Impact of crystallinity on orbital torque generation in ferromagnets

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.