Equiatomic Compounds Research Articles

New equiatomic quaternary Heusler compounds without transition metals KCaBX (X = S and Se): Robust half-metallicity and optical properties

It is known that high spin-polarization and magnetism can be found even in materials with neither transition metals nor rare earths. In this paper, we report results of the structural design, electronic structure, magnetic and optical properties of new equiatomic quaternary Heusler (EQH) KCaBX (X = S and Se) compounds. Electron exchangecorrelation interactions are described by the Wu-Cohen (WC) functional and Tran-Blaha modified Becke-Johnson exchange (mBJ) potential. Ferromagnetic ordering is stable for the cubic structure of space group F43 m in which the K, Ca, B and X atoms are located at 4c, 4d, 4a and 4b Wyckoff positions, respectively. Quaternaries at hand exhibit a perfect spin-polarization around the Fermi level, which is a result of the half-metallicity with metallic spin-up channel and semiconductor spin-dn channel. The ferromagnetic half-metallic and spin-flip band gaps are 2.648(2.470) and 0.673(0.526), respectively, for KCaBS(KCaBSe). Both studied compounds have a total magnetic moment of 2.000 μB. Additionally, the strain effect on the electronic and magnetic properties is also examined. Finally, the optical properties of the KCaBX alloys are investigated for energies up to 25 eV. Optical spectra show the metallic behavior at extremely low energies and semiconductor nature at higher energies. Interestingly, KCaBS and KCaBSe exhibit prospective absorption properties with a quite large absorption coefficient in the ultraviolet regime.

Rare-earth based half-Heusler topological quantum materials: A perspective

Topological insulator (TI) materials which are conductive at the surface but insulating in the bulk have drawn much attention in the past decade due to their fascinating properties and potential application in the field of spintronics, quantum computing, topological superconductivity and next generation electronics. In the search of three-dimensional TIs, half-Heusler compounds are the new entrants. Half-Heusler compounds are equiatomic ternary compounds with cubic symmetry. Due to the availability of a large pool of elements in the half Heusler family, the physical properties of these materials can be tuned by choosing the desired combination of elements. In this perspective, we have briefly discussed the development of structural relations, the quantum hall effect, Landau quantization, and topological properties of a few representative systems in the half-Heusler family, including methods by which they are studied and characterized such as Angle Resolved Photoemission Spectroscopy, Shubnikov-de-Hass Oscillations and Nuclear Magnetic Resonance.

Crystal Structure of LiRh‐type IrZn Determined by Powder X‐ray and Selected Area Electron Diffraction

The LiRh‐type crystal structure of the equiatomic intermetallic compound IrZn turns out to be a competitor to the otherwise energetically favored CsCl‐type structures of congeneric transition metal zinc phases, thus enlightening the structural impact of element‐specific factors beyond the Hume‐Rothery concept.

Complete spin gapless semiconductivity in equiatomic quarternary Heusler material TiZrMnAl

A new equiatomic quarternary Heusler compound TiZrMnAl has been theoretically predicted. Two possible structural configurations are considered under the site preference rule and their equilibrium lattices have been derived. Electronic band structures identify TiZrMnAl compound as the spin gapless semiconductors in both two structure types. The total magnetic moments of TiZrMnAl are both equal to zero with the partial moments of Ti and Zr antiparallelly aligned to that of Mn. This integral value of total moment follows the Slater-Pauling rule in the form of Mt = Zt − 18, where Mt represents the total magnetic moment and Zt is the total number of valence electrons. The mechanical and dynamic stabilities of TiZrMnAl have also been validated and strong elastic anisotropy is further revealed with the calculated directional dependent Young’s modulus and shear modulus for both structures. Besides, the effect of random swap disorder between Ti and Zr atoms has been examined and it is found that through the whole swap range, TiZrMnAl maintains its fully compensated spin gapless semiconductivity, which is a very good property for possible applications in spintronic devices. Lastly, the uniform and tetragonal strains have been accessed and their effects on the electronic and magnetic properties have been studied. This systematic study can provide a comprehensive reference for the further development of Ti and Zr based quarternay Heusler compounds and even inspire other relative studies for the exploration of new spin gapless semiconductors.

