Orbital-to-Spin Ratio of Dynamic Magnetization in Co/Pt

Recent theoretical work suggests that ferromagnets can generate orbital angular momentum through resonant magnetization dynamics involving both spin and orbital components [1,2]. X-ray magnetic circular dichroism measure the ratio of orbital-to-spin moment (μ_L/μ_S) in the dynamic magnetization of Co/Pt, a material combination known for strong spin-orbit coupling. We aim to explore whether orbital and spin magnetization dynamics can be distinct.

The dynamic X-ray transmission response of the FMR-driven Co magnetization precession at various photon energies was recorded, yielding the dynamic XMCD at Co L_3,2 edges. By applying the XMCD sum rule analysis, we find (μ_L/μ_S)_dynamic = 0.107 ± 0.002. This is about 5% more than (μ_L/μ_S)_static = 0.102 ± 0.002 obtained from the static XMCD. The error bars for (μ_L/μ_S)_dynamic and (μ_L/μ_S)_static account for uncertainty due to background signal processing. The smaller (μ_L/μ_S)_static compared to (μ_L/μ_S)_dynamic could imply faster spin dissipation than orbital dissipation in Co interfaced with Pt. Our research contributes to understanding non-equilibrium orbital and spin angular moment, crucial for advanced "spin-orbitronic" devices.

[1] D. Go et al., arXiv:2309.14817 (2023).

[2] H. Hayashi et al., Nature Electronics, 1-7 (2024).