arXiv subjects

Takahide Kubota

Publications and source records attributed to Takahide Kubota.

15 recordsLinked to original sources

arXiv2025

Large Spin Hall Effect in High-Entropy Alloy/CoFeB Bilayers

High-entropy alloys (HEAs) exhibit various physical properties, such as high microhardness for structured materials and high efficiency for catalysis. These features are recognized as a cocktail effect of five or more elements that stabilize a single-phase solid solution due to a high configurational entropy. HEAs may also exhibit short-range orders and microdistorshons, which cause the local symmetry breaking of systems. Systems with local symmetry breaking are of interest for spin-dependent transport, such as for spin Hall effects. In this study, sputtered-film samples of an HEA, Mn--Nb--Mo--Ta--W, and related alloys, were fabricated; these films were layered with ferromagnetic CoFeB. All samples exhibited x-ray diffraction peaks originating only from a body-centered cubic (bcc) phase, and transmission electron microscopy images indicated the absence of secondary phases and uniform elemental distributions. The spin Hall magnetoresistance (SMR) was investigated, and clear resistance changes were observed in all the samples. Quantitative analysis of SMR revealed that the spin Hall angle ($θ_\mathrm{SH}$) was 0.12 $\pm$ 0.002 and 0.14 $\pm$ 0.037 for the HEA and Nb--Mo--Ta--W alloy (medium-entropy alloy (MEA)), respectively. The $θ_\mathrm{SH}$s of the HEA and MEA were comparable to that of Pt which is a typical heavy element with relatively large $θ_\mathrm{SH}$. The newly developed HEA and MEA films are attractive spin Hall materials with the bcc phase and are suitable for spintronic applications using magnetic tunnel junctions with CoFeB and an MgO barrier.

cond-mat.mtrl-sci↗

arXiv2023

Perpendicular magnetic anisotropy of an ultrathin Fe layer grown on NiO(001)

The magnetic anisotropy and magnetic interactions at the interface between Fe and NiO(001) were investigated. Depending on the growth conditions of the NiO(001) layers and the post-annealing temperature, the preferential magnetization direction of the ultrathin Fe layer grown on a NiO(001) layer changed from in-plane to a direction perpendicular to the film plane. The lattice constant of the NiO(001) layers parallel to the growth direction increased with O$_2$ flow rate, while that parallel to the in-plane were locked onto the MgO(001) substrate regardless of the growth conditions of the NiO layers. Moreover, perpendicular magnetization was observed only when the NiO layer was grown with O$_2$ flow rates higher than 2.0 sccm corresponding to oxygen-rich NiO. X-ray magnetic circular dichroism measurements revealed an enhancement in anisotropic orbital magnetic moments similar to the origin of perpendicular magnetic anisotropy at the Fe/MgO(001) interface. The interfacial magnetic anisotropy energies were 0.93 and 1.02 mJ/m$^2$ at room temperature and at 100 K, respectively, indicating less temperature dependence. In contrast, the coercivity $H_c$ exhibited a significant temperature dependence. Although no signature of exchange bias or unidirectional loop shift was observed, $H_c$ was strongly dependent on the NiO layer thickness, indicating that the exchange interaction at the interface between the ferromagnetic and antiferromagnetic layers was not negligible, despite the NiO(001) being a spin-compensated surface.

