Co-based full-Heusler Alloy Research Articles

Ab-initio prédiction of the structural ,electro-magnetic and thermodynamic properties of Co-based Full Heusler alloy

The structural, thermodynamic, electronic, magnetic and elasticproperties of the Full-Heusler Co2MnTi compoundon the cobalt (Co) are determined byusing first-principles calculations based on density functional theory (DFT). We have used the linear Muffin-Tin Orbitals method (FP-LMTO) combined with the generalized gradient approximation (GGA) with spin. The obtained results demonstrate that our material is ferromagnetic and exhibits metallic behavior around the Fermi level .The study of the elastic properties show that Co2MnTi is mechanically stable and is also classified as a ductile material. The GIBBS program is used to comput the thermodynamic properties such as the modulus of compressibility, the heat capacity volume and pressur, the thermal expansion, the Debye temperature and the volume variation at constants. This type of material is considered as promising for spintronic applications.

Grain diameter-dependent tuning of exchange anisotropy in the ion-beam sputtered Co-based full Heusler alloy coupled with antiferromagnet

We report an investigation into the substantially large and customizable exchange anisotropy (HEA) and coercivity (HC) in a set of bottom-pinned Ir7Mn93/Co2FeAl bilayer heterostructures fabricated using ion-beam sputtering at room temperature (RT) in the presence of an in-situ in-plane static magnetic field of 1 kOe. This modulation is achieved by controlling the microstructural parameter (i.e., grain diameter) of the antiferromagnetic (AF) Ir7Mn93 (IrMn) layer. These bilayers revealed strong positive exchange anisotropy (PEA) at RT, while negative exchange anisotropy (NEA) became evident when field-cooled to 15 K in the presence of 3 kOe. By systematically controlling the AF grain diameter from ∼5.39 to ∼6.94 nm, the PEA and NEA were found to increase by a factor of ∼2.1 and ∼1.8, respectively. However, once the AF grain diameter exceeded the necessary threshold for thermal stability, further enhancement in grain diameter above ∼6.94 nm led to a reduction in both HEA and HC. This decrease was attributed to a reduction in pinning centers at the AF/FM (ferromagnet) interface. The training data are fitted by utilizing various theoretical models, such as thermal relaxation, Binek's model, and spin relaxation model. The spin relaxation model was found to be applicable to fit the complete range of training data, encompassing both thermal and athermal decay, within the context of frozen and rotatable spins.

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.

Large Improvement of Thermoelectric Performance by Magnetism in Co-Based Full-Heusler Alloys.

Full-Heusler alloys (fHAs) exhibit high mechanical strength with earth-abundant elements, but their metallic properties tend to display small electron diffusion thermopower, limiting potential applications as excellent thermoelectric (TE) materials. Here, it is demonstrated that the Co-based fHAs Co2 XAl (X = Ti, V, Nb) exhibit relatively high thermoelectric performance due to spin and charge coupling. Thermopower contributions from different magnetic mechanisms, including spin fluctuation and magnon drag are extracted. A significant contribution to thermopower from magnetism compared to that from electron diffusion is demonstrated. In Co2 TiAl, the contribution to thermopower from spin fluctuation is higher than that from electron diffusion, resulting in an increment of 280µWm-1 K-2 in the power factor value. Interestingly, the thermopower contribution from magnon drag can reach up to -47µVK-1 , which is over 2400% larger than the electron diffusion thermopower. The power factor of Co2 TiAl can reach 4000µWm-1 K-2 which is comparable to that of conventional semiconducting TE materials. Moreover, the corresponding figure of merit zT can reach ≈0.1 at room temperature, which is significantly larger than that of traditional metallic materials. The work shows a promising unconventional way to create and optimize TE materials by introducing magnetism.

Effect of substitution of 3d, 4d and 5d elements on structural, electronic, magnetic properties and XMCD spectra of Co-based full Heusler alloys: A DFT study

