arXiv subjects

Koki Takanashi

Publications and source records attributed to Koki Takanashi.

At least 19 recordsLinked to original sources

arXiv2025

Fe contribution to the magnetic anisotropy of $L{1_0}$-ordered FePt thin films studied by angle-dependent x-ray magnetic circular dichroism

Among magnetic thin films with perpendicular magnetic anisotropy (PMA), $L1_0$-ordered FePt has attracted significant attention because of its exceptionally strong PMA. However, the microscopic origin of its strong PMA has not been elucidated experimentally. We have investigated the contribution of the Fe $3d$ electrons to its magnetic anisotropy energy by angle-dependent x-ray magnetic circular dichroism at the Fe $L_{2,3}$ edge. By this technique, one can deduce the magnetic dipole moment $m_\text{T}$, which represents the anisotropic spatial distribution of spin-polarized electrons, and the orbital moment anisotropy (OMA) of Fe $3d$ electrons. Detected finite $m_\text{T}$ indicates that the spin-polarized Fe $3d$ electrons are distributed preferentially in the out-of-plane direction of the films. This $m_\text{T}$ of Fe overwhelms the positive contribution of OMA to PMA, and reduces the PMA of $L1_0$-ordered FePt thin films, consistent with a previous first-principles calculation. The present result implies that a large positive contribution of the non-magnetic element Pt rather than Fe governs the PMA of $L1_0$-ordered FePt thin films.

cond-mat.mtrl-sci↗

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↗

arXiv2024

Quantitative measurement of figure of merit for transverse thermoelectric conversion in Fe/Pt metallic multilayers

This study presents a measurement method for determining the figure of merit for transverse thermoelectric conversion ($ z_\mathrm{T}T $) in thin film forms. Leveraging the proposed methodology, we comprehensively investigate the transverse thermoelectric coefficient ($ S_\mathrm{T} $), in-plane electrical conductivity ($ σ_{yy} $), and out-of-plane thermal conductivity ($ κ_{xx} $) in epitaxial and polycrystalline Fe/Pt metallic multilayers. The $ κ_{xx} $ values of multilayers with a number of stacking repetitions ($ N $) of 200 are lower than those of FePt alloy films, indicating that the multilayer structure effectively contributes to the suppression of $ κ_{xx} $. $ z_\mathrm{T}T $ is found to increase with increasing $ N $, which remarkably reflects the $ N $-dependent enhancement of the $ S_\mathrm{T} $ values. Notably, $ S_\mathrm{T} $ and $ σ_{yy} $ are significantly larger in the epitaxial multilayers than those in the polycrystalline counterparts, whereas negligible differences in $ κ_{xx} $ are observed between the epitaxial and polycrystalline multilayers. This discrepancy in $ σ_{yy} $ and $ κ_{xx} $ with respect to crystal growth is due to the different degree of anisotropy in electron transport between epitaxial and polycrystalline multilayers, and epitaxial growth can lead to an enhancement of $ z_\mathrm{T}T $ in the multilayers. This study is the first demonstration in the evaluation of $ z_\mathrm{T}T $ in thin film forms, and our proposed measurement technique reveals the transverse thermoelectric properties inherent to multilayers.

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

Chirality-dependent spin current generation in a helimagnet: zero-field probe of chirality

In a magnetic texture, the spin of a conduction electron is forced to be aligned to the localized moment. As a result, the topology of the magnetic texture affects the electron dynamics in nontrivial ways. A representative example is the topological Hall effect in noncoplanar spin textures with finite spin chirality. While propagating in the noncoplanar spin texture, electrons acquire Berry phase, and their motion is deflected as if they were in a magnetic field. Here, we report a distinct Berry phase effect in a coplanar helimagnet: the spin moment of the conduction electron is polarized under electric currents depending on the chirality of the helimagnet. The accumulated spin polarization works as a source of spin current, and the chirality can be detected by the inverse spin Hall mechanism. The functionality allows us to read out the chirality without magnetic fields, and therefore paves the way to future helimagnet-based spintronics.

cond-mat.mes-hall↗

arXiv2022

Sm-Co-based amorphous alloy films for zero-field operation of transverse thermoelectric generation

Transverse thermoelectric generation using magnetic materials is essential to develop active thermal engineering technologies, for which the improvements of not only the thermoelectric output but also applicability and versatility are required. In this study, using combinatorial material science and lock-in thermography technique, we have systematically investigated the transverse thermoelectric performance of Sm-Co-based alloy films. The high-throughput material investigation revealed the best Sm-Co-based alloys with the large anomalous Nernst effect (ANE) as well as the anomalous Ettingshausen effect (AEE). In addition to ANE/AEE, we discovered unique and superior material properties in these alloys: the amorphous structure, low thermal conductivity, and large in-plane coercivity and remanent magnetization. These properties make it advantageous over conventional materials to realize heat flux sensing applications based on ANE, as our Sm-Co-based films can generate thermoelectric output without an external magnetic field. Importantly, the amorphous nature enables the fabrication of these films on various substrates including flexible sheets, making the large-scale and low-cost manufacturing easier. Our demonstration will provide a pathway to develop flexible transverse thermoelectric devices for smart thermal management.