Epitaxially grown Cu2Sb-type MnGaGe films with large perpendicular magnetic anisotropy

A perpendicularly magnetized film is essentially important for spintronics applications, such as spin-transfer-torque type magnetoresistive random access memories, STT-MRAMs, and large perpendicular anisotropy energy, Ku, with small saturation magnetization, Ms, is required for reducing the critical current for the STT induced magnetization switching phenomenon. In this study, an equiatomic intermetallic compound, MnGaGe, was investigated in epitaxially grown film samples. Perpendicularly magnetized films with an Ms value of about 260 emu/cm3 at 300 K were fabricated onto MgO (100) single crystal substrates. The maximum value of Ku was 8.1 × 106 erg/cm3 at room temperature. Microstructure analysis using a scanning transmission electron microscope atomically revealed the Cu2Sb type crystal structure in the film sample. First principles calculations were also carried out to discuss the electronic structure. Using the analysis of the projected density of states for 3d-orbitals, the hybridization of the 3d-orbitals was suggested as a possible factor for promoting magnetocrystalline anisotropy. The MnGaGe film with the large Ku and relatively small Ms would be potentially used for spintronics applications.

Examining the half-metallicity and thermoelectric properties of new equiatomic quaternary Heusler compound CoVRhGe under pressure

Structural, electronic, magnetic and thermoelectric properties of a new equiatomic quaternary Heusler (EQH) compound CoVRhGe are studied using the full-potential linearized augmented plane-wave (FP-LAPW) approach and Boltzmann transport theory. The structural optimization is carried out by considering different configurations of atomic positions at non-magnetic and ferromagnetic states. Obtained results show that the considered compound is half-metallic with a spin-down semiconducting band gap of 0.812 eV and its magnetic properties do follow the Slater-Pauling rule. CoVRhGe quaternary has high Seebeck coefficient in the spin-down (spin-dn) state and high electrical conductivity in the spin-up state resulting from the semiconductor and metallic nature, respectively. Finally, the pressure effect on the electronic, magnetic and thermoelectric properties of the studied material is also systematically examined.

Structural configuration and tetragonal phase stability in the equiatomic quaternary Heusler compound TiZnMnSi.

Heusler materials have aroused great scientific research interest during recent years due to their special electronic and magnetic properties. Especially for the equiatomic quaternary Heusler compounds, they exhibit very high composition flexibility and structure tunability. In this work, we have carried out a systematic study on the structural configuration and tetragonal stability for the Heusler compound TiZnMnSi by first-principles calculations. Results reveal the type-A structure with ferromagnetic state possesses the lowest total energy and thus should be the ground state configuration. Based on the equilibrium lattice constant, the electronic band structures and magnetic moments have been computed. The tetragonal phase transformation is then investigated by using the total energy variation under different tetragonal strains, and the stability analysis of the mechanical and dynamic properties indicates that TiZnMnSi exhibits a strong tendency for the tetragonal phase. These findings could provide reference data for relative experiments as well as a very helpful theoretical reference for this fascinating class of materials.

Computational prediction of the spin-polarized semiconductor equiatomic quaternary Heusler compound MnVZrP as a spin-filter.

