Full Heusler Research Articles

CALCULATIONS OF STRUCTURAL, ELASTIC AND MAGNETIC PROPERTIES OF THE NOVEL FULL HEUSLER ALLOYS RU2XY (X = NB, MN) AND (Y = TE, SB)

CALCULATIONS OF STRUCTURAL, ELASTIC AND MAGNETIC PROPERTIES OF THE NOVEL FULL HEUSLER ALLOYS RU2XY (X = NB, MN) AND (Y = TE, SB)

A first-principles investigation of band inversion in topologically nontrivial Na2AgX (X= As, Sb and Bi) full Heusler compounds

Topological nontrivial nature are the latest phases to be discovered in condensed matter physics with insulating bulk band gaps and topologically protected metallic surface states; they are one of the current hot topics because of their unique properties and potential applications. In this paper, we have highlighted a first-principles study of the structural stability and electronic behavior of the Na2AgX (X = As, Sb and Bi) full Heusler compounds, using the Full-Potential Linear Muffin-Tin Orbital (FP-LMTO) method. We have originated that the Hg2CuTi structure is appropriate in all studied materials. The negative values of the calculated formation energies mean that these compounds are energetically stable. The band structure is studied for the two cases relating the existence and the absence of spin-orbital couplings, where all materials are shown to be topologically non-trivial compounds. Spin orbital couplings were noticed to have no significant effect on the electronic properties such as the topological order.

Prediction of structural, electronic and magnetic properties of full Heusler alloys Ir2YSi (Y = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) via first-principles calculation

First principles electronic structural calculation of full Heusler alloys Ir2YSi (Y= Sc to Ni) in the L21 (Cu2MnAl) and Xa (Hg2CuTi) structures have been studied using full-potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). From the total energy calculations, it has been observed that all these alloys are stable in L21 structure than Xa structure and also it is found that the alloys Ir2YSi (Y =V to Co) are ferromagnetically stable whereas the Ir2YSi (Y = Sc, Ti, and Ni) alloys are non-magnetic in their stable L21 structure. Spin polarized band structure calculations reveal that in Ir2YSi (Y= V to Co) alloys there is spin splitting of energy states around the Fermi level (EF) indicating ferromagnetism and moreover in Ir2YSi (Y = V, Cr, and Mn) alloys majority electrons have metallic behavior while minority electrons have semiconducting nature exhibiting half metallic ferromagnetic (HMF) nature. HMF property in Ir2YSi (Y = V, Cr, and Mn) alloys have been further confirmed from the integer total magnetic moment value of 3.0 µB, 4.0 µB and 5.0 µB per formula unit respectively. Spin polarization mainly arises from the interaction between 3d electrons of V/Cr/Mn atom and 5d electrons of Ir atom. This strong d-d hybridization between the transition atoms like Ir and V/Cr/Mn composing Heusler alloys is essential for the formation of gap at the EF between the valence and conduction bands. Our results show that Ir2YSi (Y= V, Cr, and Mn) will be suitable for ferromagnetic and spintronics applications.

Structural, electronic, elastic, magnetic and thermodynamic properties of Mn2LuZ (Z = B, Al, Ga and In) Heusler compounds: A first‐principle study

AbstractThe study of the structural, electronic, thermodynamic, elastic and magnetic properties of rare‐earth based full Heusler alloys Mn2LuZ (Z = B, Al, Ga and In) have been carried out using the full‐potential linearized muffin‐tin orbital method (FP‐LMTO), within density functional theory (DFT) and generalized gradient approximation (GGA) for exchange and correlation energy. The electronic properties studied have shown that the compounds have a metallic behavior. We have also computed the mechanical properties where we found these full Heusler compounds are mechanically stable. Using the quasi‐harmonic Debye model, the variations of bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy with pressures covering the 0–40 GPa interval and temperatures ranging from 0 to 1500 K were also investigated and in‐depth discussed. The calculated properties represent a solid prediction for the Mn2LuZ (Z = B, Al, Ga and In) Heusler compounds and are awaiting for experimental concretization.

