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電子の電荷に加えてスピン角運動量を用いて新たな機能デバイスの実現や物理現象の解明を目指すスピントロニクスにおいて，電流からスピン角運動量の流れであるスピン流への変換は極めて重要な現象の一つである．そのような中で本研究は，スピン軌道相互作用大きい4f軌道を持ちながら今まで注目されていなかった希土類金属における電流-スピン流変換を系統的に調べ，f軌道電子由来した電流-スピン流変換の理解を目指し，さらに電子密度変化を通して電流スピン流変換効率の最大化を目標して行った．
本研究の本研究の実績として以下が挙げられる．
本研究で希土類金属として注目したHo,Er,GdおよびNdについて，薄膜の作製条件を決定し，XPS測定およびXRD測定によってそれらの化学的な物性や結晶生を定量した．また，電子輸送特性や磁性についても調べ，それらの強磁性転移温度および反強磁性転移温度であるキュリー温度とネール温度が薄膜化したことで低温側にシフトし，さらに電気抵抗に対して影響を与えることを明らかにした．また，強磁性体/Hf/希土類金属のヘテロ構造における電流スピン流生成について調べた．Hfは希土類金属の磁気近接効果を排除するために用いている．この測定から，電流スピン流変換現象においてf電子が電流に寄与する割合が大きいHoが最もスピン流生成功率が高いことが明らかになった．また，希土類金属中のスピン流が強磁性体に与える影響が強磁性体の種類に依存している可能性が示唆されたため今後さらに研究を行う予定である．さらに，スピン流生成功率が高かったHoについて電流スピン流電流変換現象の温度依存性を測定し，電気抵抗率に対してスピン流変換が遷移金属と同様のスケーリング則に従うことがわかった．これらの結果について2021年度の日本物理学会 第77回年次大会にて口頭の成果報告を行った．現在はPZT薄膜を使用し，希土類金属中におけるスピン流生成効率の電気的な制御を目指して取り組んでいる．
In spintronics, which aims at realizing new functional devices and elucidating physical phenomena using spin angular momentum in addition to electron charge, the conversion from charge current to spin current, which is the flow of spin angular momentum, is one of the most important phenomena. This study systematically examined the charge current-spin current conversion in rare-earth metals, which had not been noticed until now although they had 4f orbitals with large spin-orbit interaction, and aimed at understanding the charge current-spin current conversion derived from f orbital electrons, and also aimed at maximizing the conversion efficiency through electron density change.
The results of this work include the following.
For the rare earth metals Ho, Er, Gd, and Nd, which were focused on in this study, the evaporation conditions of the thin films were determined, and their chemical properties and crystal growth were determined by XPS and XRD measurements. Electron transport characteristics and magnetism were also examined, and it was clarified that the Curie temperature and Neel temperature, which are the ferromagnetic transition temperature and antiferromagnetic transition temperature, were shifted to the low temperature compared to the bulk, which further affected the electrical resistance. Spin current generation in ferromagnetic/Hf/rare-earth-metal heterostructures were also investigated. Hf layer was used to eliminate the magnetic proximity effect of rare earth metals. From this measurement, it was clarified that Ho, in which the ratio of f electrons contributing to the current is large in the charge current - spin current conversion phenomenon, has the highest spin current generation efficiency. In addition, it was suggested that the effect of spin current in rare-earth metals on ferromagnetic materials may depend on the type of ferromagnetic material, and further research will be conducted to comprehend this result in the future. In addition, the temperature dependence of the charge current - spin current conversion was measured for Ho, and it was proven that the spin current conversion obeyed the scaling law similar to the transition metal for the electrical resistivity. These results were reported orally at The physical society of Japan 2022 Annual Meeting(77th). Currently, I'm working on electrical control of spin current generation efficiency in rare-earth metals using PZT thin films.
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