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Current-spin conversion in Dirac semimetal thin film heterostructures

池, 震棟 東京大学 DOI:10.15083/0002006661

2023.03.24

概要

論文審査の結果の要旨
氏名

池 震 棟

電子は電荷とスピンという 2 つの自由度を持つ。その両方、特にスピンの性
質を巧みに使うことによって新しいエレクトロニクスを生み出そうとする研
究分野をスピントロニクスと呼ぶ。その中で重要な目標となっているのは、強
磁性体の磁化方向を、磁場ではなく、電流あるいは電場で制御する技術の開発
と応用である。このような技術は、低消費電力・高速・高密度のデバイスの実
現を可能にする。中でも、電流誘起スピントランスファートルクは、強磁性金
属薄膜の磁化を電流で制御する手法として注目されてきた。
近年、重金属と強磁性薄膜との接合や、トポロジカル絶縁体と強磁性薄膜と
の接合のようなヘテロ構造において、スピン軌道トルクと呼ばれる、スピント
ランスファートルクとは異なる新しい種類の電流誘起トルクが報告されてい
る。このトルクは、スピンホール効果やラシュバ・エデルシュタイン効果によ
って生み出されるヘテロ構造界面における電流誘起スピン蓄積によって引き
起こされる。したがって、電流-スピン流変換効率の高い材料を探索し、その
起源を明らかにすることは、これらの材料の理解を深め、応用可能なスピン軌
道トルクを実現する上で重要である。
本論文は、ビスマス(Bi)を含む材料と強磁性薄膜から構成されるヘテロ構
造において、スピン軌道トルクを評価することによって、電流-スピン流変換
効率を系統的に調査したものである。本論文は 6 章からなる。
第 1 章は導入部であり、スピントランスファートルクの説明の後、スピン軌
道トルクとその例が示される。さらに、Bi とその合金についての紹介が行われ
る。Bi は、重元素であるためスピン軌道相互作用が大きいことと、バンド構造
の L 点において有限の質量を持つディラック電子(massive Dirac 電子)が存在
するという 2 つの特徴を持っている。その後、本論文の目的と概要が示される。
第 2 章では、Bi およびその合金材料と強磁性薄膜のヘテロ構造薄膜の作製
方法、薄膜の特性評価、磁気特性の測定、素子の作製、電気的なスピン軌道ト
ルクの評価など、実験手法を紹介する。本研究では、Bi 合金において幅広い組
成比を実現するために、独自開発の交互超薄膜蒸着法(alternating ultrathin layer
deposition)を用いた。また、スピン軌道トルクの評価には「高調波ホール電圧
測定法」を用いており、本論文で議論される中心的な内容はこの手法によって
1

得られた測定結果である。この手法の原理が詳しく紹介される。
第 3 章では、Bi1-xSbx 合金薄膜と CoFeB 強磁性薄膜からなる接合における電
流-スピン流変換について調査した結果を議論している。Bi1-xSbx 合金薄膜で
は、室温で効率が 1 を超える電流-スピン流変換を観測した。さらに、Bi1-xSbx
合金薄膜の膜厚・組成・面方向・温度を系統的に変えながら効率を調べた。そ
の結果、大きな変換効率が Bi のバンド構造に基づく内因性スピンホール効果
によることが示唆された。この結果は理論計算とも整合する。一方、トポロジ
カルな表面状態については、たとえ存在したとしても、電流-スピン流変換効
率にはほとんど寄与しないことも分かった。さらに、合金のスピンホール伝導
率の温度依存性を調べ、Bi のバンド構造の L 点における有限質量を持つディ
ラック電子が変換効率に重要な役割を果たしていることを明らかにした。
第 4 章では、ノンドープの Bi 薄膜およびキャリアドープされた Bi 薄膜と
CoFeB 強磁性薄膜との接合における電流-スピン流変換効率を調べた。Bi へ
のキャリアドーピングは、Bi を Te または Sn で置換することで行われる。単
体 Bi では約 2.7 というこれまでで最大の変換効率を記録した。また、Bi 薄膜
のスピンホール伝導率は、フェルミ準位がディラック点に近いときにプラトー
を示し、電子またはホールのドーピングによって急激に低下することがわかっ
た。一方、単体 Bi のスピンホール伝導率は、薄膜の結晶方位や抵抗率の変化
に対して影響をうけにくいことが示された。これらの結果は、Bi における電流
-スピン流変換がフェルミ準位の位置と連動していることを意味しており、内
因性スピンホール効果が重要であることの証拠である。
第 5 章では、Pt1-xBix/Co/MgO 三層膜において、Bi ドープ量 x を変化させな
がら、膜構造、磁気異方性、電流-スピン流変換を調べた。Pt が多い場合、Pt1xBix の抵抗率はドープ量とともに増加するのに対し、スピンホール伝導率がド
ープ量の影響をほとんど受けないことから、Bi ドーピングは Pt の電流-スピ
ン流変換効率を高める戦略として有効であることが示唆された。
第 6 章はまとめと展望が述べられている。
本論文は、有限のベリー曲率を持つ系のスピンホール効果とディラック性の
相関を初めて実験的に明らかにした。今後、より広い範囲のディラック系物質
におけるスピンホール効果の研究に新たな可能性を開くものである。
この研究の物性物理学としての価値と独創性は十分と認められ、博士(理学)
の学位論文としてふさわしい内容をもつものと認定し、審査員全員で合格と判
定した。なお、本論文は、共同研究者らとの共同研究であるが、論文提出者が
主体となって実験の遂行や結果の解析を行ったもので、論文提出者の寄与が十
分であると判断した。
したがって、博士(理学)の学位を授与できると認める。
2

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