In the present work, we have probed the role of substitution of 3d, 4d and 5d elements (i.e., with electronic configuration (EC): ndy, where n = 3–5; y = number of electrons in the d level) at the B site on structural, magnetic and electronic properties of a set of full Heusler alloys (FHAs) namely, Co2BAl, using density functional theory (DFT) based electronic structure (ES) calculations. We have optimized all the 30 combinations studied here in Fm3̄m (A2BC type, conventional FHA structure) and F4̄3m (ABAC type, inverse FHA structure) space groups using VASP code. Thereafter, ES calculations have been carried out using DFT based FPLAPW method as implemented in WIEN2k code with an exchange-correlation term (PBE + mBJ + SOC), which is beyond GGA and includes the spin-orbit-coupling (SOC). Furthermore, XMCD calculations have been performed using single particle approximation and Fermi’s Golden rule as implemented in WIEN2k code. FHAs with atoms having EC nd1−6,10 (n = 3–5) at B site have Fm3̄m as the lowest energy structure with the exception of 4d6 and 5d6. Calculated magnetic moments (MM) of these alloys with atoms with EC nd1−5 are found to be in agreement with the Slater-Pauling (SP) rule, with an integer or close to integer value along with 3d6. However, alloys having atoms at B site with nd7−9 (along with 4d6 and 5d6) are found to be in F4̄3m space group and their values of the MM are not in agreement with the SP rule. XMCD spectra corresponding to various atomic sites help in understanding the impact of the local environment. Hence, calculations of the same have been performed to understand the low values of Co MM with atoms at B site having EC 3d3−5 and the negative MM of the Ti and Zr atoms at B site.

Enhanced Curie temperature and spin polarization in Co-based compounds under pressure: A first principles investigation

In the present study, we report a systematic first principles study which investigates and tunes the percentage of spin polarization in few Co-based full Heusler alloys Co2CrX (X = Al, Ga and In) under hydrostatic strain. At ambient condition, Co2CrAl exhibits half metallic nature while Co2CrGa possesses nearly half metallic nature and Co2CrIn shows metallic nature. Mechanical and dynamical stabilities of all the investigated compounds are confirmed using elastic tensor analysis and phonon dispersion plots respectively at ambient as well as under compression. Hydrostatic compressive strain is applied to all the systems and Co2CrGa turned out to be a half metal at compressive strain corresponding to V/V0 = 0.85 with 100% spin polarization and the percentage of spin polarization for Co2CrIn gets enhanced as a function of compressive strain. Further, an electronic topological transition (ETT) is also observed in Co2CrGa as a function of compressive strain corresponding to V/V0 = 0.85, driving the system towards the half-metallic nature. In addition, ferromagnetic nature of all these compounds is explained by the exchange interactions at both ambient and under compression. Also, the positive pressure dependence of Curie Temperature (TC) as a function of V/V0 for all the compounds is studied and the highest TC = 646.1 K for Co2CrAl is observed under compression corresponding to V/V0 = 0.85.

Perpendicular magnetic anisotropy in Pt/Co-based full Heusler alloy/MgO thin-film 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.

Exploring half-metallic Co-based full Heusler alloys using a DFT+U method combined with linear response approach.

A density functional theory (DFT)+U method based on linear response (LR) theory was applied to investigate the electronic structures of a Co-based ternary full Heusler alloy Co2YSi to explore half-metallic (HM) ferromagnets with a wide HM gap. The LR-based DFT+U calculations tend to obtain a reasonable correlation parameter for the Y site, while the correlation of the Co site misdirects to the unphysical ground state due to the overestimated parameter value that arises from the delocalized electronic structure of Co. Furthermore, we found that the HM gap of Co2MnSi originates from the Co orbital in the conduction state and the Co–Mn hybridizing t2g orbital in the valence state around the Fermi energy. This means that the HM gap is a tunable property by selecting the Y element and/or mixing several elements into the Y site through t2g atomic-orbital coupling. Our LR-based DFT+U method was extended to other ternary Co2YSi and quaternary Co2(Y,Mn)Si. We found that Co2(Ti0.25,Mn0.75)Si and Co2(Fe0.25,Mn0.75)Si show HM nature, with the Fermi energy being at almost the center of the minority band gap, which leads to high thermal stability.

Mechanical and Thermodynamic Properties of Half-metallic Ferromagnetism Containing Cobalt and Titanium

We have studied the structural, electronic, elastic, magnetic and Thermodynamic properties of Co-based full-Heusler alloys Co2TiSi and Co2TiGe. have been studied by first-principles full-potential linearized augmented plane wave (FPLAPW) method with the Generalized Gradient Approximation (GGA) based on density functional theory (DFT). This shows that the magnetic properties of the compound are dependent on electron concentration of main group element and all compunds are magnetic in their equilibrium L21 structure. The electronic structure report that, our compounds have half-metallic (HM) nature. The mechanical results show that these compounds are mechanically stable. and exhibit 100% spin polarization at the Fermi level where it can be shifted within the energy-gap. In addition, the quasi-harmonic Debye model is applied to determine the thermal properties of the alloy.

