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2021-02-22 | High T$_C$ ferromagnetic inverse Heusler alloys: A comparative study of Fe$_2$RhSi and Fe$_2$RhGe | We report the results of experimental investigations on structural, magnetic,
resistivity, caloric properties of Fe$_2$RhZ (Z=Si,Ge) along with
\textit{ab-initio} band structure calculations using first principle
simulations. Both these alloys are found to crystallize in inverse Heusler
structure but with disorder in tetrahedral sites between Fe and Rh. Fe$_2$RhSi
has saturation moment of 5.00 $\mu_B$ and while its counterpart has 5.19
$\mu_B$. Resistivity measurement reveals metallic nature in both of them.
Theoretical simulations using generalized gradient approximation(GGA) predict
inverse Heusler structure with ferromagnetic ordering as ground state for both
the alloys. However it underestimates the experimentally observed moments.
GGA+$U$ approach, with Hubbard $U$ values estimated from density functional
perturbation theory helps to improve the comparison of the experimental
results. Fe$_2$RhSi is found to be half metallic ferromagnet while Fe$_2$RhGe
is not. Varying $U$ values on Fe and Rh sites does not change the net moment
much in Fe$_2$RhSi, unlike in Fe$_2$RhGe. Relatively small exchange splitting
of orbitals in Fe$_2$RhGe compared to that of Fe$_2$RhSi is the reason for not
opening the band gap in the minority spin channel in the former. High ordering
temperature and moment make Fe$_2$RhSi useful for spintronics applications. | 2102.10967v1 |
2004-07-21 | Orbital magnetism in the half-metallic Heusler alloys | Using the fully-relativistic screened Korringa-Kohn-Rostoker method I study
the orbital magnetism in the half-metallic Heusler alloys. Orbital moments are
almost completely quenched and they are negligible with respect to the spin
moments. The change in the atomic-resolved orbital moments can be easily
explained in terms of the spin-orbit strength and hybridization effects.
Finally I discuss the orbital and spin moments derived from X-ray magnetic
circular dichroism experiments. | 0407560v1 |
2014-09-23 | The shifts X-Ray Mn Kα and 2p spectra of Mn-Heusler alloys | X-ray emission K{\alpha}1,2 spectra of Mn in Heusler alloys Co2MnMe (Me = Al,
Ga, Sb), Ni2MnIn, Cu2MnAl were studied. Shifts of Mn K{\alpha}1,2 lines
relatively pure Mn in high-energy region and low-energy shifts of binding
energy Mn 2p XPS is detected. X-ray emission and XPS shifts are in qualitative
agreement. | 1409.6375v1 |
2014-10-08 | Direct Measurement of Magnetocaloric Effect in Metamagnetic Ni43Mn37.9In12.1Co7 Heusler Alloy | The magnetocaloric effect in the metamagnetic Ni43Mn37.9In12.1Co7 Heusler
alloy is directly studied experimentally under the adiabatic and
quasi-isothermal conditions in a magnetic field with induction of up to 14 T. | 1410.2139v1 |
2019-10-29 | Perpendicular magnetic anisotropy in Pt/Co-based full Heusler alloy/MgO thin films structures | Perpendicular magnetic anisotropy (PMA) in ultrathin magnetic structures is a
key ingredient for the development of electrically controlled spintronic
devices. Due to their relatively large spin-polarization, high Curie
temperature and low Gilbert damping the Co-based full Heusler alloys are of
special importance from a scientific and applications point of view. Here, we
study the mechanisms responsible for the PMA in Pt/Co-based full Heusler
alloy/MgO thin films structures. We show that the ultrathin Heusler films
exhibit strong PMA even in the absence of magnetic annealing. By means of
ferromagnetic resonance experiments, we demonstrate that the effective
magnetization shows a two-regime behavior depending on the thickness of the
Heusler layers. Using Auger spectroscopy measurements, we evidence
interdiffusion at the underlayer/Heusler interface and the formation of an
interfacial CoFe-rich layer which causes the two-regime behavior. In the case
of the ultrathin films, the interfacial CoFe-rich layer promotes the strong PMA
through the electronic hybridization of the metal alloy and oxygen orbitals
across the ferromagnet/MgO interface. In addition, the interfacial CoFe-rich
layer it is also generating an increase of the Gilbert damping for the
ultrathin films beyond the spin-pumping effect. Our results illustrate that the
strong PMA is not an intrinsic property of the Heusler/MgO interface but it is
actively influenced by the interdiffusion, which can be tuned by a proper
choice of the underlayer material, as we show for the case of the Pt, Ta and Cr
underlayers. | 1910.13107v1 |
2017-11-01 | Tunable magnetization relaxation of Fe_{2}Cr_{1-x}Co_{x}Si half-metallic Heusler alloys by band structure engineering | We report a systematic investigation on the magnetization relaxation
properties of iron-based half-metallic Heusler alloy
Fe$_{2}$Cr$_{1-x}$Co_${x}$Si (FCCS) thin films using broadband angular-resolved
ferromagnetic resonance. Band structure engineering through Co doping (x)
demonstrated by first-principles calculations is shown to tune the intrinsic
magnetic damping over an order of magnitude, namely 0.01-0.0008. Notably, the
intrinsic damping constants for samples with high Co concentration are among
the lowest reported for Heusler alloys and even comparable to magnetic
insulator yttrium iron garnet. Furthermore, a significant reduction of both
isotropic and anisotropic contributions of extrinsic damping of the FCCS alloys
was found in the FCCS films with x=0.5-0.75, which is of particular importance
for applications. These results demonstrate a practical recipe to tailor
functional magnetization for Heusler alloy-based spintronics at room
temperature | 1711.00406v1 |
2022-04-13 | Influence of Co and Mn on Electronic and Magnetic properties of Ni2MnGa Heusler alloy | The ferromagnetic Heusler alloy $Ni_2MnGa$ had been of major interest in the
past few years because of its magnetic properties which can be easily tuned.
The $Ni_2MnGa$ Heusler alloys are intermetallic alloy with $L2_1$ structure.
Here we report a detailed investigation of the effect of doping of Co and Mn in
Ni2MnGa. Magnetic properties and electronic structure of
$Ni_{2-x}Co_xMnGa_{1-y}Mn_y$ Heusler alloys have been studied by using Green's
function-based KKR-CPA method based DFT calculations. We will show the
magnetization can be tuned depending on the Co and Mn occumencies. We will also
discuss the critical temperature, magnetic interactions and magnetic stability
of the systems. | 2204.06140v4 |
2023-06-25 | First-principles study of disordered half-Heusler alloys \textit{X}Fe$_{0.5}$Ni$_{0.5}$Sn (\textit{X} = Nb, Ta) as thermoelectric prospects | High lattice thermal conductivity in half-Heusler alloys has been the major
bottleneck in thermoelectric applications. Disordered half-Heusler alloys could
be a plausible alternative to this predicament. In this paper, utilizing
first-principles simulations, we have demonstrated the low lattice thermal
conductivity in two such phases, NbFe$_{0.5}$Ni$_{0.5}$Sn and
TaFe$_{0.5}$Ni$_{0.5}$Sn, in comparison to well-known half-Heusler alloy
TiCoSb. We trace the low thermal conductivity to their short phonon lifetime,
originating from the interaction among acoustic and low-lying optical phonons.
We recommend nanostructuring as an effective route in further diminishing the
lattice thermal conductivity. We further predict that these alloys can be best
used in the temperature range 400-600~K and carrier concentration of less than
10$^{21}$ carriers cm$^{-3}$. We found $\sim$35\% and $\sim$17\% enhancement in
$ZT$ for NbFe$_{0.5}$Ni$_{0.5}$Sn and TaFe$_{0.5}$Ni$_{0.5}$Sn, respectively,
as compared to TiCoSb. We are optimistic of the findings and believe these
materials would attract experimental investigations. | 2306.14234v1 |
2014-11-27 | $Ab$ $intito$ study on some new spin-gapless semiconductors: The Zr-based quanternary Heusler alloys | Employing $ab$ $intito$ electronic structure calculations, we have
investigated electronic and magnetic properties of the Zr-based quanternary
Heusler alloys: ZrCoVIn, ZrFeVGe, ZrCoFeP, ZrCoCrBe and ZrFeCrZ (Z=In and Ga).
Our $ab$ $intito$ calculation results show that all the alloys are (or nearly)
spin-gapless semiconductors. All the alloys have large band gaps, indicating
the stability of them at room temperature. The Slater-Pauling behaviours of
these alloys are discussed as well. The values of Curie temperature of all the
alloys are estimated. And it is found that the values of the Curie temperature
for all our calculated quanternary Heusler alloys are higher than that of room
temperature. | 1411.7570v2 |
2002-11-14 | Preparation and structural properties of thin films and multilayers of the Heusler compounds Cu2MnAl, Co2MnSn, Co2MnSi and Co2MnGe | We report on the preparation of thin films and multilayers of the
intermetallic Heusler compound CuMnAl, Co2MnSn, Co2MnSi and Co2MnGe by
rf-sputtering on MgO and Al2O3 substrates. Cu2MnAl can be grown epitaxially
with (100)-orientation on MgO (100) and in (110)-orientation on Al2O3 a-plane.
The Co based Heusler alloys need metallic seedlayers to induce high quality
textured growth. We also have prepared multilayers with smooth interfaces by
combining the Heusler compounds with Au and V. An analysis of the ferromagnetic
saturation magnetization of the films indicates that the Cu2MnAl-compound tends
to grow in the disordered B2-type structure whereas the Co-based Heusler alloy
thin films grow in the ordered L21 structure. All multilayers with thin layers
of the Heusler compounds exhibit a definitely reduced ferromagnetic
magnetization indicating substantial disorder and intermixing at the
interfaces. | 0211271v1 |
2015-07-21 | Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases | Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of
transition-group d-metals and main-group p-elements. Here, we report the
realization of FSMAs in Heusler phases that completely consist of d metals. By
introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is
obtained and the martensitic transformation temperature is decreased
efficiently. Strong ferromagnetism is established by further doping Co atoms
into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the
magnetic-field-induced martensitic transformations, collective multifunctional
properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only
facilitate the formation of B2-type Heusler phases, but also establish strong
ferromagnetic coupling and offer the possibility to tune the martensitic
transformation. | 1507.05907v1 |
2015-01-03 | First-principles study of four quaternary Heusler alloys ZrMnVZ and ZrCoFeZ (Z=Si, Ge) | We investigate the electronic structure and magnetic properties of four
quaternary Heusler alloys ZrMnVZ and ZrCoFeZ (Z=Si, Ge) by using
first-principle calculations. It is shown that ZrMnVSi, ZrMnVGe and ZrCoFeSi
are half-metallic ferromagnets with considerable half-metallic gaps of 0.14,
0.18 and 0.22 eV, respectively. ZrCoFeGe is a nearly half-metallic, the changes
of properties for this alloy under pressure is investigated, the spin
polarization of this alloy is 98.99\% at equilibrium lattice constant. | 1501.00542v1 |
2010-10-14 | Pressure dependence of Curie temperature and resistivity in complex Heusler alloys | Using first-principles electronic structure calculations, we have studied the
dependence of the Curie temperature on external hydrostatic pressure for random
Ni2MnSn Heusler alloys doped with Cu and Pd atoms, over the entire range of
dopant concentrations. The Curie temperatures are calculated by applying
random-phase approximation to the Heisenberg Hamiltonian whose parameters are
determined using the linear response and multiple scattering methods, based on
density-functional theory. In (Ni1-x,Pdx)2MnSn alloys, the Curie temperature is
found to increase with applied pressure over the whole concentration range. The
crossover from the increase to the decrease of the Curie temperature with
pressure takes place for Cu concentrations larger than about 70% in
(Ni1-x,Cux)2MnSn Heusler alloys. The results for the reference Ni2MnSn Heusler
alloy agree well with a previous theoretical study of E. Sasioglu, L. M.
Sandratskii and P. Bruno Phys. Rev. B 71 214412 (2005) and also reasonably well
with available experimental data. Results for the spin-disorder-induced part of
the resistivity in (Ni1-x,Pdx)2MnSn Heusler alloys, calculated by using the
disordered local moment model, are also presented. Finally, a qualitative
understanding of the results, based on Anderson's superexchange interaction and
Stearn's model of the indirect exchange interaction between localized and
itinerant d electrons, is provided. | 1010.3025v2 |
2019-03-28 | Inducing half metallicity with alloying in Heusler Compound CoFeMnSb | First principles studies were performed in order to find out the possibility
of inducing half-metallicity in Heusler Compound CoFeMnSb, by means of alloying
it with 3d-transition metal elements. Proper alloying element is selected
through the calculations of formation energies. These calculations were tested
with different concentrations of alloying elements at different atomic sites.
