Half-metallic Heusler Alloys Research Articles

First Principles Study of Structural, Electronic, Mechanical and Density of State Properties of the Half - heusler Alloy NaCrGe

In this study, the investigated structural parameters revealed that NaCrGe is stable at beta phase. We obtained that the material is a narrow bandgap semiconductor half-Heusler alloys with measured gap of 1.050 eV. The structure NaCrGe conduction band minimum (CBM) position at gamma(Γ) and the valence band maximum (VBM) located at X-point of the Brillouin zone. This indicates that the alloy NaCrGe has indirect bandgap semiconductors. The material NaCrGe is considered hybrids between metals and semiconductors. Hence, NaCrGe is Half-metallic heusler alloy. The calculated mechanical properties indicate that NaCrGe possesses good mechanical stability, making it suitable for structural applications. B/G ratio for NaCrGe is 2.40. This implies that NaCrGe is “ductile” in nature at ambient condition. Also NaCrGe is confirmed “ductile” in nature at positive value of C11 – C44 (+48.07). PDOS shows that Na-4p, Cr-4p and Ge- 2p has the highest orbital contribution for Na, Cr and Ge atoms respectively. At fermi energy both spin up and spin down is at zero point in the plot of projected density of state (PDOS) against energy. This revealed that NaCrGe is a half metallic heusler alloy.

Surface effects for half-metallic Heusler alloy CrYCoAl

Magnetic tunnel junctions are the core component of spintronic devices, and good magnetic tunnel junctions require high tunnel resistance, which is determined by the spin polarization of the thin films of ferromagnetic layers that make up their structure. Therefore, the design of ferromagnetic films with high spin polarization is a current research hotspot in this field. In the current study, we taken the half-metallic CrYCoAl as an object, and studied the influence of surface effect on the half-metallic behavior. Especially, we constructed the CrYCoAl (100), (110), and (111) surfaces to study their stability, electronic and magnetic properties. Results show that the YY and YAl (100) surfaces, and Co (111) surface retain the half-metallic behavior and structural stability of bulk, which provide a basis for CrYCoAl application as a thin films in magnetic tunnel junctions thus, they are likely to be used in the spintronic devices.

Density functional study of electrode material for magnetic tunnel junction designed using Co2TiZ (Z = Ge, Si) Heusler alloys

Some of the important applications in the field of spintronics are spin current driven logic devices having logic gates and in read heads in the hard disk drives, magnetic random access memory (MRAM) containing non-volatile memory units and extremely complex magnetic sensor. In spintronic applications, a magnetic tunnel junction (MTJ) is considered the back bone of all such devices. With this motivation, in present study we have predicted very low resistance area (RA) product value and high tunnel magneto resistance (TMR) of two designed MTJs by using full Heusler alloys (FHA) Co2TiZ (Z = Ge, Si) as electrode and MgO as barrier layer. These used half metallic (HM) Heusler alloys (HA), have been investigated using Wien2K code, via primary standard calculations and studied electronic and magnetic properties followed by structural and magnetic phase stability. Spin based band structure along with density of state (DOS) profiles are determined by considering both generalized gradient approximation (GGA) + modified Becke–Johnson functional (mBJ) functional and GGA potentials. For the estimation of the Curie temperature (Tc) mean field approximation (MFA) was considered. After determining physical properties of considered FHAs, spin polarised transport properties are calculated by using PWCOND code embedded in Quantum ESPRESSO (QE) for designed MTJs and obtained extremely low resistance-area product (RA) product and a high value of TMR which is found very suitable for spintronic device applications.

Spin dependent tunneling and strain sensitivity in a Co2MnSb/HfIrSb magnetic tunneling junction: a first-principles study.