Perpendicular magnetic anisotropy in half-metallic thin-film Co2CrAl

Magnetocrystalline anisotropy (MCA) is one of the key parameters investigated in spin-based electronics (spintronics), e.g. for memory applications. Here, we employ first-principles calculations to study MCA in thin film full Heusler alloy Co2CrAl. This material was studied in the past, and has been reported to exhibit half-metallic electronic structure in bulk geometry. In our recent work, we showed that it retains a 100% spin-polarization in thin-film geometry, at CrAl atomic surface termination. Here, we show that the same termination results in a perpendicular magnetic anisotropy, while Co surface termination not only destroys the half-metallicity, but also results in in-plane magnetization orientation. In addition, for films thicker than around 20 nm the contribution from magnetic shape anisotropy may become decisive, resulting in in-plane magnetization orientation. To the best of our knowledge, this is one of the first reports of half-metallic thin-film surfaces with perpendicular magnetic anisotropy. This result may be of interest for potential nano-device applications, and may stimulate a further experimental study of this and similar materials.

Structural, electronic, magnetic and thermoelectric properties of full heusler alloys Co2YZ (Z = S, Ge, Se): A first principles calculation

Density functional theory (DFT) calculations are used to find physical characteristics of full Heusler (FH) alloys Co2YZ (Z = S, Ge, Se). Full potential-linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof functional is used. The results of phase stability have confirmed the stability of ferromagnetic (FM) phase as compared to anti-ferromagnetic (AFM), and nonmagnetic (NM) phases. The electronic properties show the half metallic (HM) character of Co2YS and Co2YSe by GGA. The role of on-site coulomb interaction (U) is also checked by GGA + U approach for Co2YGe which confirmed its HM nature. The band diagrams are in accordance to density of states (DOS). The total magnetic moment (MM) of all alloys is 3μB which is in agreement to Slater-Pauling Rule (SPR). The spin polarization (SP) indicated that these materials are completely spin polarized. The Seebeck coefficient (S) and electrical conductivity (σ) are also calculated and discussed.

Insight view of Hf2CrZ (Z = B, Ga, In, Si, Ge, Sn) Heusler materials via DFT calculations: A study on structural, electronic and magnetic properties

In this study, we have investigated the structural, electronic and magnetic properties of the Hf2CrZ (Z = B, Ga, In, Si, Ge, Sn) full Heusler materials in both Hg2CuTi and Cu2MnAl type structures. These investigations have been carried out by employing the first-principles computational method - full potential (FP) linearized (L) augmented plane-wave plus local orbital (APW + lo) - framed within the density functional theory (DFT) and embodied in WIEN2k computational package. Our investigations show that Cu2MnAl-type structures of the Hf2CrZ (Z = B,Ga,In, Si, Ge, Sn) Heusler compounds are more stable than that of Hg2CuTi-type ones. Our investigations also show that Hg2CuTi type structure of Hf2CrZ Heusler alloys demonstrates half-metallic character for Hf2CrZ (Z = B,Ga, In) with a value of the magnetic moment 1 μB and lower value for Hf2CrZ (Z = Si, Ge, Sn).

Itinerant Magnetism in Ga‐Rich Ga–Mn–Co Full Heusler Alloys

Herein, the crystal structure and the magnetic and electrical properties of two Ga‐rich, off‐stoichiometric full Heusler‐type compositions with x = 0.2 and 0.4 are investigated. The innate site disorder in the crystal structure is clarified using synchrotron X‐ray diffraction (XRD) studies. Experimentally, it is observed that a control over magnetic ordering temperature () and total magnetic moment can be achieved by tuning the Ga:Mn ratio in these alloys. Consequently, for the resultant compositions, and , Curie temperatures of ≈245 and 375 K, respectively, are obtained, lying in the vicinity of room temperature. The atomic site disorder arising out of excess Mn leads to spin fluctuation scattering which is reflected in its magnetic and transport properties. Temperature dependence of resistivity shows an anomalous occurrence of a maximum, reflecting a change in the sign of the temperature coefficient of resistance, coincidental with . Furthermore, the magnetization results indicate an itinerant‐type ferromagnetic ground state over the Heisenberg‐type state, with the moment values as low as 0.35 .

