Observation of large spin conversion anisotropy in bismuth.

While the effective -factor can be anisotropic due to the spin-orbit interaction (SOI), its existence in solids cannot be simply asserted from a band structure, which hinders progress on studies from such viewpoints. The effective -factor in bismuth (Bi) is largely anisotropic; especially for holes at -point, the effective -factor perpendicular to the trigonal axis is negligibly small (<0.112), whereas the effective -factor along the trigonal axis is very large (62.7). We clarified in this work that the large anisotropy of effective -factor gives rise to the large spin conversion anisotropy in Bi from experimental and theoretical approaches. Spin-torque ferromagnetic resonance was applied to estimate the spin conversion efficiency in rhombohedral (110) Bi to be 17 to 27%, which is unlike the negligibly small efficiency in Bi(111). Harmonic Hall measurements support the large spin conversion efficiency in Bi(110). A large spin conversion anisotropy as the clear manifestation of the anisotropy of the effective -factor is observed. Beyond the emblematic case of Bi, our study unveiled the significance of the effective -factor anisotropy in condensed-matter physics and can pave a pathway toward establishing novel spin physics under -factor control.