cond-mat.mtrl-sci↗

arXiv2022

Magnetoelastic anisotropy in Heusler-type Mn$_{2-δ}$CoGa$_{1+δ}$ films

Perpendicular magnetization is essential for high-density memory application using magnetic materials. High-spin polarization of conduction electrons is also required for realizing large electric signals from spin-dependent transport phenomena. Heusler alloy is a well-known material class showing the half-metallic electronic structure. However, its cubic lattice nature favors in-plane magnetization and thus minimizes the perpendicular magnetic anisotropy (PMA), in general. This study focuses on an inverse-type Heusler alloy, Mn$_{2-δ}$CoGa$_{1+δ}$ (MCG) with a small off-stoichiometry ($δ$) , which is expected to be a half-metallic material. We observed relatively large uniaxial magnetocrystalline anisotropy constant ($K_\mathrm{u}$) of the order of 10$^5$ J/m$^3$ at room temperature in MCG films with a small tetragonal distortion of a few percent. A positive correlation was confirmed between the $c/a$ ratio of lattice constants and $K_\mathrm{u}$. Imaging of magnetic domains using Kerr microscopy clearly demonstrated a change in the domain patterns along with $K_\mathrm{u}$. X-ray magnetic circular dichroism (XMCD) was employed using synchrotron radiation soft x-ray beam to get insight into the origin for PMA. Negligible angular variation of orbital magnetic moment ($Δm_\mathrm{orb}$) evaluated using the XMCD spectra suggested a minor role of the so-called Bruno's term to $K_\mathrm{u}$. Our first principles calculation reasonably explained the small $Δm_\mathrm{orb}$ and the positive correlation between the $c/a$ ratio and $K_\mathrm{u}$. The origin of the magnetocrystalline anisotropy was discussed based on the second-order perturbation theory in terms of the spin--orbit coupling, claiming that the mixing of the occupied $\uparrow$- and the unoccupied $\downarrow$-spin states is responsible for the PMA of the MCG films.

cond-mat.mtrl-sci↗

arXiv2021

Perpendicular magnetic anisotropy in ultra-thin Cu$_2$Sb-type (Mn-Cr)AlGe films onto thermally oxidized silicon substrates

Perpendicularly magnetized films showing small saturation magnetization, $M_\mathrm{s}$, are essential for spin-transfer-torque writing type magnetoresistive random access memories, STT-MRAMs. An intermetallic compound, {(Mn-Cr)AlGe} of the Cu$_2$Sb-type crystal structure was investigated, in this study, as a material showing the low $M_\mathrm{s}$ ($\sim 300$ kA/m) and high-perpendicular magnetic anisotropy, $K_\mathrm{u}$. The layer thickness dependence of $K_\mathrm{u}$ and effects of Mg-insertion layers at top and bottom (Mn-Cr)AlGe$|$MgO interfaces were studied in film samples fabricated onto thermally oxidized silicon substrates to realize high-$K_\mathrm{u}$ in the thickness range of a few nanometer. Optimum Mg-insertion thicknesses were 1.4 and 3.0 nm for the bottom and the top interfaces, respectively, which were relatively thick compared to results in similar insertion effect investigations on magnetic tunnel junctions reported in previous studies. The cross-sectional transmission electron microscope images revealed that the Mg-insertion layers acted as barriers to interdiffusion of Al-atoms as well as oxidization from the MgO layers. The values of $K_\mathrm{u}$ were about $7 \times 10^5$ and $2 \times 10^5$ J/m$^3$ at room temperature for 5 and 3 nm-thick (Mn-Cr)AlGe films, respectively, with the optimum Mg-insertion thicknesses. The $K_\mathrm{u}$ at a few nanometer thicknesses is comparable or higher than those reported in perpendicularly magnetized CoFeB films which are conventionally used in MRAMs, while the $M_\mathrm{s}$ value is one third or less smaller than those of the CoFeB films. The developed (Mn-Cr)AlGe films are promising from the viewpoint of not only the magnetic properties, but also the compatibility to the silicon process in the film fabrication.