cond-mat.mtrl-sci↗

arXiv2022

Spin Hall effect driven by the spin magnetic moment current in Dirac materials

The spin Hall effect of a Dirac Hamiltonian system is studied using semiclassical analyses and the Kubo formula. In this system, the spin Hall conductivity is dependent on the definition of spin current. All components of the spin Hall conductivity vanish when spin current is defined as the flow of spin angular momentum. In contrast, the off-diagonal components of the spin Hall conductivity are non-zero and scale with the carrier velocity (and the effective $g$-factor) when spin current consists of the flow of spin magnetic moment. We derive analytical formula of the conductivity, carrier mobility and the spin Hall conductivity to compare with experiments. In experiments, we use Bi as a model system that can be characterized by the Dirac Hamiltonian. Te and Sn are doped into Bi to vary the electron and hole concentration, respectively. We find the spin Hall conductivity ($σ_\mathrm{SH}$) takes a maximum near the Dirac point and decreases with increasing carrier density ($n$). The sign of $σ_\mathrm{SH}$ is the same regardless of the majority carrier type. The spin Hall mobility, proportional to $σ_\mathrm{SH}/n$, increases with increasing carrier mobility with a scaling coefficient of $\sim$1.4. These features can be accounted for quantitatively using the derived analytical formula. The results demonstrate that the giant spin magnetic moment, with an effective $g$-factor that approaches 100, is responsible for the spin Hall effect in Bi.

cond-mat.mes-hall↗

arXiv2022

Room-temperature chirality switching in a helimagnetic thin film

Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality (or helicity) corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control sequence have to be solved in practical applications. Another issue is the thin film fabrication that enables the development of spintronic devices based on helimagnets. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu$_2$ helimagnetic conductor. The result demonstrates the feasibility of helimagnet-based spintronics that can overcome all the above problems.

cond-mat.mes-hall↗

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↗

arXiv2022

Intercorrelated anomalous Hall and spin Hall effect in kagome-lattice Co$_3$Sn$_2$S$_2$-based shandite films

Magnetic Weyl semimetals (mWSMs) are characterized by linearly dispersive bands with chiral Weyl node pairs associated with broken time reversal symmetry. One of the hallmarks of mWSMs is the emergence of large intrinsic anomalous Hall effect. On heating the mWSM above its Curie temperature, the magnetism vanishes while exchange-split Weyl point pairs collapse into doubly-degenerated gapped Dirac states. Here, we reveal the attractive potential of these Dirac nodes in paramagnetic state for efficient spin current generation at room temperature via the spin Hall effect. Ni and In are introduced to separately substitute Co and Sn in a prototypal mWSM Co$_3$Sn$_2$S$_2$ shandite film and tune the Fermi level. Composition dependence of spin Hall conductivity for paramagnetic shandite at room temperature resembles that of anomalous Hall conductivity for ferromagnetic shandite at low temperature; exhibiting peak-like dependence centering around the Ni-substituted Co$_2$Ni$_1$Sn$_2$S$_2$ and undoped Co$_3$Sn$_2$S$_2$ composition, respectively. The peak shift is consistent with the redistribution of electrons' filling upon crossing the ferromagnetic-paramagnetic transition, suggesting intercorrelation between the two Hall effects. Our findings highlight a novel strategy for the quest of spin Hall materials, guided by the abundant experimental anomalous Hall effect data of ferromagnets in the literature.

cond-mat.mes-hall↗

arXiv2021

Highly fcc-textured Pt-Al alloy films grown on MgO(001) showing enhanced spin Hall efficiency

We report on a systematic comparative study of the spin Hall efficiency between highly face-centered cubic (fcc)-textured Pt-Al alloy films grown on MgO(001) and poorly-crystallized Pt-Al alloy films grown on SiO$_2$. Using CoFeB as the detector, we show that for Al compositions centering around $x = 25$, mainly L1$_{2}$ ordered Pt$_{100-x}$Al$_x$ alloy films grown on MgO exhibit outstanding charge-spin conversion efficiency. For Pt$_{78}$Al$_{22}$/CoFeB bilayer on MgO, we obtain damping-like spin Hall efficiency as high as $ξ_\textrm{DL} \sim +0.20$ and expect up to seven-fold reduction of power consumption compared to the polycrystalline bilayer of the same Al composition on SiO$_2$. This work demonstrates that improving the crystallinity of fcc Pt-based alloys is a crucial step for achieving large spin Hall efficiency and low power consumption in this material class.

