Yuya Sakuraba

Bio: Yuya Sakuraba is an academic researcher from National Institute for Materials Science. The author has contributed to research in topics: Magnetoresistance & Giant magnetoresistance. The author has an hindex of 32, co-authored 176 publications receiving 4217 citations. Previous affiliations of Yuya Sakuraba include Tohoku University & National Presto Industries.

Giant tunneling magnetoresistance in Co2MnSi∕Al–O∕Co2MnSi magnetic tunnel junctions

Abstract: Magnetic tunnel junctions (MTJs) with a stacking structure of Co2MnSi∕Al–O∕Co2MnSi were fabricated using magnetron sputtering system. Fabricated MTJ exhibited an extremely large tunneling magnetoresistance (TMR) ratio of 570% at low temperature, which is the highest TMR ratio reported to date for an amorphous Al–O tunneling barrier. The observed dependence of tunneling conductance on bias voltage clearly reveals the half-metallic energy gap of Co2MnSi. The origins of large temperature dependence of TMR ratio were discussed on the basis of the present results.

Large tunnel magnetoresistance in magnetic tunnel junctions using a Co2MnSi Heusler alloy electrode and a MgO barrier

Abstract: A large tunnel magnetoresistance (TMR) ratio of 753% has been observed at 2 K in a magnetic tunnel junction (MTJ) using a Co2MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. This TMR ratio is the largest reported to date in MTJs using a Heusler alloy electrode. Moreover, we have observed a large TMR ratio of 217% at room temperature (RT). This TMR at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio is still large because of inelastic tunneling in the antiparallel magnetic configuration.

Huge Spin-Polarization of L21-Ordered Co2MnSi Epitaxial Heusler Alloy Film

Abstract: Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial Co2MnSi/Al–O barrier/poly-crystalline Co75Fe25 were fabricated using an ultrahigh vacuum sputtering system. The epitaxial Co2MnSi bottom electrode exhibited highly ordered L21 structure and very smooth surface morphology. Observed magnetoresistance (MR) ratios of 70% at room temperature (RT) and 159% at 2 K are the highest values to date for MTJs using a Heusler alloy electrode. A high spin-polarization of 0.89 at 2 K for Co2MnSi obtained from Julliere's model coincided with the half-metallic band structure that was predicted by theoretical calculations.

Mechanism of large magnetoresistance in Co 2 MnSi / Ag / Co 2 MnSi devices with current perpendicular to the plane

Abstract: Fully epitaxial current-perpendicular-to-plane giant magnetoresistance (MR) devices with half-metallic ${\text{Co}}_{2}\text{MnSi}$ (CMS) electrodes and a Ag spacer were fabricated to investigate the relationship between the chemical ordering in CMS and its MR properties, including bulk and interface spin-asymmetry coefficients $\ensuremath{\beta}$ and $\ensuremath{\gamma}$. CMS/Ag/CMS annealed at $550\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ shows the largest MR ratio: 36.4% and 67.2% at RT and 110 K, respectively. An analysis based on Valet-Fert's model reveals large spin asymmetry $(\ensuremath{\gamma}g0.8)$ at the CMS/Ag interface, which contributes predominantly to the large MR ratio observed. First-principles ballistic conductance calculations for (001)-CMS/Ag/CMS predict a high majority-spin electron conductance, which could be the origin of the large $\ensuremath{\gamma}$ observed in this study.

Spin Seebeck effect in thin films of the Heusler compound Co 2 MnSi

Abstract: The recently discovered spin Seebeck effect (SSE) which generates spin voltage due to a temperature gradient in ferromagnets, was systematically studied in half-metallic Heusler compound ${\mathrm{Co}}_{2}\mathrm{MnSi}$ (CMS)/Pt thin films to investigate the effect of spin polarization of ferromagnetic layer on SSE. An epitaxial thin film of CMS with an almost perfect $B$2-ordered structure was prepared directly on a MgO(001) substrate. The measurement was performed at room temperature for various temperature differences, \ensuremath{\Delta}$T$ $=$ 0--20 K between higher (300 K$+$\ensuremath{\Delta}$T$) and lower (300 K) temperature ends along the film. The clear sign reversal of the thermally induced spin voltage due to SSE at the higher and lower temperature ends of the CMS film was detected by means of inverse spin-Hall effect in a Pt wire. The SSE was also investigated in a Py thin film deposited on a MgO(001) substrate and compared to that with CMS to verify the effect of spin polarization on SSE. Comparable signals of SSE in CMS and Py thin films suggested that thermal excitation of magnons might have more vital effects in SSE than the degree of spin polarization in ferromagnetic metals.

A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction

Abstract: Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L1(0)-ordered (Co, Fe)-Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB-MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 microA.

Spin Caloritronics

Abstract: This is a brief overview of the state of the art of spin caloritronics, the science and technology of controlling heat currents by the electron spin degree of freedom (and vice versa)

Spintronics: a challenge for materials science and solid-state chemistry.

Abstract: Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.