Theoretical Study of the Electronic and Thermodynamic Properties of Co2CrZ (Z=Ga,Al)

The present work includes a first principle study of the electronic structure, elastic, magnetic and thermal properties of Co-based ternary full-Heusler alloys Co2CrX (x=Al,Ga). The lattice constant, bulk modulus, magnetic moment and density of states are studied using the full-potential linearized augmented plane wave method with the Generalized Gradient Approximation (GGA) as functional of exchange and correlation. This shows that the magnetic properties of the compound are dependent on electron concentration of main group element and all concentrations are magnetic in their equilibrium L21 structure. The calculations show that the alloys with all concentration are true half-metallic materials and exhibit 100% spin polarization at the Fermi level where it can be shifted within the energy-gap. In addition, the quasi-harmonic Debye model is applied to determine the thermal properties of the alloy.

ChemInform Abstract: Perpendicular Magnetic Anisotropy in Co‐Based Full Heusler Alloy Thin Films

AbstractReview: 59 refs.

Probing the possibility of coexistence of martensite transition and half-metallicity in Ni and Co-based full-Heusler alloys: Anab initiocalculation

Using first-principles calculations based on density functional theory, we have studied the mechanical, electronic, and magnetic properties of Heusler alloys, namely, Ni$_{2}BC$ and Co$_{2}BC$ ($B$ = Sc, Ti, V, Cr and Mn as well as Y, Zr, Nb, Mo and Tc; $C$ = Ga and Sn). On the basis of electronic structure (density of states) and mechanical properties (tetragonal shear constant), as well as magnetic interactions (Heisenberg exchange coupling parameters), we probe the properties of these materials in detail. We calculate the formation energy of these alloys in the (face-centered) cubic austenite structure to probe the stability of all these materials. From the energetic point of view, we have studied the possibility of the electronically stable alloys having a tetragonal phase lower in energy compared to the respective cubic phase. A large number of the magnetic alloys is found to have the cubic phase as their ground state. On the other hand, for another class of alloys, the tetragonal phase has been found to have lower energy compared to the cubic phase. Further, we find that the values of tetragonal shear constant show a consistent trend : a high positive value for materials not prone to tetragonal transition and low or negative for others. In the literature, materials, which have been seen to undergo the martensite transition, are found to be metallic in nature. We probe here if there is any Heusler alloy which has a tendency to undergo a tetragonal transition and at the same time possesses a high spin polarization at the Fermi level. From our study, it is found that out of the four materials, which exhibit a martensite phase as their ground state, three of these, namely, Ni$_{2}$MnGa, Ni$_{2}$MoGa and Co$_{2}$NbSn have a metallic nature; on the contrary, Co$_{2}$MoGa exhibits a high spin polarization.

Perpendicular Magnetic Anisotropy in Co-Based Full Heusler Alloy Thin Films

Half-metallic Co-based full Heusler alloys have been qualified as promising functional materials in spintronic devices due to their high spin polarization. The lack of perpendicular magnetic anisotropy (PMA) is one of the biggest obstacles restricting their application in next generation ultrahigh density storage such as magnetic random access memory (MARM). How to induce the PMA in Co-based full Heusler alloy thin films has attracted much research interest of scientists. This paper presents an overview of recent progress in this research area. We hope that this paper would provide some guidance and ideas to develop highly spin-polarized Co-based Heusler alloy thin films with PMA.

ChemInform Abstract: Spin Transfer Torque Switching and Perpendicular Magnetic Anisotropy in Full Heusler Alloy CO2FeAl‐Based Tunnel Junctions

AbstractReview: 57 refs.

Perpendicular magnetic anisotropy and magnetization dynamics in oxidized CoFeAl films.