Probing the martensite transition and thermoelectric properties of Co x TaZ (Z = Si, Ge, Sn and x = 1, 2): a study based on density functional theory

In this work, using density functional theory based electronic structure calculations, we carry out a comparative study of geometric, mechanical, electronic, magnetic, and thermoelectric properties of Co x TaZ alloys, where Z = Si, Ge and Sn and x = 1 and 2. In the present study, a systematic approach has been taken to perform calculations to probe the possibility of existence of a tetragonal (martensite) phase in these alloys and also to perform a comparative study of various physical properties of the six systems, mentioned above, in the cubic and possible tetragonal phases. From our calculations, a tetragonal phase has been found to be stable up to about 400 K in case of Co2TaSi and Co2TaGe alloys, and up to about 115 K for Co2TaSn, indicating the presence of room temperature cubic phase in the latter alloy unlike the former two. Further, the results based on the energetics and electronic structure have been found to corroborate well with the elastic properties. All the above-mentioned full Heusler alloys (FHAs) show magnetic behavior with metallicity in both the phases. However, their half Heusler counterparts exhibit non-magnetic semi-conducting behavior in the cubic phase. We calculate and compare the thermoelectric properties, in detail, of all the materials in the cubic and possible tetragonal phases. In the cubic phase, the half Heusler alloys exhibit improved thermoelectric properties compared to the respective FHAs. Furthermore, it is observed that the FHAs exhibit higher (by about an order of magnitude) values of Seebeck coefficients in their cubic phases, compared to those in the tetragonal phases (which are of the order of only a few micro-volts/Kelvin). The observed behaviors of the transport properties of the probed materials have been analyzed using the topology of the Fermi surface.

Spin gapless semiconducting behavior in inverse Heusler Mn2-xCo1+xAl (0[formula omitted]x[formula omitted]1.75) thin films

We correlate the structural, electrical, and magnetotransport properties of co-sputtered Mn2-xCo1+xAl full Heusler alloy thin films (0≤x≤1.75) in terms of Co/Mn concentration variation concerning the spin gapless semiconducting (SGS) behavior. The alloy thin films are found to stabilize in B2 order for near stoichiometric films, i.e. (x = 0 and x = 1), with the gradual change in the ordering and lattice parameter through Mn concentration variation. Magnetization measurements in Mn2-xCo1+xAl thin films reveal the ferromagnetic and ferrimagnetic character for x = 1.75, 1.5, 1.25 & 1, and x = 0, 0.5 & 0.75, respectively. The longitudinal resistivity measurement revealed that the films exhibit semiconducting behavior with a change in sign of the temperature coefficient of resistance with temperature. The anomalous Hall conductivity values for the Mn2-xCo1+xAl thin films are extracted from the Anomalous Hall effect (AHE) measurements. The non-saturating positive MR (linear in H) is being reported for the first time in the Mn2CoAl thin films. The value of the AHE coefficient and positive MR together serve as a piece of experimental evidence for the SGS character in the thin film. The SGS behavior becomes predominant at higher Mn concentration. Highly resistive thin films with ferromagnetic (ferrimagnetic) character in Co2MnAl (Mn2CoAl) could be beneficial for semiconductor spintronics, where we need a good resistive element to match up with Silicon base substrate.

