Co2MnSi Layer Research Articles

Micromagnetic study of exchange bias effect in sub-micron dots of Co2MnSi interfaced with uncompensated IrMn

Owing to its crucial role in spintronics devices, the exchange bias (EB) phenomenon has been extensively investigated in various ferromagnet (FM) and antiferromagnet (AFM) bilayers since its discovery in Co/CoO core–shell nanoparticles. In this study, we present the emergence of negative EB for the first time in the Co2MnSi Heusler alloy interfacing with an uncompensated AFM, exhibiting analogous anisotropy to the IrMn. Due to the high pinning and IrMn anisotropy values, EB is stronger here. Investigation into the influence of ferromagnetic layer thickness (tFM) on exchange bias reveals an inverse relationship, while coercivity displays a non-monotonic increase. The analysis of spin canting angles suggests the presence of a maximum canting angle in the Co2MnSi layer close to the interface. We thoroughly analyze the spin configurations at the interface as well as away from it in the Co2MnSi (25 nm)/IrMn (5 nm) bilayer to better understand the mechanism of magnetization reversal. Interestingly, our findings unveiled distinct spin behaviors for the first and second reversals. In cases of small AFM thicknesses (tAFM), the exchange field is proportionate to the tAFM, contrasting with large tAFM, where it scales as 1/tAFM. Notably, coercivity demonstrates an increasing behavior across all tAFM variations. The angular dependence of the Heusler alloy revealed a four-fold symmetry indicative of cubic anisotropy and a two-fold symmetry representative of uniaxial anisotropy. The angular dependency study of exchange bias indicated similar clockwise (CW) and counterclockwise (CCW) rotations, with cos (θ) unidirectional dependence. However, loop shifting revealed that the lower pinning ability at 0° was due to a low Meiklejohn-Bean parameter (R) value. Additionally, through the manipulation of the R-parameter, we can tune the magnitude of the coercive field and EB. All these results are crucial for the utilization of the Co2MnSi/IrMn heterostructures for various applications in spintronics-based devices.

Room-temperature two-terminal magnetoresistance ratio reaching 0.1% in semiconductor-based lateral devices with L21-ordered Co2MnSi

We report on the highest two-terminal magnetoresistance (MR) ratio at room temperature in semiconductor-based lateral spin-valve devices. From first-principles calculations, we predict energetically stable ferromagnet–semiconductor heterointerfaces consisting of Co2MnSi (CMS) and Ge(111) upon insertion of Fe atomic layers. Using low-temperature molecular beam epitaxy, we demonstrate L21-ordered CMS epilayers at 80 °C on Ge(111), where the CMS layer can be utilized as a spin injector and detector. Two-terminal MR ratios as high as 0.1% are achieved in n-Ge-based lateral spin-valve devices with CMS/Fe/Ge Schottky tunnel contacts annealed at 200 °C. This study will open a path for semiconductor-based spintronic devices with a large MR ratio at room temperature.

Transport properties of epitaxial films for superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields

Transport properties were investigated for epitaxial films of superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields up to 17T. The superconducting transition temperature Tc of NbN/Co2MnSi/Au trilayer films was determined to be 16.1K in the absence of magnetic field. Temperature dependence of the resistivity ρ(T) was measured in both magnetic fields parallel and perpendicular to the surface of NbN/Co2MnSi/Au trilayer films. The activation energy U0(H) for vortex motion of the trilayer films in both magnetic fields was well fitted above 5T by the similar model with the exponents in the field dependence of the pinning force density. From the resistivity ρ(T) measurements under high magnetic fields, the upper critical field Hc2(0) at 0K was also deduced to be μ0Hc2∥(0)=23.2T for the parallel magnetic filed and μ0Hc2⊥(0)=15.8T for the perpendicular magnetic field, respectively. The experimental results under magnetic fields revealed the superconductivity of the NbN layer was affected by the interplay between the superconducting NbN layer and the half-metallic Co2MnSi layer.

Spin polarization and magnetic characteristics at C6H6/Co2MnSi(001) spinterface.

