a semiconductor nature. The CoVSb and NiVSb present
half-metallic behaviour with total magnetic moment of $1\mu_\text{B}$ and
$2\mu_\text{B}$ in good agreement with Slater-Pauling rule. These alloys seem
to be a potential candidate of spintronic devices. | 1812.08559v1 |
2019-05-23 | Thermodynamic and Thermoelectric Properties of CoFeYGe (Y= Ti, Cr) Quaternary Heusler Alloys: First Principle Calculations | Utilizing a material in thermoelectric applications requires a mechanical,
thermal, and lattice stability as well a high figure of merit (ZT). In this
work, we present the structural, electronic, magnetic, mechanical,
thermodynamic, dynamic, and thermoelectric properties of CoFeYGe (Y = Ti, Cr)
quaternary Heusler compounds using the density functional theory (DFT). The
calculated mechanical properties and phonon dispersions reveal that the
structures of these compounds are stable. Both CoFeCrGe and CoFeTiGe compounds
show a ferromagnetic and ferrimagnetic half-metallic behavior with band gaps of
0.41 and 0.38 eV, respectively. The lattice thermal conductivity (\k{appa}L)
exhibits low values that reach 3.01 W/(m.K) (3.47 W/(m.K)) for CoFeCrGe
(CoFeTiGe) at 1100 K. The optical phonon modes have a large contribution of
60.2% (70.9 %) to \k{appa}L value for CoFeCrGe (CoFeTiGe). High ZT values of
0.71 and 0.65 were obtained for CoFeCrGe and CoFeTiGe, respectively. Based on
our calculations, CoFeCrGe and CoFeTiGe combine both good spintronic and
thermoelectric behaviors that may be used in spin injection applications. | 1905.09854v1 |
2020-09-02 | Phase transition in the magnetocrystalline anisotropy of tetragonal Heusler alloys: Rh$_2T$Sb, $T=$ Fe, Co | This work reports on first principles calculations of the electronic and
magnetic structure of tetragonal Heusler compounds with the composition
Rh$_2$Fe$_{x}$Co$_{1-x}$Sb ($0\leq x\leq1$). It is found that the magnetic
moments increase from 2 to 3.4~$\mu_B$ and the Curie temperature decreases from
500 to 464~K with increasing Fe content $x$. The $3d$ transition metals make
the main contribution to the magnetic moments, whereas Rh contributes only
approximately 0.2~$\mu_B$ per atom, independent of the composition. The paper
focuses on the magnetocrystalline anisotropy of the borderline compounds
Rh$_2$FeSb, Rh$_2$Fe$_{0.5}$Co$_{0.5}$Sb, and Rh$_2$CoSb. A transition from
easy-axis to easy-plane anisotropy is observed when the composition changes
from Rh$_2$CoSb to Rh$_2$FeSb. The transition occurs at an iron concentration
of approximately 40\%. | 2009.00920v1 |
2021-02-23 | Strain glass versus antisite disorder induced ferromagnetic state in Fe doped Ni-Mn-In Heusler martensites | Fe doping in Ni$_2$Mn$_{1.5}$In$_{0.5}$ results in suppression of the
martensitic phase via two contrasting routes. In
Ni$_2$Mn$_{1.5-x}$Fe$_{x}$In$_{0.5}$, the martensitic phase is converted to a
strain glassy phase, while in Ni$_{2-y}$Fe$_y$Mn$_{1.5}$In$_{0.5}$, a cubic
ferromagnetic phase results at the expense of the martensite. Careful studies
of magnetic and structural properties reveal the presence of the impurity
$\gamma -$(Fe,Ni) phase as the reason for the emergence of non-ergodic strain
glassy phase when Fe is sought to be doped at Y/Z (Mn) sites of X$_2$YZ Heusler
alloy. Whereas attempts to dope Fe in the X (Ni) sublattice result in an A2
type antisite disorder that promotes a ferromagnetic ground state. | 2102.11611v1 |
2023-04-06 | Formation of Core-Shell Precipitates in off-stochiometric Ni-Mn-Sn Heusler alloys probed through the induced Sn-moment | The Shell-ferromagnetic effect originates from the segregation process in
off-stochiometric Ni-Mn-based Heusler. In this work, we investigate the
precipitation process of L2$_1$-ordered Ni$_2$MnSn and L1$_0$-ordered NiMn in
off-stochiometric Ni$_{50}$Mn$_{45}$Sn$_{5}$ during temper annealing, by X-ray
diffraction (XRD) and $^{119}$Sn M\"ossbauer spectroscopy. While XRD probes
long-range ordering of the lattice structure, M\"ossbauer spectroscopy probes
nearest-neighbour interactions, reflected in the induced Sn magnetic moment. As
shown in this work, the induced magnetic Sn moment can be used as a detector
for microscopic structural changes and is, therefore, a powerful tool for
investigating the formation of nano-precipitates. Similar research can be
performed in the future, for example, on different pinning type magnets like
Sm-Co or Nd-Fe-B. | 2304.03033v1 |
2002-09-24 | Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni2.19Mn0.81Ga Heusler alloy | In this paper we report the existence of a giant magnetocaloric effect (MCE)
in a intermetallic compound non-containing rare-earth. This effect is
associated with the concomitant occurrence of a structural and a magnetic
transition. The result has been compared with that obtained in a parent
compound in which magnetic and structural transition occur separately. | 0209564v2 |
2003-08-07 | The effect of the spin-orbit interaction on the band gap of half-metals | The spin-orbit interaction can cause a nonvanishing density of states (DOS)
within the minority-spin band gap of half-metals around the Fermi level. We
examine the magnitude of the effect in Heusler alloys, zinc-blende half metals
and diluted magnetic semiconductors, using first-principles calculations. We
find that the ratio of spin-down to spin-up DOS at the Fermi level can range
from below 1% (e.g. 0.5% for NiMnSb) over several percents (4.2% for (Ga,Mn)As)
to 13% for MnBi. | 0308146v1 |
2006-03-10 | Model Hamiltonian parameters for half-metallic ferromagnets NiMnSb and CrO2 | Using the recently developed Nth-order muffin-tin-orbital (NMTO) based
downfolding technique we revisit the electronic properties of half-metallic
ferromagnets, the semi-Heusler NiMnSb and rutile CrO2. The NMTO Wannier
orbitals for the Mn-d and Cr-t2g manifolds are constructed and the mechanism of
chemical bonding is discussed. The effective hopping Hamiltonian parameters are
calculated using a NMTO downfolded basis set. We propose model Hamiltonian
parameters with possibly minimal basis sets for both half-metallic
ferromagnetic alloys. | 0603305v2 |
2007-11-28 | Tailoring magnetic and magnetocaloric properties of martensitic transitions in ferromagnetic Heusler alloys | Ni$_{50}$Mn$_{34}$In$_{16}$ undergoes a martensitic transformation around 250
K and exhibits a field induced reverse martensitic transformation and
substantial magnetocaloric effects. We substitute small amounts Ga for In,
which are isoelectronic, to carry these technically important properties to
close to room temperature by shifting the martensitic transformation
temperature. | 0711.4506v1 |