cond-mat.mtrl-sci↗

arXiv2020

Interlayer exchange coupling through Ir-doped Cu spin Hall material

Metallic superlattices where the magnetization vectors in the adjacent ferromagnetic layers are antiferromagnetically coupled by the interlayer exchange coupling through nonmagnetic spacer layers are systems available for the systematic study on antiferromagnetic (AF) spintronics. As a candidate of nonmagnetic spacer layer material exhibiting remarkable spin Hall effect, which is essential to achieve spin-orbit torque switching, we selected the Ir-doped Cu in this study. The AF-coupling for the Co / Cu$_{95}$Ir$_{5}$ / Co was investigated, and was compared with those for the Co / Cu / Co and Co / Ir / Co. The maximum magnitude of AF-coupling strength was obtained to be 0.39 mJ/m$^{2}$ at the Cu$_{95}$Ir$_{5}$ thickness of about 0.75 nm. Furthermore, we found a large spin Hall angle of Cu$_{95}$Ir$_{5}$ in Co / Cu$_{95}$Ir$_{5}$ bilayers by carrying out spin Hall magnetoresistance and harmonic Hall voltage measurements, which are estimated to be 3 ~ 4 %. Our experimental results clearly indicate that Cu$_{95}$Ir$_{5}$ is a nonmagnetic spacer layer allowing us to achieve moderately strong AF-coupling and to generate appreciable spin-orbit torque via the spin Hall effect.

cond-mat.mtrl-sci↗

arXiv2020

Scaling of quadratic and linear magnetooptic Kerr effect spectra with L2$_1$ ordering of Co$_2$MnSi Heusler compound

The Heusler compound Co$_2$MnSi provides a crystallographic transition from B2 to L2$_1$ structure with increasing annealing temperature $T_a$, being a model system for investigating the influence of crystallographic ordering on structural, magnetic, optic, and magnetooptic (MO) properties. Here, we present quadratic magnetooptic Kerr effect (QMOKE) spectra depending on $M^2$ in addition to the linear magnetooptic Kerr effect (LinMOKE) spectra being proportional to $M$, both in the extended visible spectral range of light from 0.8\,eV to 5.5\,eV. We investigated a set of Co$_2$MnSi thin films deposited on MgO(001) substrates and annealed from 300$^\circ$C to 500$^\circ$C. The amplitude of LinMOKE and QMOKE spectra scales linearly with $T_a$, and this effect is well pronounced at the resonant peaks below 2.0\,eV of the QMOKE spectra. Furthermore, the spectra of the MO parameters, which fully describe the MO response of Co$_2$MnSi up to the second order in $M$, are obtained dependend on $T_a$. Finally, the spectra are compared to ab-initio calculations of a purely L2$_1$ ordered Co$_2$MnSi Heusler compound.

physics.app-ph↗

arXiv2019

Spin-charge conversion in NiMnSb Heusler alloy films

Half-metallic Heusler alloys are attracting considerable attention because of their unique half-metallic band structures which exhibit high spin polarization and yield huge magnetoresistance ratios. Besides serving as ferromagnetic electrodes, Heusler alloys also have the potential to host spin-charge conversion which has been recently demonstrated in other ferromagnetic metals. Here, we report on the spin-charge conversion effect in the prototypical Heusler alloy NiMnSb. Spin currents were injected from Y3Fe5O12 into NiMnSb films by spin pumping, and then the spin currents were converted to charge currents via spin-orbit interactions. Interestingly, an unusual charge signal was observed with a sign change at low temperature, which can be manipulated by film thickness and ordering structure. It is found that the spin-charge conversion has two contributions. First, the interfacial contribution causes a negative voltage signal, which is almost constant versus temperature. The second contribution is temperature dependent because it is dominated by minority states due to thermally excited magnons in the bulk part of the film. This work provides a pathway for the manipulation of spin-charge conversion in ferromagnetic metals by interface-bulk engineering for spintronic devices.

cond-mat.mtrl-sci↗

arXiv2017

Mn$_2$VAl Heusler alloy thin films: Appearance of antiferromagnetism and an exchange bias in a layered structure with Fe

Mn$_2$VAl Heusler alloy films were epitaxially grown on MgO(100) single crystal substrates by means of ultra-high-vacuum magnetron sputtering. A2 and L2$_1$ type Mn$_2$VAl order was controlled by the deposition temperatures. A2-type Mn$_2$VAl films showed no spontaneous magnetization and L2$_1$-type Mn$_2$VAl films showed ferrimagnetic behavior with a maximum saturation magnetization of 220 emu/cm$^3$ at room temperature. An antiferromagnetic reflection was observed with neutron diffraction at room temperature for an A2-type Mn$_2$VAl film deposited at 400$^\circ$C. A bilayer sample of the antiferromagnetic A2 Mn$_2$VAl and Fe showed an exchange bias of 120 Oe at 10 K.