Organic materials with mechanical flexibility, low cost, chemical engineering, and long spin lifetime attract considerable attention for building spintronic devices. Here, a C6H6/Co2MnSi(001) spinterface is investigated by first-principles calculations and spin-polarized scanning tunneling microscopy simulations. Several high symmetry adsorption sites are discussed, together with two possible surface terminations of Co2MnSi(001). An inversion of the spin polarization is induced near EF even in the case of an external electric field, indicating that C6H6 can act as a spin filter to exploit the spin injection efficiency in organic spintronic devices. Unlike previous studies on molecule/ferromagnet interfaces, this inversion is closely related to the electronic structure of the atoms in the subsurface layer of Co2MnSi according to the orbital symmetry analysis. Furthermore, the magnetic moment and magnetic anisotropic energy (MAE) in the outermost Co2MnSi layer are studied. Particularly, in the most stable configuration, the sign of MAE is inversed due to hybridization between C p and Co dz2 orbitals, which suggests that a greater modification on MAE can be achieved by the use of a highly chemically reactive organic molecule. These findings improve the study on the engineering of magnetic properties at molecule/ferromagnetic interfaces through a single π-conjugated organic molecule.

Interface formation of epitaxial MgO/Co2MnSi(001) structures: Elemental segregation and oxygen migration

The interface formation in epitaxial MgO/Co2MnSi(001) films was studied using in-situ X-ray photoelectron spectroscopy (XPS). MgO was deposited on single crystal Co2MnSi(001) layers using e-beam evaporation: a technique which is expected to oxidize the Co2MnSi layer somewhat due to the rise in oxygen partial pressure during MgO deposition while leaving the deposited MgO oxygen deficient. Not unexpectedly, we find that e-beam evaporation of MgO raises the oxygen background in the deposition chamber to a level that readily oxidizes the Co2MnSi surface, with oxygen bonding preferentially to Mn and Si over Co. Interestingly, this oxidation causes an elemental segregation, with Mn-Si effectively moving toward the surface, resulting in an MgO/Co2MnSi interface with a composition significantly differing from the original surface of the unoxidized Co2MnSi film. As MgO is deposited on the oxidized Co2MnSi, the Mn-oxides are reduced, while the Si oxide remains, and is only somewhat reduced after additional annealing in ultrahigh vacuum. Annealing after the MgO is grown on Co2MnSi causes oxygen to move away from the oxidized Co2MnSi interface toward the surface and into the MgO. This observation is consistent with an increase in the tunneling magnetoresistance ratio with post-growth annealing measured in fabricated magnetic tunnel junctions (MTJs). The findings are discussed in light of fabrication of MgO/Heusler based MTJs, where the exponential decay of tunneling probability with contact separation exemplifies the importance of the ferromagnet/tunnel barrier interface.

The microstructure and magnetic properties of Co2MnSi thin films deposited on Si substrate

Co2MnSi (CMS) thin films with different thickness were deposited on Si substrate and annealed at different temperatures to investigate the evolution of microstructure and magnetic properties. When Ta was fixed at 500 °C, due to the enhancement of B2-ordering, the increasing of CMS thickness to 100 nm will induce the increasing of Ms to 862 emu/cc. When CMS film thickness was fixed at 75 nm, the abnormal decrease of the Ms value from 782 emu/cc to 614 emu/cc were observed with increasing Ta to 700 °C which may be attributed to the intermixing between CMS layer and Ta layer.

Interfacial contributions to perpendicular magnetic anisotropy in Pd/Co2MnSi/MgO trilayer films

Heusler alloy Co2MnSi is widely selected as the ferromagnetic layer to achieve a giant tunneling magnetic resistance (TMR). It is also one of the most promising materials for potential spintronic applications of magnetic random access memory (MRAM) due to the high spin polarization, in which the configuration of perpendicular magnetic anisotropy (PMA) possesses great advantages over the in-plane ones. Therefore, it is highly desirable to investigate the PMA effects of the Co2MnSi layer with a suitable stack structure. In this work, a strong PMA (1.61 × 106 erg cm−3) is demonstrated in the system of Pd/Co2MnSi/MgO trilayer films. The contributions of the interfaces beside the ferromagnetic Co2MnSi layer were quantitatively clarified. The interfacial anisotropy Ks,MgO of 0.79 erg cm−2 at the Co2MnSi/MgO interface is larger than the Ks,Pd value of 0.26 erg cm−2 at the Pd/Co2MnSi interface. Due to the dual interfacial effects, the strong PMA can be sustained at the high annealing temperature with a thick Co2MnSi layer of about 4.9 nm, which is favorable to the potential spintronic application. The Mn–O bonding was also found to be enriched at the Co2MnSi/MgO interface for the annealed Pd/Co2MnSi (3.4 nm)/MgO film with the large PMA, showing an experimental evidence for the theoretical results of the Mn–O bonding contribution to PMA.