2009-02-18 | Majority-spin non-quasiparticle states in half-metallic ferrimagnet Mn$_2$VAl | The density of non-quasiparticle states in the ferrimagnetic full-Heuslers
Mn$_2$VAl alloy is calculated from first principles upon appropriate inclusion
of correlations. In contrast to most half-metallic compounds, this material
displays an energy gap in the majority-spin spectrum. For this situation,
non-quasiparticle states are located below the Fermi level, and should be
detectable by spin-polarized photoemission. This opens a new way to study
many-body effects in spintronic-related materials. | 0902.3109v1 |
2009-06-04 | Giant diamagnetism in half-metallic Co$_{2}$CrAl Heusler alloy | A giant diamagnetism in the Co$_{2}$CrAl compounds, in both bulk and thin
film, below a certain temperature ($T_z$) was observed. Above $T_z$, the
compound behaves as an ordinary ferromagnet. The diamagnetic alignment might be
initiated by the Landau diamagnetism because of the half-metallic properties
and the pinning of the diamagnetism is preserved by the peculiar electronic
structures. | 0906.0824v1 |
2010-01-04 | Rare-earth impurities in Co$_2$MnSi: an opportunity to improve Half-Metallicity at finite temperatures | We analyse the effects of doping Holmium impurities into the full-Heusler
ferromagnetic alloy Co$_2$MnSi. Experimental results, as well as theoretical
calculations within Density Functional Theory in the "Local Density
Approximation plus Hubbard U" framework show that the holmium moment is aligned
antiparallely to that of the transition metal atoms. According to the
electronic structure calculations, substituting Ho on Co sites introduces a
finite density of states in the minority spin gap, while substitution on the Mn
sites preserves the half-metallic character. | 1001.0480v1 |
2011-12-09 | Spin polarized tunneling in MgO-based tunnel junctions with superconducting electrodes | We prepared magnetic tunnel junctions with one ferromagnetic and one
superconducting Al-Si electrode. Pure cobalt electrodes were compared with a
Co-Fe-B alloy and the Heusler compound Co2FeAl. The polarization of the
tunneling electrons was determined using the Maki-Fulde-model and is discussed
along with the spin-orbit scattering and the total pair-breaking parameters.
The junctions were post-annealed at different temperatures to investigate the
symmetry filtering mechanism responsible for the giant tunneling
magnetoresistance ratios in Co-Fe-B/ MgO/ Co-Fe-B junctions. | 1112.2110v1 |
2012-10-27 | Structural, magnetic, magnetocaloric and magneto-transport properties in Ge doped Ni-Mn-Sb Heusler Alloys | The effect of Ge substitution on the magnetic, magnetocaloric and transport
properties of Ni45Co5Mn38Sb12-xGex (x=0-3) has been investigated. The decrease
in the exchange interaction brought by Ge substitution can be seen from the
reduction in the magnetization of austenite phase and the increase in the
martensitic transition temperature. Large magnetocaloric effect and
magnetoresistance have been observed at room temperature, making it a potential
material system for various applications. | 1210.7297v2 |
2014-01-08 | Tri-Dirac Surface Modes in Topological Superconductors | We propose a new type of topological surface modes having cubic dispersion in
three-dimensional topological superconductors. Lower order dispersions are
prohibited by the threefold rotational symmetry and time-reversal symmetry.
Cooper pairing in the bulk changes sign under improper rotations, akin
to$^{3}$He-B. The surface manifestations are a divergent surface density of
states at the Fermi level and isospins that rotate three times as they circle
the origin in momentum space. We propose that Heusler alloys with band
inversion are candidate materials to harbor the novel topological
superconductivity. | 1401.1823v1 |
2014-09-24 | Direct magnetocaloric effect measurement technique in alternating magnetic fields | A method for direct measurement of the magnetocaloric effect (MCE) in
alternating magnetic fields is offered. Main advantages of the method compared
to a classical direct one are the high temperature sensitivity (better than
10-3 K); the ability of measuring of MCE in weak magnetic fields (by several
tens oersteds and higher); the ability of measuring of MCE on small-sized
samples (1x1x 0.01 mm3 and larger); the ability of measuring of MCE in
alternating magnetic fields up to 50 Hz of frequency. The results on
measurement of MCE on Gd and Ni-Mn-In Heusler alloy are reported. | 1409.6898v1 |
2016-02-01 | Anti-sites disordering suppression of the possible phase transition in Mn2CrGa | Theoretical and experimental characterizations of Mn2CrGa compound in regard
to the possibility of phase transformation have been carried out in this work.
Under a high ordering L21 structure, this compound has the potential to be a
martensite phase transition material. However, experimental results show a
severe disordering took place in this system, which forbids the occurring of
the phase transition. This work provides important reference for the design of
new phase transition materials in Heusler alloys. | 1602.00397v1 |
2016-11-16 | Perpendicular magnetic anisotropy in Co$_2$MnGa | We report perpendicular magnetic anisotropy in the ferromagnetic Heusler
alloy Co$_2$MnGa in a MgO/Co$_2$MnGa/Pd trilayer stack for Co$_2$MnGa
thicknesses up to 3.5 nm. There is a thickness- and temperature-dependent spin
reorientation transition from perpendicular to in-plane magnetic anisotropy
which we study through the anomalous Hall effect. From the temperature
dependence of the anomalous Hall effect, we observe the expected scaling of
$\rho_{xy}^{AHE}$ with $\rho_{xx}$, suggesting the intrinsic and side-jump
mechanisms are largely responsible for the anomalous Hall effect in this
material. | 1611.05110v1 |
2017-07-26 | Interfacial exchange interactions and magnetism of Ni2MnAl/Fe bilayers | Based on a multi-scale calculations, combining ab-initio methods with spin
dynamics simulations, we perform a detailed study of the magnetic behavior of
Ni2MnAl/Fe bilayers. Our simulations show that such a bilayer exhibits a small
exchange bias effect when the Ni2MnAl Heusler alloy is in a disordered B2
phase. Additionally, we present an effective way to control the magnetic
structure of the Ni2MnAl antiferromagnet, in the pseudo-ordered B2-I as well as
the disordered B2 phases, via a spin-flop coupling to the Fe layer. | 1707.08651v1 |
2019-10-29 | Possible martensitic transformation in Pd2MnTi and Pt2MnTi: First-principles investigation | The martensitic transformation in new-type all-d-metal Heusler alloys Pd2MnTi
and Pt2MnTi have been investigated based on first-principles investigations.