Half-metallic Co-based full Heusler alloys have captured considerable attention of researchers in the realm of spintronic applications, owing to their remarkable characteristics such as exceptionally high spin polarization at the Fermi level, ultra-low Gilbert damping, and a high Curie temperature. In this comprehensive study, employing the density functional theory, we delve into the electronic stability and ballistic spin transport properties of a magnetic tunneling junction (MTJ) comprising a Co2MnSb/HfIrSb interface. An in-depth investigation of k-dependent spin transmissions uncovers the occurrence of coherent tunneling for the Mn-Mn/Ir interface, particularly when a spacer layer beyond a certain thickness is employed. It has been found that the Co-terminated Co2MnSb/HfIrSb interface shows perpendicular magnetic anisotropy, while those with Mn-Sb and Mn-Mn termination exhibit in-plane magnetic anisotropy. Furthermore, our spin-dependent transmission calculations demonstrate that the Mn-Mn/Ir interface manifests strain-sensitive transmission properties under both compressive and tensile strain and yields a remarkable three-fold increase in majority spin transmission under tensile strain conditions. We find a tunnel magnetoresistance of ∼500% under a bi-axial strain of -3%, beyond which the tunnel resistance is found to be theoretically infinite. These compelling outcomes place the Co2MnSb/HfIrSb junction among the highly promising candidates for nanoscale spintronic devices, emphasizing the potential significance of the system in the advancement of the field.

Structural stability, opto-electronic, magnetic and thermoelectric properties of half-metallic ferromagnets quaternary Heusler alloys CoFeXAs (X = Mn, Cr and V)

The Quaternary Heusler alloys CoFeXAs (X = Cr, Mn and V) fully spin-polarized with half-metallic stability, show low direct band gap, high absorption in the ultraviolet light, high spin polarization and adequate Seebeck coefficient. CoFeCrAs, CoFeMnAs and CoFeVAs have an integer magnetic moment of 4 μB, 5 μB and 3 μB according to the Slater-Pauling rule. The obtained minimal energy favors the type I CoFeVAs alloy over the type III CoFeCrAs and CoFeMnAs alloys. For CoFeCrAs and CoFeMnAs alloys, there is no band gap close to the Fermi level in the minority-spin, suggesting that they are direct Г–Г band gap semiconductors. Three-dimensional Co, Fe, and Mn atoms hybridized strongly, with little input from V and As atoms. Co, Fe, Cr, and Mn all have parallel magnetic moments, which results in ferromagnetic interactions between these atoms and gives these elements their ferromagnetic character.

Combinatorial optimization for high spin polarization in Heusler alloy composition-spread thin films by anisotropic magnetoresistance effect

Half-metallic Heusler alloys are promising candidates for spintronic applications due to their high spin polarization. However, the spin polarization strongly depends on the atomic composition, which is time-consuming to optimize from various compositional combinations. Here, we demonstrate a high-throughput compositional optimization method for high spin polarization in Co2(Mn, Fe)Ge Heusler alloys by combining composition-spread films and anisotropic magnetoresistance (AMR) measurement. Two types of composition-spread films of polycrystalline Co2(Mn1−xFex)Ge and (Co2Mn0.5Fe0.5)1−yGey are fabricated on SiO2/Si substrates by combinatorial sputtering deposition, followed by post-annealing. The compositional dependence of AMR shows the largest negative AMR ratio of −0.13% and the smallest temperature dependence of the resistance change of AMR for y = 0.25 in the (Co2Mn0.5Fe0.5)1−yGey composition-spread film, suggesting the highest spin polarization and the closest nature to the ideal half-metal at this composition ratio. To verify this, we also develop a new technique to measure the compositional dependence of spin polarization by measuring the spin accumulation signals of nonlocal spin-valve devices fabricated on the composition-spread films and observe the highest spin polarization of 82% for y = 0.24. This confirms a clear qualitative correlation between the large negative AMR ratio and high spin polarization. Our combinatorial method using the composition-spread films and the AMR measurement proves to be a facile way for optimizing the fabrication conditions of half-metallic Heusler alloys with high spin polarization.