In this work, a new equiatomic quaternary Heusler (EQH) compound, MnVZrP, is predicted using first principles calculations. Simulations show the good stability of the material, suggesting experimental realization. Results show that MnVZrP is a magnetic semiconductor material, exhibiting semiconductor characteristics in both spin channels, however, with strong spin-polarization. Electronic band gaps of 0.97 and 0.47 eV are obtained in the spin-up and spin-dn states, respectively. Mainly the d–d coupling regulates the electronic band structure around the Fermi level. Strain effects on the electronic properties of the proposed compound are also investigated. Simulations give the total magnetic moment of 3 μB satisfying the Slate-Pauling rule. The main magnetic contributions are given by the Mn and V constituents. The results presented here suggest the promising applicability of EQH MnVZrP as a spin-filter. Additionally, the elastic property calculations indicate the mechanical stability and elastic anisotropy. The work may be useful in the magnetic Heusler alloys field, introducing a new member to the small group of magnetic semiconductor EQH compounds for spin-filter applications.

Theoretical Study of the Electronic, Magnetic, Mechanical and Thermodynamic Properties of the Spin Gapless Semiconductor CoFeMnSi

CoFeMnSi has been both experimentally and theoretically proven as a novel spin-gapless semiconductor and resulted in a new research direction in equiatomic full Heusler compounds. Using the first-principles calculation method, we investigated the electronic, magnetic and mechanical properties of CoFeMnSi material in this study. The obtained lattice constant under the LiMgPdSn-type Heusler structure is 5.611 Å and it is fairly consistent with previous experimental results and theoretical calculations. Furthermore, the achieved total magnetic moment of 4 μB follows the Slater–Pauling rule as Mtotal = Ztotal − 24, where Mtotal is the total magnetic moment per formula unit and Ztotal is the total valence electron number, i.e., 28 for CoFeMnSi material. We have also examined the mechanical properties of CoFeMnSi and computed its elastic constants and various moduli. Results show CoFeMnSi behaves in a ductile fashion and its strong elastic anisotropy is revealed with the help of the 3D-directional-dependent Young’s and shear moduli. Both mechanical and dynamic stabilities of CoFeMnSi are verified. In addition, strain effects on the electronic and magnetic properties of CoFeMnSi have been investigated, including both uniform and tetragonal strains, and we found that the spin-gapless feature is easily destroyed with both strain conditions, yet the total magnetic moment maintains a good stability. Furthermore, the specific behaviors under various temperatures and pressures have been accessed by the thermodynamic properties with a quasi-harmonic Debye model, including bulk modulus, thermal expansion coefficient, Grüneisen constant, heat capacity and Debye temperature. This comprehensive study can offer a very helpful and valuable reference for other relative research works.

Computational study of the electronic, magnetic, mechanical and thermodynamic properties of new equiatomic quaternary heusler compounds TiZrRuZ (Z = Al, Ga, In)

The electronic, magnetic, and mechanical properties of the new equiatomic quarternary Heusler compounds TiZrRuZ (Z = Al, Ga, In) have been studied with the first-principles calculations. Three types of different atomic configurations have been examined and results show the atomic site occupation follows the general site preference rule. The equilibrium lattice constants have been determined and also the total and atom-resolved magnetic moments have been calculated. These new equiatomic quarternary Heusler compounds TiZrRuZ (Z = Al, Ga, In) are ferromagnetic half metals at the ground states. In particular for the total magnetic moment (MT), an integral number of 1 µB is obtained with the main contribution from Ti atoms and it follows the general Slater-Pauling rule, i.e. MT = ZT -18, where ZT is the total valence electron number, 19 for TiZrRuZ. Furthermore, the derived mechanical properties have shown that these materials behave in a strong ductile manner with a high elastic anisotropy, which is further confirmed by the calculated 3D Young's modulus and shear modulus. Their mechanical and dynamic stabilities have also been verified with the generalized elastic criteria and phonon dispersion spectrum. Afterwards, the effects of the uniform and tetragonal strains have been studied. The variation of the conduction band minimum and the valence band maximum with both strains have been revealed and the materials show good robustness of the half metallicity. Also, the total magnetic moment maintains constant of 1 µB against both strains while the atomic magnetic moments vary. In the last, the thermodynamic properties have also been analyzed with the quasi-harmonic Debye model under pressure 0 to 10 GPa and temperature from 0 to 500 K. These materials can be good candidates for the future spintronic application and can even inspire other relative work for material research in this field.

