arXiv subjects

K. Takanashi

Publications and source records attributed to K. Takanashi.

12 recordsLinked to original sources

arXiv2026

Influence of chemical ordering on magnetocrystalline anisotropy and magnetoelastic properties in Weyl magnetic semimetal Co2MnGa thin films

The magnetic anisotropy and magnetoelastic properties of the magnetic Weyl semimetal Co2MnGa were investigated in the Co2MnGa thin films with different chemical orderings. Their properties change, in line with the increase in the contents of the highly-ordered B2 and L21 phases. The most significant changes of both the magnetic anisotropy and the magnetoelastic properties were correlated with the appearance of the L21 ordered phase of Heusler alloy, which is also responsible for topological properties of these compounds. The magnetoelastic properties of L21 ordered Co2MnGa are discussed in comparison with the Co2FexMn1-xSi system with a similar degree of ordering, which has similar crystallographic structure but it is not Weyl semimetal. In particular, significant differences between the magnetoelastic properties of ordered Co2MnGa and Co2MnSi were found. In the case of L21-ordered Co2MnGa, the giant anisotropy of the magnetoelastic properties was observed.

cond-mat.mtrl-sci↗

arXiv2022

Lattice constants and magnetism of L10-ordered FePt under high pressure

We studied the relationship between the lattice constant and magnetism of L10-ordered FePt under high pressure by means of first-principles calculations and synchrotron x-ray measurements. Based on our calculations, we found that the c/a ratio shows an anomaly at ~ 20 GPa and that the Pt magnetic moment is sharply suppressed at ~ 60 GPa. As for the c/a, we experimentally verified the anomaly at ~ 20 GPa by powder x-ray diffraction. We also measured the x-ray magnetic circular dichroism at the Pt L edge up to ~ 20 GPa. Any significant change of the Pt magnetic moment was not observed, in agreement with the calculations. These results thus indicate the possibility that novel magnetic states can be created in L10-ordered FePt by lattice deformation under high pressure.

cond-mat.mtrl-sci↗

arXiv2020

Enhancement of anomalous Nernst effect in Ni/Pt superlattice

We report an enhancement of the anomalous Nernst effect (ANE) in Ni/Pt (001) epitaxial superlattices. The transport and magneto-thermoelectric properties were investigated for the Ni/Pt superlattices with various Ni layer thicknesses (${\it t}$). The anomalous Nernst coefficient was increased up to more than 1 $μ$V K$^{-1}$ for 2.0 nm ${\leq}$ ${\it t}$ ${\leq}$ 4.0 nm, which was the remarkable enhancement compared to the bulk Ni. It has been found that the large transverse Peltier coefficient ($α$$_{xy}$), reaching $α$$_{xy}$ = 4.8 A K$^{-1}$ m$^{-1}$ for ${\it t}$ = 4.0 nm, plays a prime role for the enhanced ANE of the Ni/Pt (001) superlattices.

cond-mat.mtrl-sci↗

arXiv2020

Perpendicularly Magnetized Ni / Pt (001) Epitaxial Superlattice

A perpendicularly magnetized ferromagnetic layer is an important building block for recent/future highdensity spintronic memory applications. This paper reports on the fabrication of perpendicularly magnetized Ni / Pt superlattices and the characterization of their structures and magnetic properties. The optimization of film growth conditions allowed us to grow epitaxial Ni / Pt (001) superlattices on SrTiO$_{3}$ (001) single crystal substrates. We investigated their structural parameters and magnetic properties as a function of the Ni layer thickness, and obtained a high uniaxial magnetic anisotropy energy of 1.9 x 10$^{6}$ erg/cm$^{3}$ for a [Ni (4.0 nm) / Pt (1.0 nm)] superlattice. In order to elucidate the detailed mechanism on perpendicular magnetic anisotropy for the Ni / Pt (001) superlattices, the experimental results were compared with the first-principles calculations. It has been found that the strain effect is a prime source of the emergence of perpendicular magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2019

Magnetization reversal, damping properties and magnetic anisotropy of L10-ordered FeNi thin films

L10 ordered magnetic alloys such as FePt, FePd, CoPt and FeNi are well known for their large magnetocrystalline anisotropy. Among these, L10-FeNi alloy is economically viable material for magnetic recording media because it does not contain rare earth and noble elements. In this work, L10-FeNi films with three different strengths of anisotropy were fabricated by varying the deposition process in molecular beam epitaxy system. We have investigated the magnetization reversal along with domain imaging via magneto optic Kerr effect based microscope. It is found that in all three samples, the magnetization reversal is happening via domain wall motion. Further ferromagnetic resonance (FMR) spectroscopy was performed to evaluate the damping constant and magnetic anisotropy. It was observed that the FeNi sample with moderate strength of anisotropy exhibits low value of damping constant ~ 4.9X10^-3. In addition to this, it was found that the films possess a mixture of cubic and uniaxial anisotropies.

physics.app-ph↗

arXiv2018

Spin-Hall and Anisotropic Magnetoresistance in Ferrimagnetic Co-Gd / Pt layers

We present the Co-Gd composition dependence of the spin-Hall magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) for ferrimagnetic Co100-xGdx / Pt bilayers. With Gd concentration x, its magnetic moment increasingly competes with the Co moment in the net magnetization. We find a nearly compensated ferrimagnetic state at x = 24. The AMR changes sign from positive to negative with increasing x, vanishing near the magnetization compensation. On the other hand, the SMR does not vary significantly even where the AMR vanishes. These experimental results indicate that very different scattering mechanisms are responsible for AMR and SMR. We discuss a possible origin for the alloy composition dependence.

