Spin–orbit torque (SOT) is a promising approach to manipulate the magnetization for high-performance spintronic applications. In conventional SOT heterostructures with heavy metal (HM)/ferromagnet layers, the SOT efficiency is determined by the charge-to-spin conversion, characterized by the spin Hall angle θSH of the HM layer. Researchers have investigated various HMs with different θSH to enhance the SOT efficiency while it is still limited because of the HM’s intrinsic properties. In this study, we employ a rare-earth holmium (Ho) layer on top of a ferromagnetic Co layer (Pt/Co/Ho) to enhance the SOT efficiency. An increased damping-like SOT efficiency up to 200% is achieved at an optimized thickness of 2-nm Ho, corresponding to a lower switching current density, which is 60% less compared to the sample without a Ho layer. The damping-like torque efficiency per current density is estimated at around 0.256 for Pt/Co/Ho heterostructures. Our results, herein, demonstrate that inserting a rare-earth metal affords an additional spin current and/or improves the spin transparency to enhance the SOT efficiency, providing a route for energy-efficient spintronic devices.

中文翻译：

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通过插入 Ho 层提高 Pt/Co/Ho 异质结构中的自旋轨道扭矩效率

自旋轨道扭矩（SOT）是一种用于操纵高性能自旋电子应用磁化的有前途的方法。在具有重金属 (HM)/铁磁体层的传统 SOT 异质结构中，SOT 效率由电荷到自旋转换决定，由 HM 层的自旋霍尔角 θSH 表征。研究人员研究了具有不同 θSH 的各种 HM 以提高 SOT 效率，但由于 HM 的固有特性，它仍然受到限制。在这项研究中，我们在铁磁 Co 层 (Pt/Co/Ho) 的顶部采用稀土钬 (Ho) 层来提高 SOT 效率。在 2 nm Ho 的优化厚度下，实现了高达 200% 的类似阻尼的 SOT 效率提高，对应于较低的开关电流密度，与没有 Ho 层的样品相比，开关电流密度降低了 60%。对于 Pt/Co/Ho 异质结构，每个电流密度的类似阻尼的扭矩效率估计约为 0.256。我们的结果表明，插入稀土金属提供了额外的自旋电流和/或提高了自旋透明度以提高 SOT 效率，为节能自旋电子器件提供了一条途径。