Among the selected transition metal elements Sc and Ti are proposed to be
excellent alloying elements particularly at Mn site. By using these alloying
elements complete half metallic behaviour is obtained in CoFeMn0.25Sc0.75Sb,
CoFeMn0.75Ti0.25Sb, CoFeMn0.625Ti0.375Sb, CoFeMn0.50Ti0.50Sb,
CoFeMn0.25Ti0.75Sb and CoFeTiSb alloys. Shifting of Co-Fe d-states towards
lower energy region leads to zero density of states at Fermi level for the spin
minority channel. Alloying effects on the electronic structure and
magnetization are discussed in details. Thermodynamical stability of these new
alloys are major part of this study. The Curie temperatures of
CoFeMn0.25Sc0.75Sb and CoFeMn0.75Ti0.25Sb were found to be 324.5 K and 682 K;
respectively, showing good candidature for spintronics applications. For
understanding the bonding nature of constituent atom of CoFeMnSb, crystal
orbital Hamiltonian populations have been analysed. | 1903.11799v1 |
2007-09-26 | Role of defects and disorder in the half-metallic full-Heusler compounds | Half-metallic ferromagnets and especially the full-Heusler alloys containing
Co are at the center of scientific research due to their potential applications
in spintronics. For realistic devices it is important to control accurately the
creation of defects in these alloys. We review some of our late results on the
role of defects and impurities in these compounds. More precisely we present
results for the following cases (i) doping and disorder in Co$_2$Cr(Mn)Al(Si)
alloys, (ii) half-metallic ferrimagnetism appeared due to the creation of
Cr(Mn) antisites in these alloys, (iii) Co-doping in Mn$_2$VAl(Si) alloys
leading to half-metallic antiferromagnetism, and finally (iv) the occurrence of
vacancies in the full-Heusler alloys containing Co and Mn. These results are
susceptible of encouraging further theoretical and experimental research in the
properties of these compounds. | 0709.4183v2 |
2005-07-28 | Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys | We study the exchange interactions in half-metallic Heusler alloys using
first-principles calculations in conjunction with the frozen-magnon
approximation. The Curie temperature is estimated within both mean-field (MF)
and random-phase-approximation (RPA) approaches. For the half-Heusler alloys
NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In
this case the MF and RPA estimations differ strongly. The RPA approach provides
better agreement with experiment. The exchange interactions are more complex in
the case of full-Heusler alloys Co$_2$MnSi and Co$_2$CrAl where the dominant
effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms
and between Co atoms at different sublattices. For these compounds we find that
both MF and RPA give very close values of the Curie temperature slightly
underestimating experimental quantities. We study the influence of the lattice
compression on the magnetic properties. The temperature dependence of the
magnetization is calculated using the RPA method within both quantum mechanical
and classical approaches. | 0507697v2 |
2007-02-19 | Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn--based Heusler alloys | Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having
the formula Co$_2$MnZ where Z a sp element, are among the most studied Heusler
alloys due to their stable ferromagnetism and the high Curie temperatures which
they present. Using state-of-the-art electronic structure calculations we show
that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these
Mn atoms have spin moments antiparallel to the other transition metal atoms.
The ferrimagnetic compounds, which result from this procedure, keep the
half-metallic character of the parent compounds and the large
exchange-splitting of the Mn impurities atoms only marginally affects the width
of the gap in the minority-spin band. The case of [Co$_{1-x}$Mn$_x$]$_2$MnSi is
of particular interest since Mn$_3$Si is known to crystallize in the Heusler
$L2_1$ lattice structure of Co$_2$MnZ compounds. Robust half-metallic
ferrimagnets are highly desirable for realistic applications since they lead to
smaller energy losses due to the lower external magnetic fields created with
respect to their ferromagnetic counterparts. | 0702437v1 |
2010-02-08 | Quantitative analysis of atomic disorders in full-Heusler Co2FeSi alloy thin films using x-ray diffraction with Co-Ka and Cu-Ka sources | The authors developed a new analysis technique for atomic disorder structures
in full-Heusler alloys using x-ray diffraction (XRD) with Co-Ka and Cu-Ka
sources. The developed technique can quantitatively evaluate all the atomic
disorders for the exchanges between X, Y, and Z atoms in full-Heusler X2YZ
alloys. In particular, the technique can treat the DO3 disorder that cannot be
analyzed by ordinary Cu-Ka XRD. By applying this technique to full-Heusler
Co2FeSi alloy thin films formed by rapid thermal annealing (RTA),
RTA-temperature (TA) dependence of the atomic disorders was revealed. The site
occupancies of Co, Fe, and Si atoms on their original sites were 98 %, 90 %,
and 93 %, respectively, for the film formed at TA = 800 degree C, indicating
that the RTA-formed Co2FeSi film had the L21 structure with the extremely high
degree of ordering. | 1002.1564v1 |
2015-05-28 | Uncovering High Thermoelectric Figure of Merit in (Hf,Zr)NiSn Half-Heusler Alloys | Half-Heusler alloys (MgAgSb structure) are promising thermoelectric
materials. RNiSn half-Heusler phases (R=Hf, Zr, Ti) are the most studied in
view of their thermal stability. The highest dimensionless figure of merit (ZT)
obtained is ~1 in the temperature range ~450-900oC, primarily achieved in
nanostructured alloys. Through proper annealing, ZT~1.2 has been obtained in a
previous ZT~1 n-type (Hf,Zr)NiSn phase without the nanostructure. There is an
appreciable increase in the power factor, decrease in charge carrier density,
and increase in carrier mobility. The findings are attributed to the
improvement of structural order. Present approach may be applied to optimize
the functional properties of Heusler-type alloys. | 1505.07773v2 |
2020-04-02 | Reversible adiabatic temperature change in the shape memory Heusler alloy Ni2.2Mn0.8Ga: An effect of structural compatibility | The large magnetocaloric effect (MCE) observed in Ni-Mn based shape-memory
Heusler alloys put them forward to use in magnetic refrigeration technology. It
is associated with a first-order magnetostructural (martensitic) phase
transition. We conducted a comprehensive study of the MCE for the
off-stoichiometric Heusler alloy Ni$_{2.2}$Mn$_{0.8}$Ga in the vicinity of its
first-order magnetostructural phase transition. We found a reversible MCE under
repeated magnetic field cycles. The reversible behavior can be attributed to
the small thermal hysteresis of the martensitic phase transition. Based on the
analysis of our detailed temperature dependent X-ray diffraction data, we
demonstrate the geometric compatibility of the cubic austenite and tetragonal
martensite phases. This finding directly relates the reversible MCE behavior to
an improved geometric compatibility condition between cubic austenite and
tetragonal martensite phases. The approach will help to design shape-memory
Heusler alloys with a large reversible MCE taking advantage of the first-order
martensitic phase transition. | 2004.00949v1 |
2017-06-14 | Temperature-dependent Gilbert damping of Co2FeAl thin films with different degree of atomic order | Half-metallicity and low magnetic damping are perpetually sought for in
spintronics materials and full Heusler alloys in this respect provide
outstanding properties. However, it is challenging to obtain the well-ordered
half-metallic phase in as-deposited full Heusler alloys thin films and theory
has struggled to establish a fundamentals understanding of the temperature
dependent Gilbert damping in these systems. Here we present a study of the
temperature dependent Gilbert damping of differently ordered as-deposited
Co2FeAl full Heusler alloy thin films. The sum of inter- and intraband electron
scattering in conjunction with the finite electron lifetime in Bloch states
govern the Gilbert damping for the well-ordered phase in contrast to the
damping of partially-ordered and disordered phases which is governed by
interband electronic scattering alone. These results, especially the ultralow
room temperature intrinsic damping observed for the well-ordered phase provide
new fundamental insights to the physical origin of the Gilbert damping in full
Heusler alloy thin films. | 1706.04670v2 |
2021-08-04 | Theory of Huge Thermoelectric Effect Based on Magnon Drag Mechanism: Application to Thin-Film Heusler Alloy | To understand the unexpectedly high thermoelectric performance observed in
the thin-film Heusler alloy Fe$_2$V$_{0.8}$W$_{0.2}$Al, we study the magnon
drag effect, generated by the tungsten based impurity band, as a possible
source of this enhancement, in analogy to the phonon drag observed in FeSb$_2$.
Assuming that the thin-film Heusler alloy has a conduction band integrating
with the impurity band, originated by the tungsten substitution, we derive the
electrical conductivity $L_{11}$ based on the self-consistent t-matrix
approximation and the thermoelectric conductivity $L_{12}$ due to magnon drag,
based on the linear response theory, and estimate the temperature dependent
electrical resistivity, Seebeck coefficient and power factor. Finally, we
compare the theoretical results with the experimental results of the thin-film
Heusler alloy to show that the origin of the exceptional thermoelectric
properties is likely to be due to the magnon drag related with the
tungsten-based impurity band. | 2108.01880v1 |
2023-05-04 | Experimental observation of spin glass state in highly disordered quaternary Heusler alloy FeRuMnGa | The realization of spin-glass (S-G) state in Heusler alloys is very rare
despite the presence of inherent structural and elemental disorder in those
compounds. Although a few half and full Heusler alloys are known to exhibit S-G
state, there is hardly any manifestation of the same in cases of quaternary
Heusler compounds. Here we report the observation of S-G state in a highly
disordered equiatomic quaternary Heusler compound: FeRuMnGa, where the S-G
state is in between of canonical S-G and cluster glass. Different intricate
features of S-G state including non-equilibrium magnetic dynamics at low
temperature in the compound are unveiled through our comprehensive magnetic,
heat capacity and neutron diffraction studies. The structural disorder in the
sample is neither conventional \textit{A2}- nor \textit{B2}-type while those
two types are commonly observed for Heusler compounds. The presence of disorder
also plays a significant role in electron transport properties of the alloy,
which is reflected in its exhibition of semi-metallic behavior and anomalous
Hall effect at low temperature. | 2305.03093v1 |
2021-11-09 | Effect of substitutional doping and disorder on the phase stability, magnetism, and half-metallicity of Heusler alloys | Spintronics is the fast growing field that will play a key role in optimizing
power consumption, memory, and processing capabilities of nanoelectronic
devices. Heusler alloys are potential candidates for application in spintronics
due to their room temperature (RT) half-metallicity, high Curie temperature,
low lattice mismatch with most substrates, and strong control on electronic
density of states at Fermi level. In this work, we investigate the effect of
{substitutional doping and disorder} on the half-metallicity, phase stability,
and magnetism of Heusler alloys using density functional theory methods. Our
study shows that electronic and magnetic properties of half/full-Heusler alloys
can be tuned by changing electron-count through controlled variation of
chemical compositions of alloying elements. We provide a detailed discussion on
the effect of substitutional doping and disorder on the tunability of
half-metallic nature of Co$_{2}$MnX and NiMnX based Heusler alloys, where X
represents group 13\textendash 16 and period 3\textendash 6 elements of the
periodic table. {Based on the idea of electron count and disorder, we predicted
a possible existence of thermodynamically stable half-metallic multicomponent
bismuthides, for example, (CuNi$_{3}$)Mn$_{4}$Bi$_{4}$ and
(ZnNi$_{7}$)Mn$_{8}$Bi$_{8}$, through substitution doping at Ni site by
specific Cu and Zn composition in half-Heusler NiMnBi.} We believe that the
design guide {based on electron-counts} presented for half-metals will play a
key role in electronic-structure engineering of novel Heusler alloys for
spintronic application, which will accelerate the development and synthesis of
novel materials. | 2111.05179v1 |
2016-11-24 | Thermophysical and magnetic properties of p- and n-type Ti-Ni-Sn based half-Heusler alloys | A total of 5 different half-Heusler alloys, two p-type and two n-type with
the fifth a charge compensated alloy have been designed and synthesized. The
thermophysical properties of these alloys have been investigated in the range
10 K to 1000 K while the magnetic behavior has been studied up to 300 K. The
electrical resistivity of all the alloys varies within the range 0.06 to 5
m{\Omega} cm indicating that they are in the degenerate semiconductor limits.
The temperature dependence of p-type alloys exhibits a transition from metallic
to semiconducting behavior, typical of topological insulators. The transition
is found to occur in the range 300 K to 500 K. The n-type and compensated
alloys exhibit a weak metallic behavior in the complete temperature range. The
Seebeck coefficient in the p-type alloys increases with temperature reaching a
maximum value of 50 microV K-1 while that of the n-type alloys increases
continuously reaching a value of 45 microV K-1 at ~ 800 K. The corresponding
power factor of the n-type alloy reaches 900 microW m-1 K-2 at ~ 900 K compared
to a maximum of ~ 250 microW m-1 K-2 at 700 K for the p-type alloy.
