Orbital currents induced by electric fields in transition metals have been experimentally reported in various material systems to date, in the form of orbital torques and orbital accumulation. However, most of these studies have been conducted on polycrystalline systems, and there has been virtually no discussion on their relation to first-principles electronic band structure calculations. In this study, we investigated the orbital Hall effect using epitaxial thin films of Ti, a 3d transition metal in which the spin Hall effect is negligibly small and the orbital Hall effect is dominant. Our first-principles calculations revealed that, within the $\mathrm{Ti}\left(1\overline{1}00\right)$ plane, the magnitude of the orbital Hall conductivity differs depending on whether the electric field is applied along the $\left[0001\right]$ or $\left[11\overline{2}0\right]$ direction. Based on this, we fabricated epitaxial $\mathrm{Ti}\left(1\overline{1}00\right)$ films and detected orbital torque using Ni, which has negligible magnetocrystalline anisotropy. As a result, we observed a clear anisotropy in the orbital torque generation efficiency depending on the direction of the applied electric field, consistent with the calculations. Furthermore, when using this orbital torque to reverse the magnetization of Ni, we found that the critical current density required for magnetization switching also varied according to the magnitude of the orbital torque generation efficiency. In this way, our study discusses the anisotropy of the orbital Hall effect in epitaxial transition metals with an aspect not observed in previously reported polycrystalline systems and offers a new direction in research on the orbital Hall effect.

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• Magnetotransport
• Spin-orbit coupling
• Spintronics