Site preference, magnetic and electronic properties of half-metallic Vanadium-based full Heusler alloys

We reported a new atomic site preference occupation for Vanadium-based X2YZ full Heusler alloys. It tells that alloys with less 24 valence electrons form the L21-type structure, while others form the XA-type structure. This fact means that the site preference rule for Vanadium-based alloys depends on the total number of valence electrons instead of the electropositive between X and Y atoms. The magnetic moments of XA-type alloys follow the Slater-Pauling rules, with two forms, Mt = NV − 24 and Mt = NV − 18 corresponding to different origins of the band gaps. XA-type V2YZ (Y = Cr, Mn and Fe; Z = Ga, Ge and As), V2CoGa, V2CoGe and V2NiGa alloys have half-metallic property. L21-type V2YZ (Y = Cu and Zn; Z = Ga, Ge and As) alloys show no spin polarization and no half-metallic property due to the structural symmetry and the non-magnetism of copper and zinc atoms.

Effect on Annealing Temperature (Ta) of Ternary Full Fe2CrSi Heusler Alloy Nanoparticles for Spin-Based Device Applications

The ternary full Heusler alloy nanoparticle Fe2CrSi compounds were prepared using mechanical alloy method. The as-prepared sample was observed as amorphous in nature and further increases various annealing temperatures (like 700 °C, 800 °C, and 900 °C) in nitrogen gas atmosphere for 5 h in tubular furnace using quartz tube. X-ray diffraction studies confirmed the formation of Cu2MnAl-type structure instead of the conventional L21 disordered structure. Through Williamson-Hall plot (W-H plot), the uniform deformation model was used to get low value of strain-induced broadening. In the surface morphology, elemental composition studies were carried out using HRSEM with EDS. The presence of Fe2CrSi with the particle size of ~ 35 nm was also observed using high-resolution scanning electron microscopy and elemental composition study confirmed the presence of Fe, Cr, and Si. The magnetic properties of Fe2CrSi nanoparticles were studied showing ferromagnetic behavior and low coercivity of soft magnetic nature. Half metallic ferromagnetic Heusler alloys have wide range of applications from medical imaging to data storage for its profound performance on spintronics.

Interfacial structure and strain accommodation in two-phase NbCo1.2Sn Heusler intermetallics

Multiphasic intermetallic materials are attractive candidates for functional applications where the constituent phases alone do not meet the property targets. The interfaces at the phase boundaries in these materials are central to their macroscopic properties, with the details of the atomic structure underpinning local thermomechanical, electrical, and magnetic behavior. Here, we characterize the interface structure in the biphasic Nb-Co-Sn system consisting of $\mathrm{Nb}{\mathrm{Co}}_{2}\mathrm{Sn}$ full Heusler (FH) precipitates embedded in a NbCoSn half Heusler (HH) matrix, which exhibits semicoherent interfaces with a 3.3% lattice parameter misfit that determines the constitution of the FH/HH interface on the nanometer scale. We use detailed transmission electron microscopy (TEM) and atomistic calculations to precisely determine the dislocation content and structure at the semicoherent interface, which modulates the strain fields in a semiregular pattern. We find that the interface forms regularly spaced paired partial dislocations with a joint Burgers vector of $a/2\ensuremath{\langle}110\ensuremath{\rangle}$, favored by misfit energy relief and chemical ordering. The interface exhibits numerous interface steps (disconnections) which in turn determine the precipitate morphology. Overall, the two phases show full coherency except in the vicinity of the misfit dislocation cores located every 11 nm, leading to a modulated strain field parallel to the interface, and no long-range strain fields as the interfacial dislocations accommodate the misfit strain.

