Co2FeSi Heusler Alloy Research Articles

Strain-induced specific orbital control in a Heusler alloy-based interfacial multiferroics

For the development of spintronic devices, the control of magnetization by a low electric field is necessary. The microscopic origin of manipulating spins relies on the control of orbital magnetic moments (morb) by strain; this is essential for the high performance magnetoelectric (ME) effect. Herein, electric-field induced X-ray magnetic circular dichroism (XMCD) is used to determine the changes in morb by piezoelectric strain and clarify the relationship between the strain and morb in an interfacial multiferroics system with a significant ME effect; the system consists of the Heusler alloy Co2FeSi on a ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 substrate. Element-specific investigations of the orbital states by operando XMCD and the local environment via extended X-ray absorption fine structure (EXAFS) analysis show that the modulation of only the Fe sites in Co2FeSi primarily contributes to the giant ME effect. The density functional theory calculations corroborate this finding, and the growth of the high index (422) plane in Co2FeSi results in a giant ME effect. These findings elucidate the element-specific orbital control using reversible strain, called the ‘orbital elastic effect,’ and can provide guidelines for material designs with a giant ME effect.

Patterns of Grain Fragmentation During Plastic Deformation of Metals at Small to Medium Strains (Brief Review)

Due to high carrier mobility and long spin diffusion length graphene is a promising material for spintronics applications. In order to achieve effective spin transport and increase spin injection efficiency, graphene interfaces with highly spin-polarised materials, such as Heusler alloys, are needed. In this work, first-principles calculations of graphene/Co2FeSi electronic structure are done in the frame of density functional theory. It is shown that the high percent of spin polarization in this system is combined with the linear dispersion of the π-states of graphene. The results suggest that the Co2FeSi Heusler alloy is a promising candidate for graphene-based spintronic devices.

Surface Morphology and Exchange Bias Anisotropy Studies in Large Area Deposited Co2FeSi Ir50Mn50 Multi Layers For Spintronic Applications

Surface morphology and magnetic properties of ferromagnetic Heusler alloy Co2FeSi thin films and their multi-layers with anti-ferromagnetic Ir50Mn50, which find applications in spintronic devices were investigated. The sputtering process flow for large area deposition of thin films on 3 inch size thermally oxidized single crystal Si(100)/SiO2 substrates have been developed by optimizing the sputtering geometry and other process parameters. A uniform film composition, thickness, smooth surface, good crystallinity and magnetic properties have been achieved in the films over 3-inch size wafers. The isotropic magnetic properties such as saturation/remanent magnetizations, coercivity were achieved in Co2FeSi films deposited on 3-inch size Si(100)/SiO2 wafers with 15 nm Cr buffer layer. An exchange bias anisotropy has been established in Co2FeSi/IrMn multilayer by magnetic annealing process using in-house made magnetic annealing set up. A maximum exchange bias anisotropy field, Hex of 178 Oe and low coercivity, Hc of 85 Oe has been achieved in the Co2FeSi/IrMn multilayer stacks suitable for magnetic tunnel junctions for spintronic applications.

Growth temperature-controlled Gilbert damping and anisotropies in PLD grown epitaxial Co2FeSi Heusler alloy thin film

The utilization of spin rather than charge as a functional aspect in the electronic devices will result in an energy-efficient, thermally stable, and fast operation. Cobalt-based Heusler alloy Co2FeSi (CFS) offers excellent possibilities due to its high curie temperature, large spin polarization, and low Gilbert damping (α). Small damping makes the alloy fascinating for spin transfer torque magnetoresistive random-access memory (STT-MRAM) devices where a low α causes a low value of switching current. The present investigation explored the effect of deposition temperature on the chemical, structural and magnetic properties of the pulsed laser deposition grown single-crystal CFS film of thickness 50 nm on a lattice-matched (001) MgO substrate. The XRD data suggest that Co site atomic ordering and overall film crystallinity improved with temperature, resulting in a minimal value of α obtained from ferromagnetic resonance (FMR) experiment in the film grown at 600°C. The azimuthal angular variation (φ) of FMR field data gives a fourfold anisotropy pattern that confirms the single-crystal CFS film epitaxially grown on the MgO substrate. Two in-plane anisotropies, namely cubic (four-fold) and uniaxial, were obtained in all the samples. This investigation is crucial in the fabrication of fast spintronics based memory and processing devices.

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.