Magnetically the p-type and n-type alloys are found to be paramagnetic while
the compensated alloy exhibits a ferromagnetic behavior. | 1611.08148v1 |
2014-08-19 | Phase formation characteristics and magnetic properties of bulk Ni2MnGe Heusler alloy | We have systemically studied the effects of annealing temperature and alloy
composition on the structural and magnetic properties of bulk Ni$_{2}$MnGe and
Ni$_{2.1}$Mn$_{0.9}$Ge Heusler alloys. We have observed that both annealing
temperature and the alloy composition drastically alter the phases found in the
samples due to the presence of competing ternary phases. Annealing at 900 and
950 $^{\circ}$C for both alloy compositions facilitate the formation of
L2$_{1}$ Heusler phase. Nevertheless, formation of Ni$_{5}$Mn$_{4}$Ge$_{3}$ and
Ni$_{16}$Mn$_{6}$Ge$_{7}$ phases cannot be prevented for Ni$_{2}$MnGe and
Ni$_{2.1}$Mn$_{0.9}$Ge alloys, respectively. In order to estimate the magnetic
contribution of the Ni$_{5}$Mn$_{4}$Ge$_{3}$ impurity phase to that of the
parent Ni$_{2}$MnGe, we have also synthesized pure Ni$_{5}$Mn$_{4}$Ge$_{3}$
alloy. Antiferromagnetic nature of Ni$_{5}$Mn$_{4}$Ge$_{3}$ with low
magnetization response allows us to reveal the magnetic response of the
stoichiometric bulk Ni$_{2}$MnGe. Bulk Ni$_{2}$MnGe shows simple ferromagnetic
behavior with a Curie temperature of 300 K, in agreement with the previous
results on thin films. Despite the divergence of magnetization curves between
field cooled (FC) and field heated (FH) modes, stoichiometric Ni$_{2}$MnGe
alloy does not undergo a martensitic phase transition based on our variable
temperature x-ray diffraction experiments. | 1408.4241v1 |
2015-11-30 | Band structure and transport studies of half Heusler compound DyPdBi: An efficient thermoelectric material | The discovery of Heusler alloys has revolutionized the research field of
intermetallics due to the ease with which one can derive potential candidates
for multifunctional applications. During recent years, many half Heusler alloys
have been investigated for their thermoelectric properties. The f electron
based rare earth ternary half Heusler compound DyPdBi has its f energy levels
located close to the Fermi energy level. Other research efforts have emphasized
that such materials have good thermoelectric capabilities. We have explored
using first principles the electronic band structure of DyPdBi by use of
different exchange correlation potentials in the density functional theoretical
framework. Transport coefficients that arise in the study of thermoelectric
properties of DyPdBi have been calculated and illustrate its potential as an
efficient thermoelectric material. Both the theoretically estimated Seebeck
coefficient and the power factor agree well with the available experimental
results. Our calculations illustrate that it is essential to include spin-orbit
coupling in these models of f electron half Heusler materials. | 1511.09187v1 |
2006-10-30 | Spin-polarization and electronic properties of half-metallic Heusler alloys calculated from first-principles | Half-metallic Heusler alloys are amongst the most promising materials for
future magnetoelectronic applications. We review some recent results on the
electronic properties of these compounds. The origin of the gap in these
half-metallic alloys and its connection to the magnetic properties are well
understood. Changing the lattice parameter shifts slightly the Fermi level.
Spin-orbit coupling induces states within the gap but the alloys keep a very
high degree of spin-polarization at the Fermi level. Small degrees of doping
and disorder as well as defects with low formation energy have little effect on
the properties of the gap, while temperature effects can lead to a quick loss
of half-metallicity. Finally we discuss two special issues; the case of
quaternary Heusler alloys and the half-metallic ferrimagnets. | 0610827v1 |
2008-11-10 | Tuning the magnetic properties of half-metallic semi-Heusler alloys by sp-electron substitution: The case of AuMnSn$_{1-x}$Sb$_x$ quaternary alloys | We study the electronic and magnetic properties of the quaternary
AuMnSn$_{1-x}$Sb$_{x}$ Heusler alloys using first principles calculations. We
determine their magnetic phase diagram and we show that they present a phase
transition from a ferromagnetic to an antiferromagnetic state with increasing
Sb concentration. For large Sb concentrations the antiferromagnetic
superexchange coupling dominates over the ferromagnetic RKKY-like exchange
mechanism. This behavior is similar to the one demonstrated by the isovalent
Ni$_{1-x}$Cu$_x$MnSb alloy studied recently by the authors [I. Galanakis et al,
Phys. Rev. B. \textbf{77}, 214417 (2008)]. Thus the variation of the
concentration of the \textit{sp}-electrons (Sn and Sb atoms) and the variation
of the concentration of the non-magnetic \textit{3d} atoms (Cu) lead to a
similar tuning of the the magnetic properties of the Heusler alloys. We show
that the inclusion of correlation effects does not alter the phase diagram.
Calculated results are in good agreement with the available experimental data. | 0811.1408v1 |
2011-10-25 | Half-metallic properties for the Ti2YZ (Y=Fe,Co,Ni,Z=Al,Ga,In) Hesuler alloys: A first-principles study | Using the full-potential local orbital minimum-basis method,the Ti2-based
full-Heusler alloys are studied. The results show that these compounds exhibit
a half-metallic behavior, however, in contrast to the conventional full-Heusler
alloys,the full-Heusler alloys show a Slater-Pauling behavior and the total
spin magnetic moment per unit cell(Mt) following the rule Mt=Zt-18. The origin
of the gap in these half-metallic alloys are well understood. It is found that
the half-metallic properties of Ti2-based compounds are insensitive to the
lattice distortion and a fully spin polarization can be obtained within a wide
range of lattice parameters. This is favorable in practical application. | 1110.5411v1 |
2014-10-07 | Anti-site disorder and improved functionality of Mn$_{2}$Ni{\it X} ({\it X}= Al, Ga, In, Sn) inverse Heusler alloys | Recent first-principles calculations have predicted Mn$_{2}$Ni{\it X} ({\it
X}=Al, Ga, In, Sn) alloys to be magnetic shape memory alloys. Moreover,
experiments on Mn$_{2}$NiGa and Mn$_{2}$NiSn suggest that the alloys deviate
from the perfect inverse Heusler arrangement and that there is chemical
disorder at the sublattices with tetrahedral symmetry. In this work, we
investigate the effects of such chemical disorder on phase stabilities and
magnetic properties using first-principles electronic structure methods. We
find that except Mn$_{2}$NiAl, all other alloys show signatures of martensitic
transformations in presence of anti-site disorder at the sublattices with
tetrahedral symmetry. This improves the possibilities of realizing martensitic
transformations at relatively low fields and the possibilities of obtaining
significantly large inverse magneto-caloric effects, in comparison to perfect
inverse Heusler arrangement of atoms. We analyze the origin of such
improvements in functional properties by investigating electronic structures
and magnetic exchange interactions. | 1410.1690v1 |
2019-12-23 | Peculiarities of electronic transport and magnetic state in half-metallic ferromagnetic and spin gapless semiconducting Heusler alloys | A brief survey of experimental and theoretical studies of half-metallic
ferromagnets (HMFs) and spin gapless semiconductors is given, the possible
candidates being the X$_2$YZ (X = Mn, Fe, Co; Y = Ti, V, Cr, Mn, Fe, Co, Ni; Z
= Al, Si, Ga, Ge, In, Sn, Sb) Heusler alloys. The data on the electrical
resistivity, normal and anomalous Hall Effect, and magnetic properties are
presented. It is shown that the Co$_2$FeZ alloys demonstrate properties of
conventional ferromagnets, the HMF properties being also manifested at the
variation of the Z-component. The Fe$_2$YAl and Mn$_2$YAl alloys show at the
variation of the Y-component both metallic and semiconducting electronic
characteristics, the magnetic properties, changing from the ferromagnetic to
compensated ferrimagnetic state. The HMF and spin gapless semiconductor states
are supposed to exist in these Heusler alloys systems. | 1912.10771v1 |
2020-06-05 | Anomalous dependence of thermoelectric parameters on carrier concentration and electronic structure in Mn-substituted Fe2CrAl Heusler alloy | We investigate the high temperature thermoelectric properties of Heusler
alloys Fe2-xMnxCrAl (0<x<1). Substitution of 12.5% Mn at Fe-site (x = 0.25)
causes a significant increase in high temperature resistivity (\r{ho}) and an
enhancement in the Seebeck coefficient (S), as compared to the parent alloy.
However, as the concentration of Mn is increased above 0.25, a systematic
decrement in the magnitude of both parameters is noted. These observations have
been ascribed (from theoretical analysis) to a change in band gap and
electronic structure of Fe2CrAl with Mn-substitution. Due to absence of mass
fluctuations and lattice strain, no significant change in thermal conductivity
is seen across this series of Heusler alloys. Additionally, S drastically
changes its magnitude along with a crossover from negative to positive above
900 K, which has been ascribed to the dominance of holes over electrons in high
temperature regime. In this series of alloys, S and \r{ho} shows a strong
dependence on the carrier concentration and strength of d-d hybridization
between Fe/Mn and Cr atoms. | 2006.03234v2 |
2002-09-25 | Magnetic properties of Ni2.18Mn0.82Ga Heusler alloys with a coupled magnetostructural transition | Polycrystalline Ni2.18Mn0.82Ga Heusler alloys with a coupled
magnetostructural transition are studied by differential scanning calorimetry,
magnetic and resistivity measurements. Coupling of the magnetic and structural
subsystems results in unusual magnetic features of the alloy. These uncommon
magnetic properties of Ni2.18Mn0.82Ga are attributed to the first-order
structural transition from a tetragonal ferromagnetic to a cubic paramagnetic
phase. | 0209574v1 |
2006-03-30 | Effect of doping and disorder on the half-metallicity of full Heusler alloy | Heusler alloys containing Co and Mn are amongst the most heavily studied
half-metallic ferromagnets for future applications in spintronics. Using
state-of-the-art electronic structure calculations, we investigate the effect
of doping and disorder on their electronic and magnetic properties. Small
degrees of doping by substituting Fe or Cr for Mn scarcely affect the
half-metallicity. A similar effect is also achieved by mixing the sublattices
occupied by the Mn and sp atoms. Thus the half-metallicity is a robust property
of these alloys. | 0603811v1 |
2023-02-13 | Half-metallicity in $Ni_2XMn$ Heusler alloys ($X=Fe,Co,Cr$): ab-initio calculations | Lattice parameter, bulk modulus, formation energy and magnetism equilibrium
structure in all-d transition metals $Ni_2XMn$ (X=Co,Cr,Fe) for Full and
Inverse Heusler structure are studied by first-principles calculations.
Tetragonal distortion is calculated for the equilibrium volume of each alloy
and possible new equilibrium structures are reported. Density of electronic
states are analyzed and the results lead to half-metallicity for this type of
alloys. | 2302.06450v1 |
2006-11-22 | Magnetism and structure of magnetic multilayers based on the fully spin polarized Heusler alloys Co2MnGe and Co2MnSn | Our Introduction starts with a short general review of the magnetic and
structural properties of the Heusler compounds which are under discussion in
this book. Then, more specifically, we come to the discussion of our
experimental results on multilayers composed of the Heusler alloys Co2MnGe and
Co2MnSn with V or Au as interlayers. The experimental methods we apply combine
magnetization and magneto-resistivity measurements, x-ray diffraction and
reflectivity, soft x-ray magnetic circular dichroism and spin polarized neutron
reflectivity. We find that below a critical thickness of the Heusler layers at
typically dcr = 1.5 nm the ferromagnetic order is lost and spin glass order
occurs instead. For very thin ferromagnetic Heusler layers there are
peculiarities in the magnetic order which are unusual when compared to
conventional ferromagnetic transition metal multilayer systems. In [Co2MnGe/Au]
multilayers there is an exchange bias shift at the ferromagnetic hysteresis
loops at low temperatures caused by spin glass ordering at the interface. In
[Co2MnGe/V] multilayers we observe an antiferromagnetic interlayer long range
ordering below a well defined Neel temperature originating from the dipolar
stray fields at the magnetically rough Heusler layer interfaces. | 0611590v1 |
2013-01-27 | Spintronic properties and stability of the half-Heusler alloys LiMnZ (Z=N, P, Si) | Li-based half-Heusler alloys have attracted much attention due to their
potential applications in optoelectronics and because they carry the
possibility of exhibiting large magnetic moments for spintronic applications.
Due to their similarities to metastable zinc blende half-metals, the
half-Heusler alloys $\beta$-LiMnZ (Z = N, P and Si) were systematically
examined for their electric, magnetic and stability properties at optimized
lattice constants and strained lattice constants that exhibit half-metallic
properties. Other phases of the half-Heusler structure ($\alpha$ and $\gamma$)
are also reported here, but they are unlikely to be grown. The magnetic moments
of these stable Li-based alloys are expected to reach as high as 4
$\mu_{\mathrm{B}}$ per unit cell when Z = Si and 5 $\mu_{\mathrm{B}}$ per unit
cell when Z = N and P, however the antiferromagnetic spin configuration is
energetically favored when Z is a pnictogen. $\beta$-LiMnSi at a lattice
constant 14\% larger than its equilibrium lattice constant is a promising
half-metal for spintronic applications due to its large magnetic moment and
vibrational stability. The modified Slater--Pauling rule for these alloys is
determined. Finally, a plausible method for developing half-metallic Li$_x$MnZ
at equilibrium, by tuning $x$, is investigated, but, unlike tetragonalization,
this type of alloying introduces local structural changes that destroy the
half-metallicity. | 1301.6367v3 |
2022-08-29 | Substantial enhancement in thermoelectric figure-of-merit of half Heusler ZrNiPb alloys | Ternary half Heusler alloys are under intense investigations recently towards
achieving high thermoelectric figure-of-merit (ZT). Of particular interest is
the ZrNiPb based half Heusler (HH) alloy where an optimal value of ZT = 0.7 at
773 K has been achieved by co-doping Sn and Bi at Pb site. In this work, we
identify an excellent ZT of 1.3 in ZrNi1+xPb0.38Sn0.6Bi0.02 (x= 0.03, at 773 K)
composite alloy. This is achieved by synergistic modulation of electronic as
well as thermal properties via introduction of minor phase of full Heusler (FH)
in the HH matrix through compositional tuning approach. These Ni-rich
ZrNi1+xPb0.38Sn0.6Bi0.02 alloys were synthesized via Arc melting followed by
consolidation via Spark Plasma Sintering (SPS). These alloys were characterized
by XRD and SEM that shows formation of nanocomposites comprising of HH matrix
phase and FH secondary minor phases. Enhancement in ZT is mainly attributed to
a synchronized increase in power factor and about 25% decrease in its thermal
conductivity. The thermoelectric compatibility factor (S) is also calculated
for all samples. The theoretically calculated thermoelectric device efficiency
of best performing sample ZrNi1.03Pb0.38Sn0.6Bi0.02 is estimated to be 13.6%.