Extremely Large Non-equilibrium Tunnel Magnetoresistance Ratio in CoRhMnGe Based Magnetic Tunnel Junction by Interface Modification.

Equiatomic quaternary Heusler compounds (EQHCs) generally have the advantages of high Curie temperature, large spin polarization and long spin diffusion length, and they are regarded as one of the most promising candidates for spintronics devices. Herein, we report a theoretical investigation on an EQHC CoRhMnGe based magnetic tunnel junction (MTJ) with (i) MnGe-terminated interface and (ii) modified pure Mn terminated interface, i.e., MnMn-terminated interface. By employing first principle calculations combined with non-equilibrium Green's function, the local density of states (LDOS), transmission coefficient, spin-polarized current, tunnel magnetoresistance (TMR) ratio and spin injection efficiency (SIE) as a function of bias voltage are studied. It reveals that when the MTJ under equilibrium state, TMR ratio of MnGe-terminated structure is as high as 3,438%. When the MTJ is modified to MnMn-terminated interface, TMR ratio at equilibrium is enhanced to 2 × 105%, and spin filtering effects are also strengthened. When bias voltage is applied to the MTJ, the TMR ratio of the MnGe-terminated structure suffers a dramatic loss. While the modified MnMn-terminated structure could preserve a large TMR value of 1 × 105%, even bias voltage rises up to 0.1 V, showing a robust bias endurance. These excellent spin transport properties make the CoRhMnGe a promising candidate material for spintronics devices.

Investigation on new equiatomic quaternary Heusler compound CoCrIrSi via FP-LAPW calculations

In this work, we have studied the structural, elastic, electronic and magnetic properties of CoCrIrSi EQH compound basing on the first principles calculations. CoCrIrSi is found to be energetically favorable in face-centered cubic (FCC) structure with space group F4‾3m, in which Co, Cr, Ir and Si atoms are located at 4c, 4b, 4d and 4a Wyckoff positions, respectively. Our results show that CoCrIrSi is mechanically stable and ductile. Electronic calculations indicate that CoCrIrSi is half-metallic with relatively large spin-down band gap of 0.741 eV. Otherwise, magnetic properties of CoCrIrSi do obey Slater-Pauling rule. The effect of uniform strain and tetragonal deformation on the half-metallicity is also studied. Finally, the thermodynamic properties of CoCrIrSi are calculated under large range of temperature up to 1200 K and pressure up to 30 GPa, using Debye quasi-harmonic model, and the obtained results will be discussed in detail.

Magnetocaloric effect and critical behavior in ternary equiatomic magnesium compounds REPtMg (RE = Tb, Dy and Ho)

The magnetocaloric effect (MCE) and critical behavior in the REPtMg (RE = Tb, Dy and Ho) intermetallic compounds around Curie temperatures TC were investigated by magnetization measurements. All the REPtMg compounds exhibit pronounced MCE performance with the maximum magnetic entropy change (−ΔSMmax) values up to 5.1, 7.2, 10.2 J/kg K under the magnetic field change (ΔH) of 0–5 T around TC of 58, 29, and 20 K for RE = Tb, Dy and Ho, respectively. The reasonable and accurate critical exponents for the present REPtMg series compounds for RE = Tb, Dy and Ho obtained by the ΔH dependence of ΔSM method are evaluated to be β= 0.4691, 0.4681, 0.4141, γ= 1.0412, 1.0482, 1.1067 and δ= 3.2196, 3.2393, 3.6728 around TC, which are verified by the Widom scaling law and scaling equations, respectively.