Half-metallic Co-based full-Heusler alloys with perpendicular magnetic anisotropy (PMA), such as Co2FeAl in contact with MgO, are receiving increased attention recently due to its full spin polarization for high density memory applications. However, the PMA induced by MgO interface can only be realized for very thin magnetic layers (usually below 1.3 nm), which would have strong adverse effects on the material properties of spin polarization, Gilbert damping parameter, and magnetic stability. In order to solve this issue, we fabricated oxidized Co50Fe25Al25 (CFAO) films with proper thicknesses without employing the MgO layer. The samples show controllable PMA by tuning the oxygen pressure (PO2) and CFAO thickness (tCFAO), large perpendicular anisotropy field of ~8.0 kOe can be achieved at PO2 = 12% for the sample of tCFAO = 2.1 nm or at PO2 = 7% for tCFAO = 2.8 nm. The loss of PMA at thick tCFAO or high PO2 results mainly from the formation of large amount of CoFe oxides, which are superparamagnetic at room temperature but become hard magnetic at low temperatures. The magnetic CFAO films, with strong PMA in a relatively wide thickness range and small intrinsic damping parameter below 0.028, would find great applications in developing advanced spintronic devices.

Magnetic damping constant in Co-based full heusler alloy epitaxial films

Co-based full-Heusler alloys, such as Co2MnSi and Co2MnGe, are expected to be used as half-metallic ferromagnetic material, which has complete spin polarization. They are the most promising materials for realizing half-metallicity at room temperature due to their high Curie temperature. The optimization of the magnetic damping constant of ferromagnetic materials is extremely important for achieving high-speed magnetization switching and reducing critical current density for spin torque transfer switching. We have systematically investigated the magnetic damping constant in Co-based full Heusler alloy epitaxial films. We found that the Gilbert damping constant seems to be roughly proportional to the total density of states at the Fermi level (EF) by first principle calculation. A very small magnetic damping constant of 0.003 in the Co2Fe0.4Mn0.6Si epitaxial film was demonstrated. The small magnetic damping constant in Co2FexMn1−xSi films with x < 0.6 can be attributed to the half-metallicity of Heusler alloys. Co-based full Heusler alloys with both half-metallicity and small magnetic damping will be very useful for future applications based on spintronic devices.

Effect of Tungsten Seed Layer on Perpendicular Magnetic Anisotropy for Co2FeAl Full-Heusler Half-Metal Based Perpendicular Magnetic Tunnel Junctions

Recently, the perpendicular-spin-transfer-torque magnetic-random-access-memory (p-STT MRAM) with full-heusler alloy based perpendicular-magnetic-tunnel-junctions (p-MTJs) is being researched due to unlimited tunnel magnetoresistance (TMR) ratio, high thermal stability and low power consumption [1]. Half metals with Co-based full-heusler alloys have been expected to be optimal material for p-MTJs with high thermal stability and low damping constant (α) of around 0.001. Co2FeAl (CFA) has the full-heusler alloy composition with high Curie temperature (TC ) of 980oC and stable energy band gap on minority-spin conduction band. Furthermore, the hybridization of electron orbitals makes perpendicular magnetic anisotropy (PMA) characteristic at the interface between CFA and MgO [2]. We studied about the p-MTJs using CFA material with PMA characteristic.In our experiments, all stacks were fabricated on 12-inch thermal oxidized Si substrate and the films of stacks were deposited by ultra-vacuum magnetron sputtering system with a pressure of 10– 8 torr at room temperature. After film deposition process, ex-situ annealing was progressed at a temperature ranging from 275oC to 450oC in a vacuum around 10– 6 torr for 1 hr under 3 Tesla out-of-plane magnetic field.To verify the PMA characteristic of CFA p-MTJs, we fabricated the pseudo-spin-valve (P-SV) stack consists of SiO2 / MgO(2.0) / Pt(2.0) / CFA(0.9) / MgO(1.8) / CFA(0.9) / Pt(2.0) where numbers are nominal thickness in nanometers. For the crystal texturing, the stack was ex-situ annealed at Tex = 450oC. Figure 1(a) shows the schematic structure of P-SV and its magnetic moment versus external magnetic field (M-H) curves. The PMA characteristic of CFA was definitely indicated in out-of-plane M-H curve as shown in Fig. 1(a). Nevertheless the conventional-spin-valve (C-SV) with CFA p-MTJs and [Co/Pd]n multilayer based synthetic anti-ferromagnetic coupled (SyAF) layer [3] has low TMR ratio of 1.7% despite the CFA free layer has PMA characteristic as shown in the inset of Fig. 1(b).For understanding this unexpected result, we analyzed high-resolution transmission-electron-microscopy (HR-TEM) image of the PMA multilayer-stack structure of SiO2 / MgO(2.0) / Pt(2.0) / CFA(0.9) / MgO(0.9) / Pt(2.0) annealed at Tex = 450oC as shown in Fig. 2. To obtain high TMR ratio, MgO tunneling barrier system p-MTJs has to satisfy two important conditions. At first, magnetic anisotropy characteristic is required for maximizing resistance difference between parallel state and anti-parallel state. The next thing is body centered cubic (bcc) (100) crystalline linearity for Δ1 coherence tunneling with MgO tunneling barrier. However Pt seed layer was grown as face centered cubic (fcc) (111) orientation and it was transited to CFA during the processes of sputtering and annealing. As a result, this crystalline linearity disturbs Δ1 coherence tunneling and brings the lowering of TMR ratio.It suggests p-MTJs using bcc (100) crystal structure seed layer to realize the good Δ1 coherence tunneling at CFA. Especially, we used W that can be the good assistant for p-MTJs due to the low resistivity and low diffusivity to MgO tunnel barrier. Figure 3 shows the M-H curves of the PMA multilayer-stacks structure of SiO2 / MgO(1.2) / W(3.0) / CFA(1.2) / MgO(1.2) with different ex-situ annealing temperatures. It was obviously obtained that the PMA characteristic was appeared at an ex-situ annealing temperature of higher than 400oC as shown in Fig. 3(d). It is result of that the large thermal energy is required for CFA crystallization. In other words, it is the greatest merit that CFA material has a large thermal endurance to back-end heat budget of semiconductor fabrication process such as a metallization.In our presentation, we present the effects of W seed layer on CFA crystalline linearity affect to TMR ratio. In addition, we will review the magnetic properties of CFA grown on W seed layer using VSM, XRD and HR-TEM, etc.* This work was supported by the IT R&D program of MOTIE/KEIT [No.10043398 and 10041608] and the Brain Korea 21 plus Project in 2014, Korea.Reference[1] W. Wang et al., Phys. Rev. B 81 140402(R) (2010).[2] Z.Wen et al., Appl. Phys. Lett. 98 242507, (2011).[3] G. Hu et al., IEEE Magn. Lett. 4 3000104 (2013).