Experimental and first-principles investigation on magnetic properties and electronic structure in half-metallic Mn-Co-V-Al Heusler alloy

We report a systematic experimental and theoretical study of half-metallic Heusler alloys Mn2V1−xCoxAl (x = 0, 0.25, 0.5, 0.75, 1). In this study, using the arc-melting method and optimizing annealing temperatures, we succeed to synthesize Mn2V1−xCoxAl (x = 0, 0.25, 0.5, 0.75, 1) alloys with evident L21 (or XA) order. A near compensated ferrimagnetic moment of 0.02 μB/f.u. at 5 K is observed with the Co concentration of 0.5 which is consistent with the S-P rule. In addition, we systematacially investigate the effects of Co concentration on electronic structure of Heusler alloys Mn2V1−xCoxAl (x = 0, 0.25, 0.5, 0.75, 1) using first-principles calculations, and found that the half-metallic characteristic persists for all Co concentrations. In line with our experimental results, employing appropriate atomic site occupation, we obtained a full compensated ferrimagnetic moment of 0 μB/f.u. for stoichiometric Mn2V0.5Co0.5Al alloy. In addition, the robustness of half-metallic properties for compensated ferrimagnet Mn2V0.5Co0.5Al with the variation of atomic disorder are investigated and the results indicate that the Mn-Al swapping weakly destroys the half-metallic properties and the compensated ferrimagnet Mn2V0.5Co0.5Al alloy maintains the high spin polarization under the effect of B2 disorder.

Study of structural, electronic, elastic, magnetic and thermal properties of the new half-metallic Heusler alloys Cr2LuSn1−xPbx: by DFT approach

Study of structural, electronic, elastic, magnetic and thermal properties of the new half-metallic Heusler alloys Cr2LuSn1−xPbx: by DFT approach

Soft magnetism in single phase Fe3Si thin films deposited on SrTiO3(001) by pulsed laser deposition

The development of devices relying on spin phenomena requires of an ideal spin polarized electron source. This can be achieved by taking advantage of half-metallic full Heusler alloy thin films. However, their implementation requires a controlled growth of stoichiometric films with large activation volumes. In this work, we report on the growth of epitaxial Fe3Si ultra-thin films by pulsed laser deposition on SrTiO3(001) substrates, analyzing the effect of deposition temperature in the structural, morphological and magnetic properties of the deposited films. We conclude that optimal compromise between phase purity and interface quality is obtained at 200 ºC, obtaining the best magnetic response under this condition.

Magnetization dynamics of a CoFe/Co2MnSi magnetic bilayer structure

Half-metallic Heusler alloys are receiving significant attention for spintronic applications utilizing magnetic tunnel junctions and requiring large spin polarization. Co2MnSi (CMS) is one of the most promising candidates for this purpose. Here, we report the magnetization dynamics of a thin, epitaxial CMS film in a magnetic CoFe/CMS bilayer structure sputtered on an MgO substrate. The magnetic precession frequency response of the CoFe/CMS bilayer shows a fourfold symmetry with respect to the azimuthal applied field angle, reflecting the crystal symmetry of the CMS layer and not the underlying CoFe film. Moreover, the effective Gilbert damping parameter exhibits inhomogeneous broadening at lower applied magnetic fields. At large fields, however, the azimuthal angle dependence disappears, and the intrinsic Gilbert damping is observed. This study provides insight into the dynamics of a magnetic bilayer structure that forms an integral element in spintronic applications.