Effect of Mn Doping on the Electronic and Magnetic Properties of Full Heusler Alloy Co2FeSi

Heusler compounds are one of the largest families of ternary intermetallics with broad range of applications. In this work, we inspected the electronic and magnetic properties of full Heusler alloy Co2FeSi and the effect induced by doping Mn atom on those properties. We employed plane wave pseudo-potential method based on DFT framework implemented in Quantum ESPRESSO code using PBE type of functional for exchange correlation energy. The calculation demonstrates that Co2FeSi exhibits metallic nature in the majority spin state and semi-conducting nature in minority spin state with indirect energy gap of ≈ 0.132 eV and shows chances of weakened half-metallicity as indicated by the presence of band gap below fermi level. The calculated total magnetic moment is 5.44 μB/cell which is found to be deviated from Slater-Pauling rule M = NV − 24 for full Heusler compounds. The site selection for doping was based on the empirical rule for atomic occupation established for Heusler alloys and the calculation of formation energy. The calculation of formation energy at B and D site showed B site as more favorable for doping for both 25% and 50% concentration. At 25% Mn concentration, calculation shows Co2FeSi has typical metallic behavior for spin-up electrons, but semi-conducting behavior with decreased gap below fermi level for spin-down electrons showing no half-metallicity whereas for 50% Mn concentration the gap gets broadened by 0.089645 eV. The calculated magnetic moment for 25% and 50% Mn doping is 21.93 μB/cell and 21.69 μB/cell respectively. Our calculation depicts that the chances of half-metallicity decreases for 25% concentration of Mn atom and increases for 50% Mn concentration.

Anomalous magnetic behavior in half-metallic Heusler Co2FeSi alloy glass-coated microwires with high Curie temperature

In the current study, we have succeeded to fabricate Co2FeSi Heusler alloy glass-covered microwires with magnetic core nucleus diameter d = 4.36 µm and total diameter D = 17.55 µm, with high Curie temperature (Tc>1100 K) and well-defined magnetic anisotropy for high temperature spintronic devices application. The magnetic properties of as-prepared and annealed at different temperature (873 K, 973 K and 1073 K for 1 h) of Co2FeSi Heusler alloy glass-covered microwires have been investigated. Strong dependence of the magnetic properties on the annealing conditions has been indicated. Anomalous magnetic behavior for annealed samples at 873 K and 973 K has been found and structural properties of such samples have been analyzed. Critical temperatures 155 K and 250 K have been detected for annealed samples at 873 K and 973 K, respectively, where the behavior of M-H loops and coercivity changed. Below the critical point the M-H curve shows “kink or wasp-waisted” magnetic behavior and complex magnetic reversal mechanism is supposed. The anomalous magnetic behavior is due to the martensitic phases induced by annealing conditions below the room temperature. This unusual magnetization behavior provides opportunities to understand the phenomena of different types of magnetic domain structures in the preferred crystallographically oriented Co2FeSi Heusler alloy glass-coated micro-wires, essentially helpful for designing the devices based on magnetization switching.

Longitudinal fluctuations of Co spin moments and their impact on the Curie temperature of the Heusler alloy Co2FeSi

The magnetism of the full Heusler alloy Co2FeSi with its high magnetic ordering temperature is studied on a first-principles basis employing the disordered local moment approximation, the magnetic force theorem and single-site spin fluctuation theory as formulated recently in the framework of the Local Spin Density Approximation. We find that the magnetic moments of Fe and Co in Co2FeSi exhibit a quite distinctive behavior at high temperatures. The Fe moments are well localized and keep their magnitude unchanged with temperature, whereas the Co moments are itinerant and show a temperature dependence. We find that the effects of magnetic disorder strongly renormalize the Fe-Co inter-atomic exchange interactions. Our results suggest a deficiency of the classical Heisenberg model with rigid localized atomic spin moments for the description of magnetism in Co2FeSi. An accurate estimation of the Curie temperature is obtained by taking into the account the thermal longitudinal fluctuations the Co moments.

Fabrication and Magneto-Structural Properties of Co2-Based Heusler Alloy Glass-Coated Microwires with High Curie Temperature