Impact of CoFe buffer layers on the structural and electronic properties of the Co2MnSi/MgO interface

The latest improvement of MgO-based magnetic tunnel junctions has been achieved by the combination of CoFe buffer layers and potentially half-metallic ultrathin Co2MnSi electrodes. By this, tunnel magnetoresistance ratios of almost 2000% could be obtained. However, a complete understanding of the underlying processes leading to this enhancement is not yet given. We present a comprehensive study regarding the structural and electronic spin properties of the CoFe(30 nm)-buffered Co2MnSi(3 nm)/MgO(2 nm) buried interface identical to the one formed in actual devices. Low energy electron diffraction experiments show that the ultrathin Co2MnSi layer adopts the lattice constant of the underlying CoFe buffer layer, leading to improved structural conditions at the interface to MgO. In contrast, the Co2MnSi/MgO interface spin polarization at the Fermi level is not affected by the magnetic CoFe buffer layer, as found by interface-sensitive spin-resolved extremely low energy photoemission spectroscopy.

Magnetic anisotropy of [Co2MnSi/Pd]n superlattice films prepared on MgO(001), (110), and (111) substrates

Superlattice films with full-Heusler Co2MnSi (CMS) alloy and Pd layers prepared on Pd-buffered MgO(001), (110), and (111) substrates were investigated. Crystal orientation and epitaxial relationship of Pd and CMS layers were analyzed from x-ray diffraction, pole figure measurements, and transmission electron microscope observation. Formation of the L21-ordered structure in the CMS layers was confirmed by observation of CMS(111) diffraction. Perpendicular magnetic anisotropy (PMA) was obtained in the [CMS (0.6 nm)/Pd (2 nm)]6 superlattice film formed using MgO(111) substrates although other superlattice films prepared using MgO(001) and (110) substrates showed in-plane and isotropic magnetic anisotropy, respectively. The perpendicular magnetic anisotropy energy constant K for the superlattice films prepared using MgO(111) substrate was estimated to be 2.3 Mergs/cm3, and an interfacial anisotropy constant Ki per one CMS-Pd interface in the superlattice films was estimated to be 0.16 ergs/cm2. Ki in superlattice films with various crystal orientations showed positive values, indicating that Pd/CMS interfaces had an ability to induce PMA regardless of their crystal orientation.

Structural, electronic and magnetic properties of Co2MnSi/Ag(1 0 0) interface

The performance of advanced current-perpendicular-to-plane giant magnetoresistance (MR) built of ferromagnetic (FM) electrodes and nonmagnetic metals as a spacer depends decisively on the properties of the FM/metal interface. Here we investigate the interface character between Co2MnSi and Ag by using the first-principles density functional simulations. To simulate the actual cases, we build two types of interfaces, namely tO and bO interfaces by connecting the most favorable MnSi-termination of nine ordered Co2MnSi layers to the top of Ag and the bridge site between two Ag atoms of seven atomic layers, respectively. Our calculations indicate that the bO interface is more probable to form in the growth than the tO interface due to the lower formation energy and interface free energy, and the interface states occur in the minority-spin gap, leading to the destruction of half-metallicity of the ordered Co2MnSi. Further, taking into account the atomic disorder of the Co2MnSi layers near the interface, we show that the A2 disorder is most likely to occur in the layers close to the interface due to the lower formation energy and interface free energy, and the interface states occurring in the spin-minority gap are strengthened and the spin polarization is further degraded by the A2 disorder of Co2MnSi layers. Therefore, the spin-polarization reduction induced by interface itself and the disorder of Co2MnSi layers close to the interface, together with the interface roughness indicated by the calculations, may be main causes of very low MR ratio.