Our results imply that controlled fine tuning in HH compounds through
compositional tuning approach would lead to novel off-stoichiometric HH phases
with enhanced ZT value for efficient thermoelectric device fabrication. | 2208.13563v1 |
2002-04-03 | Surface Properties of the Half- and Full-Heusler Alloys | Using a full-potential \textit{ab-initio} technique I study the electronic
and magnetic properties of the (001) surfaces of the half-Heusler alloys,
NiMnSb, CoMnSb and PtMnSb and of the full-Heusler alloys Co$_2$MnGe, Co$_2$MnSi
and Co$_2$CrAl. The MnSb terminated surfaces of the half-Heusler compounds
present properties similar to the bulk compounds and, although the
half-metallicity is lost, an important spin-polarisation at the Fermi level. In
contrast to this the Ni terminated surface shows an almost zero net
spin-polarisation. While the bulk Co$_2$MnGe and Co$_2$MnSi are almost
half-ferromagnetic, their surfaces lose the half-metallic character and the net
spin-polarisation at the Fermi level is close to zero. Contrary to these
compounds the CrAl terminated (001) surface of Co$_2$CrAl shows a spin
polarisation of about 84%. | 0204083v1 |
2005-06-29 | Anomalous vibrational effects in non-magnetic and magnetic Heusler alloys | First-principles calculations are used in order to investigate phonon
anomalies in non-magnetic and magnetic Heusler alloys. Phonon dispersions for
several systems in their cubic L2$\mathrm{_1}$ structure were obtained along
the [110] direction. We consider compounds which exhibit phonon instabilities
and compare them with their stable counterparts. The analysis of the electronic
structure allows us to identify the characteristic features leading to
structural instabilities. The phonon dispersions of the unstable compounds show
that, while the acoustic modes tend to soften, the optical modes disperse in a
way which is significantly different from that of the stable structures. The
optical modes that appear to disperse at anomalously low frequencies are Raman
active, which is considered an indication of a stronger polarizability of the
unstable systems. We show that phonon instability of the TA$_{2}$ mode in
Heusler alloys is driven by interaction(repulsion) with the low energy optical
vibrations. The optical modes show their unusual behavior due to covalent
interactions which are additional bonding features incommensurate with the
dominating metallicity in Heusler compounds. | 0506774v1 |
2009-08-09 | Synthetic antiferromagnet with Heusler alloy Co2FeAl ferromagnetic layers | Heusler alloy Co2FeAl was employed as ferromagnetic layers in
Co2FeAl/Ru/Co2FeAl synthetic antiferromagnet structures. The experimental
results show that the structure with a Ru thickness of 0.45 nm takes on
strongly antiferromagnetic coupling, which maintains up to 150 oC annealing for
1 hour. The structure has a very low saturation magnetization Ms of 425 emu/cc,
a low switching field Hsw of 4.3 Oe and a high saturation field Hs of 5257 Oe
at room temperature, which are favorable for application in ultrahigh density
magnetic read heads or other magnetic memory devices. XRD study testifies that
the as-deposited Co2FeAl film is in B2 phase. Therefore Heusler alloys can be
used to fabricate SyAF and it is possible to make "all-Heusler" spin-valves or
magnetic tunneling junctions with better magnetic switching properties and high
magnetoresistance. | 0908.1215v1 |
2018-06-13 | Minimizing hysteresis in martensite phase transforming magnetocaloric Heusler alloys | The large magnetocaloric effect in Heusler alloys showing martensite phase
transformation puts them forward as efficient materials for magnetic
refrigeration. However, irreversibility of the magnetocaloric cooling cycle is
a major challenge for real applications. This irreversibility is directly
linked to the thermal hysteresis at the first-order martensite phase
transition. Therefore, minimizing the hysteresis is essential in order to
achieve reversibility. Here we show a large reduction in the thermal hysteresis
at the martensite transition in the Ni$_{2}$Mn$_{1.4}$In$_{0.6}$ and
Ni$_{1.8}$Co$_{0.2}$Mn$_{1.4}$In$_{0.6}$ Heusler alloys upon the application of
hydrostatic pressure. Our pressure dependent X-ray diffraction study on
Ni$_{2}$Mn$_{1.4}$In$_{0.6}$ reveals that with increasing pressure the lattice
parameters of the two crystallographic phases (austenite and martensite) change
in such a way that they increasingly satisfy the geometric compatibility
(co-factor) condition. These results provide an opportunity to overcome the
hysteresis problem and hence the irreversible behavior in Heusler materials
using pressure as an external parameter. | 1806.05075v1 |
2017-06-06 | Designing magnetism in Fe-based Heusler alloys: a machine learning approach | Combining material informatics and high-throughput electronic structure
calculations offers the possibility of a rapid characterization of complex
magnetic materials. Here we demonstrate that datasets of electronic properties
calculated at the ab initio level can be effectively used to identify and
understand physical trends in magnetic materials, thus opening new avenues for
accelerated materials discovery. Following a data-centric approach, we utilize
a database of Heusler alloys calculated at the density functional theory level
to identify the ideal ions neighbouring Fe in the $X_2$Fe$Z$ Heusler prototype.
The hybridization of Fe with the nearest neighbour $X$ ion is found to cause
redistribution of the on-site Fe charge and a net increase of its magnetic
moment proportional to the valence of $X$. Thus, late transition metals are
ideal Fe neighbours for producing high-moment Fe-based Heusler magnets. At the
same time a thermodynamic stability analysis is found to restrict $Z$ to main
group elements. Machine learning regressors, trained to predict magnetic moment
and volume of Heusler alloys, are used to determine the magnetization for all
materials belonging to the proposed prototype. We find that Co$_2$Fe$Z$ alloys,
and in particular Co$_2$FeSi, maximize the magnetization, which reaches values
up to 1.2T. This is in good agreement with both ab initio and experimental
data. Furthermore, we identify the Cu$_2$Fe$Z$ family to be a cost-effective
materials class, offering a magnetization of approximately 0.65T. | 1706.01840v1 |
2023-02-22 | Influence of Gd-rich precipitates on the martensitic transformation, magnetocaloric effect and mechanical properties of Ni-Mn-In Heusler alloys -- A comparative study | A multi-stimuli cooling cycle can be used to increase the cyclic caloric
performance of multicaloric materials like Ni-Mn-In Heusler alloys. However,
the use of a uniaxial compressive stress as an additional external stimulus to
a magnetic field requires good mechanical stability. Improvement of mechanical
stability and strength by doping has been shown in several studies. However,
doping is always accompanied by grain refinement and a change in transition
temperature. This raises the question of the extent to which mechanical
strength is related to grain refinement, transition temperature, or
precipitates. This study shows a direct comparison between a single-phase
Ni-Mn-Sn and a two-phase Gd-doped Ni-Mn-In alloy with the same transition
temperature and grain size. It is shown that the excellent magnetocaloric
properties of the Ni-Mn-In matrix are maintained with doping. The isothermal
entropy change and adiabatic temperature change are reduced by only 15% in the
two-phase Ni-Mn-In-Heusler alloy compared to the single-phase alloy, which is
resulting from a slight increase in thermal hysteresis and the width of the
transition. Due to the same grain size and transition temperature, this effect
can be directly related to the precipitates. The introduction of Gd
precipitates leads to a 100% improvement in mechanical strength, which is
significantly lower than the improvement observed for Ni-Mn-In alloys with
grain refinement and Gd precipitates. This reveals that a significant
contribution to the improved mechanical stability in Gd-doped Heusler alloys is
related to grain refinement. | 2302.11439v1 |
2006-07-25 | Doping and disorder in the Co$_2$MnAl and Co$_2$MnGa half-metallic Heusler alloys | We expand our study on the full-Heusler compounds [I. Galanakis \textit{et
al.}, Appl. Phys. Lett. \textbf{89}, 042502 (2006)] to cover also the case of
doping and disorder in the case of Co$_2$MnAl and Co$_2$MnGa half-metallic
Heusler alloys. These alloys present a region of very small minority density of
states instead of a real gap. Electronic structure calculations reveal that
doping with Fe and Cr in the case of Co$_2$MnAl retains the half-metallicity
contrary to the Co$_2$MnGa compound. Cr impurities present an unusual behavior
and the spin moment of the Cr impurity scales almost linearly with the
concentration of Cr atoms contrary to the Co$_2$MnZ (Z= Si, Ge, Sn) where it
was almost constant. Half-metallicity is no more preserved for both Co$_2$MnAl
and Co$_2$MnGa alloys when disorder occurs and there is either excess of Mn or
$sp$ atoms. | 0607652v1 |
2015-06-22 | Two successive magneto-structural transformations and their relation to enhanced magnetocaloric effect for Ni55.3Mn19.7Ga25 Heusler alloy | In the present work, two successive magneto-structural transformations (MSTs)
consisting of martensitic and intermartensitic transitions have been reported
in polycrystalline Ni55.8Mn18.1Ga26.1 Heusler alloy. Benefiting from the
additional latent heat contributed from intermediate phase, this alloy exhibits
a large transition entropy change {\Delta}Str with the value of ~28 J/kg K.