In this work, we were able to produce Co2FeSi Heusler alloy glass-covered microwires with a metallic nucleus diameter of about 4.4 µm and total sample diameter of about 17.6 μm by the Taylor–Ulitovsky Technique. This low cost and single step fabrication process allowed the preparation of up to kilometers long glass-coated microwires starting from a few grams of high purity inexpensive elements (Co, Fe and Si), for a wide range of applications. From the X-ray diffraction, XRD, analysis of the metallic nucleus, it was shown that the structure consists of a mixture of crystalline and amorphous phases. The single and wide crystalline peak was attributed to a L21 crystalline structure (5.640 Å), with a possible B2 disorder. In addition, nanocrystalline structure with an average grain size, Dg = 17.8 nm, and crystalline phase content of about 52% was obtained. The magnetic measurements indicated a well-defined magnetic anisotropy for all ranges of temperature. Moreover, soft magnetic behavior was observed for the temperature measuring range of 5–1000 K. Strong dependence of the magnetic properties on the applied magnetic field and temperature was observed. Zero field cooling and field cooling magnetization curves showed large irreversibility magnetic behavior with a blocking temperature (TB = 205 K). The in-plane magnetization remanence and coercivity showed quite different behavior with temperature, due to the existence of different magnetic phases induced from the internal stress created by the glass-coated layer. Moreover, a high Curie temperature was reported (Tc ≈ 1059 K), which predisposes this material to being a suitable candidate for high temperature spintronic applications.

An investigation on the novel granular composite of Co2FeSi Heusler alloy with Mn and Al2O3: Structural and magnetic properties

The magnetic-field-induced resistance switching of a granular composite containing Co 2 FeSi with Mn and Al 2 O 3 was investigated for the first time. Field emission scanning electron microscopy (FESEM) images showed a mean particle size of about 100–150 nm. Different compositions of materials showed different Magnetoresistance (MR). Positive and negative MR responses were observed in the samples while the possible mechanisms were also discussed. The electric resistance of the samples increased by the Al 2 O 3 amount and tunneling became the dominant electron drift mechanism. The electron conductance percolation threshold of the composite was found to be around 20 to 30 wt% Al 2 O 3 in which the MR reached a maximum value. This paper is part of a series of systematic investigations on these novel composites publishing for the first time. • The sharp resistance increase at Al 2 O 3 ratios shows the behavior of the sample changes from metal to insulator. • This research indicates the effect of initial magnetization on the MR behavior of the granular composites. • This investigation can pave the way for the development of granular magnetoresistive elements with novel composites.

Giant converse magnetoelectric effect in a multiferroic heterostructure with polycrystalline Co2FeSi

To overcome a bottleneck in spintronic applications such as those of ultralow-power magnetoresistive random-access memory devices, the electric-field control of magnetization vectors in ferromagnetic electrodes has shown much promise. Here, we show the giant converse magnetoelectric (CME) effect in a multiferroic heterostructure consisting of the ferromagnetic Heusler alloy Co2FeSi and ferroelectric-oxide Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) for electric-field control of magnetization vectors. Using an in-plane uniaxial magnetic anisotropy of polycrystalline Co2FeSi film grown on PMN-PT(011), the nonvolatile and repeatable magnetization vector switchings in remanent states are demonstrated. The CME coupling coefficient of the polycrystalline Co2FeSi/PMN-PT(011) is over 1.0 × 10−5 s/m at room temperature, comparable to those of single-crystalline Fe1-xGax/PMN-PT systems. The giant CME effect has been demonstrated by the strain-induced variation in the magnetic anisotropy energy of Co2FeSi with an L21-ordered structure. This approach can lead to a new solution to the reduction in the write power in spintronic memory architectures at room temperature.

Spin–orbit torque magnetization switching in a perpendicularly magnetized full Heusler alloy Co2FeSi

To optimize the writing and reading performance of magnetic random-access memory (MRAM) devices, achieving current-induced spin–orbit torque (SOT) magnetization switching in perpendicularly magnetized full Heusler alloys is vitally important. For conventional SOT-metal bilayer systems, heavy metals (HMs) with a large spin Hall angle (θSH) are generally used for generating a spin current, which is injected into the adjacent ferromagnet (FM) layer and exerts a torque on the magnetization to switch it. However, the large resistivity of generally used HMs such as β-Ta and β-W can increase the Ohmic loss. In this article, we achieve full SOT switching in Heusler alloy Co2FeSi using low-resistivity Pd as a spin current generation source. The critical switching current density is found to be 3.7 × 107 A cm−2, which is in the same order of magnitude as that required for conventional HM/FM systems even though Pd has a smaller θSH than that of generally used HMs. Using harmonic Hall measurements, the damping-like and field-like effective fields per unit current density are estimated to be 56.9 (10−7 Oe A−1 cm2) and 39.8 (10−7 Oe A−1 cm2), respectively. This high efficiency can be attributed to the excellent lattice matching between Co2FeSi and Pd (only 2% mismatch), to a slight Pd diffusion, and possibly to the additional SOTs induced by the in-plane spin component generated in the Co2FeSi layer. Our finding will advance the development of SOT-MRAM devices with both better reading and writing performance.