Ab initio studies of disorder in the full Heusler alloy Co2FexMn1−xSi

Co-based full Heusler alloys have been predicted to be half-metals, with 100% spin polarisation at the Fermi level, yet this has yet to be realised in practice. Heusler thin-films often exhibit a degree of atomic disorder, and this is believed to be one cause of the low magnetoresistance in Heusler-based spin valves' devices. We present an ab initio density functional theory + U investigation into the effect of disorder on the electronic properties of Co2Fe0.5Mn0.5Si. It is found that the half-metallicity depends strongly on the kind of disorder present, with low or even reversed spin-polarisation when the Co sublattice mixes with either the Fe/Mn or Si sublattice, but keeps the high spin polarisation when the Mn/Fe and Si sublattices are mixed. Calculations of the formation energy show that this latter kind of disorder is by far the most likely to occur, an encouraging result which means that it may not be necessary to produce perfectly ordered Heusler alloys in order to achieve 100% spin polarisation.

Tuning Fermi level within half-metallic gap in Co-based Heusler alloys

We systematically study the tuning of Fermi level in half-metal gap for Co-based full-Heusler alloys in the frame of first-principles calculations. It is found that the position of the Fermi level within whole gap can be specified in compounds Co2MnZ1−x1 Zx2 (Z1 and Z2 are the III, IV or V main group element). As one of examples, Co2MnAl doped by As is further explored in detail, of which the Fermi level falls into the expectative position of the minority spin gap at proper concentration of As doping (0.5 < x < 0.75). This can effectively suppress the so-called spin-flip excitation and promise a good candidate for spintronics application.

Effects of interfacial noncollinear magnetic structures on spin-dependent conductance in Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions: A first-principles study

We investigate the effects of spin-flip scattering on tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) with half-metallic Co${}_{2}$MnSi (CMS) and MgO on the basis of the first-principles calculations. We found that noncollinear magnetic structures of interfacial Co spin moments resulting from the thermal fluctuations cause spin-flip scattering, leading to a significant reduction of the TMR. Interface states originating from a projection of the majority-spin ${\ensuremath{\Delta}}_{1}$ states of CMS in the minority-spin half-metallic gap because of the interfacial noncollinear magnetic structures play an important role in the spin-flip process. From these results, together with an estimated interfacial exchange stiffness constant, we conclude that the TMR ratio at room temperature in MTJs with half-metallic Co-based full-Heusler alloys can be attributed to the spin-flip scattering by the interfacial noncollinear magnetic structures as a result of the thermal fluctuation.