We investigate the impact of crystallinity on the generation of orbital torques in Ni, which is predicted to exhibit the strongest orbital response among conventional $3d$ ferromagnetic metals. We show that the current-induced torques in Hf/Ni bilayers are primarily dominated by the orbital torque, arising from the orbital Hall effect in the Hf layer. We find that the orbital torque efficiency is enhanced by a factor of 2 when the stacking order of the Hf/Ni bilayer is altered. Through the examination of bulk and interfacial structural properties of the Ni and Hf layers, and by quantifying the torque efficiency in a symmetric Hf/Ni/Hf trilayer, we show that the orbital torque efficiency is strongly dependent on the crystallinity of the Ni layer. This dependence is in stark contrast to conventional spin-orbit torques, which arise from the spin Hall effect and are typically insensitive to the crystallinity of the ferromagnetic layer. These findings highlight the significant role of the crystalline structure of the ferromagnetic layer in its orbital response and illustrate the potential of crystal structure engineering in optimizing orbital torques.

中文翻译：

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结晶度对铁磁体轨道扭矩产生的影响

我们研究了 Ni 中结晶度对轨道扭矩产生的影响，预计 Ni 会表现出传统材料中最强的轨道响应$3d$铁磁金属。我们表明，Hf/Ni 双层中的电流感应扭矩主要由轨道扭矩主导，该轨道扭矩由 Hf 层中的轨道霍尔效应引起。我们发现，当 Hf/Ni 双层的堆叠顺序改变时，轨道扭矩效率提高了 2 倍。通过检查 Ni 和 Hf 层的体积和界面结构特性，并通过量化对称 Hf/Ni/Hf 三层中的扭矩效率，我们表明轨道扭矩效率强烈依赖于 Ni 层的结晶度。这种依赖性与传统的自旋轨道扭矩形成鲜明对比，传统的自旋轨道扭矩由自旋霍尔效应产生，并且通常对铁磁层的结晶度不敏感。这些发现强调了铁磁层晶体结构在其轨道响应中的重要作用，并说明了晶体结构工程在优化轨道扭矩方面的潜力。