Asymmetric magnetization reversal of the Heusler alloy Co2FeSi as free layer in an CoFeB/MgO/Co2FeSi magnetic tunnel junction

We report the in-plane magnetization reversal behavior of the Co2FeSi Heusler alloy free layer in a bottom-pinned magnetic tunnel junction film with 149% TMR. Using magneto-optic indicator film imaging, we visualize the magnetic domain dynamics of this buried layer integrated within a full magnetic tunnel junction stack. The magnetic domain dynamics reveal anisotropic magnetization reversal within Co2FeSi under applied in-plane magnetic fields. While the reversal behavior under in-plane fields applied perpendicular to the in-plane exchange bias direction indicates smooth, coherent rotation away from the easy axis, asymmetric magnetic reversal behavior is observed along the easy axis. Reversed domains propagate from the interior of the film laterally outward toward the edges as the free layer magnetization switches into parallel alignment with the pinned synthetic antiferromagnetic reference layer (IrMn/CoFe/Ru/CoFeB). On the other hand, domains propagate from the film edges inward for the free layer transition into antiparallel alignment with the reference layer. These results have important implications for Heusler magnetic tunnel junction device performance.

Magneto-optical Kerr microscopy investigation of magnetization reversal in Co2FeSi Heusler alloy thin films

Magneto-optical Kerr effect microscopy investigation of magnetocrystalline anisotropy and the magnetization reversal (MR) process in Co2FeSi (CFS) Heusler alloy thin films reveals the following: Regardless of the strength of the anti-site atomic disorder, all CFS films of fixed thickness (t = 50 nm) exhibit variations in the remanent magnetization (Mr) and coercive field (Hc) with the “in-plane” magnetic field (H) angle, φH, that are characteristic of the “in-plane” uniaxial anisotropy (UA) with an easy axis along φH = 0°. The observed variations with the field-angle are well described by the two-phase pinning (TP) model. Exceptions to this rule are the films with t ≥ 25 nm, having a maximum B2 atomic order, which show completely different angular variations in Mr and Hc. In such exceptional cases, we find that the TP model reproduces Mr(φH) and Hc(φH) only when it takes into account two mutually exclusive UAs, UA1 and UA2, with easy axes perpendicular to one another and UA1 ≫ UA2. When H points along the easy axis (φH = 0°), MR in all CFS films proceeds through the nucleation of reverse domains and their subsequent growth by domain wall movement as H increases. Atomic disorder has essentially no effect on the MR process, but the domain wall pinning at defects/imperfections affects the magnitudes of Hc and the UA field, Hk. At φH ≃ 45°, MR takes place through the formation of ripple domains. As H increases, the reverse domains grow at the expense of ripple domains, and a single domain configuration is established at high fields. When φH = 90°, MR in all films involves nucleation and subsequent growth of reverse domains by the field-induced movement of the 180° domain walls.

Uniaxial anisotropy, intrinsic and extrinsic damping in Co2FeSi Heusler alloy thin films

Ferromagnetic resonance (FMR) technique has been used to study the magnetization relaxation processes and magnetic anisotropy in two different series of the Co2FeSi (CFS) Heusler alloy thin films, deposited on the Si (1 1 1) substrate by ultra high vacuum magnetron sputtering. While the CFS films of fixed (50 nm) thickness, deposited at different substrate temperatures (TS) ranging from room temperature (RT) to 600°C, constitute the series-I, the CFS films with thickness t varying from 12 nm to 100 nm and deposited at 550°C make up the series-II. In series-I, the CFS films deposited at TS = RT and 200°C are completely amorphous, the one at C is partially crystalline, and those at C, 550°C and 600°C are completely crystalline with B2 order. By contrast, all the CFS films in series-II are in the fully-developed B2 crystalline state. Irrespective of the strength of disorder and film thickness, angular variation of the resonance field in the film plane unambiguously establishes the presence of global ‘in-plane’ uniaxial anisotropy. The uniaxial anisotropy field decreases as the crystalline order in the films increases and goes through a minimum at t = 50 nm as a function of film thickness. Landau–Lifshitz–Gilbert damping and two-magnon scattering dominantly contribute to the line-broadening in both ‘in-plane’ and ‘out-of-plane’ configurations. The two-magnon scattering has larger magnitude in the amorphous films than in the crystalline ones. Angular variation of the linewidth in the film plane reveals that, in the CFS thin films of varying thickness, a crossover from the ‘in-plane’ local four-fold symmetry (cubic anisotropy) to local two-fold symmetry (uniaxial anisotropy) occurs as t exceeds 50 nm. Gilbert damping constant decreases monotonously from 0.047 to 0.0078 with decreasing disorder strength (increasing TS) and jumps from 0.008 for the CFS film with t = 50 nm to 0.024 for the film with t = 75 nm. Such variations of with TS and t are understood in terms of the changes in the total (spin-up and spin-down) density of states at the Fermi level caused by the disorder and film thickness. Spin pumping across the Co2FeSi film/Ta cap-layer interface makes negligible contribution to . We propose that disorder and/or the film thickness can be used as control parameters to tune , whose value is decisive in choosing a ferromagnetic film for a given spintronics application.