Spin polarized carrier injection from full Heusler alloy Co2MnSi into superconducting NbN

The spin-polarized current injection (SPCI) experiments, where the spin-polarized current (IS) is injected into a 25 μm wide superconducting NbN bridge from a ferromagnetic Co2MnSi layer by tunneling through MgO barrier, show a large (≈67%) suppression of superconducting critical current (IC) due to IS injection at T/TC ≈ 0.4. This corresponds to a large dynamic gain of ≈36 at 3 K, which is 40 times higher than the gain at 6.5 K. Such a rapid rise of gain at lower temperatures strongly suggests a dominant role of SPCI in IC suppression as compared to the effect of Joule-heating.

High power radio frequency oscillation by spin transfer torque in a Co2MnSi layer: Experiment and macrospin simulation

We experimentally and numerically investigated rf oscillation induced by spin transfer torque in a current-perpendicular-to-plane giant magnetoresistance (GMR) device with full-Heusler Co2MnSi (CMS) layers. High output power (Prf) of 1 nW was experimentally achieved owing to the large GMR effect resulting from the half-metallic feature of the CMS layers. However, the high power rf oscillation was observed only in the narrow dc current (Idc) region. Macrospin simulation suggested that the high spin polarization of CMS layers led to narrowing the optimum Idc region for the rf oscillation.

High-power rf oscillation induced in half-metallic Co2MnSi layer by spin-transfer torque

The rf oscillation induced in a current-perpendicular-to-plane device with Co2MnSi (CMS) layers by spin-transfer torque was investigated to enhance the rf output power due to the large magnetoresistance (MR) ratio. A large MR ratio of 12.5% was obtained due to the large spin-polarization of CMS, and fundamental and second harmonic rf oscillations were clearly observed in the CMS layer. A high rf output power of 1.1 nW was achieved in spite of a small precession angle of 8.6°.

Spin pumping efficiency from half metallic Co2MnSi

We present spin pumping using a Heusler alloy Co2MnSi/Pt bilayer film. A spin current is produced by a ferromagnetic resonance (FMR) technique. The pure spin current injected into the Pt layer from the Co2MnSi layer is detected by the inverse spin-Hall effect (ISHE), which converts the spin current into an electric current. We estimated a damping constant of the Co2MnSi/Pt bilayer film from an angular dependence of FMR spectra. Using the damping constant efficiency of spin pumping from the Co2MnSi layer is evaluated. We found that a mixing conductance at the Co2MnSi/Pt interface is comparable to that at a permalloy/Pt interface.

Preparation of Highly-Oriented Co2MnSi Films on a Non-Single-Crystalline Substrate Using a Titanium–Nitride Buffer Layer

Highly-oriented titanium–nitride films were prepared on a non-single-crystalline substrate as a buffer layer of a Co2MnSi film. Co2MnSi films prepared on the titanium–nitride buffer layer showed a high (001)-orientation and at least a B2-ordered phase even for room temperature preparation. The magnetization curve of the Co2MnSi film annealed at 600 °C showed an extremely low coercivity of 1.4 Oe in addition to a high saturation magnetization, which indicated no interdiffusion between a Co2MnSi layer and a buffer layer. These results indicate that titanium nitride is a potential material for applications based on Co2MnSi.

Structure and transport properties of current-perpendicular-to-plane spin valves using Co2FeAl0.5Si0.5 and Co2MnSi Heusler alloy electrodes

We report the structure and transport properties of current-perpendicular-to-plane spin valves (CPP SVs) with Co2FeAl0.5Si0.5 (CFAS) or Co2MnSi (CMS) Heusler alloy magnetic layers and an Ag or Cu spacer layer. A multilayer stack of sub/Cr/Ag/Heusler/(Ag or Cu)/ Heusler/Co75Fe25/Ir22Mn78/Ru was deposited on a MgO(001) single crystalline substrate by magnetron sputtering. Transmission electron microscopy observations showed epitaxial growth from the substrate to the top Heusler layer. The CPP SV with a CFAS/Ag/CFAS trilayer showed relatively large magnetoresistance (MR) ratios of 12% at room temperature and 31% at 12 K, with monotonous temperature dependence. However, the MR values of the SV with the CMS/Ag/CMS trilayer showed a different temperature dependence with a maximum value of 22% at 100 K. This might be related to the 90° couplings between the two CMS layers.

