Orientational anisotropy of magnetic damping in Ta/CoFeB/MgO heterostructures

Magnetic damping controls the performance and operational speed of many spintronics devices. Being a tensor quantity, the damping in magnetic thin films often shows anisotropic behavior with the magnetization orientation, known as orientational anisotropy. Here, we studied the orientational anisotropy of damping in Ta/CoFeB/MgO heterostructures deposited on thermally oxidized Si substrate. By performing ferromagnetic resonance (FMR) measurements based on spin pumping and inverse spin Hall effect (ISHE), we extract the damping parameter in those films and find that the orientational anisotropy of damping contains four-fold and two-fold anisotropy terms. We infer that four-fold anisotropy originates from two-magnon scattering (TMS). By studying reference Ta/CoFeB/MgO films deposited on LiNbO 3 substrates, we find that the two-fold anisotropy is correlated with in-plane magnetic anisotropy (IMA) of the films, suggesting its origin as the anisotropy in bulk spin-orbit coupling (SOC) of CoFeB film. We conclude that when IMA is very small, it has too little influence on two-fold anisotropy to be experimentally identified. However, as IMA increases, it starts to interfere with two-fold anisotropy in damping. These results will be beneficial for designing future spintronics devices.