The calculated results indicate that the martenstic transformation have great
possibility to occur in both Pd2MnTi and Pt2MnTi. The energy differences
between the cubic and tetragonal phases are 215.12 meV and 329.45 meV for
Pd2MnTi and Pt2MnTi, respectively. The analysis of the electronic structure of
cubic and tetragonal phases also support this conclusion. The magnetic
properties are also investigated for the two compounds. | 1910.13075v1 |
2016-03-13 | Perpendicular magnetic anisotropy in Co$_2$Fe$_{0.4}$Mn$_{0.6}$Si | We report perpendicular magnetic anisotropy (PMA) in the half-metallic
ferromagnetic Heusler alloy Co$_2$Fe$_{0.4}$Mn$_{0.6}$Si (CFMS) in a
MgO/CFMS/Pd trilayer stack. PMA is found for CFMS thicknesses between 1 and 2
nm, with a magnetic anisotropy energy density of $K_U = 1.5\times 10^6$
erg/cm$^3$ for t$_{\tiny \textrm{CFMS}} = 1.5$ nm. Both the MgO and Pd layer
are necessary to induce the PMA. We measure a tunable anomalous Hall effect,
where its sign and magnitude vary with both the CFMS and Pd thickness. | 1603.04072v1 |
2019-03-26 | Quantum topological transitions and spinons in metallic ferro- and antiferromagnets | An effective Hamiltonian describing fluctuation effects in the magnetic
phases of the Hubbard model in terms of spinon excitations is derived. A
comparison of spin-rotational Kotliar-Ruckenstein slave boson and Ribeiro-Wen
dopon representations is performed. The quantum transition into the
half-metallic ferromagnetic state with vanishing of spin-down Fermi surface is
treated as the topological Lifshitz transition in the quasimomentum space. The
itinerant-localized magnetism transitions and Mott transition in
antiferromagnetic state are considered in the topological context. Related
metal-insulator transitions in Heusler alloys are discussed. | 1903.11003v1 |
2022-12-15 | Griffiths' phase behavior of the Weyl semimetal CrFeVGa | We report a combined theoretical and experimental study of a new topological
semimetal CrFeVGa with an emphasis on the role of atomic disorder on the
magnetoelectronic properties and its applications.CrFeVGa belongs to the
quaternary Heusler alloy family and crystallizes in the cubic structure.
Synchrotron XRD measurement confirms B2 disorder, which plays a crucial role in
dictating the electronic and magnetic properties of the system. | 2212.07576v1 |
2023-09-29 | Micromagnetics of ferromagnetic/antiferromagnetic nanocomposite materials. Part I: Towards the mesoscopic approach | In the first of two articles, we present here a novel mesoscopic
micromagnetic approach for simulating materials composed of ferromagnetic and
antiferromagnetic phases. Starting with the atomistic modeling of quasi
one-dimensional systems, we explicitly show how the material parameters for the
mesoscopic model of an antiferromagnet can be derived. The comparison between
magnetization profiles obtained in atomistic and mesoscopic calculations (using
a Heusler alloy as an example) proves the validity of our method. This approach
opens up the possibility to recover the details of the magnetization
distribution in ferromagnetic/antiferromagnetic materials with the resolution
of a few nanometers covering length scales up to several hundreds of
nanometers. | 2309.17131v1 |
2013-05-15 | Structural ordering driven anisotropic magnetoresistance, anomalous Hall resistance and its topological overtones in full-Heusler Co2MnSi thin films | We report the evolution of crystallographic structure, magnetic ordering and
electronic transport in thin films of full-Heusler alloy Co$_2$MnSi deposited
on (001) MgO with annealing temperatures ($T_A$). By increasing the $T_A$ from
300$^\circ$C to 600$^\circ$C, the film goes from a disordered nanocrystalline
phase to $B2$ ordered and finally to the $L2_1$ ordered alloy. The saturation
magnetic moment improves with structural ordering and approaches the
Slater-Pauling value of $\approx 5.0 \mu_B$ per formula unit for $T_A$ =
600$^\circ$C. At this stage the films are soft magnets with coercive and
saturation fields as low as $\approx$ 7 mT and 350 mT, respectively. Remarkable
effects of improved structural order are also seen in longitudinal resistivity
($\rho_{xx}$) and residual resistivity ratio. A model based upon electronic
transparency of grain boundaries illucidates the transition from a state of
negative $d\rho/dT$ to positive $d\rho/dT$ with improved structural order. The
Hall resistivity ($\rho_{xy}$) derives contribution from the normal scattering
of charge carriers in external magnetic field, the anomalous effect originating
from built-in magnetization and a small but distinct topological Hall effect in
the disordered phase. The carrier concentration ($n$) and mobility ($\mu$) have
been extracted from the high field $\rho_{xy}$ data. The highly ordered films
are characterized by $n$ and $\mu$ of 1.19$\times$ 10$^{29}$ m$^{-3}$ and 0.4
cm$^2V^{-1}s^{-1}$ at room temperature. The dependence of $\rho_{xy}$ on
$\rho_{xx}$ indicates the dominance of skew scattering in our films, which
shows a monotonic drop on raising the $T_A$. The topological Hall effect is
analyzed for the films annealed at 300$^\circ$C. ...... | 1305.3453v1 |
2014-08-02 | Tunnel magnetoresistance and spin-transfer-torque switching in polycrystalline Co2FeAl full-Heusler alloy magnetic tunnel junctions on Si/SiO2 amorphous substrates | We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based
magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate.
Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a
flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance
(TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on
a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching
was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a
switching layer. Using a thermal activation model, the intrinsic critical
current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower
than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett.