cond-mat.mtrl-sci↗

arXiv2018

Conversion of Heat into Charge Current by the Spin Wave Anomalous Nernst Effect

A novel process of spin conversion from a temperature gradient to a transverse voltage is addressed in this paper, viz. the anomalous Nernst effect (ANE) in a ferromagnetic metal. We report that an additional voltage is superposed on the conventional anomalous Nernst voltage in FePt crystalline thin films. The dynamics of the local magnetization is modulated by the heat current and excites spin waves. These generate a conduction electron spin current via s-d coupling, which flows along the temperature gradient, and the spin current is converted to a Nernst voltage by the inverse spin Hall effect.

cond-mat.mtrl-sci↗

arXiv2017

Visualization of Anomalous Ettingshausen Effect in a Ferromagnetic Film: Direct Evidence of Different Symmetry from Spin Peltier Effect

Spatial distribution of temperature modulation due to anomalous Ettingshausen effect (AEE) is visualized in a ferromagnetic FePt thin film with in-plane and out-of-plane magnetizations using the lock-in thermography technique. Comparing the AEE of FePt with the spin Peltier effect (SPE) of a Pt / yttrium iron garnet junction provides direct evidence of different symmetries of AEE and SPE. Our experiments and numerical calculations reveal that the distribution of heat sources induced by AEE strongly depends on the direction of magnetization, leading to the remarkable different temperature profiles in the FePt thin film between the in-plane and perpendicularly magnetized configurations.

cond-mat.mes-hall↗

arXiv2017

Magnetic anisotropy of $L1_0$-ordered FePt thin films studied by Fe and Pt $L_{2,3}$-edges x-ray magnetic circular dichroism

The strong perpendicular magnetic anisotropy of $L{\rm1_0}$-ordered FePt has been the subject of extensive studies for a long time. However, it is not known which element, Fe or Pt, mainly contributes to the magnetic anisotropy energy (MAE). We have investigated the anisotropy of the orbital magnetic moments of Fe 3$d$ and Pt 5$d$ electrons in $L{\rm1_0}$-ordered FePt thin films by Fe and Pt $L_{2,3}$-edge x-ray magnetic circular dichroism (XMCD) measurements for samples with various degrees of long-range chemical order $S$. Fe $L_{2,3}$-edge XMCD showed that the orbital magnetic moment was larger when the magnetic field was applied perpendicular to the film than parallel to it, and that the anisotropy of the orbital magnetic moment increased with $S$. Pt $L_{2,3}$-edge XMCD also showed that the orbital magnetic moment was smaller when the magnetic field was applied perpendicular to the film than parallel to it, opposite to the Fe $L_{2,3}$-edge XMCD results although the anisotropy of the orbital magnetic moment increases with $S$ like the Fe edge. These results are qualitatively consistent with the first-principles calculation by Solovyev ${\it et\ al.}$ [Phys. Rev. B $\bf{52}$, 13419 (1995).], which also predicts the dominant contributions of Pt 5$d$ to the magnetic anisotropy energy rather than Fe 3$d$ due to the strong spin-orbit coupling and the small spin splitting of the Pt 5$d$ bands in $L{\rm1_0}$-ordered FePt.

cond-mat.mtrl-sci↗

arXiv2016

Resonant Magnetization Switching Conditions of an Exchange-coupled Bilayer under Spin Wave Excitation

We systematically investigated the spin wave-assisted magnetization switching (SWAS) of a L10-FePt / Ni81Fe19 (Permalloy; Py) exchange-coupled bilayer using a pulse-like rf field (hrf) and mapped the switching events in the magnetic field (H) - hrf frequency (f) plane to reveal the switching conditions. The switching occurred only in a limited region following the dispersion relationship of the perpendicular standing spin wave (PSSW) modes in Py. The results indicate that SWAS is a resonant magnetization switching process, which is different from the conventional microwave assisted switching, and has the potential to be applied to selective switching for multilevel recording media.

cond-mat.mtrl-sci↗

arXiv2010

Surface-assisted Spin Hall Effect in Au Films with Pt Impurities

We show, both experimentally and theoretically, a novel route to obtain giant room temperature spin Hall effect due to surface-assisted skew scattering. In the experiment, we report the spin Hall effect in Pt-doped Au films with different thicknesses $t_N$. The giant spin Hall angle $γ_S$ = $0.12 \pm 0.04$ is obtained for $t_N$ = 10 nm at room temperature, while it is much smaller for $t_N$ = 20 nm sample. Combined ab initio and quantum Monte Carlo calculations for the skew scattering due to a Pt impurity show $γ_S$ $\cong$ 0.1 on the Au (111) surface, while it is small in bulk Au. The quantum Monte Carlo results show that the spin-orbit interaction of the Pt impurity on the Au (111) surface is enhanced, because the Pt 5d levels are lifted to the Fermi level due to the valence fluctuation. In addition, there are two spin-orbit interaction channels on the Au (111) surface, while only one in bulk Au.

cond-mat.mtrl-sci↗

arXiv1999

Effect of Coulomb blockade on STM current through a granular film

The electron transport through an array of tunnel junctions consisting of an STM tip and a granular film is studied both theoretically and experimentally. When the tunnel resistance between the tip and a granule on the surface is much larger than those between granules, a bottleneck of the tunneling current is created in the array. It is shown that the period of the Coulomb staircase(CS) is given by the capacitance at the bottleneck. Our STM experiments on Co-Al-O granular films show the CS with a single period at room temperature. This provides a new possibility for single-electron-spin-electronic devices at room temperature.

cond-mat.mes-hall↗