Magnetic structure and field-dependent magnetic phase diagram of Ni2In-type PrCuSi

The magnetic structure of the ternary equiatomic intermetallic compound PrCuSi is investigated using neutron powder diffraction experiments in 0 T as well as in external magnetic fields up to 2 T. The PrCuSi compound crystallizes in the hexagonal Ni2In-type structure, in the space group P63/mmc. In this structure, cationic ordering of Cu and Si takes place. The antiferromagnetic phase transition in the Pr sublattice takes place at K in 0 T. Under an external magnetic field of 2 T, a field-induced ferromagnetic phase is observed. Magnetoelastic coupling is evidenced by an increase in the unit cell volume. Clear signatures of a mixed antiferromagnetic and ferromagnetic phase in weak, intermediate fields, 0.4–0.8 T, are obtained from the present study. Using the present set of experimental data, we construct the H − T phase diagram of PrCuSi.

Strain Control of the Tunable Physical Nature of a Newly Designed Quaternary Spintronic Heusler Compound ScFeRhP

Recently, an increasing number of rare-earth-based equiatomic quaternary compounds have been reported as promising novel spintronic materials. The rare-earth-based equiatomic quaternary compounds can be magnetic semiconductors (MSs), spin-gapless semiconductors (SGSs), and half-metals (HMs). Using first-principle calculations, we investigated the crystal structure, density of states, band structure, and magnetic properties of a new rare-earth-based equiatomic quaternary Heusler (EQH) compound, ScFeRhP. The results demonstrated that ScFeRhP is a HM at its equilibrium lattice constant, with a total magnetic moment per unit cell of 1 μB. Furthermore, upon introduction of a uniform strain, the physical state of this compound changes with the following transitions: non-magnetic-semiconductor-(NMS) → MS → SGS → HM → metal. We believe that these results will inspire further studies on other rare-earth-based EQH compounds for spintronic applications.

First-Principles Investigation of Half-Metallic Ferromagnetism of a New 1:1:1:1 Type Quaternary Heusler Compound YRhTiSi

First-principles theory was applied to study the electronic, magnetic, and half-metallic behaviors of a new Y-based equiatomic quaternary Heusler compound YRhTiSi. We will come to study the physical properties of this compound and find it has the half-metallic nature in the ferromagnetic (FM) state. The total magnetic moment of this compound YRhTiSi is 2 μB, which follows the famous Slater-Pauling rule, Mt = Zt − 18, Mt is the total magnetic moment and Zt is the total number of valence electrons. Based on its equilibrium and strained lattice constants, the magnetic and half-metallic behaviors of this compound have been discussed in detail. And we find that the H–M gap and the band gap can be regulated through the effect of uniform strain and the tetragonal deformation. A large H–M gap of 0.48 eV has been found in our work, which is nearly larger than any previous study. Furthermore, the formation and cohesive energies have also been discussed in our work to examine the phase stability of YRhTiSi. This theoretical investigation provides further insight into the application of Y-based compounds as spintronic materials.

Electronic, Magnetic, Half-Metallic, and Mechanical Properties of a New Equiatomic Quaternary Heusler Compound YRhTiGe: A First-Principles Study.

We apply First-principles theory to study the electronic structure as well as the magnetic and mechanical characteristics of YRhTiGe, a newly-designed Y-based quaternary equiatomic Heusler compound. This compound is half-metallic in nature with a ferromagnetic ground state. The total magnetic moment of YRhTiGe is 2 μB and it obeys the Slater-Pauling rule, Mt = Zt − 18, where Mt and Zt are the total magnetic moment and total number of valence electrons, respectively. The magnetic and half-metallic behaviors at its equilibrium and strained lattice constants have been discussed in detail. In addition, for FM-type YRhTiGe, its polycrystalline mechanical features such as Poisson’s ratio, Lame constants, Kleinman parameter and hardness, are also computed according to the well-known Voigt-Reuss-Hill approximation. We investigate the mechanical anisotropy of YRhTiGe using the directional dependences of the Young’s modulus and the shear modulus. Finally, we prove this compound is structurally and mechanically stable. This theoretical investigation provides further insight into the application of Y-based compounds as spintronic materials.