physics.app-ph↗

arXiv2017

Nonlinear electric field effect on perpendicular magnetic anisotropy in Fe/MgO interfaces

The electric field effect on magnetic anisotropy was studied in an ultrathin Fe(001) monocrystalline layer sandwiched between Cr buffer and MgO tunnel barrier layers, mainly through post-annealing temperature and measurement temperature dependences. A large coefficient of the electric field effect of more than 200 fJ/Vm was observed in the negative range of electric field, as well as an areal energy density of perpendicular magnetic anisotropy (PMA) of around 600 uJ/m2. More interestingly, nonlinear behavior, giving rise to a local minimum around +100 mV/nm, was observed in the electric field dependence of magnetic anisotropy, being independent of the post-annealing and measurement temperatures. The insensitivity to both the interface conditions and the temperature of the system suggests that the nonlinear behavior is attributed to an intrinsic origin such as an inherent electronic structure in the Fe/MgO interface. The present study can contribute to the progress in theoretical studies, such as ab initio calculations, on the mechanism of the electric field effect on PMA.

cond-mat.mtrl-sci↗

arXiv2016

Voltage control of magnetic anisotropy in epitaxial Ru/Co2FeAl/MgO heterostructures

Voltage control of magnetic anisotropy (VCMA) in magnetic heterostructures is a key technology for achieving energy-efficiency electronic devices with ultralow power consumption. Here, we report the first demonstration of the VCMA effect in novel epitaxial Ru/Co2FeAl(CFA)/MgO heterostructures with interfacial perpendicular magnetic anisotropy (PMA). Perpendicularly magnetized tunnel junctions with the structure of Ru/CFA/MgO were fabricated and exhibited an effective voltage control on switching fields for the CFA free layer. A large VCMA coefficient of 108 (139) fJ/Vm for the CFA film was achieved at room temperature (4 K). The interfacial stability in the heterostructure was confirmed by repeating measurements. Temperature dependences of both the interfacial PMA and the VCMA effect were also investigated. It is found that the temperature dependences follow power laws of the saturation magnetization with an exponent of ~2. The significant VCMA effect observed in this work indicates that the Ru/CFA/MgO heterostructure could be one of the promising candidates for spintronic devices with voltage control.

cond-mat.mtrl-sci↗

arXiv2015

Modelling of the Peltier effect in magnetic multilayers

We model the charge, spin, and heat currents in ferromagnetic metal$|$normal metal$|$normal metal trilayer structures in the two current model, taking into account bulk and interface thermoelectric properties as well as Joule heating. Results include the temperature distribution as well as resistance-current curves that reproduce the observed shifted parabolic characteristics. Thin tunneling barriers can enhance the apparent Peltier cooling. The model agrees with experimental results for wide multilayer pillars, but the giant effects observed for diameters $\lesssim 100\,$nm are still under discussion.

cond-mat.mes-hall↗

arXiv2015

Direct measurement of the magnetic anisotropy field in Mn--Ga and Mn--Co--Ga Heusler films