Half metallic Heusler alloys XMnGe (X = Ti, Zr, Hf) for spin flip and thermoelectric device application – Material computations

The ground state properties of XMnGe (X = Ti, Zr, Hf) magnetic half Heusler alloys have been investigated through density functional theory. Two different exchange correlation functionals (GGA and mBJ) are used to study the reported materials for comparison. By using these approximation methods, we have calculated corresponding lattice constants and ground state energies. The band structure and density of state calculations have been done by computing dispersion curves. From mBJ potential, the reported alloys are predicted to be half metals with semiconducting nature in spin up state and metallic nature in spin down state. The respective band gap for XMnGe (X = Ti, Zr, Hf) is 1.06 e V, 1.13 eV and 1.03 eV. The studied cubic alloys show ductile character and also exhibit directional property. The calculated Seebeck coefficient of XMnGe (X = Ti, Zr, Hf) is −109.4 μV/K, 101.9 μV/K and 128.5 μV/K at 1100 K, 1300 K and 1100 K respectively and the corresponding power factor is 2 × 1012 W/m.K [2].s, 1.4 × 1012 W/m.K [2].s and 1.3 × 1012 W/m.K2.s at respective same temperature. Since, these alloys exhibit half metallic behaviour and also have good thermoelectric nature, they can be used for spin-based electronics and thermoelectric energy applications.

Low magnetic damping constant in half-metallic Co2FeAl Heusler alloy thin films grown by molecular beam epitaxy

Half-metallic Co2FeAl Heusler alloy thin films with various thickness were prepared by MBE method and its magnetic and crystalline properties were investigated. At the thickness of 20 nm and above, good magnetic properties and high B2-order parameter was obtained. In addition, magnetic damping constant was evaluated from peak-to-peak linewidth measured by FMR and ultra-low magnetic damping constant below 0.001 was obtained at the film thickness of 30 nm. These results are thanks to the excellent half-metallicity in the Co2FeAl films prepared by MBE.

Superconducting fluctuation effect on epitaxially layered films of superconductor NbN and half-metallic Heusler alloy Co2MnSi

Superconducting properties were investigated in epitaxially layered films consisting of superconductor NbN and half-metallic Heusler alloy Co2MnSi (CMS). Temperature dependence of the electrical resistivity ρ(T) was measured by applying perpendicular magnetic fields to the surface of NbN/CMS films. With the increase of the CMS thickness dCMS, the upper critical field μ0Hc2 decreased monotonically, but the superconducting transition temperature Tc had the minimum of 10.1 K at dCMS ≈ 5 nm. The Tc behavior was in qualitative agreement with the theory of the π-coupling. The pair-breaking parameter δ determined by the superconducting fluctuation theory took the maximum at dCMS = 3 ∼ 5 nm, which would be related to the minimum of Tc. The experimental results reveal that the superconductivity of the NbN layer in NbN/CMS films is affected by the interplay between the superconducting NbN layer and the half-metallic CMS layer.

Continuous modulation of charge-spin conversion by electric field in Pt/Co2FeSi/Pb(Mg1/3Nb2/3)O3-Pb0.7Ti0.3O3 heterostructures

The half-metallic Heusler alloy Co2FeSi is an ideal material in spintronic devices due to its higher spin polarization, higher Curie temperature and lower damping parameters. In this work, the dynamic magnetism of Co2FeSi is modulated by electric field and it is demonstrated that the charge-spin conversion efficiency ξ is continuous and controllable by the electric field. We further find an extremely high ξ in ferromagnetic/ferroelectric (FM/FE) heterostructures, which could be ascribed to interfacial effect in FM/FE interface. Moreover, we investigate that the charge-spin conversion efficiency varies with the electric field in a butterfly-like behavior, which accords with the S–E curve of Pb(Mg1/3Nb2/3)O3-Pb0.7Ti0.3O3(PMN-PT) and could be attributed to strain effect. The modulation of charge-spin conversion efficiency in FM/FE heterostructures via electric field presents a profound potential for next-generation spintronic devices and applications of current-induced magnetization switching.

Half-Metallic Heusler Alloy/MoS2 Based Magnetic Tunnel Junction.

