AI 中文总结
本研究通过多种测量方法探究Bi及Bi/Ag层的电荷-自旋转换，发现插入Ag间隔层可大幅提升SOT效率，明确自旋电流产生源于体Bi效应，为Bi基SOT器件设计提供了原则。
AI 中文摘要
Bi被预测为自旋轨道转矩（SOT）的高效发生器，其电荷-自旋转换效率可与Ta、W、Pt等典型重金属相媲美。然而，实验报道的转换效率差异巨大，且Bi/Ag双层中转换信号较大的起源仍存在争议。本研究通过磁光克尔效应磁力计测量类阻尼SOT，辅以结构与光谱分析及谐波霍尔电阻测量，探究外延与多晶Bi基磁性异质结构中的电荷-自旋转换。结果表明，在Bi(001)与金属铁磁体（FeCo或Ni）之间插入Ag间隔层，可使SOT效率提升一个数量级以上，达到约2×10^5 (ℏ/2e) S/m的有效自旋霍尔电导率，与体Bi的理论预期高度吻合。这种增强可由Bi结构与化学完整性的保留合理解释——直接沉积铁磁覆盖层会破坏Bi的完整性，而Bi/Ag界面的Rashba自旋轨道耦合并非原因。对超出已知破坏Bi(001)表面态的氧剂量的外延、多晶及故意表面氧化Bi薄膜的对比研究显示，结构无序对SOT效率有负面影响，且Bi/Ag异质结构中自旋电流产生主要源于体效应，对应的有效Bi自旋霍尔角约为1。本研究建立了原子尺度完整性与电荷-自旋转换的直接关联，为提高Bi基SOT器件的可靠性提供了设计原则，并为解释Bi及Bi/Ag系统中的自旋-电荷转换提供了可靠框架。
英文摘要
Bi is predicted to be an efficient generator of spin-orbit torques (SOTs), with charge-to-spin conversion efficiency comparable to those of prototypical heavy metals, such as Ta, W, and Pt. However, experimental reports provide widely scattered interconversion efficiencies, while the origin of the large conversion signal in Bi/Ag bilayers remains controversial. Here, we investigate charge-to-spin conversion in epitaxial and polycrystalline Bi-based magnetic heterostructures by measuring the damping-like SOT using magneto-optic Kerr effect magnetometry, complemented by structural and spectroscopic analyses and harmonic Hall resistance measurements. We show that inserting an Ag spacer between Bi(001) and metallic ferromagnets (FeCo or Ni) enhances the SOT efficiency by more than one order of magnitude, reaching an effective spin Hall conductivity of approximately $2 \times 10^5 (\hbar/2e)$ S/m, in excellent agreement with theoretical expectations for bulk Bi. This enhancement can be consistently explained by the preservation of the structural and chemical integrity of Bi, otherwise compromised by the direct deposition of a ferromagnetic overlayer, rather than by Rashba spin-orbit coupling at the Bi/Ag interface. Comparative studies across epitaxial, polycrystalline, and intentionally surface-oxidized Bi films, beyond oxygen doses known to destroy Bi(001) surface states, reveal that structural disorder has a negative impact on the SOT efficiency and indicate a dominant bulk contribution to spin-current generation in Bi/Ag heterostructures, yielding an effective Bi spin Hall angle of approximately 1. By establishing a direct correlation between atomic-scale integrity and charge-to-spin conversion, this study provides design principles to improve the reliability of Bi-based SOT devices and offers a robust framework for interpreting spin-charge interconversion in Bi and Bi/Ag systems.
Journal refPhys. Rev. Mater. 10, 074406 (2026)