100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using
ferromagnetic resonance measurements for the polycrystalline CFA film was
~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0
for the polycrystalline MTJ was mainly attributed to the low damping of the CFA
layer compared with the value in the epitaxial one (~0.04). | 1408.0341v1 |
2018-05-18 | Reentrant cluster glass and stability of ferromagnetism in Ga2MnCo Heusler alloy | We present here a detailed investigation into the magnetic ordering of full
Heusler alloy Ga$_2$MnCo using dc, ac magnetization measurements, neutron
diffraction and neutron depolarization experiments. Crystal structure at room
temperature was first confirmed to be L2$_1$ using the highly intense
synchrotron X-ray diffraction (XRD) technique. Temperature dependent
magnetization reveals that Ga$_2$MnCo enters a ferromagnetic (FM) state at $T_C
= $154 K, characterized by a sharp increase in magnetization and a plateau-like
region hereafter. As the temperature is decreased further, a sharp drop in
magnetization is observed at $T_f$ = 50 K, hinting towards an antiferromagnetic
(AFM) phase change. Neutron diffraction (ND) recorded over the range of
temperature from 6 to 300 K, provides combined information regarding crystal as
well as magnetic structure. Accordingly, an increase in the intensity of the ND
pattern is seen at 150 K, signaling onset of long range FM order. However,
there is no sign of appearance of superlattice reflections corresponding to the
AFM phase, in the patterns recorded below 50 K. An unusual discontinuity in the
unit cell volume is seen around $T_f$ indicating a coupling of this second
transition with the contraction of the lattice. Attempts to unravel this
interesting magnetic behaviour using ac susceptibility measurements lead to the
existence of glassy magnetism below $T_f$. Systematic analysis of the
susceptibility results along with neutron depolarization measurement,
identifies the low temperature phase as a reentrant cluster glass. | 1805.07320v1 |
2019-11-15 | Fermi level tuning and atomic ordering induced giant anomalous Nernst effect in Co2MnAl1-xSix Heusler alloy | Co2MnAl has been predicted to have Weyl points near Fermi level which is
expected to give rise to exotic transverse transport properties such as large
anomalous Hall(AHE) and Nernst effects(ANE) due to large Berry curvature. In
this study, the effect of Fermi level position and atomic ordering on AHE and
ANE in Co2MnAl1-xSix were studied systematically. The Co2MnAl film keeps
B2-disordred structure regardless of annealing temperature, which results in
much smaller anomalous Hall conductivity sigma_xy and transverse Peltier
coefficient sigma_xy than those calculated for L21-ordered Co2MnAl. Our newly
performed calculation of sigma_xy with taking B2 disordering into account well
reproduces experimental result, thus it was concluded that Berry curvature
originating from Weyl points is largely reduced by B2 disordering. It was also
revealed Al substitution with Si shifts the position of Fermi level and
improves the L21-atomic ordering largely, leading to strong enhancement of
sigma_xy, which also agreed with our theoretical calculation. The highest
thermopower of ANE of 6.1uV, which is comparable to the recent reports for
Co2MnGa, was observed for Co2MnAl0.63Si0.37 because of dominant contribution of
sigma_xy. This study clearly shows the importance of both Fermi level tuning
and high atomic ordering for obtaining the effect of topological feature in
Co-based Heusler alloys on transverse transport properties. | 1911.07741v1 |
2021-07-01 | Dopant-segregation to grain boundaries controls electrical conductivity of n-type NbCo(Pt)Sn half-Heusler alloy mediating thermoelectric performance | Science-driven design of future thermoelectric materials requires a deep
understanding of the fundamental relationships between microstructure and
transport properties. Grain boundaries in polycrystalline materials influence
the thermoelectric performance through the scattering of phonons or the
trapping of electrons due to space-charge effects. Yet, the current lack of
careful investigations on grain boundary-associated features hinders further
optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler
alloys, which were synthesized by ball milling and spark plasma sintering
(SPS). Post-SPS annealing was performed on one sample, leading to improved
low-temperature electrical conductivity. The microstructure of both samples was
examined by electron microscopy and atom probe tomography. The grain size
increases from ~230 nm to ~2.38 {\mu}m upon annealing. Pt is found within
grains and at grain boundaries, where it locally reduces the resistivity, as
assessed by in situ four-point-probe electrical conductivity measurement. Our
work showcases the correlation between microstructure and electrical
conductivity, providing opportunities for future microstructural optimization
by tuning the chemical composition at grain boundaries. | 2107.00326v1 |
2018-10-17 | Investigation of the structural, electronic, transport and magnetic properties of Co$_2$FeGa Heusler alloy nanoparticles | We report the structural, transport, electronic, and magnetic properties of
Co$_2$FeGa Heusler alloy nanoparticles. The Rietveld refinements of x-ray
diffraction (XRD) data with the space group Fm$\bar {3}$m clearly demonstrates
that the nanoparticles are of single phase. The particle size (D) decreases
with increasing the SiO$_2$ concentration. The Bragg peak positions and the
inter-planer spacing extracted from high-resolution transmission electron
microscopy image and selected area electron diffraction are in well agreement
with data obtained from XRD. The coercivity initially increases from 127~Oe to
208~Oe between D = 8.5~nm and 12.5~nm, following the D$^{-3/2}$ dependence and
then decreases with further increasing D up to 21.5~nm with a D$^{-1}$
dependence, indicating the transition from single domain to multidomain regime.
The effective magnetic anisotropic constant behaves similarly as coercivity,
which confirms this transition. A complex scattering mechanisms have been
fitted to explain the electronic transport properties of these nanoparticles.
In addition we have studied core-level and valence band spectra using
photoemission spectroscopy, which confirm the hybridization between $d$ states
of Co/Fe. Further nanoparticle samples synthesized by co-precipitation method
show higher saturation magnetization. The presence of Raman active modes can be
associated with the high chemical ordering, which motivates for detailed
temperature dependent structural investigation using synchrotron radiation and
neutron sources. | 1810.07660v1 |
2019-07-13 | Magnetocaloric properties and critical behavior of Co$_2$Cr$_{1-x}$Mn$_x$Al Heusler alloys | We study the magnetocaloric effect and critical behavior of
Co$_2$Cr$_{1-x}$Mn$_x$Al ($x=$ 0.25, 0.5, 0.75) Heusler alloys across the
ferromagnetic (FM) transition (T$_{\rm C}$). The Rietveld refinement of x-ray
diffraction patterns exhibit single phase cubic structure for all the samples.
The temperature dependent magnetic susceptibility $\chi$(T) data show a
systematic enhancement in the Curie temperature and effective magnetic moment
with Mn concentration, which is consistent with the Slater-Pauling behavior.
The M(H) isotherms also exhibit the FM ordering and the analysis of $\chi$(T)
data indicates the nature of the phase transition to be a second order, which
is further supported by scaling of the entropy curves and Arrott plot.
Interestingly, the Mn substitution causes an increase in the magnetic entropy
change and hence large relative cooling power for multi-stage magnetic
refrigerator applications. In order to understand the nature of the magnetic
phase transition we examine the critical exponents $\beta$, $\gamma$, $\delta$
for the $x=$ 0.75 sample by the modified Arrott plot and the critical isotherm
analysis, which is further confirmed by Kouvel-Fisher method and Widom scaling
relation, respectively. The estimated values of $\beta=$ 0.507, $\gamma=$
1.056, $\delta=$ 3.084 are found to be close to the mean field theoretical
values. The renormalized isotherms (m vs h) corresponding to these exponent
values collapse into two branches, above and below T$_{\rm C}$ that validates
our analysis. Our results suggest for the existence of long-range FM
interactions, which decays slower than power law as $J(r)\sim r^{-4.5}$ for a 3
dimensional mean field theory. | 1907.06114v1 |
2019-10-16 | Modeling magnetic evolution and exchange hardening in disordered magnets: The example of Mn$_{1-x}$Fe$_x$Ru$_2$Sn Heusler alloys | We demonstrate how exchange hardening can arise in a chemically-disordered
solid solution from a first-principles statistical mechanics approach. A
general mixed-basis chemical and magnetic cluster expansion has been developed,
and applied to the Mn$_{1-x}$Fe$_x$Ru$_2$Sn Heusler alloy system; single-phase
solid solutions between antiferromagnetic \ch{MnRu2Sn} and ferromagnetic
\ch{FeRu2Sn} with disorder on the Mn/Fe sublattice that exhibit unexpected
exchange hardening. Monte Carlo simulations applied to the cluster expansion
are able to reproduce the experimentally measured magnetic transition
temperatures and the bulk magnetization as a function of composition. The
magnetic ordering around a site is shown to be dependent not only on bulk
composition, but also on the identity of the site and the local composition
around that site. The simulations predict that local antiferromagnetic
orderings form inside a bulk ferromagnetic region at intermediate compositions
that drives the exchange hardening. Furthermore, the antiferromagnetic regions
disorder at a lower temperature than the ferromagnetic regions, providing an
atomistic explanation for the experimentally-observed decrease in exchange
hardening with increasing temperature. These effects occur on a length scale
too small to be resolved with previously-used characterization techniques. | 1910.07543v1 |
2019-06-15 | Effect of L21 and XA ordering on phase stability, half-metallicity and magnetism of Co2FeAl Heusler Alloy: GGA and GGA+U approach | The generalized gradient approximation (GGA) scheme in the first-principles
calculations are used to study the effect of L21 and XA ordering on the phase
stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy.
Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II were
prepared under the conventional L21 and inverse XA phases by altering the
atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of
CFA is the most stable phase energetically among all the structures. The
electronic structure calculations without U show the presence of half-metallic
(HM) ground state only in L21-1 structure and the other structures are found to
be metallic. However, the electronic structures of CFA are significantly
modified in the presence of U, although the total magnetic moments per cell
remained the same and consistent with the Slater-Pauling (SP) rule. The
metallic ground states of CFA in L21-II and XA-II structures are converted into
the half-metallic ground states in presence of U but remained the same
(metallic) in XA-I structure. The results indicate that the electronic
structures are not only dependent on the L21 and XA ordering of the atoms but
also depend on the choice of U values. So experiments may only verify the
superiority of GGA+U to GGA. | 1906.06516v2 |
2021-03-06 | Observation of inverse magnetocaloric effect in magnetic-field-induced austenite phase of Heusler Alloys Ni50-xCoxMn31.5Ga18.5 (x = 9 and 9.7) | Magnetocaloric effect (MCE), magnetization, specific heat, and
magnetostriction measurements were performed in both pulsed and steady high
magnetic fields to investigate the magnetocaloric properties of Heusler alloys
Ni50-xCoxMn31.5Ga18.5 (x = 9 and 9.7). From direct MCE measurements for
Ni41Co9Mn31.5Ga18.5 up to 56 T, a steep temperature drop was observed for
magnetic-field-induced martensitic transformation (MFIMT), designated as
inverse MCE. Remarkably, this inverse MCE is apparent not only with MFIMT, but
also in the magnetic-field-induced austenite phase. Specific heat measurements
under steady high magnetic fields revealed that the magnetic field variation of
the electronic entropy plays a dominant role in the unconventional
magnetocaloric properties of these materials. First-principles based
calculations performed for Ni41Co9Mn31.5Ga18.5 and Ni45Co5Mn36.7In13.3 revealed
that the magnetic-field-induced austenite phase of Ni41Co9Mn31.5Ga18.5 is more
unstable than that of Ni45Co5Mn36.7In13.3 and that it is sensitive to slight
tetragonal distortion. We conclude that the inverse MCE in the
magnetic-field-induced austenite phase is realized by marked change in the
electronic entropy through tetragonal distortion induced by the externally
applied magnetic field. | 2103.04143v1 |
2021-03-18 | Giant spin Hall angle in the Heusler alloy Weyl ferromagnet Co$_2$MnGa | Weyl semimetals are playing a major role in condensed matter physics due to
exotic topological properties, and their coexistence with ferromagnetism may
lead to enhanced spin-related phenomena. Here, the inverse spin Hall effect
(ISHE) in the ferromagnetic Weyl-semimetal Heusler alloy Co$_2$MnGa was
investigated at room temperature by means of electrical spin injection in
lateral spin valve structures. Spin transport properties such as spin
polarization and spin diffusion length in this material were precisely
extracted in order to estimate the spin Hall angle $\theta_{\textrm{SH}}$,
which was found to be $-0.19\pm0.04$ and is among the highest reported for a
ferromagnet. Although this value is on the same order of magnitude of known
heavy metals, the significantly higher resistivity of Co$_2$MnGa implies an
improvement on the magnitude of detection voltages, while its ferromagnetic
nature allows controlling the intensity of SHE through the magnetization
direction. It was also shown that Onsager's reciprocity does not hold for this
system, which is in part attributable to a different spin-dependent Hall
conductivity for spin-up and spin-down carriers. | 2103.10188v1 |
2022-03-20 | Deposition temperature dependence of thermo-spin and magneto-thermoelectric conversion in Co$_2$MnGa films on Y$_3$Fe$_5$O$_{12}$ and Gd$_3$Ga$_5$O$_{12}$ | We have characterized Co$_2$MnGa (CMG) Heusler alloy films grown on
Y$_3$Fe$_5$O$_{12}$ (YIG) and Gd$_3$Ga$_5$O$_{12}$ (GGG) substrates at
different deposition temperatures and investigated thermo-spin and
magneto-thermoelectric conversion properties by means of a lock-in thermography
technique. X-ray diffraction, magnetization, and electrical transport
measurements show that the deposition at high substrate temperatures induces
the crystallized structures of CMG while the resistivity of the CMG films on
YIG (GGG) prepared at and above 500 {\deg}C (550 {\deg}C) becomes too high to
measure the thermo-spin and magneto-thermoelectric effects due to large
roughness, highlighting the difficulty of fabricating highly ordered continuous
CMG films on garnet structures. Our lock-in thermography measurements show that
the deposition at high substrate temperatures results in an increase in the
current-induced temperature change for CMG/GGG and a decrease in that for
CMG/YIG. The former indicates the enhancement of the anomalous Ettingshausen
effect in CMG through crystallization. The latter can be explained by the
superposition of the anomalous Ettingshausen effect and the spin Peltier effect
induced by the positive (negative) charge-to-spin conversion for the amorphous
(crystallized) CMG films. These results provide a hint to construct
spin-caloritronic devices based on Heusler alloys. | 2203.10566v2 |
2016-03-01 | Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys | Weyl fermions have recently been observed in several time-reversal-invariant
semimetals and photonics materials with broken inversion symmetry. These
systems are expected to have exotic transport properties such as the chiral
anomaly. However, most discovered Weyl materials possess a substantial number
of Weyl nodes close to the Fermi level that give rise to complicated transport
properties. Here we predict, for the first time, a new family of Weyl systems
defined by broken time-reversal symmetry, namely, Co-based magnetic Heusler
materials XCo2Z (X = IVB or VB; Z = IVA or IIIA). To search for Weyl fermions
in the centrosymmetric magnetic systems, we recall an easy and practical
inversion invariant, which has been calculated to be -1, guaranteeing the
existence of an odd number of pairs of Weyl fermions. These materials exhibit,
when alloyed, only two Weyl nodes at the Fermi level - the minimum number
possible in a condensed matter system. The Weyl nodes are protected by the
rotational symmetry along the magnetic axis and separated by a large distance
(of order 2$\pi$) in the Brillouin zone. The corresponding Fermi arcs have been
calculated as well. This discovery provides a realistic and promising platform
for manipulating and studying the magnetic Weyl physics in experiments. | 1603.00479v2 |
2018-04-17 | Strain and order-parameter coupling in Ni-Mn-Ga Heusler alloys from resonant ultrasound spectroscopy | Resonant ultrasound spectroscopy and magnetic susceptibility experiments have
been used to characterize strain coupling phenomena associated with structural
and magnetic properties of the shape-memory Heusler alloy series
Ni$_{50+x}$Mn$_{25-x}$Ga$_{25}$ ($x=0$, 2.5, 5.0, and 7.5). All samples exhibit
a martensitic transformation at temperature $T_M$ and ferromagnetic ordering at
temperature $T_C$, while the pure end member ($x=0$) also has a premartensitic
transition at $T_{PM}$, giving four different scenarios: $T_C>T_{PM}>T_M$,
$T_C>T_M$ without premartensitic transition, $T_C\approx T_M$, and $T_C<T_M$.