Integration of half-metallic materials and 2D spacers into vertical magnetoresistive spin valves may pave the way for effective low-power consumption storage and memory technologies. Driven by the recent successful growth of graphene/half-metallic Co2Fe(Ge1/2Ga1/2) (CFGG) heterostructure, here we report a theoretical investigation of magnetic tunnel junction (MTJ) based on the ferromagnetic CFGG Heusler alloy and the MoS2 spacer of different thicknesses. Using ab initio approach, we demonstrate that the inherent ferromagnetism of CFGG is preserved at the interface, while its half-metallicity is recovering within few atomic layers. Ballistic transport in CFGG/MoS2/CFGG MTJ is studied within the nonequilibrium Green's function formalism, and a large magnetoresistance value up to ∼105% is observed. These findings support the idea of effective spintronics devices based on half-metallic Heusler alloys and highly diversified transition metal dichalcogenide family.

Large enhancement of magnetic damping in half-metallic Co2MnSi Heusler alloy thin films with heavy metal Pt layer observed by the all-optical method

Manipulation of magnetic moments through spin transport is determined by the effective spin mixing conductance g eff ↑↓ in the ferromagnetic metal/heavy metal heterostructure. Magnetization dynamics detection and evaluation of the Gilbert damping constant α is usually employed to estimate g eff ↑↓. An all-optical pump-probe method is a powerful method for investigating α with high accuracy because an extrinsic contribution on α can be suppressed at a high precession frequency with an increasing external magnetic field. A large voltage signal was recently observed in half-metallic Heusler alloy Co2MnSi thin film due to spin current injection; however, that voltage signal may also include contributions from thermal effect and spin Hall angle in the heavy metal layer. In this study, a large enhancement of α was observed in Co2MnSi/Pt heterostructure by using the all-optical method. g eff ↑↓ for Co2MnSi/Pt was 2 times larger than that for Fe/Pt. This large g eff ↑↓ is promising for spintronic device applications.

Machine learning study of highly spin-polarized Heusler alloys at finite temperature

A huge magnetoresistance (MR) ratio exceeding 2000% at cryogenic temperature that was reported for half-metallic Heusler alloy based magnetic tunnel junctions showed large degradation at room temperature, which impedes practical application of Heusler alloy based MR devices. This motivates us to explore alternative Heusler alloys that show high spin polarization at finite temperatures. Here, we propose half-metallic Heusler alloys based on finite-temperature first-principles calculation via the disordered local moment method together with machine learning. We found several prospective materials at room temperature such as ${\mathrm{Co}}_{2}{\mathrm{MnGa}}_{0.2}{\mathrm{As}}_{0.8}$ and ${\mathrm{Co}}_{2}{\mathrm{FeAl}}_{0.4}{\mathrm{Sn}}_{0.6}$. We also investigated two combinatorial series, ${\mathrm{Co}}_{2}{\mathrm{MnGa}}_{y}{\mathrm{As}}_{1\text{\ensuremath{-}}y}$ and ${\mathrm{Co}}_{2}{\mathrm{FeAl}}_{y}{\mathrm{Sn}}_{1\text{\ensuremath{-}}y}$, to understand the effect of alloy mixing on temperature dependence and found that Fermi level tuning significantly improved the spin polarization and its temperature dependence, especially in ${\mathrm{Co}}_{2}{\mathrm{FeAl}}_{y}{\mathrm{Sn}}_{1\text{\ensuremath{-}}y}$.

Selective electronic excitations in nearly half-metallic Heusler alloy NiFeMnSn—A Raman spectroscopic study

Half-metallic ferromagnetic full Heusler alloy NiFeMnSn is a promising material in spintronic device fabrication as it carries high spin polarization and high Curie temperature (Tc = 405 K). Understanding electronic excitations in a spin-polarized band structure is essential for the further use of this material in spin-based devices and to optimize the spin-dependent electronic structure in related compounds. In this Letter, we report electronic Raman scattering of NiFeMnSn with spectral signatures at a higher wavenumber than expected from the calculated phonon modes of the system. Temperature-dependent Raman spectroscopy demonstrates a non-monotonic behavior of the Raman shift with temperature across Tc. The orbital resolved electronic density of states and electronic band structure for both spin channels of the system, as obtained from the first principles density functional theory calculations, suggest that the observed Raman signatures originate from the Fe d orbital and its splitting due to the crystal field near the Fermi level. Furthermore, a strong magnetic field dependence of the spectral profile is observed. The study not only exhibits electronic Raman scattering in a Heusler alloy compound, which was unexplored to date, but also establishes Raman scattering as a promising probe to study the orbital-resolved partial density of states in the band structure near the Fermi level of a Heusler alloy.