Moreover, the magnetocaloric effect (MCE) has been also evaluated in terms of
Maxwell relation. For the magnetic field change of 3 T, it is found that the
calculated value of refrigeration capacity for Ni55.8Mn18.1Ga26.1 attains to
~72 J/kg around room temperature, which significantly surpasses those obtained
in many Ni-Mn based Heusler alloys. The mechanism underlying the enhanced MCE
is believed to be responsible for these multiple transformations, which can
sustain the pronounced isothermal entropy change {\Delta}ST over a relatively
wide temperature interval. | 1506.06517v2 |
2017-05-17 | High Thermoelectric Figure of Merit by Resonant Dopant in Half-Heusler Alloys | Half-Heusler alloys have been one of the benchmark high temperature
thermoelectric materials owing to their thermal stability and promising figure
of merit ZT. Simonson et al. early showed that small amounts of vanadium doped
in Hf0.75Zr0.25NiSn enhanced the Seebeck coefficient and correlated the change
with the increased density of states near the Fermi level. We herein report a
systematic study on the role of vanadium (V), niobium (Nb), and tantalum (Ta)
as prospective resonant dopants in enhancing the ZT of n-type half-Heusler
alloys based on Hf0.6Zr0.4NiSn0.995Sb0.005. The V doping was found to increase
the Seebeck coefficient in the temperature range 300-1000 K, consistent with a
resonant doping scheme. In contrast, Nb and Ta act as normal n-type dopants, as
evident by the systematic decrease in electrical resistivity and Seebeck
coefficient. The combination of enhanced Seebeck coefficient due to the
presence of V resonant states and the reduced thermal conductivity has led to a
state-of-the-art ZT of 1.3 near 850 K in n-type
(Hf0.6Zr0.4)0.99V0.01NiSn0.995Sb0.005 alloys. | 1705.06100v1 |
2016-03-28 | Ab initio Studies on Electronic and Magnetic Properties of X$_{2}$PtGa (X = Cr, Mn, Fe, Co) Heusler Alloys | Using first-principles calculations based on density functional theory, we
probe the electronic and magnetic properties of X$_{2}$PtGa (X being Cr, Mn,
Fe, Co) Heusler alloys. Our calculations predict that all these systems possess
inverse Heusler alloy structure in their respective ground states. Application
of tetragonal distortion leads to lowering of energy with respect to the cubic
phase for all the materials. The equilibrium volumes of both the phases are
nearly the same. These results of our calculations indicate that all these
materials are prone to undergo martensite transition, as has been recently
shown theoretically for Mn$_{2}$PtGa in the literature. Ground state with a
tetragonal symmetry of these materials is supported by the observation of soft
tetragonal shear constants in their cubic phase. By comparing the energies of
various types of magnetic configurations of these alloys we predict that
Cr$_{2}$PtGa and Mn$_{2}$PtGa possess ferrimagnetic configuration whereas
Fe$_{2}$PtGa and Co$_{2}$PtGa possess ferromagneic configuration in their
respective ground states. | 1603.08350v2 |
2023-09-06 | Development of a self-consistent thermodynamically optimized database along with phase transition experiments in Ni-Mn-Ga system for magnetocaloric applications | Magnetocaloric materials have received significant attention of research
community as they can minimize the use of harmful gases (CFCs, HFCs) and render
eco-friendly refrigeration. Heusler alloys (Ni2MnGa) are known for their
magnetocaloric effects, which make them useful as energy efficient and
eco-friendly refrigerating materials. Magnetocaloric properties significantly
depend on the composition of these alloys. Ni-Mn-Ga is one of the interesting
Heusler systems, which exhibits magnetocaloric properties. In the present
study, we performed the thermodynamic optimization of two sub binaries of the
Ni-Mn-Ga system: Mn-Ga and Ni-Ga, using CALPHAD approach. A Modified
Quasichemical Model (MQM) was used to describe the thermodynamic properties of
the liquid solutions in both the binaries. Both the binaries were combined with
Mn-Ni to develop a self-consistent thermodynamic database for Ni-Mn-Ga. In
order to resolve the existing experimental discrepancies in the Mn-Ga and Ni-Ga
system, few alloy compositions were prepared and analyzed using differential
thermal analysis. Finally, the developed thermodynamic database was used to
calculate the ternary isothermal section of the Ni-Mn-Ga (Heusler alloy) system
at 1073 K with a proposed phase region for magnetocaloric applications. | 2309.02694v1 |
2016-04-26 | First principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys | Heusler alloys have been intensively studied due to the wide variety of
properties that they exhibit. One of these properties is of particular interest
for technological applications, i.e. the fact that some Heusler alloys are
half-metallic. In the following, a systematic study of the magnetic properties
of three different Heusler families $\textrm{Co}_2\textrm{Mn}\textrm{Z}$,
$\text{Co}_2\text{Fe}\text{Z}$ and $\textrm{Mn}_2\textrm{V}\textrm{Z}$ with
$\text{Z}=\left(\text{Al, Si, Ga, Ge}\right)$ is performed. A key aspect is the
determination of the Gilbert damping from first principles calculations, with
special focus on the role played by different approximations, the effect that
substitutional disorder and temperature effects. Heisenberg exchange
interactions and critical temperature for the alloys are also calculated as
well as magnon dispersion relations for representative systems, the
ferromagnetic $\textrm{Co}_2\textrm{Fe}\textrm{Si}$ and the ferrimagnetic
$\textrm{Mn}_2\textrm{V}\textrm{Al}$. Correlations effects beyond standard
density-functional theory are treated using both the local spin density
approximation including the Hubbard $U$ and the local spin density
approximation plus dynamical mean field theory approximation, which allows to
determine if dynamical self-energy corrections can remedy some of the
inconsistencies which were previously reported for these alloys. | 1604.07552v1 |
2023-01-02 | Double Half-Heusler Alloys X$_2$Ni$_2$InSb (X= Zr/Hf) with promising Thermoelectric Performance: Role of varying structural phases | Double half-heusler alloys are the new class of compounds which can be seen
as transmuted version of two single half-heusler with higher flexibility of
tuning their properties. Here, we report a detailed study of thermoelectric
(TE) properties of two double half-heusler (HH) alloys X$_2$Ni$_2$InSb
(X=Hf/Zr), using first-principles calculation. These alloys exhibit a rich
phase diagram with the possibility of tetragonal, cubic and solid solution
phase at different temperature range. As such, a comparative study of TE
properties of all these phases is performed. The ordered phases show quite
favorable electronic transport as compared to the disordered ones in both
compounds. Lattice thermal conductivity of double HH alloys is lower than their
ternary counter-part, making them most promising for TE application. Simulated
band gap, obtained using hybrid functional, of ordered phases of
Hf$_2$Ni$_2$InSb and Zr$_2$Ni$_2$InSb lie in the range 0.24-0.4 eV and
0.17-0.59 eV respectively, while for disordered phase, it is 0.05- 0.06 eV.
Hf$_2$Ni$_2$InSb shows a reasonably high ZT value of $\sim$ 2.19, while
Zr$_2$Ni$_2$InSb yields 2.46 at high temperature for n-type conduction in
tetragonal phase. The ZT value for p-type conduction is also quite promising
($\sim$ 1.35 and $\sim$ 2.19 for Hf- and Zr-based compounds). In both the
compounds, electronic transport (Seebeck and electrical conductivity) plays the
dominant role for the high ZT-value. Keeping in mind the promising TE
performance, we propose immediate attention from experimentalists to synthesize
and cross validate our findings for these new candidate materials. | 2301.00598v1 |
2013-02-04 | Disorder-induced cubic phase in Fe$_2$-based Heusler alloys | Based on first-principles electronic structure calculations, we analyze the
chemical and magnetic mechanisms stabilizing the cubic phase in Fe$_2$-based
Heusler materials, which were previously predicted to be tetragonal when being
chemically ordered. In agreement with recent experimental data, we found that
these compounds crystallize within the so-called "inverted" cubic Heusler
structure perturbed by a certain portion of the intrinsic chemical disorder.
Understanding these mechanisms is a necessary step to guide towards the
successful future synthesis of the stable Fe$_2$-based tetragonal phases, which
are very promising candidates for the fabrication of rare-earth-free permanent
magnets. | 1302.0713v1 |
2014-01-24 | Performance Analysis of Spin Transfer Torque Random Access Memory with cross shaped free layer using Heusler Alloys by using micromagnetic studies | We investigated the performance of spin transfer torque random access memory
(STT-RAM) cell with cross shaped Heusler compound based free layer using
micromagnetic simulations. We designed the free layer using Cobalt based
Heusler compounds. Here in this paper, simulation results predict that
switching time from one state to other state is reduced. Also it is examined
that critical switching current density to switch the magnetization of free
layer of STT RAM cell is reduced. | 1401.6971v1 |
2017-03-24 | Probing the degree of spin polarization of a ferromagnet with ferromagnet/superconductor proximity effect | Superconductor/ferromagnet proximity effect has been studied for
Pb/Co$_2$Cr$_{1-x}$Fe$_x$Al bilayers. Different substrate temperatures allowed
us to prepare the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al films with different
degree of spin polarization of conduction band of the Heusler layer. We obtain
a strong correlation between the dependence of the superconducting transition
temperature on the Pb-layer thickness at fixed thickness of the Heusler layer
and the degree of spin polarization in the ferromagnetic layer. | 1703.08422v1 |
2011-08-18 | Comparison of two structures for transition-metal-based half Heusler alloys exhibiting fully compensated half metallicity | We search for new fully compensated half metals, in which only one electronic
spin channel is conducting and there exists no net magnetic moment. We focus on
half Heusler alloys and we examine the physical consequence of different
crystal structures found in the literature for these compounds, XMnZ, with a
transition metal element, such as Cr, Mn, and Fe for X and a nonmetallic
element, such as P, Sb and Si for Z. The structures differ in the placement of
voids in the L2$_1$ structure of the full Heulser alloy. One structure has the
void at (1/4, 3/4, 1/4)a and the other places the void at (0.0, 0.0, 1/2)a. The
first structure is expected to have greater d-p hybridization between Mn and
the Z atom. The other exhibits strong d-d hybridization between the nearest
neighboring transition metal elements. Five XMnZ compounds are considered along
with the previously studied CrMnSb in the second structure, which serves as a
reference. Besides the CrMnSb, only one other alloy, MnMnSi, shows fully
compensated half metallic properties in both structures. Both these alloys obey
the Slater-Pauling electron counting rule for half Hesuler alloys. The
differences between CrMnSb and MnMnSi in the two structures are discussed based
on their atomic properties. In the search for fully compensated half metals in
transition metal-based half Heusler alloys, we suggest using the counting rule
as a guide. | 1108.3651v1 |
2012-02-08 | Evolution of microstructural and mechanical properties of nanocrystalline Co2FeAl Heusler alloy prepared by mechanical alloying | Mechanical alloying (MA) has been used to fabricate the Co2FeAl Heusler alloy
with a nanocrystalline structure. The formation mechanism of the alloy has been
investigated. Rietveld analysis showed that all samples that were milled for
more than 15 hours had an L21 structure with a space group of Fm3m. The
crystallite size and internal strain of the samples were calculated using the
Williamson-Hall equation. With mechanical alloying of up to 20 hours the
crystallite size of Co2FeAl increased, after which the crystallite size started
to decrease. In contrast, internal strain first decreased during the process
and then increased with the increase of milling time. The powder obtained after
20 hours of MA was split into three parts and separately annealed at 300, 500
and 700 oC for 5 hours. A considerable increase was observed in the hardness
value of powder particles with the increase of annealing temperature up to 500
oC. However, the hardness value of the sample annealed at 700 oC decreased. It
seems that this feature is related to parameters such as increase of
crystallite size, enhancement of lattice ordering, change in density of defects
and impurities and nonstoichiometric effects. | 1202.1754v1 |
2014-11-13 | Electrical Control of Carriers Spin Orientation in FeVTiSi Heusler Alloy | The direct control of carriers spin by electric field under room temperature
is one of the most important challenges in the field of spintronics. For this
purpose, we here propose a quaternary Heusler alloy FeVTiSi. Based on first
principles calculations, FeVTiSi alloy is found to be an intrinsic bipolar
magnetic semiconductor in which the valence band and conduction band approach
the Fermi level through opposite spin channels. Thus FeVTiSi alloy can conduct
completely spin-polarized currents with tunable spin-polarization direction
simply by applying a gate voltage. Furthermore, by Monte Carlo simulations
based on the classical Heisenberg Hamiltonian, the Curie temperature of FeVTiSi
alloy is predicted to be as high as 1293 K, far above the room temperature. The
bipolar magnetic semiconducting character and the high Curie temperature endow
the FeVTiSi alloy great potentials in developing electrically controllable
spintronic devices working at room temperature. | 1411.3426v1 |
2018-12-11 | Half-metallicity in quaternary Heusler alloys with 3$d$ and 4$d$ elements: observations and insights from DFT calculations | In this work, we provide important insights into the evolution of
half-metallicity in quaternary Heusler alloys. Employing {\it ab initio}
electronic structure methods we study 18 quaternary Heusler compounds having
the chemical formula CoX$^\prime$Y$^\prime$Al, where Y$^\prime$ = Mn, Fe; and
X$^\prime$ a 4$d$ element. Along with the search for new materials for
spintronics applications, the trends in structural, electronic, magnetic
properties and Curie temperature were investigated. We have made comparative
studies with the compounds in the quaternary series
CoX$^{\prime}$Y$^{\prime}$Si with X$^{\prime}$ materials from 3$d$ and 4$d$
transition metal series in the periodic table. We observe that the
half-metallic behaviour depends primarily on the crystal structure type based
on atomic arrangements and the number of valence electrons. As long as these
two are identical, the electronic structures and the magnetic exchange
interactions bear close resemblances. Consequently, the materials exhibit
identical electronic properties, by and large. We analysed the roles of
different transition metal atoms in affecting hybridisations and correlated
them with the above observations. This work, therefore, provides important
perspectives regarding the underlying physics of half-metallic behaviour in
quaternary Heusler compounds which goes beyond specifics of a given material.
This, thus, paves way for smart prediction of new half-metals. This work also
figures out an open problem of understanding how different ternary Heuslers
with different electronic behaviour may lead to half-metallic behaviour in
quaternary Heuslers with 4$d$ transition metal elements. | 1812.04477v1 |
2022-06-23 | A general rule for predicting the magnetic moment of Cobalt-based Heusler compounds using compressed sensing and density functional theory | We propose a general rule for estimating the magnetic moments of Co$
2$(cobalt)-based Heusler alloys, especially when doped with late transition
metals. We come up with a descriptor that can characterise both pure Co$_2$YZ
compounds and the doped ones with the chemical formula Co$_2$Y$_{1-x}$M$_x$Z (M
is the dopant) using online data for magnetic moments of Heusler alloys with
Co$_2$YZ structure and compressive sensing approach. The newly proposed
descriptor not only depends on the number of valence electrons of the compound
also it depends on the number of unoccupied d-electrons in the doping site. A
comparison of the performance of the proposed descriptor and the Slater-Pauling
rule is made. Unlike the Slater-Pauling rule, which is only effective for
half-metallic Heusler compounds, our machine-learning approach is more generic
since it applies to any Co$_2$YZ Heusler compounds, regardless of whether they
are half-metals or not. We use this new rule to estimate the magnetic moments
of a few yet-to-be-discovered Heusler compounds and compare the results to
density functional theory (DFT) based calculations. Finally, we use DFT and
machine learning investigations to prove their stability. | 2206.11635v3 |
2022-10-25 | Conventional Half-Heusler Alloys Advance State-of-the-Art Thermoelectric Properties | Half-Heusler (HH) phases have garnered much attention as thermally stable and
non-toxic thermoelectric materials for power conversion. The most studied
alloys to date utilize Hf, Zr, and Ti as the base components. These alloys can
achieve a moderate dimensionless figure of merit, ZT, near 1. Recent studies
have advanced the thermoelectric performance of HH alloys by employing
nanostructures and novel compositions to achieve larger ZT, reaching as high as
1.5. Herein, we report that traditional alloying techniques applied to the
conventional HfZr-based half-Heusler alloys can also lead to exceptional ZT.