The static and dynamic magnetic properties of tetragonally distorted Mn--Ga based alloys were investigated. Static properties are determined in magnetic fields up to 6.5~T using SQUID magnetometry. For the pure Mn$_{1.6}$Ga film, the saturation magnetisation is 0.36~MA/m and the coercivity is 0.29~T. Partial substitution of Mn by Co results in Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$. The saturation magnetisation of those films drops to 0.2~MA/m and the coercivity is increased to 1~T. Time-resolved magneto-optical Kerr effect (TR-MOKE) is used to probe the high-frequency dynamics of Mn--Ga. The ferromagnetic resonance frequency extrapolated to zero-field is found to be 125~GHz with a Gilbert damping, $α$, of 0.019. The anisotropy field is determined from both SQUID and TR-MOKE to be 4.5~T, corresponding to an effective anisotropy density of 0.81~MJ/m$^3$. Given the large anisotropy field of the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film, pulsed magnetic fields up to 60~T are used to determine the field strength required to saturate the film in the plane. For this, the extraordinary Hall effect was employed as a probe of the local magnetisation. By integrating the reconstructed in--plane magnetisation curve, the effective anisotropy energy density for Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ is determined to be 1.23~MJ/m$^3$.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic dichroism study on Mn$_{1.8}$Co$_{1.2}$Ga thin film using a combination of X-ray absorption and photoemission spectroscopy

Using circularly polarised radiation and a combination of bulk-sensitive hard X-ray photoelectron spectroscopy and X-ray-absorption spectroscopy (XAS) we studied the electronic and magnetic structure of epitaxial Mn$_{1.8}$Co$_{1.2}$Ga thin films. Spin resolved Bloch spectral functions, density of states as well as charge and magnetisation densities were investigated by a first-principles analysis of full potential, fully relativistic Korringa--Kohn--Rostoker calculations of the electronic structure. The valence states were experimentally investigated by using linear dichroism in the angular distribution and comparing the results to spin-resolved densities of states. The linear dichroism in the valence band enabled a symmetry analysis of the contributing states. The spectra were in good agreement with the theoretical partial density of states. The element-specific, spin-resolved, unoccupied densities of states for Co and Mn were analysed by using XAS and X-ray magnetic circular dichroism (XMCD) at the $L_{3,2}$ edges. The spectra were influenced by strong correlation effects. XMCD was used to extract the site resolved magnetic moments. The experimental values of $m_{\rm Mn}=0.7\:μ_B$ and $m_{\rm Co}=1.05\:μ_B$ agree very well with the calculated magnetic moments. Magnetic circular dichroism in angle-resolved photoelectron spectroscopy at the Mn and Co $2p$ core level exhibited a pronounced magnetic dichroism and confirmed the localised character of the Mn $d$ valence states.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic and transport properties of tetragonal- or cubic-Heusler-type Co-substituted Mn-Ga epitaxial thin films

The composition dependence of the structural, magnetic, and transport properties of epitaxially grown Mn-Co-Ga films were investigated. The crystal structure was observed to change from tetragonal to cubic as the Co content was increased. In terms of the dependence of saturation magnetization on the Co content, relatively small value was obtained for the Mn$_{2.3}$Co$_{0.4}$Ga$_{1.3}$ film at a large {\it K}$_\textrm u$ value of 9.2 Merg/cm$^3$. Electrical resistivity of Mn-Co-Ga films was larger than that of pure Mn-Ga film. The maximum value of the resistivity was 490 $μΩ$cm for Mn$_{2.2}$Co$_{0.6}$Ga$_{1.2}$ film. The high resistivity of Mn-Co-Ga might be due to the presence of localized electron states in the films due to chemical disordering caused by the Co substitution.

cond-mat.mtrl-sci↗

arXiv2012

Perpendicularly magnetized Mn-Co-Ga-based thin films with high coercive field

Mn$_{3-x}$Co$_{x}$Ga epitaxial thin films were grown on MgO substrates by magnetron co-sputtering. Structures were tetragonal or cubic depending on Co content. Composition dependence of saturation magnetization and uniaxial magnetic anisotropy $K_u$ of the films were investigated. A high $K_u$ (1.2 MJ m$^{-3}$) was achieved for the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film with the magnetic moment 0.84$μ_B$. Valence band spectra were obtained by hard X-ray photoelectron spectroscopy. Sharp peaks in the cubic case, which were absent in the tetragonal case, prove that a van Hove singularity causes a band Jahn-Teller effect with tetragonal distortion. Observations agree well with the first-principles calculations.

cond-mat.mtrl-sci↗