Fundamental differences in elastic properties i.e., stiffening versus
softening, are explained in terms of coupling of shear strains with three
discrete order parameters relating to magnetic ordering, a soft mode and the
electronic instability responsible for the large strains typical of martensitic
transitions. Linear-quadratic or biquadratic coupling between these order
parameters, either directly or indirectly via the common strains, is then used
to explain the stabilities of the different structures. Acoustic losses are
attributed to critical slowing down at the premartensite transition, to the
mobility of interphases between coexisting phases at the martensitic transition
and to mobility of some aspect of the twin walls under applied stress down to
the lowest temperatures at which measurements were made. | 1804.06183v1 |
2018-04-20 | Presence of atomic disorder and its effect on magnetic and electronic properties of NiCrGa half Heusler alloy | In this work, polycrystalline NiCrGa half Heusler alloy, which is predicted
to be half-metallic ferromagnet from first principles calculations, has been
synthesized by arc meting technique and its structural, magnetic as well as the
electronic properties have been studied. The measured x-ray diffraction (XRD)
pattern shows the signature of a disordered structure. From the magnetization
measurements, there is no evidence of ferromagnetic ordering observed in this
system down to the lowest temperature studied. Instead, the system shows the
signature of an antiferromagnetic ordering at very low temperature. The
experimentally observed structural and magnetic properties are found to be
significantly different from the theoretically predicted properties of the
ordered cubic C1b structure. To probe the possible disorder present in the
system and its effect on the magnetic properties, we have carried out first
principles calculations using the spin-polarized-relativistic
Korringa-Kohn-Rostoker method (SPR-KKR). Using a combination of XRD,
photoelectron spectroscopy, magnetization measurements and first principles
calculations, we conclude that NiCrGa has significant amount of atomic
disorder. Although, the ordered structure is energetically more stable than the
disordered structures, we find that after synthesis, the system tends to
stabilize in a disordered structure. With this atomic disorder present in the
sample, the ferromagnetic ordering is disturbed and the calculated spin
polarization is consequently reduced. | 1804.07613v1 |
2019-03-01 | Linear-response-based DFT+U method for exploring half-metallic Co-based full Heusler alloys | The density functional theory (DFT)+U method based on the linear response
(LR) theory was applied to investigate the electronic structures of Co-based
ternary full Heusler alloy Co$_2Y$Si for exploring half-metallic (HM)
ferromagnets with a wide HM gap. The LR-based DFT+U calculations tend to obtain
a reasonable correlation parameter for $Y$ site, while the correlation of Co
site misleads to the unphysical ground state due to the overestimated parameter
value that arises from the delocalized electronic structure of Co. Furthermore,
we found that the HM gap of Co$_2$MnSi originates from Co $e_u$ orbital in the
conduction state and Co-Mn hybridizing $t_{2g}$ orbital in the valence state
around the Fermi energy. This means that the HM gap is a tunable property by
selecting the $Y$ element and/or mixing several elements into the $Y$ site
through $t_{2g}$ atomic-orbital coupling. Our LR-based DFT+U method was
extended to other ternary Co$_2Y$Si and quaternary Co$_2$($Y$,Mn)Si. We found
that Co$_2$(Ti$_{0.25}$,Mn$_{0.75}$)Si and Co$_2$(Fe$_{0.25}$,Mn$_{0.75}$)Si
show HM nature, with the Fermi energy being at almost the center of the
minority band gap, which leads to high thermal stability. | 1903.00180v2 |
2019-05-10 | Magnetic tunnel junctions with a B2-ordered CoFeCrAl equiatomic Heusler alloy | The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for
spin-gapless semiconductors (SGSs). However, to date, there have been no
experimental attempts at fabricating a junction device. This paper reports a
fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with
CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction
measurements show that the (001) CoFeCrAl electrode films with atomically flat
surfaces have a $B2$-ordered phase. The saturation magnetization is 380
emu/cm$^3$, almost the same as the value given by the Slater--Pauling--like
rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87%
and 165%, respectively. Cross-sectional electron diffraction analysis shows
that the MTJs have MgO interfaces with fewer dislocations. The temperature- and
bias-voltage-dependence of the transport measurements indicates magnon-induced
inelastic electron tunneling overlapping with the coherent electron tunneling.
X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic
arrangement of the Co and Fe magnetic moments of $B2$-ordered CoFeCrAl, in
contrast to the ferrimagnetic arrangement predicted for the $Y$-ordered state
possessing SGS characteristics. Ab-initio calculations taking account of the
Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect
of the Cr-Fe swap disorder on the ability for electronic states to allow
coherent electron tunneling is discussed. | 1905.04070v1 |
2021-02-21 | Experimental method to determine specific heat and transition enthalpy at a first order phase transition: fundamentals and application to a NiMnIn Heusler alloy | A new method that characterizes thermal properties during a first-order phase
transition is described. The technique consists in exciting the sample by a
series of constant frequency thermal pulses which one in every N pulses (N is a
small number like four) being exceedingly large in amplitude. This pulse
induces phase transformation which is inhibited during the following smaller
pulses due to thermal hysteresis. That way the specific heat for a given
mixture of phases can be determined. The results obtained are independent of
experimental parameters like the rate and the amplitude of the pulses, unlike
what happens in other calorimetric techniques. The method also provides the
enthalpy excess by analyzing the energy balance between the dissipated heat and
the heat flowing during each pulse of measurement.