Specifically, we present the well-studied p-type Hf0.3Zr0.7CoSn0.3Sb0.7,
previously reported to have a ZT~0.8, resonantly doped with less than 1 at. %
metallic Al on the Sn/Sb site, touting a remarkable ZT near 1.5 at 980 K. This
is achieved through a significant increase in power factor, by ~65%, and a
notable but smaller decrease in thermal conductivity, by ~13%, at high
temperatures. These favorable thermoelectric properties are discussed in terms
of a local anomaly in the density of states near the Fermi energy designed to
enhance the Seebeck coefficient, as revealed by first-principles calculations,
as well as the emergence of a highly heterogeneous grain structure that can
scatter phonons across different length scales, effectively suppressing the
thermal conductivity. Consequently, the effective mass is significantly
enhanced from ~ 7 to 10me within a single parabolic band model, consistent with
the result from first-principles calculations. The discovery of high ZT in a
commonly studied half-Heusler alloy through a conventional and non-complex
approach opens a new path for further discoveries in similar types of alloys.
Furthermore, it is reasonable to believe that the study will reinvigorate
effort in exploring high thermoelectric performance in conventional alloy
systems. | 2210.13949v1 |
2012-10-22 | Generalized Slater-Pauling rule for the inverse Heusler compounds | We present extensive first-principles calculations on the inverse
full-Heusler compounds having the chemical formula X$_2$YZ where (X = Sc, Ti,
V, Cr or Mn), (Z = Al, Si or As) and the Y ranges from Ti to Zn. Several of
these alloys are identified to be half-metallic magnets. We show that the
appearance of half-metallicity is associated in all cases to a Slater-Pauling
behavior of the total spin-magnetic moment. There are three different variants
of this rule for the inverse Heusler alloys depending on the chemical type of
the constituent transition-metal atoms. Simple arguments regarding the
hybridization of the d-orbitals of neighboring atoms can explain these rules.
We expect our results to trigger further experimental interest on this type of
half-metallic Heusler compounds. | 1210.5816v1 |
2013-11-08 | Electronic, magnetic and transport properties of full and half-metallic thin film Heusler alloys | The electronic and magnetic bulk properties of half-metallic Heusler alloys
such as Co$_{2}$FeSi, Co$_{2}$FeAl, Co$_{2}$MnSi and Co$_{2}$MnAl are
investigated by means of {\em ab initio} calculations in combination with Monte
Carlo simulations. The electronic structure is analyzed using the plane wave
code Quantum Espresso and magnetic exchange interactions are determined using
the KKR method. From the magnetic exchange interactions the Curie temperature
is obtained via Monte Carlo simulations. In addition, electronic transport
properties of the trilayer systems consisting of two semi-infinite platinum
leads and a Heusler layer in between are obtained from the fully relativistic
KKR method by employing the Kubo-Greenwood formalism. The focus is on
thermoelectric properties, namely the Seebeck effect and its spin dependence.
It turns out that already thin Heusler layers provide highly polarized currents
within the systems. This is attributed to the recovery of half-metallicity with
increasing thickness. The absence of electronic states of the spin down
electrons around the Fermi level suppresses the contribution of this spin
channel to the total conductivity. This strongly influences the thermoelectric
properties of such systems and results in polarized thermoelectric currents. | 1311.1950v1 |
2015-03-10 | Vanadium sacrificial layers as a novel approach for the fabrication of freestanding Heusler Shape Memory Alloys | In this study we report a method for the preparation of freestanding
magnetocaloric thin films. Non-stoichiometric Heusler alloys Ni-Mn-Sn,
Ni-Co-Mn-Sn and Ni-Co-Mn-Al are prepared via sputter deposition. A sacrificial
vanadium layer is added between the substrate and the Heusler film. By means of
selective wet-chemical etching the vanadium layer can be removed. Conditions
for the crystallization of Vanadium layers and epitaxial growth of the Heusler
films are indicated. Magnetic and structural properties of freestanding and
as-prepared films are compared in detail. The main focus of this study is on
the influence of substrate constraints on the Martensitic transistion. | 1503.02987v1 |
2018-05-07 | Electronic, Elastic and X-ray spectroscopic properties of direct and inverse full Heusler compounds: DFT+U study | Half-metallicity, low magnetic damping and high curie temperature (TC) are
crucial for application in spintronics and full Heusler alloys in this regard
exhibit remarkable properties. Herein, we have considered Co2FeAl (CFA) and
Fe2CoAl (FCA) as a representative of direct and indirect full Heusler compounds
which crystallises in L21 and C1b phases, respectively. The theory of L21 type
Heusler alloys has been well established, however the fundamental understanding
of Fe2CoAl is still under developed. In this paper, we have employed density
functional theory (DFT) to study the electronic, elastic and X-ray
spectroscopic properties of Co2FeAl and Fe2CoAl. The electron exchange
correlation were treated within a generalized gradient approximation (GGA) as
PBE-scheme. In order to include the impact of valence electrons an onsite
Coulomb potential is added to GGA as GGA+U. Within both GGA and GGA+U, CFA
shows a half-metallic behaviour but FCA is metallic. The calculated values of
magnetic moment and TC are in close agreement with the experimental data. | 1805.02618v1 |
2018-09-04 | Increasing the performance of the superconducting spin valve using a Heusler alloy | We have studied superconducting properties of the spin-valve thin layer
heterostructures CoO$_x$/F1/Cu/F2/Cu/Pb where the ferromagnetic F1 layer was
standardly made of Permalloy whereas for the F2 layer we have taken a specially
prepared film of the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al with a small degree
of spin polarization of the conduction band. The heterostructures demonstrate a
significant superconducting spin-valve effect, i.e. a complete switching on and
off of the superconducting current flowing through the system by manipulating
the mutual orientations of the magnetization of the F1 and F2 layers. The
magnitude of the effect is doubled in comparison with the previously studied
analogous multilayers with the F2 layer made of the strong ferromagnet Fe.
Theoretical analysis shows that a drastic enhancement of the switching effect
is due to a smaller exchange field in the heterostructure coming from the
Heusler film as compared to Fe. This enables to approach almost ideal
theoretical magnitude of the switching in the Heusler-based multilayer with the
F2 layer thickness of $\sim 1$\,nm. | 1809.00876v1 |
2019-01-27 | Correlation effects on ground-state properties of ternary Heusler alloys: first-principles study | The strongly constrained and appropriately normed (SCAN) semi-local
functional for exchange-correlation is deployed to study the ground-state
properties of ternary Heusler alloys transforming martensitically. The
calculations are performed for ferromagnetic, ferrimagnetic, and
antiferromagnetic phases. Comparisons between SCAN and generalized gradient
approximation (GGA) are discussed. We find that SCAN yields smaller lattice
parameters and higher magnetic moments compared to the GGA corresponding values
for both austenite and martensite phases. Furthermore, in the case of
ferromagnetic and non-magnetic Heusler compounds, GGA and SCAN display similar
trends in the total energy as a function of lattice constant and tetragonal
ratio. However, for some ferrimagnetic Mn-rich Heusler compounds, different
magnetic ground states are found within GGA and SCAN. | 1901.09460v1 |
2020-05-07 | Structural and magnetic behavior of Cr$_2$Co$_{(1-x)}$Cr$_x$Al inverse Heusler alloys | We report the structural and magnetic behavior of single phase inverse
Heusler alloys Cr$_2$Co$_{(1-x)}$Cr$_x$Al ($x = $ 0, 0.2, 0.4) using x-ray
diffraction (XRD), Raman spectroscopy, isothermal magnetization, and magnetic
susceptibility measurements. Interestingly, the Rietveld refinement of XRD data
with the space group I$\bar{4}m2$ reveal a tetragonal distortion with c/a ratio
around 1.38 in these inverse Heusler structures. The bulk compositions have
been confirmed by energy dispersive x-ray spectroscopy measurements. The active
Raman mode F$_{2g}$ is observed at 320~cm$^{-1}$, which confirms the X-type
Heusler structure as the A2 and B2 type structures are known to be not Raman
active. The area of F$_{2g}$ mode decreases with Cr concentration, which
indicate the origin of this mode due to Co vibrations. The isothermal
magnetization data confirm the magnetic moment close to zero ($\le$0.02
$\mu_B/f.u.$) at $\approx$70~kOe and negligible coercive field suggest the
fully compensated ferrimagnetic nature of these samples. The susceptibility
behavior indicates irreversibility between zero-field and field-cooled curves
and complex magnetic interactions at low temperatures. | 2005.03528v1 |
2016-12-19 | How to enable bulk-like martensitic transformation in epitaxial films | The present study is dedicated to the influence of different substrate and
buffer layer materials on the martensitic transformation in sputter deposited
epitaxial shape memory Heusler alloys. For this, the magnetocaloric Heusler
alloy Ni-Co-Mn-Al \cite{Teichert2015b} is grown on MgO(001),
MgAl$_{2}$O$_{4}$(001), and MgO(001)/V substrates, which exhibit a lattice
misfit to the Ni-Co-Mn-Al between $-1.2\%$ and $3.6\%$. By temperature
dependent X-ray diffraction measurements it is shown that the optimum buffer
layer for shape memory Heusler films is not one with minimum lattice misfit,
but one with minimum Young's modulus and moderate misfit because an elastic
buffer layer can deform during the martensitic transformation of the Heusler
layer. Furthermore, epitaxial strain caused by a moderate lattice misfit does
not significantly change the martensitic transformation temperatures. | 1612.06077v1 |
2023-06-26 | Origin of magnetic ordering in half-Heusler RuMnGa | The half-Heusler alloy RuMnGa having valence electron count (VEC) 18 has
recently been theoretically proposed to exhibit compensated ferrimagnetic
(CFiM) character instead of the expected nonmagnetic ground state. On the other
hand, a preliminary experimental study proposed ferromagnetic (FM) ordering. As
no half-Heusler system with VEC 18 is known to exhibit magnetic ordering, we
have investigated the details of crystal structure and magnetic properties of
RuMnGa using a combination of experimental tools, viz., x-ray and neutron
diffraction techniques, dc and ac susceptibility, isothermal magnetisation,
heat capacity, resistivity and neutron depolarisation measurements. Rietveld
refinements of x-ray and neutron diffraction data suggest single phase nature
of the compound with elemental composition RuMn$_{0.86}$Ga$_{1.14}$. We have
shown that the system exhibits FM-type ordering owing to the inherent presence
of this minor off-stoichiometry, showing very low magnetic moment. The system
also exhibits reentrant canonical spin-glass behaviour, which is rarely
observed in half-Heusler alloys. The temperature coefficient of resistivity
changes its sign from negative to positive and further to negative as the
temperature decreases. | 2306.14836v1 |
2012-11-01 | Prediction of topological insulating behavior in Hg2CuTi-type Heusler compounds from first principles | The topological band structures of the X2YZ Heusler compounds with the
Hg2CuTi structure are investigated by using first-principles calculations
within density functional theory. Our results clearly show that a large number
of the Hg2CuTi type Heusler compounds naturally exhibit distinct band-inversion
feature, which is mainly controlled by the Y-Z zinc blende sublattice. Similar
to the half-Heusler family, the topological band order in Hg2CuTi type Heusler
compounds is sensitive to the variation of lattice constant, and most of them
possess a negative formation energy, which makes them more suitable in material
growth and could easily achieve the topological insulating behavior by alloying
or proper strain. | 1211.0190v1 |
2015-04-13 | Evidence for Localized Moment Picture in Mn-based Heusler Compounds | X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism
(XMCD) were used to probe the oxidation state and element specific magnetic
moments of Mn in Heusler compounds with different crystallographic structure.
The results were compared with theoretical calculations, and it was found that
in full Heusler alloys, Mn is metallic (oxidation state near 0) on both
sublattices. The magnetic moment is large and localized when octahedrally
coordinated by the main group element, consistent with previous theoretical
work, and reduced when the main group coordination is tetrahedral. By contrast,
in the half Heusler compounds the magnetic moment of the Mn atoms is large and
the oxidation state is +1 or +2. The magnetic and electronic properties of Mn
in full and half Heusler compounds are strongly dependent on the structure and
sublattice, a fact that can be exploited to design new materials. | 1504.03088v1 |
2002-03-07 | Resonant x-ray emission spectra of Mn based Heusler alloys | The Mn L_2,L_3 x-ray spectra of the Cu_2MnAl and Co_2MnZ (Z=Al,Ga, Sn, Sb)
Heusler alloys have been investigated by the Resonant X-ray Emission
Spectroscopy (RXES) using linearly polarized monochromatic synchrotron
radiation for. The interplay between the half-metallic character of the Mn 3d
electronic structure in connection with the local magnetic moment $\mu_{Mn}$
and Mn $2p\to 3d$ x-ray emission spectra is discussed. | 0203169v1 |
2004-06-24 | Electronic Structure and Magnetic Exchange Coupling in Ferromagnetic Full Heusler Alloys | Density-functional studies of the electronic structures and exchange
interaction parameters have been performed for a series of ferromagnetic full
Heusler alloys of general formula Co$_2$MnZ (Z = Ga, Si, Ge, Sn), Rh$_2$MnZ (Z
= Ge, Sn, Pb), Ni$_2$MnSn, Cu$_2$MnSn and Pd$_2$MnSn, and the connection
between the electronic spectra and the magnetic interactions have been studied.