Temperature dependent partially compensated to nearly fully compensated magnetic state in half-metallic full Heusler alloy, Mn1.2Fe1.18V0.62Al

We report experimental and theoretical studies of magnetization compensation phenomenon in quaternary Heusler alloy Mn1.2Fe1.18V0.62Al by employing dc magnetization, neutron depolarization, neutron diffraction, specific heat, electrical resistivity, and electronic structure calculation. In this ferrimagnetic alloy system with ordering temperature TC = 360 K, the asymmetric thermal variations of the site magnetic moments provide an evidence of the partially compensated magnetic state of the system down to ∼ 48 K. However, below 48 K, the system exhibits its magnetic ground state with the negligibly small ordered moment of 0.05(5) μB/f.u. at 10 K, as obtained from the neutron diffraction study. Nearly full recovery of neutron beam polarization at 4 K in the neutron depolarization study also infers a nearly fully compensated magnetization state in this Heusler alloy. Temperature dependent specific heat and resistivity measurements also corroborate with the dc magnetization study representing 48 K as a turning point at which the system reaches its ground state. The electronic structure calculations, using the SPR-KKR Green’s function approach, verify the half-metallic nature of the present system. The net magnetic moment (0.06 μB/f.u.) obtained from the theoretical calculations as well as neutron diffraction experiments (0.05 μB/f.u.) is in good agreement with the Slater-Pauling rule (0.06 μB/f.u.). This small magnetic moment of the system (Mn0.82Fe1.18)(Mn0.38V0.62)Al proclaims to be a nearly compensated ferrimagnetic system with 23.94 valence electrons. Such magnetically compensated systems with finite spin polarization could be favorable for spintronics applications.

Prediction of half-metallic gap formation and Fermi level position in Co-based Heusler alloy epitaxial thin films through anisotropic magnetoresistance effect

We have investigated the temperature $(T)$ dependence of the anisotropic magnetoresistance (AMR) effect of ${\mathrm{Co}}_{2}{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) epitaxial thin films having different compositions and atomic orders to examine the relation between AMR and the half-metallic electronic structure based on a developed theoretical model. The $T$ dependence of the resistance change (\ensuremath{\Delta}\ensuremath{\rho}) of the AMR normalized at 10 K is minimal in the CFGG films having a standard composition and high atomic order. In contrast, the films having a large atomic disorder due to the Co-rich composition exhibit a large reduction of \ensuremath{\Delta}\ensuremath{\rho} with $T$. Our theoretical model of AMR well explains this behavior; namely, the half-metallic ferromagnets having the Fermi level $({E}_{\mathrm{F}})$ around the center of the half-metallic gap are predicted to show a small $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} because of no/few localized $d\ensuremath{\downarrow}$ states at ${E}_{\mathrm{F}}$. On the contrary, a large change of \ensuremath{\Delta}\ensuremath{\rho} with $T$ is expected for the half-metallic materials having ${E}_{\mathrm{F}}$ close to gap edges or large in-gap states because of the contribution of thermally excited s-d scattering involving the $d\ensuremath{\downarrow}$ states. Moreover, we have also investigated the variation of \ensuremath{\Delta}\ensuremath{\rho} with $T$ for the thin films of various half-metallic Co-based Heusler alloys and found the behavior that agrees with our theoretical prediction. The present study proves that the formation of a half-metallic gap and the position of ${E}_{\mathrm{F}}$ in a half-metallic material are readily predictable from the $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} of the AMR effect, which can be a facile way for efficient screening of half-metallic materials.