Oscillatory interlayer exchange coupling in epitaxial Co2MnSi∕Cr∕Co2MnSi trilayers

Interlayer exchange coupling (IEC) in trilayers, which consist of a full Heusler Co2MnSi (CMS) phase as ferromagnetic layers separated by a Cr spacer layer, has been investigated. The shape of magnetization loops shows unusual oscillatory behavior with the thickness of Cr. The oscillation period is about 3.3–3.5nm. The charecteristics of magnetization curves show that 90° coupling plays a dominant role in IEC between CMS layers. Moreover, the strength of 90° coupling turns out to be very high (up to −1.85ergs∕cm2) around the first oscillation peak.

The structure of sputter-deposited Co2MnSi thin films deposited on GaAs(001)

The structure of Co2MnSi thin films on GaAs(001) has been characterized by transmission electron microscopy in order to evaluate the feasibility of achieving spin injection into GaAs from such electrodes. The films were dc-magnetron sputtered and varied in thickness between 15 and 260 nm with substrate temperatures during growth of 250, 300, and 374 °C. All films exhibited a polycrystalline structure with mainly an L21 type crystallographic symmetry, and a high degree of preferred orientation with the GaAs. A reaction with the GaAs substrate, rich in Mn and As, occurs for deposition even of the 15 nm thick film, creating zones that exhibit an epitaxial relation with the substrate. Between this reaction zone and the film, a continuous interlayer forms, which is rich in Ga, and several nanometers thick. Films thicker than 35 nm were found to be stoichiometric in chemical composition, while thinner films were deficient in Mn and richer in Si. Decreasing the substrate temperature resulted in reduction of the extent of the reaction with the substrate, but also reduced the crystallographic ordering of the Co2MnSi layer. Finally, both kinematic and dynamic simulations of selected-area electron diffraction patterns demonstrate that this technique may not be a sensitive methodology to detect Co-Mn antisite defects and off-stoichiometry compositions. These defects may be responsible for the approximately 55% spin polarization measured in these films, rather than the full spin polarization expected from this theoretically predicted half-metal.

Ｃｏ２ＭｎＳｉ（１１０）エピタキシャル薄膜を用いた強磁性トンネル接合の作製

According to Julliere’s model, magnetic tunnel junctions (MTJs) with half-metallic electrodes lead to a large tunnel magnetoresistance (TMR) ratio. A Co2MnSi Heusler alloy is theoretically expected to exhibit half-metallicity. We fabricated (110)-oriented epitaxial Co2MnSi electrodes on sapphire substrates using W and Ta/W/Cr buffer layers. With the W buffer layer, we found that the W and Co2MnSi layers formed a twin structure. However, with the Ta/W/Cr multi-buffer layers, we succeeded in fabricating a high-quality Co2MnSi(110) epitaxial electrode. We fabricated a MTJ with the high-quality Co2MnSi(110) electrode and investigated TMR effects in the MTJ. As a result, we observed a TMR ratio of about 40% at room temperature and 120% at 2 K.

Co 2 Mn Si Heusler alloy as magnetic electrodes in magnetic tunnel junctions

As a consequence of the growing theoretical predictions of 100% spin-polarized half- and full-Heusler compounds over the past six years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated Co2MnSi, as a representative of the full-Heusler compound family, as one magnetic electrode into magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co2MnSi at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer, subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co2MnSi, and (3) homogenizes the AlOx barrier. It is shown that the magnetic switching of the ferromagnetic electrodes is well controlled from room temperature down to 10K. The resulting tunnel magnetoresistance-effect amplitude of the Co2MnSi containing magnetic tunnel junctions has been determined as a function of temperature and the spin polarization of the Co2MnSi Heusler compound has been estimated to be 61% at 10K. Thus, the spin polarization of the Co2MnSi layer at 10K exceeds that of conventional transition metals.