Different mechanisms contributing to the exchange coupling are revealed. The
band dependence of the exchange parameters, their dependence on volume and
valence electron concentration have been thoroughly analyzed within the Green
function technique. | 0406588v1 |
2006-03-23 | Inverted spin polarization of Heusler alloys for new spintronic devices | A new magnetic logic overcomes the major limitations of field programmable
gate arrays while having a 50% smaller unit cell than conventional designs
utilizing magnetic tunnel junctions with one Heusler alloy electrode. These
show positive and negative TMR values at different bias voltages at room
temperature which generally adds an additional degree of freedom to all
spintronic devices. | 0603616v1 |
2006-09-20 | Towards a full Heusler alloy showing room temperature half-metallicity at the surface | In this article we investigate the surface spin polarization in a 100 nm
Co2Cr0.6Fe0.4Al (CCFA) film grown ex situ epitaxially on MgO(100) with a 10 nm
Fe buffer layer by means of spin resolved photoemission. We show that a careful
in situ preparation of the sample surface leads to values for the room
temperature spin polarization up to 45% at the Fermi level. To our knowledge,
this is the highest value measured so far at the surface region of a full
Heusler alloy at room temperature. | 0609504v1 |
2007-11-05 | Magnetization easy-axis in martensitic Heusler alloys estimated by strain measurements under magnetic-field | We study the temperature dependence of strain under constant magnetic-fields
in Ni-Mn based ferromagnetic Heusler alloys in the form Ni-Mn-$X$ ($X$: Ga, In,
Sn, Sb) which undergo a martensitic transformation. We discuss the influence of
the applied magnetic-field on the nucleation of ferromagnetic martensite and
extract information on the easy-axis of magnetization in the martensitic state. | 0711.0639v1 |
2018-03-30 | Boosting Material Modeling Using Game Tree Search | We demonstrate a heuristic optimization algorithm based on the game tree
search for multi-component materials design. The algorithm searches for the
largest spin polarization of seven-component Heusler alloys. The algorithm can
find the peaks quickly and is more robust against local optima than Bayesian
optimization approaches using the expected improvement or upper confidence
bound approaches. We also investigate Heusler alloys including anti-site
disorder and show that
[Fe$_{0.9}$Co$_{0.1}$]$_{2}$Cr$_{0.95}$Mn$_{0.05}$Si$_{0.3}$Ge$_{0.7}$ has the
potential to be a high spin polarized material with robustness against
anti-site disorder. | 1803.11338v2 |
2018-06-18 | Large anomalous Nernst effect across the magneto-structural transition in bulk Ni-Co-Mn-Sn full Heusler alloy | We report on the observation of temperature and field dependent anomalous
Nernst effect (ANE) in Ni-rich bulk Ni-Co-Mn-Sn full Heusler alloy. A large
change in the transverse Nernst coefficient (N) is obtained across the first
order magnetostructural transition from a tetragonal martensite to a cubic
austenite phase. The saturation of ANE and magnetic data appear to depend
largely on the magnetic anisotropy of the device. Such change in the Nernst
co-efficient may prove to be useful for switching applications controlled by
temperature and magnetic field changes. | 1806.06675v1 |
2018-11-09 | Anomaly in anomalous Nernst effect at low temperature for C1b-type NiMnSb half-Heusler alloy thin film | The anomaly in the anomalous Nernst effect (ANE) was observed for a C1b-type
NiMnSb half-Heusler alloy thin film deposited on a MgO (001) substrate. The
Nernst angle ({\theta}ANE) showed maximum peak with decreasing temperature and
reached 0.15 at 80 K, which is considered to be brought by the cross-over from
half-metal to normal ferromagnet in NiMnSb at low temperature. This anomaly was
also observed for the transport properties, that is, both the resistivity and
the anomalous Hall resistivity in the same temperature range. | 1811.03791v1 |
2020-01-17 | Fermi Level Controlled Ultrafast Demagnetization Mechanism in Half-Metallic Heusler Alloy | The electronic band structure-controlled ultrafast demagnetization mechanism
in Co2FexMn1-xSi Heusler alloy is underpinned by systematic variation of
composition. We find the spin-flip scattering rate controlled by spin density
of states at Fermi level is responsible for non-monotonic variation of
ultrafast demagnetization time ({\tau}M) with x with a maximum at x = 0.4.
Furthermore, Gilbert damping constant exhibits an inverse relationship with
{\tau}M due to the dominance of inter-band scattering mechanism. This
establishes a unified mechanism of ultrafast spin dynamics based on Fermi level
position. | 2001.06217v1 |
2006-06-05 | Properties of the quaternary half-metal-type Heusler alloy Co$_2$Mn$_{1-x}$Fe$_x$Si | This work reports on the bulk properties of the quaternary Heusler alloy
Co$_2$Mn$_{1-x}$Fe$_x$Si with the Fe concentration $x=$. All samples, which
were prepared by arc melting, exhibit $L2_1$ long range order over the complete
range of Fe concentration. Structural and magnetic properties of
Co$_2$Mn$_{1-x}$Fe$_x$Si Heusler alloys were investigated by means of X-ray
diffraction, high and low temperature magnetometry, M{\"o\ss}bauer
spectroscopy, and differential scanning calorimetry. The electronic structure
was explored by means of high energy photo emission spectroscopy at about 8 keV
photon energy. This ensures true bulk sensitivity of the measurements. The
magnetization of the Fe doped Heusler alloys is in agreement with the values of
the magnetic moments expected for a Slater-Pauling like behavior of
half-metallic ferromagnets. The experimental findings are discussed on the hand
of self-consistent calculations of the electronic and magnetic structure. To
achieve good agreement with experiment, the calculations indicate that on-site
electron-electron correlation must be taken into account, even at low Fe
concentration. The present investigation focuses on searching for the
quaternary compound where the half-metallic behavior is stable against outside
influences. Overall, the results suggest that the best candidate may be found
at an iron concentration of about 50%. | 0606108v2 |
2007-12-02 | Role of the conduction electrons in mediating exchange interactions in Heusler alloys | Because of large spatial separation of the Mn atoms in Heusler alloys the Mn
3d states belonging to different atoms do not overlap considerably. Therefore
an indirect exchange interaction between Mn atoms should play a crucial role in
the ferromagnetism of the systems. To study the nature of the ferromagnetism of
various Mn-based semi- and full-Heusler alloys we perform a systematic
first-principles calculation of the exchange interactions in these materials.
The calculation of the exchange parameters is based on the frozen-magnon
approach. The calculations show that the magnetism of the Mn-based Heusler
alloys depends strongly on the number of conduction electrons, their spin
polarization and the position of the unoccupied Mn 3d states with respect to
the Fermi level. Various magnetic phases are obtained depending on the
combination of these characteristics. The Anderson's s-d model is used to
perform a qualitative analysis of the obtained results. The conditions leading
to diverse magnetic behavior are identified. If the spin polarization of the
conduction electrons at the Fermi energy is large and the unoccupied Mn 3d
states lie well above the Fermi level, an RKKY-type ferromagnetic interaction
is dominating. On the other hand, the contribution of the antiferromagnetic
superexchange becomes important if unoccupied Mn 3d states lie close to the
Fermi energy. The resulting magnetic behavior depends on the competition of
these two exchange mechanisms. The calculational results are in good
correlation with the conclusions made on the basis of the Anderson s-d model
which provides useful framework for the analysis of the results of
first-principles calculations and helps to formulate the conditions for high
Curie temperature. | 0712.0158v1 |
2012-12-01 | On the half-metallicity of Co2FeSi Heusler alloy: an experimental and ab initio study | Co2FeSi, a Heusler alloy with the highest magnetic moment per unit cell and
the highest Curie temperature, has largely been described theoretically as a
half-metal. This conclusion, however, disagrees with Point Contact Andreev
Reflection (PCAR) spectroscopy measurements, which give much lower values of
spin polarization, P. Here, we present the spin polarization measurements of
Co2FeSi by the PCAR technique, along with a thorough computational exploration,
within the DFT and a GGA+U approach, of the Coulomb exchange U-parameters for
Co and Fe atoms, taking into account spin-orbit coupling. We find that the
orbital contribution (mo) to the total magnetic moment (mT) is significant,
since it is at least 3 times greater than the experimental uncertainty of mT.
Account of mo radically affects the acceptable values of U. Specifically, we
find no values of U that would simultaneously satisfy the experimental values
of the magnetic moment and result in the half-metallicity of Co2FeSi. On the
other hand, the ranges of U that we report as acceptable are compatible with
spin polarization measurements (ours and the ones found in the literature),
which all are within approximately 40-60% range. Thus, based on reconciling
experimental and computational results, we conclude that: a) spin-orbit
coupling cannot be neglected in calculating Co2FeSi magnetic properties, and b)
Co2FeSi Heusler alloy is not half-metallic. We believe that our approach can be
applied to other Heusler alloys such as Co2FeAl. | 1212.0168v1 |
2019-02-20 | Spin-charge conversion in NiMnSb Heusler alloy films | Half-metallic Heusler alloys are attracting considerable attention because of
their unique half-metallic band structures which exhibit high spin polarization
and yield huge magnetoresistance ratios. Besides serving as ferromagnetic
electrodes, Heusler alloys also have the potential to host spin-charge
conversion which has been recently demonstrated in other ferromagnetic metals.
Here, we report on the spin-charge conversion effect in the prototypical
Heusler alloy NiMnSb. Spin currents were injected from Y3Fe5O12 into NiMnSb
films by spin pumping, and then the spin currents were converted to charge
currents via spin-orbit interactions. Interestingly, an unusual charge signal
was observed with a sign change at low temperature, which can be manipulated by
film thickness and ordering structure. It is found that the spin-charge
conversion has two contributions. First, the interfacial contribution causes a
negative voltage signal, which is almost constant versus temperature. The
second contribution is temperature dependent because it is dominated by
minority states due to thermally excited magnons in the bulk part of the film.
This work provides a pathway for the manipulation of spin-charge conversion in
ferromagnetic metals by interface-bulk engineering for spintronic devices. | 1902.07406v1 |
2022-12-02 | FeRhCrSi: A new spin semimetal with room temperature spin-valve behavior | Spin semimetals are a recently discovered new class of spintronic materials,
which exhibit a band gap in one spin channel while a semimetallic feature in
the other and thus allows for tunable spin transport. Here, we present
experimental verification of spin semimetallic behavior in FeRhCrSi, a
quaternary Heusler alloy with saturation moment 2 $\mu_B$ and Curie temperature
$>$ 400 K. It crystallises in the L2$_1$ structure with 50$\%$ antisite
disorder between Fe and Rh. Below 300 K, it shows a weakly temperature
dependent electrical resistivity with negative temperature coefficient,
indicating the normal semimetal or spin semimetal behavior. Anomalous
magnetoresistance data reveals dominant contribution from asymmetric part, a
clear signature of spin-valve nature, which is retained even at room
temperature. \textcolor{black}{The asymmetric part of magneto-resistance shows
an unusual increase with increasing temperature.} Hall measurements confirm the
anomalous nature of conductivity originating from the intrinsic Berry
curvature, with holes being the majority carriers. Ab-initio simulation
confirms a unique long-range ferrimagnetic ordering to be the ground state,
explaining the origin behind the unexpected low saturation moment. The
ferrimagnetic disordered structure confirms the spin semimetallic feature of
FeRhCrSi, as observed experimentally. | 2212.00924v2 |
2022-12-15 | Exploration of all-3d Heusler alloys for permanent magnets: an ab initio based high-throughput study | Heusler alloys have attracted interest in various fields of functional
materials since their properties can quite easily be tuned by composition.
Here, we have investigated the relatively new class of all-3d Heusler alloys in
view of its potential as permanent magnets. To identify suitable candidates, we
performed a high-throughput study using an electronic structure database to
search for X$_2$YZ-type Heusler systems with tetragonal symmetry and high
magnetization. For the alloys which passed our selection filters, we have used
a combination of density functional theory calculations and spin dynamics
modelling to investigate their magnetic properties including the
magnetocrystalline anisotropy energy and exchange interactions. The candidates
which fulfilled all the search criteria served as input for the investigation
of the temperature dependence of the magnetization and determination of Curie
temperature. Based on our results, we suggest that Fe$_2$NiZn, Fe$_2$NiTi and
Ni$_2$CoFe are potential candidates for permanent magnets with large
out-of-plane magnetic anisotropy (1.23, 0.97 and 0.82 MJ/m$^3$ respectively)
and high Curie temperatures lying more than 200 K above the room temperature.