DFT – GGA calculations of magnetic, elastic, thermodynamic and thermoelectric properties of Co2YAl (Y = Ti, V) full Heusler alloy systems

This study used first principles calculations within the generalized gradient approximation (GGA) via plane-wave pseudo-potential scheme to extensively study the magnetic, elastic, thermodynamic and thermoelectric properties of Co2TiAl and Co2VAl Heusler alloys at their optimized lattice constants. Elastic properties of these alloys were obtained by fitting the stress-strain relationship via Voigt-Reuss-Hill approximations. Thermodynamic properties were calculated within the implementation of thermo_pw package while thermoelectric properties were estimated from non-spin polarized calculations via Boltzmann semi-classical approach as implemented in BoltzTraP package at temperatures between 0 and 800 K. Calculations reaffirmed the stable electronic and magnetic states for the alloys to be half metallic ferromagnetic (HMF) states. The estimated band gaps in the minority spin channels were 0.431 and 0.399 eV for Co2TiAl and Co2VAl alloys respectively and these were justified by their corresponding density of states. The calculated magnetic moments for the respective alloys were 0.949 and 1.978 μB, which agreed with experimental, theoretical and Slater-Pauling rule determined magnetic moments. Elastic constants (C11, C12 and C44) obtained for these cubic systems fulfilled Born-Huang mechanical stability criteria, from which other elastic related quantities were extracted. Elastic calculations predicted these alloys to be metallic bonded, extremely incompressible, highly anisotropic and ductile in nature. Thermodynamic calculations showed that Debye vibrational energies, entropies and constant volume heat capacities of the alloys increased remarkably as temperature increased whereas vibrational free energies decreased steadily with increased temperature. Thermoelectric properties were also determined, with Co2TiAl having higher figure of merit (ZT) than Co2VAl within the investigative temperatures.

Magnetoelastic and magnetostrictive properties of Co2XAl Heusler compounds

We present a comprehensive first principles electronic structure study of the magnetoelastic and magnetostrictive properties in the Co-based Co$_2$XAl (X = V, Ti, Cr, Mn, Fe) full Heusler compounds. In addition to the commonly used total energy approach, we employ torque method to calculate the magnetoelastic tensor elements. We show that the torque based methods are in general computationally more efficient, and allow to unveil the atomic- and orbital-contributions to the magnetoelastic constants in an exact manner, as opposed to the conventional approaches based on second order perturbation with respect to the spin-orbit coupling. The magnetostriction constants are in good agreement with available experimental data. The results reveal that the main contribution to the magnetostriction constants, $\lambda_{100}$ and $\lambda_{111}$, arises primarily from the strained-induced modulation of the $\langle d_{x^2-y^2}|\hat{L}_z|d_{xy}\rangle$ and $\langle d_{z^2}|\hat{L}_x|d_{yz}\rangle$ spin orbit coupling matrix elements, respectively, of the Co atoms.

Electronic Structure and Magnetic Properties of the (111), (110), and (001) Surfaces for the Full Heusler Alloy Zr2VGa

In this paper, the electronic and magnetic properties of the Zr2VGa (111), (001), and (110) surfaces are studied via the first-principles calculations based on the density-functional theory. The calculations indicate that the Zr2VGa full-Heusler alloy displays a half-metallic behavior with a total spin magnetic moment of 2.0 μB at the equilibrium lattice parameter of 6.657 Ǻ. The energy band gap is 0.28 eV at the Fermi level. A weak relaxation at the terminations of the (111) and (001) surfaces is obtained which means that the terminations are stable. However, the stronger relaxations are at the terminations of the (110) surface. Examining the electronic density of states, the half-metallicity is determined which was verified in the bulk Zr2VGa and it is destroyed at the (111), (001) and (110) surfaces. Furthermore, it is discovered that the atomic spin magnetic moments at the Zr(2)Ga-terminated (001) surface is decreased compared to those of the bulk Zr2VGa, while the magnetic moments are increased at the Zr(1) and V-terminated (111) surfaces.

High Thermoelectric Performance and Defect Energetics of Multipocketed Full Heusler Compounds

Thermoelectrics are used in energy harvesting technology for power generation and refrigeration, but await breakthroughs with a higher figure of merit ($Z\phantom{\rule{0}{0ex}}T$), especially at room-to-cryogenic temperatures. Via explicit treatment of electron-phonon scattering, this study shows that multipocketed full Heusler compounds Sr${}_{2}$BiAu and Sr${}_{2}$SbAu could feature notably high theoretical $Z\phantom{\rule{0}{0ex}}T$. Stability and defects analysis also suggest that these compounds may be synthesizable and favorably $n$-type. A successful experimental realization of these compounds could pave new grounds in bulk thermoelectrics.