Energy gap formation mechanism through the interference phenomena of electrons in face-centered cubic elements and compounds with the emphasis on half-Heusler and Heusler compounds

Many face-centred cubic elements and compounds with the number of atoms per unit cell N equal to 8, 12 and 16 are known to be stabilised by forming either a band gap or a pseudogap at the Fermi level. They are conveniently expressed as cF8, cF12 and cF16, respectively, in the Pearson symbol. From the cF8 family, we worked on three tetravalent elements C (diamond), Si and Ge, SZn-type AsGa compound and NaCl-type compounds like BiLu, AsSc, etc. From the cF12 family, more than 80 compounds were selected, with a particular emphasis on ABC- and half-Heusler-type ternary equiatomic compounds. Among cF16 compounds, both the Heusler compounds ABC2 and Zintl compounds were studied. We revealed that, regardless of whether or not the transition metal (TM) and/or rare-earth (RE) elements are involved as constituent elements, the energy gap formation mechanism for cF8, cF12 and cF16 compounds can be universally discussed in terms of interference phenomenon of itinerant electrons with set of reciprocal lattice planes with = 8, 11 and 12, where refers to square of the critical reciprocal of lattice vector of an fcc lattice. The number of itinerant electrons per unit cell, e/uc, for all these band gap/pseudogap-bearing compounds is found to fall on a universal line called “3/2-power law” when plotted against on a logarithmic scale. This proves the validity of the fulfilment of the interference condition in conformity with other pseudogap compounds with different crystal symmetries and different sizes of the unit cell reported in literature.

Anisotropy of the ferromagnetic L1 phase in the Mn-Al-C alloys induced by high-pressure spark plasma sintering

The metastable τ-phase of MnAl equi-atomic compound belongs to a family of ferromagnetic alloys with L10 crystal structure. Stabilization of the phase by adding 2 at. % using manganese carbide (Mn23C6) enhances the magnetization in relation with the increase in lattice volume. It is thus a promising candidate for rare-earth-free permanent magnets. Coercivity of Mn-Al-C alloys being still weak, there is an interest to see to which extend sintering/transformation of the ϵ-phase by Spark Plasma Sintering (SPS) can increase the coercivity and the anisotropy. The structural and the magnetic properties were studied for samples sintered at 550 °C under uniaxial pressure of 100, 200, 300 and 400 MPa. Coercivity, remanence and anistotropy appears with the sintering pressure. The high pressure applied while sintering produces preferential orientation of the flake-shaped grains which influences the remanence.

Phase stability, magnetic, electronic, half-metallic and mechanical properties of a new equiatomic quaternary Heusler compound ZrRhTiIn: A first-principles investigation

In this manuscript, we have studied the electronic, magnetic, half-metallic and mechanical properties of a new Zr-based equiatomic quaternary Heusler (EQH) compound, ZrRhTiIn using first-principles calculations. The generalized gradient approximation (GGA) calculation results imply that at its equilibrium lattice constant of 6.70 Å, ZrRhTiIn is a half-metallic material (HMM) with a considerable band gap (Ebg) of 0.530 eV and a spin-filter/half-metallic band-gap (EHM) of 0.080 eV in the minority-spin channel. For ZrRhTiIn, the formation energy of −2.738 eV and the cohesive energy of 21.38 eV indicate that it is a thermodynamically stable material according to theory. The minority-spin EHM arises from the hybridization among Zr-4d, Ti-3d and Rh-4d electrons. The calculated total magnetic moment of ZrRhTiIn is 2 μB, meeting the well-known Slater-Pauling rule Mt = Zt −18. Furthermore, uniform strain and tetragonal strain were applied in this work to examine the magneto-electronic and half-metallic behaviors of the ZrRhTiIn system. Finally, we show that ZrRhTiIn is mechanically stable, ductile and anisotropic.