三维铁磁异质结构中太赫兹自旋与轨道泵浦的定量解缠
Quantitative Disentanglement of Terahertz Spin and Orbital Pumping in 3d Ferromagnetic Heterostructures
- Fudan University（复旦大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
该研究结合STES与楔形样品厚度控制，定量解缠3d铁磁异质结构中太赫兹自旋与轨道泵浦，发现轨道泵浦贡献向Ni递增且远超理论预测，建立了超快自旋与轨道泵浦的定量框架。
AI中文摘要：
自旋泵浦与轨道泵浦是指受驱动铁磁体向相邻非磁性层注入自旋角动量和轨道角动量的过程，是磁异质结构中角动量产生与输运的基础。飞秒光激发将这些现象拓展至超快 regime，其中自旋电子太赫兹发射光谱（STES）通过自旋-电荷转换与轨道-电荷转换，以非接触方式探测皮秒级角动量电流。微观理论预测，轨道泵浦效率在3d系列中从Fe到Ni递增，但这些预测在超快激发下是否成立尚不明确。核心挑战在于自旋电流与轨道电流同时产生，且对同一太赫兹发射有加性贡献，阻碍了二者的定量分离。本研究通过将STES与楔形样品厚度控制相结合，利用非磁性层（Ta、W、Nb）的自旋霍尔角与轨道霍尔角符号相反的异质结构，克服了这一局限。两种角动量通道因此呈现出不同的发射极性与厚度依赖性，实现了二者的定量分解。对Fe、Co、Ni异质结构的系统测量显示，轨道泵浦贡献向Ni逐步增加，达到自旋电流贡献的数十百分比，远超理论预测；即使Fe也会产生不可忽略的轨道电流，在非磁性层较薄的 regime 中至关重要。提取的轨道扩散长度始终短于自旋扩散长度，且随非磁性层自旋-轨道耦合强度降低而增加。这些结果为磁异质结构中的超快自旋与轨道泵浦建立了定量框架。
英文摘要：
Spin and orbital pumping - the injection of spin and orbital angular momentum from a driven ferromagnet into an adjacent nonmagnetic layer - are fundamental processes underlying angular-momentum generation and transport in magnetic heterostructures. Femtosecond optical excitation extends these phenomena into the ultrafast regime, where spintronic terahertz emission spectroscopy (STES) detects picosecond angular-momentum currents in a contact-free manner through spin-to-charge and orbital-to-charge conversion. Microscopic theory predicts that orbital-pumping efficiency increases from Fe to Ni across the 3d series, yet whether these predictions hold under ultrafast excitation remains unclear. A central challenge is that spin and orbital currents are generated simultaneously and contribute additively to the same terahertz emission, preventing their quantitative separation. Here, we overcome this limitation by combining STES with wedge-sample thickness control in heterostructures whose nonmagnetic layers (Ta, W, and Nb) have spin Hall and orbital Hall angles of opposite signs. The two angular-momentum channels therefore exhibit distinct emission polarities and thickness dependences, enabling their quantitative decomposition. Systematic measurements on Fe, Co, and Ni heterostructures reveal that the orbital-pumping contribution increases progressively toward Ni, reaching several tens of percent of the spin-current contribution - far exceeding theoretical predictions. Even Fe generates a non-negligible orbital current that becomes essential in the thin-nonmagnetic-layer regime. The extracted orbital diffusion lengths are consistently shorter than spin diffusion lengths and increase with decreasing spin-orbit coupling strength of the nonmagnetic layer. These results establish a quantitative framework for ultrafast spin and orbital pumping in magnetic heterostructures.