Manufacturing of Sputtering Composite Targets Containing Phases of Co2FeSi or Co2MnSi Heusler Alloy

The paper presents a method for manufacturing mechanically strong sputtering composite targets containing the phase of the Co2FeSi or Co2MnSi Heusler alloy of the stoichiometric composition, which can be used for fabrication of spin electronic devices by high-frequency magnetron deposition and pulsed laser deposition of thin films.

Enhanced current-perpendicular-to-plane giant magnetoresistance by improvement of atomic order of Co2FeSi Heusler alloy film through Ag doping

We investigated the effect of Ag doping on atomic order and spin polarization of Co2FeSi Heusler alloy thin films. The Co2FeSi film doped with 3at.%-Ag showed improved degree of B2- and L21-ordering after thermal annealing comparing to Co2FeSi film. Negative sign of anisotropic magnetoresistance suggests the half-metallic nature of Ag-doped Co2FeSi film after annealing at a relatively low temperature of 300 °C. The current-perpendicular-to-plane pseudo spin-valves using the Ag-doped Co2FeSi ferromagnetic (FM) electrodes showed about 1.2 times larger MR ratio than that using the Co2FeSi FM electrodes, reflecting an enhanced spin polarization in Ag-doped Co2FeSi film. Micro-structure analysis revealed the doped Ag was rejected out of Co2FeSi layer after annealing. This study suggests Ag doping to Co-based Heusler compound having no/few solubility with Ag is beneficial for realizing spin-valves with high output at moderate annealing conditions.

Epitaxial contact Andreev reflection spectroscopy of NbN/Co2FeSi layered devices

We investigated the spin polarization P of Co-based Heusler alloy Co2FeSi by epitaxial contact Andreev reflection (ECAR) spectroscopy using epitaxially grown superconductor NbN and Heusler alloy Co2FeSi layered devices. Ferromagnetic Co2FeSi possesses the highest Curie temperature (TC ≈ 1100 K) and the largest spontaneous magnetic moment (ps ≈ 6 μB) in the class of Heusler alloys. The ECAR measurements revealed that the P value of Co2FeSi was 54 ± 2% with a finite barrier parameter Z, indicating that an intrinsic P value in ECAR spectroscopy would exceed reported values in point-contact Andreev reflection spectroscopy. We therefore established not only the epitaxial integration of ferromagnetic Co2FeSi with superconductor NbN on an MgO substrate but also the fabrication and evaluation techniques of their ECAR devices. This highly versatile superconducting spintronic system enables fundamental superconducting spintronic studies, and it is also a candidate for practical superconducting spintronic devices.

Correlations Between Structural and Magnetic Properties of Co2FeSi Heusler-Alloy Thin Films

The structural and magnetic properties of the full Heusler alloy Co2FeSi are its key features for practical applications, but the exact structure-function relationship and underlying mechanism still remain unclear. In this detailed study, experiments and simulations clarify the precise correlations between magnetic parameters and the degree of structural ordering. Enhancement of both biaxial anisotropy and atomic ordering are realized with increasing substrate temperature. These findings are helpful for designing advanced spintronic devices with controllable magnetic anisotropy.

Enhancement of L21 order and spin-polarization in Co2FeSi thin film by substitution of Fe with Ti

We investigated the effect of Ti substitution for Fe in Co2FeSi Heusler alloy (Co2Fe1-xTixSi) on their electronic structure, chemical ordering, and spin-dependent transport properties. First-principles calculations of the density of states (DOS) indicated that the peak just above the Fermi level in the minority-spin DOS (the conduction band edge of half-metallic gap) shifts towards higher energies with increasing Ti, leading to the enhancement in the half-metallic gap and the spin-polarization. We found in epitaxial Co2(Fe1-xTix)Si thin films that the required annealing temperature for long range L21-ordering can be substantially reduced from 650 °C for Co2FeSi to 400 °C for x ≥ 0.2. The enhancement of spin-polarization by the substitution of Fe with Ti was experimentally confirmed from anisotropic magnetoresistance measurements and spin-accumulation signals in non-local spin valve devices with the Co2Fe1-xTixSi films.