The protocol is tested to analyze the phase transitions of a Heusler alloy Ni
50.54 Mn 33.65 In 15.82 . The paramagnetic-ferromagnetic transition for the
austenite phase is continuous and the specific heat shows a lambda anomaly. The
martensitic phase transition shows a first-order character and the specific
heat follows a step-like behaviour in contrast with previously reported
large-peak anomalies. | 2102.10653v2 |
2021-08-15 | An ab-initio study of topological and transport properties of YAuPb | In the last few decades, the study of topological materials has been carried
out on an extensive scale. Half-Heusler alloys are well known for their
topological behaviours. In this work, we present a detailed study of
topological properties of a ternary Half-Heusler alloy, YAuPb, using the
tight-binding approach. We have calculated some important topological
properties which includes$-$ finding nodes and their chiralities, Berry
curvature ($\boldsymbol\Omega$) and the surface-states. Based on the study of
these properties, we categorise the material as non-trivial topological
semimetal. Besides the topological behaviours, we present a comparative study
of temperature dependent transport properties corresponding to the chemical
potential ($\mu$) of the Fermi level and the node points. The results obtained
from the calculations of electrical conductivity per unit relaxation time
($\boldsymbol\sigma/\tau$) and the electronic part of thermal conductivity per
unit relaxation time ($\boldsymbol\kappa_0$) indicates the conducting nature of
the material to both the heat and electricity. Furthermore, the negative value
of $S$ obtained, indicates the n-type behaviour of the compound. The calculated
value of electronic specific heat (Pauli magnetic susceptibility) corresponding
to Fermi level is $\sim 0.03 \hspace{1mm}(0.18) \times 10^{-2}$ $
Jmol^{-1}K^{-1}$ ($\sim 1.21 \hspace{1mm}(1.14) \times 10^{-10}$ $
m^{3}mol^{-1}$) at 50 (300) K. This work suggests that YAuPb is a promising
candidate of non-trivial topological semimetals which can be employed in
transmission of heat and electricity, and as n-type material within the
temperature range of 50-300 K. | 2108.06678v1 |
2021-11-25 | Efficient spin current source using a half-Heusler alloy topological semimetal with Back-End-of-Line compatibility | Topological materials, such as topological insulators (TIs), have great
potential for ultralow power spintronic devices, thanks to their giant spin
Hall effect. However, the giant spin Hall angle (${\theta}_{SH}$ > 1) is
limited to a few chalcogenide TIs with toxic elements and low melting points,
making them challenging for device integration during the silicon
Back-End-of-Line (BEOL) process. Here, we show that by using a half-Heusler
alloy topological semi-metal (HHA-TSM), YPtBi, it is possible to achieve both a
giant ${\theta}_{SH}$ up to 1.6 and a high thermal budget up to 600${\deg}$C.
We demonstrate magnetization switching of a CoPt thin film using the giant spin
Hall effect of YPtBi by current densities lower than those of heavy metals by
one order of magnitude. Since HHA-TSM includes a group of three-element
topological materials with great flexibility, our work opens the door to the
third-generation spin Hall materials with both high ${\theta}_{SH}$ and high
compatibility with the BEOL process that would be easily adopted by the
industry. | 2111.12889v1 |
2023-01-24 | Ab initio Prediction of Mechanical, Electronic, Magnetic and Transport Properties of Bulk and Heterostructure of a Novel Fe-Cr based Full Heusler Chalcogenide | Using electronic structure calculations based on density functional theory,
we predict and study the structural, mechanical, electronic, magnetic and
transport properties of a new full Heusler chalcogenide, namely, Fe$_2$CrTe,
both in bulk and heterostructure form. The system shows a ferromagnetic and
half-metallic(HM) like behavior, with a very high (about 95%) spin polarization
at the Fermi level, in its cubic phase. Interestingly, under tetragonal
distortion, a clear minimum (with almost the same energy as the cubic phase)
has also been found, at a c/a value of 1.26, which, however, shows a
ferrimagnetic and fully metallic nature. The compound has been found to be
dynamically stable in both the phases against the lattice vibration. The
elastic properties indicate that the compound is mechanically stable in both
the phases, following the stability criteria of the cubic and tetragonal
phases. The elastic parameters unveil the mechanically anisotropic and ductile
nature of the alloy system. Due to the HM-like behavior of the cubic phase and
keeping in mind the practical aspects, we probe the effect of strain as well as
substrate on various physical properties of this alloy. Transmission profile of
the Fe$_2$CrTe/MgO/Fe$_2$CrTe heterojunction has been calculated to probe it as
a magnetic tunneling junction (MTJ) material in both the cubic and tetragonal
phases. Considerably large tunneling magnetoresistance ratio (TMR) of 1000% is
observed for the tetragonal phase, which is found to be one order of magnitude
larger than that of the cubic phase. | 2301.09843v1 |
2023-03-15 | Rare observation of spin-gapless semiconducting characteristics and related band topology of quaternary Heusler alloy CoFeMnSn | In this paper, we report the theoretical investigation and experimental
realization of a new spin-gapless semiconductor (SGSs) compound CoFeMnSn
belonging to the family of quaternary Heusler alloys. Through the use of
several ground-state energy calculations, the most stable structure has been
identified. Calculations of the spin-polarized band structure in optimized
structure's reveals the SGS nature of the compound. The compound form in an
ordered crystal structure and exhibit a high ferromagnetic transition
temperature (T$_{\rm C}$ = 560 K), making the material excellent for room
temperature applications. Adherence of saturation magnetization to the
Slater-Pauling rule, together with the nearly temperature-independent
resistivity, conductivity, and carrier concentration of the compound in the
temperature regime 5$-$300 K along with the low value of anomalous Hall
conductivity (AHC) further confirms the SGS nature. Theoretical calculations
also reveal the robustness of the SGS state due to lattice contraction and one
can obtain a high value of intrinsic AHC using hole doping. Combined SGS and
topological properties of the compound make CoFeMnSn suitable for spintronics
and magneto-electronics devices. | 2303.08589v2 |
2023-07-11 | Microstructure of a spark-plasma-sintered Fe2VAl-type Heusler alloy for thermoelectric application | The influence of microstructure on thermoelectricity is increasingly
recognized. Approaches for microstructural engineering can hence be exploited
to enhance thermoelectric performance, particularly through manipulating
crystalline defects, their structure, and composition. Here, we focus on a
full-Heusler Fe2VAl-based compound that is one of the most promising
thermoelectric materials containing only Earth-abundant, non-toxic elements. A
Fe2VTa0.05Al0.95 cast alloy was atomized under a nitrogen-rich atmosphere to
induce nitride precipitation. Nanometer- to micrometer-scale microstructural
investigations by advanced scanning electron microscopy and atom probe
tomography (APT) are performed on the powder first and then on the material
consolidated by spark-plasma sintering for an increasing time. APT reveals an
unexpected pick-up of additional impurities from atomization, namely W and Mo.