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↗

arXiv2021

Spin Hall effect in a spin-1 chiral semimetal

Spin-1 chiral semimetal is a new state of quantum matter hosting unconventional chiral fermions that extend beyond the common Dirac and Weyl fermions. B20-type CoSi is a prototypal material that accommodates such an exotic quasiparticle. To date, the spin transport properties in the spin-1 chiral semimetals, have not been explored yet. In this work, we fabricated B20-CoSi thin films on sapphire c-plane substrates by magnetron sputtering and studied the spin Hall effect (SHE) by combining experiments and first-principles calculations. The SHE of CoSi using CoSi/CoFeB/MgO heterostructures was investigated via spin Hall magnetoresistance and harmonic Hall measurements. First-principles calculations yield an intrinsic spin Hall conductivity (SHC) at the Fermi level that is consistent with the experiments and reveal its unique Fermi-energy dependence. Unlike the Dirac and Weyl fermion-mediated Hall conductivities that exhibit a peak-like structure centering around the topological node, SHC of B20-CoSi is odd and crosses zero at the node with two antisymmetric local extrema of opposite sign situated below and above in energy. Hybridization between Co d-Si p orbitals and spin-orbit coupling are essential for the SHC, despite the small (~1%) weight of Si p-orbital near the Fermi level. This work expands the horizon of topological spintronics and highlights the importance of Fermi-level tuning in order to fully exploit the topology of spin-1 chiral fermions for spin current generation.

cond-mat.mtrl-sci↗

arXiv2021

Two-dimensionality of metallic surface conduction in Co3Sn2S2 thin films

Two-dimensional (2D) surface of the topological materials is an attractive channel for the electrical conduction reflecting the linearly-dispersive electronic bands. By applying a reliable systematic thickness t dependent measurement of sheet conductance, here we elucidate the dimensionality of the electrical conduction paths of a Weyl semimetal Co3Sn2S2. Under the ferromagnetic phase, the 2D conduction path clearly emerges in Co3Sn2S2 thin films, indicating a formation of the Fermi arcs projected from Weyl nodes. Comparison between 3D conductivity and 2D conductance provides the effective thickness of the surface conducting region being estimated to be approximately 20 nm, which is rather thicker than 5 nm in topological insulator Bi2Se3. This large value may come from the narrow gap at Weyl point and relatively weak spin-orbit interaction of the Co3Sn2S2. The emergent surface conduction will provide a pathway to activate quantum and spintronic transport features stemming from a Weyl node in thin-film-based devices.

cond-mat.mtrl-sci↗

arXiv2020

High-temperature dependence of anomalous Ettingshausen effect in SmCo$_5$-type permanent magnets

The anomalous Ettingshausen effect (AEE) in SmCo$_5$-type permanent magnets has been investigated in the high-temperature range from room temperature to around 600 K. The anomalous Ettingshausen coefficient of the SmCo$_5$ and (SmGd)Co$_5$ magnets monotonically increases with increasing the temperature and shows the similar temperature dependence, while the coefficient of SmCo$_5$ is slightly larger than that of (SmGd)Co$_5$ at high temperatures. The dimensionless figure of merit for AEE in SmCo$_5$ at high temperatures is much greater than the previous record obtained for the anomalous Nernst effect. The observed high-temperature behavior of AEE is discussed based on the first-principles calculations of transverse transport coefficients.

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

Spin-mediated charge-to-heat current conversion phenomena in ferromagnetic binary alloys

Spin-mediated charge-to-heat current conversion phenomena, i.e., the anomalous Ettingshausen effect (AEE) and the anisotropic magneto-Peltier effect (AMPE), have been investigated in various ferromagnetic Ni-Fe, Ni-Pt, Ni-Pd, and Fe-Pt binary alloys at room temperature. When a charge current is applied to a ferromagnetic conductor, the AMPE modulates the Peltier coefficient depending on the angle between the directions of the charge current and magnetization, while the AEE generates a heat current in the direction perpendicular to both the charge current and magnetization. We observed the strong material dependence of the thermoelectric conversion coefficients and figures of merit of these phenomena. Among the ferromagnetic alloys used in this study, Ni$_{95}$Pt$_{5}$ exhibits the largest AMPE of which the anisotropy of the Peltier coefficient is $\sim 12\%$. In contrast, the magnitude of the AEE signals is moderate in Ni$_{95}$Pt$_{5}$ but largest in Ni$_{75}$Pt$_{25}$ and Ni$_{50}$Fe$_{50}$. We discuss these behaviors by exploring the relations between these charge-to-heat current conversion phenomena and other transport as well as magnetic properties. This systematic study will provide a clue for clarifying the mechanisms of the AMPE and AEE and for enhancing the thermoelectric conversion efficiency of these phenomena.

cond-mat.mtrl-sci↗

arXiv2020

Fermi Level Controlled Ultrafast Demagnetization Mechanism in Half-Metallic Heusler Alloy

The electronic band structure-controlled ultrafast demagnetization mechanism in Co2FexMn1-xSi Heusler alloy is underpinned by systematic variation of composition. We find the spin-flip scattering rate controlled by spin density of states at Fermi level is responsible for non-monotonic variation of ultrafast demagnetization time (τM) with x with a maximum at x = 0.4. Furthermore, Gilbert damping constant exhibits an inverse relationship with τM due to the dominance of inter-band scattering mechanism. This establishes a unified mechanism of ultrafast spin dynamics based on Fermi level position.

cond-mat.mtrl-sci↗