We further show that the magnitude and direction of anisotropy is very
sensitive to the strain by calculating the values of anisotropy energy for
several tetragonal phases. Thus, application of strain can be used to tune the
anisotropy in these compounds. | 2212.07845v3 |
2017-01-28 | Half-metallicity versus Symmetry in Pt, Ni and Co-based Half Heusler Alloys: A First-principles Calculation | Using first principles calculations based on density functional theory, we
study the geometric, electronic, and magnetic properties of Pt, Ni and Co-based
half Heusler alloys, namely, Pt$BC$, Ni$BC$ and Co$BC$ ($B$ = Cr, Mn and Fe;
$C$ = Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te). We calculate the
formation energy of these alloys in various crystal symmetries, which include,
the (face-centered) cubic $C1_{b}$ ($F\bar{4}$3m), orthorhombic ($Pnma$), as
well as hexagonal ($P\bar{6}2m$ and $P6_{3}/mmc$) structures. It has been
observed that out of all the 108 structures, studied here, energetically stable
cubic structure is observed for only 18 materials. These alloys are primarily
having either a $C$ atom or an $A$ atom with a high atomic number. We also
observe that along with the alloys with $C$ atoms from group IIIA, IVA and VA
-- alloys with $C$ atoms from group VIA are also found to be, by and large,
energetically stable. To examine the relative stabilities of different
symmetries in order to search for the respective lowest energy state for each
of the above-mentioned systems, as well as to find whether a material in the
ground state is half-metallic or not, we analyze the formation energy, and the
electronic density of states, in detail. Based on these analyses, the
possibility of existence of any {\it one-to-one relationship} between the {\it
cubic symmetry} and the {\it half-metallicity} in these half Heusler alloys is
probed. Subsequently, we predict about the existence of a few new {\it
non-cubic} half Heusler alloys with substantially low density of states at one
of the spin channels and reasonably {\it high spin polarization at the Fermi
level} | 1701.08282v2 |
2014-06-03 | Phase coexistence and interrupted 1st order transition in magnetic shape memory alloys | Current theoretical studies on structural and magnetic properties of
functional Ni-Mn-Z (Z = Ga, In, Sn) Heusler alloys address the origin of the
structural transition from the austenite to martensite, and also address the
dominant contribution to the latent heat associated with this
magneto-structural transition. This should help understand the origin of
kinetic arrest of 1st order magnetic transitions. | 1406.0627v1 |
2003-05-07 | Appearance of Half-Metallicity in the Quaternary Heusler Alloys | I report systematic first-principle calculations of the quaternary Heusler
alloys like Co$_2$[Cr$_{1-x}$Mn$_x$]Al, Co$_2$Mn[Al$_{1-x}$Sn$_x$] and
[Fe$_{1-x}$Co$_x$]$_2$MnAl. I show that when the two limiting cases (x=0 or 1)
correspond to a half-metallic compound, so do the intermediate cases. Moreover
the total spin moment $M_t$ in $\mu_B$ scales linearly with the total number of
valence electrons $Z_t$ (and thus with the concentration $x$) following the
relation $M_t=Z_t-24$, independently of the origin of the extra valence
electrons, confirming the Slater-Pauling behavior of the normal Heusler alloys.
Finally I discuss in all cases the trends in the atomic projected DOSs and in
the atomic spin moments. | 0305134v1 |
2009-02-09 | Analysis of L21-ordering in full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing | The authors developed a new analysis approach for evaluation of atomic
ordering in full-Heusler alloys, which is extension of the commonly used
Webster model. Our model can give accurate physical formalism for the degree of
atomic ordering in the L21 structure, including correction with respect to the
fully disordered A2 structure, i.e., the model can directly evaluate the degree
of L21-ordering under a lower ordering structure than the complete B2-ordering
structure. The proposed model was applied to full-Heusler Co2FeSi alloy thin
films formed by rapid thermal annealing. The film formed at TA = 800 C showed a
relatively high degree of L21-ordering of 83 % under a high degree of
B2-ordering of 97 %. | 0902.1438v1 |
2009-06-16 | Epitaxial Growth of a Full-Heusler Alloy Co$_{2}$FeSi on Silicon by Low-Temperature Molecular Beam Epitaxy | For electrical spin injection and detection of spin-polarized electrons in
silicon, we explore highly epitaxial growth of ferromagnetic full-Heusler-alloy
Co2FeSi thin films on silicon substrates using low-temperature molecular beam
epitaxy (LTMBE). Although in-situ reflection high energy electron diffraction
images clearly show two-dimensional epitaxial growth for growth temperatures
T_G of 60, 130, and 200 C, cross-sectional transmission electron microscopy
experiments reveal that there are single-crystal phases other than Heusler
alloys near the interface between Co_2FeSi and Si for T_G = 130 and 200 C. On
the other hand, almost perfect heterointerfaces are achieved for T_G = 60 C.
These results and magnetic measurements indicate that highly epitaxial growth
of Co_2FeSi thin films on Si is demonstrated only for T_G = 60 C. | 0906.2840v1 |
2010-06-01 | Observation of enhanced exchange bias behavior in NiCoMnSb Heusler alloys | We report the observation of large exchange bias in Ni50-xCoxMn38Sb12 Heusler
alloys with x=0, 2, 3, 4, 5, which is attributed to the coexistence of
ferromagnetic and antiferromagnetic phases in the martensitic phase. The phase
coexistence is possibly due to the supercooling of the high temperature
ferromagnetic phase and the predominant antiferromagnetic component in the
martensitic phase. The presence of exchange bias is well supported by the
observation of training effect. The exchange bias field increases with Co
concentration. The maximum value of 480 Oe at T=3K is observed in x=5 after
field cooling in 50 kOe, which is almost double the highest value reported so
far in any Heusler alloy system. Increase in the antiferromagnetic coupling
after Co substitution is found to be responsible for the increase in the
exchange bias. | 1006.0071v1 |
2012-06-11 | Crossover from antiferromagnetic to ferromagnetic ordering in semi-Heusler alloys Cu1-xNixMnSb with increasing Ni concentration | The magnetic properties and transition from an antiferromagnetic (AFM) to a
ferromagnetic (FM) state in semi Heusler alloys Cu1-xNixMnSb, with x < 0.3 have
been investigated in details by dc magnetization, neutron diffraction, and
neutron depolarization. We observe that for x < 0.05, the system Cu1-xNixMnSb
is mainly in the AFM state. In the region 0.05 \leq x \leq 0.2, with decrease
in temperature, there is a transition from a paramagnetic to a FM state and
below ~50 K both AFM and FM phases coexist. With an increase in Ni
substitution, the FM phase grows at the expense of the AFM phase and for x >
0.2, the system fully transforms to the FM phase. Based on the results
obtained, we have performed a quantitative analysis of both magnetic phases and
propose a magnetic phase diagram for the Cu1-xNixMnSb series in the region x <
0.3. Our study gives a microscopic understanding of the observed crossover from
the AFM to FM ordering in the studied semi Heusler alloys Cu1-xNixMnSb. | 1206.2243v1 |
2013-11-22 | Full-Heusler Co2FeSi alloy thin films with perpendicular magnetic anisotropy induced by MgO-interface | The authors demonstrated that L21-ordered full-Heusler Co2FeSi (CFS) alloy
film with thickness of 100 nm were formed by facing targets sputtering (FTS)
method at a substrate temperature TS = 300 deg C. Degrees of L21- and B2- order
for the film were 0.37, and 0.96, respectively. Furthermore, full-Heusler CFS
alloy thin films with perpendicular magnetic anisotropy (PMA) induced by
MgO-interface magnetic anisotropy were successfully formed by the FTS method.
The CFS/MgO stacking layers showed PMA when dCFS was 0.6 nm <= dCFS <= 1.0 nm.
The PMA in these structures resulted from the CFS/MgO interfacial perpendicular
magnetic anisotropy. | 1311.5670v2 |
2015-08-07 | Multifunctional Heusler alloy: experimental evidences of enhanced magnetocaloric properties at room temperature and half-metallicity | Heusler alloys are widely studied due to their interesting structural and
magnetic properties, like magnetic memory shape ability, coupled
magneto-structural phase transitions and half-metallicity; ruled, for many
cases, by the valence electrons number ($N_v$). The present work focuses on the
magnetocaloric potentials of half-metals, exploring the effect of $N_v$ on the
magnetic entropy change, preserving half-metallicity. The test bench is the
Si-rich side of the half-metallic series Fe$_2$MnSi$_{1-x}$Ga$_x$. From the
obtained experimental results it was possible to obtain $|\Delta
S|_{max}=\Delta H^{0.8}(\alpha+\beta N_v)$, i.e., the maximum magnetic entropy
change depends in a linear fashion on $N_v$, weighted by a power law on the
magnetic field change $\Delta H$ ($\alpha$ and $\beta$ are constants
experimentally determined). In addition, it was also possible to predict a new
multifunctional Heusler alloy, with enhanced magnetocaloric effect, Curie
temperature close to 300 K and half-metallicity. | 1508.01828v1 |
2018-09-19 | Quaternary Heusler Alloy: An Ideal Platform to Realize Triple Point Fermion | The existence of three fold rotational, mirror and time reversal symmetries
often give rise to the triply degenerate nodal point (TP) in the band structure
of a material. Based on point group symmetry analysis and first principle
electronic structure, we predict, in this article, a series of quaternary
Heusler alloys host an ideal platform for the occurrence of TP. We simulated,
the projection of these TPs onto the (111) and (100) surfaces lead to form
topological Fermi arcs, which may further be detected by scanning tunneling
spectroscopy and angle resolved photoemission spectroscopy. These Fermi arcs
arise due to the symmetry protected band degeneracies, which are robust and can
not be avoided due to the non-trivial band topology. Interestingly the TPs, in
these class of Heusler alloys are far away from the $\Gamma$ point along C$_3$
axes, which allow to overcome the experimental difficulties over previously
studied hexagonal and HgTe-type compounds. | 1809.07026v1 |
2019-11-22 | Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers | We report a comparative analysis and theoretical description of the
superconducting properties of two spin-valve-valve structures containing the
Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$ as one of two ferromagnetic (F1 or
F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb
layer. In our experiments we used the Heusler alloy layer in two roles: as a
weak ferromagnet on the place of the F2 layer and as a half-metal on the place
of the F1 layer. In the first case, we obtained a large ordinary
superconducting spin-valve effect $\Delta T_c$ assisted by the triplet
superconducting spin-valve effect $\Delta T_c^{trip}$. In the second case, we
observed a giant magnitude of $\Delta T_c^{trip}$ reaching 0.5 K. An underlying
theory based on the solution of the Usadel equations using Kupriyanov-Lukichev
boundary conditions with arbitrary material parameters for all layers and
arbitrary boundary parameters for all interfaces is presented in Appendix. We
find a good agreement between our experimental data and theoretical results. | 1911.09984v1 |
2017-04-03 | Half-Heusler alloy LiBaBi: A new topological semimetal with five-fold band degeneracy | Based on first-principles study, we report the finding of a new topological
semimetal LiBaBi in half-Heusler phase. The remarkable feature of this
nonmagnetic, inversion-symmetry-breaking material is that it consists of only
simple $s$- and $p$-block elements. Interestingly, the material is ordinary
insulator in the absence of spin-orbit coupling (SOC) and becomes nodal-surface
topological semimetal showing drumhead states when SOC is included. This is in
stark contrast to other nodal-line and nodal-surface semimetals, where the
extended nodal structure is destroyed once SOC is included. Importantly, the
linear band crossings host three-, four-, five- and six-fold degeneracies near
the Fermi level, making this compound very attractive for the study of
`unconventional' fermions. The band crossing points form a three-dimensional
nodal structure around the zone center at the Fermi level. We identify the
surface states responsible for the appearance of the drumhead states. The alloy
also shows a phase transition from topological semimetal to a trivial insulator
on application of pressure. In addition to revealing an intriguing effect of
SOC on the nodal structure, our findings introduce a new half-Heusler alloy in
the family of topological semimetals, thus creating more avenues for
experimental exploration. | 1704.00697v1 |
2018-07-17 | Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy | In this work, we report the structural, magnetic and electrical and thermal
transport properties of the Heusler-type alloy Ru2NbAl. From the detailed
analysis of magnetization data, we infer the presence of superparamagnetically
interacting clusters with a Pauli paramagnetic background, while short-range
ferromagnetic interaction is developed among the clusters below 5 K. The
presence of this ferromagnetic interaction is confirmed through heat capacity
measurements. The relatively small value of electronic contribution to specific
heat, gamma (~2.7 mJ/mol-K2), as well as the linear nature of temperature
dependence of Seebeck coefficient indicate a semi-metallic ground state with a
pseudo-gap that is also supported by our electronic structure calculations. The
activated nature of resistivity is reflected in the observed negative
temperature coefficient and has its origin in the charge carrier localization
due to antisite defects, inferred from magnetic measurements as well as
structural analysis. Although the absolute value of thermoelectric figure of
merit is rather low (ZT = 5.2*10-3) in Ru2NbAl, it is the largest among all the
reported non-doped full Heusler alloys. | 1807.06608v1 |
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