The microstructure is then correlated with local and global measurements of the
thermoelectric properties. At grain boundaries, segregation and precipitation
locally reduce the electrical resistivity, as evidenced by in-situ four-point
probe measurements. The final microstructure contains a hierarchy of structural
defects, including individual point defects, dislocations, grain boundaries,
and precipitates, that allow for a strong decrease in thermal conductivity. In
combination, these effects provide an appreciable increase in thermoelectric
performance. | 2307.05051v1 |
2023-09-18 | Coherent Tunneling and Strain Sensitivity of an All Heusler Alloy Magnetic Tunneling Junction: A First-Principles Study | Half-metallic Co-based full Heusler alloys have captured considerable
attention of the researchers in the realm of spintronic applications, owing to
their remarkable characteristics such as exceptionally high spin polarization
at Fermi level, ultra-low Gilbert damping, and high Curie temperature. In this
comprehensive study, employing density functional theory, we delve into the
stability and electron transport properties of a magnetic tunneling junction
(MTJ) comprising a Co$_2$MnSb/HfIrSb interface. Utilizing a standard model
given by Julliere, we estimate the tunnel magnetoresistance (TMR) ratio of this
heterojunction under external electric field, revealing a significantly high
TMR ratio (500%) that remains almost unaltered for electric field magnitudes up
to 0.5 V/A. In-depth investigation of K-dependent majority spin transmissions
uncovers the occurrence of coherent tunneling for the Mn-Mn/Ir interface,
particularly when a spacer layer beyond a certain thickness is employed.
Additionally, we explore the impact of bi-axial strain on the MTJ by varying
the in-plane lattice constants between -4% and +4%. Our spin-dependent
transmission calculations demonstrate that the Mn-Mn/Ir interface manifests
strain-sensitive transmission properties under both compressive and tensile
strain, and yields a remarkable three-fold increase in majority spin
transmission under tensile strain conditions. These compelling outcomes place
the Co2MnSb/HfIrSb junction among the highly promising candidates for nanoscale
spintronic devices, emphasizing the potential significance of the system in the
advancement of the field. | 2309.09755v1 |
2023-09-29 | Micromagnetics of ferromagnetic/antiferromagnetic nanocomposite materials. Part II: Mesoscopic modeling | In the second part of this publication, we present simulation results for two
three-dimensional models of Heusler-type alloys obtained by the mesoscopic
micromagnetic approach. In the first model, we simulate the magnetization
reversal of a single ferromagnetic (FM) inclusion within a monocrystalline
antiferromagnetic (AFM) matrix, revealing the evolution of the complex
magnetization distribution within this inclusion when the external field is
changed. The main result of this ``monocrystalline'' model is the absence of
any hysteretic behavior by the magnetization reversal of the FM inclusion.
Hence, this model is unable to reproduce the basic experimental result for the
corresponding nanocomposite -- hysteresis in the magnetization reversal of FM
inclusions with a vertical shift of the corresponding loops. To explain this
latter feature, in the second model we introduce a polycrystalline AFM matrix,
with exchange interactions between AFM crystallites and between the FM
inclusion and these crystallites. We show that within this model we can not
only reproduce the hysteretic character of the remagnetization process, but
also achieve a semi-quantitative agreement with the experimentally observed
hysteresis loop assuming that the concentration of FM inclusions strongly
fluctuates. These findings demonstrate the reliability of our enhanced
micromagnetic model and set the basis for its applications in future studies of
Heusler alloys and FM/AFM nanocomposites. | 2309.17129v1 |
2023-10-03 | Complex magnetic interactions and critical behavior analysis in quaternary CoFeV$_{0.8}$Mn$_{0.2}$Si Heusler alloy | We investigate the magnetic behavior and critical exponents of quaternary
CoFeV$_{0.8}$Mn$_{0.2}$Si Heusler alloy to understand the interactions across
the Curie temperature ($T_{\rm C}$). The Rietveld refinement of the x-ray
diffraction pattern with the space group F$\bar{4}$3m confirms a single-phase
cubic Y-type crystal structure. The magnetic susceptibility $\chi (T)$ data
show a ferromagnetic nature with a second-order phase transition from
paramagnetic to ferromagnetic at $446\pm1$~K. The saturation magnetization at
5~K is determined to be 2.2~$\mu_B$/f.u., which found to be close to the
Slater--Pauling rule and indicates its half-metallic nature. The values of
asymptotic critical exponents ($\beta$, $\gamma$, and $\delta$) and the $T_{\rm
C}$ are extracted through detailed analytical analysis including the Modified
Arrott plot, the Kouvel-Fisher (K--F) method, and the Widom scaling relation.
Interestingly, the estimated values of $\beta$ = 0.369 and $\gamma$ = 1.445
closely approximate the theoretical values of the 3D Heisenberg model and
second-order thermodynamic phase transition across the $T_{\rm C}$. The
obtained exponents lead to the collapse of renormalized isotherms, represented
by the relationship between the magnetization (m) and the applied magnetic
field (h), into two distinct branches above and below the $T_{\rm C}$, which
validates the reliability of the analysis. Furthermore, these exponents suggest
that the spin interaction follows a decay pattern of $J(r) \sim r^{-4.99}$,
indicating a short-range magnetic ordering, akin to the itinerant-electron 3D
Heisenberg model. | 2310.01790v1 |
2016-11-06 | Lattice thermal conductivity of Ti$_x$Zr$_y$Hf$_{1-x-y}$NiSn half-Heusler alloys calculated from first principles: Key role of nature of phonon modes | In spite of their relatively high lattice thermal conductivity
$\kappa_{\ell}$, the XNiSn (X=Ti, Zr or Hf) half-Heusler compounds are good
thermoelectric materials. Previous studies have shown that $\kappa_{\ell}$ can
be reduced by sublattice-alloying on the X-site. To cast light on how the alloy
composition affects $\kappa_\ell$, we study this system using the phonon
Boltzmann-transport equation within the relaxation time approximation in
conjunction with density functional theory.The effect of alloying through
mass-disorder scattering is explored using the virtual crystal approximation to
screen the entire ternary Ti$_x$Zr$_{y}$Hf$_{1-x-y}$NiSn phase diagram. The
lowest lattice thermal conductivity is found for the Ti$_x$Hf$_{1-x}$NiSn
compositions; in particular, there is a shallow minimum centered at
Ti$_{0.5}$Hf$_{0.5}$NiSn with $\kappa_l$ taking values between 3.2 and 4.1 W/mK
when the Ti content varies between 20 and 80\%. Interestingly, the overall
behavior of mass-disorder scattering in this system can only be understood from
a combination of the nature of the phonon modes and the magnitude of the mass
variance. Mass-disorder scattering is not effective at scattering acoustic
phonons of low energy. By using a simple model of grain boundary scattering, we
find that nanostructuring these compounds can scatter such phonons effectively
and thus further reduce the lattice thermal conductivity; for instance,
Ti$_{0.5}$Hf$_{0.5}$NiSn with a grain size of $L= 100$ nm experiences a 42\%
reduction of $\kappa_{\ell}$ compared to that of the single crystal. | 1611.01757v2 |
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