The origin of magnetic anisotropy in magnetic compounds is a longstanding issue in solid state physics, and nonmagnetic ligand ions are considered to contribute little to it. On the contrary, we show it originates from the anisotropy of the magnetic moments of O (ligand) ions in 4d ferromagnetic Weyl semimetal SrRuO₃. The individual magnetic moments of Ru and O for stoichiometric 1-10 nm-thick films were evaluated by the ratios of X-ray magnetic circular dichroism (XMCD) to X-ray absorption spectroscopy (XAS) peak intensities. Comparison of the data taken between perpendicular or in-plane configurations revealed that the O magnetic moment is anisotropic while the Ru one is isotropic for the ultra-thin films (1-2 nm), though they are both anisotropic for the thicker films (4 -10 nm). Since the coherent O K-edge Ru 4d t_2g peak intensity also decreases with decreasing thickness, indicating less Ru 4d t_2g-O 2p hybridization, we can conclude that the anisotropy in the ligand (O) determines the magnetic anisotropy of SrRuO₃. This novel concept of ligand-driven magnetic anisotropy is distinct from the conventional single-ion model, where the effects of the crystal field at the d orbitals of magnetic elements predominate [1].

 

[1] Y. K. Wakabayashi, et al., arXiv:2309.05228 (2023).