Effect of Ga substitution on structural and magnetic properties of Fe50Mn25Al25-xGax Heusler alloys

Heusler alloys possess the structure of an ordered compound while displaying most of the properties of metals. Numerous magnetic exchange mechanisms may be operating in such systems. Herein, the structural and magnetic properties have been investigated as a function of substitution of Al by Ga in Fe50Mn25Al25-xGax (x = 0, 2.5, 5.0, 7.5, 10.0) quaternary (pseudo-ternary) full Heusler alloys. The X-ray diffraction analysis confirms the substituting Al by Ga promoting the lattice expansion in the L21 crystal structure in Fe50Mn25Al25-xGax alloys. The line scanning profile using energy dispersive X-ray analysis investigations suggests the homogeneous distribution of Ga in all the alloys. The magnetic moment of the alloys was found to be affected by Ga as the antiferromagnetic counterpart increases with Ga. A decrease in the value of Curie temperature was found with increase in Ga content. A second-order magnetic phase transition was observed in the alloys above room temperature.

Noncollinear antiferromagnetic states in Ru-based Heusler compounds induced by biquadratic coupling

We investigate the magnetic properties of Ru$_{2}$Mn$Z$ ($Z$ = Sn, Sb, Ge, Si) chemically ordered full Heusler compounds for zero as well as finite temperatures. Based on first principles calculations we derive the interatomic isotropic bilinear and biquadratic couplings between Mn atoms from the paramagnetic state. We find frustrated isotropic couplings for all compounds and in case of $Z$ = Si and Sb a nearest-neighbor biquadratic coupling that favors perpendicular alignment between the Mn spins. By using an extended classical Heisenberg model in combination with spin dynamics simulations we obtain the magnetic equilibrium states. From these simulations we conclude that the biquadratic coupling, in combination with the frustrated isotropic interactions, leads to non-collinear magnetic ground states in the Ru$_{2}$MnSi and Ru$_{2}$MnSb compounds. In particular, for these alloys we find two distinct, non-collinear ground states which are energetically equivalent and can be identified as $3-q$ and $4-q$ states on a frustrated fcc lattice. Investigating the thermal stability of the non-collinear phase we find that in case of Ru$_{2}$MnSi the multiple$-q$ phase undergoes a transition to the single$-q$ phase, while in case of Ru$_{2}$MnSb the corresponding transition is not obtained due to the larger magnitude of the nearest-neighbor biquadratic coupling.

Stoichiometric and off-stoichiometric full Heusler Fe2V1−xWxAl thermoelectric systems

A series of full Heusler alloys, ${\text{Fe}}_{2}{\text{V}}_{1\ensuremath{-}x}{\text{W}}_{x}\text{Al}$, $0\ensuremath{\le}x\ensuremath{\le}0.2$, was prepared and characterized, and relevant physical properties to account for the thermoelectric performance were studied in a wide temperature range. Additionally, off-stoichiometric samples with similar compositions have been included, and a 10% improvement of the thermoelectric figure of merit was obtained. The V/W substitution causes (i) a change of the main carrier type, from holes to electrons as evidenced from Seebeck and Hall measurements, and (ii) a substantial reduction of the lattice thermal conductivity due to a creation of lattice disorder by means of a distinct different mass and metallic radius upon the V/W substitution. Moreover $ZT$ values above 0.2 have been obtained. A microscopic understanding of the experimental data observed is revealed from ab initio calculations of the electronic and phononic structure. This series of alloys constitutes the basis for thin film systems, which have recently been found to exhibit $ZT$ values beyond those reported so far in the literature.