Probing anisotropy in epitaxial Fe/Pt bilayers by spin-orbit torque ferromagnetic resonance*

Spin-orbit torques (SOTs) in novel materials have attained much attention recently as they have the potential to revolutionize highly energy-efficient storage applications. From this end, spin-orbit torque ferromagnetic resonance (STFMR) is a prominent choice for studying spin-orbit torques in multilayers. However, an overlooked aspect of STFMR has been the dynamic response driven by oscillatory SOT.

Here, we report the generation and detection of STFMR in micropatterned epitaxial Fe/Pt bilayers grown by molecular beam epitaxy. The biasing magnetic field was applied in an in-plane geometry at an angle of 45° with respect to the microwave-current direction to maximize the signal strength. The STFMR measurement was compared with broadband ferromagnetic resonance spectroscopy of the unpatterned bilayer thin films. The results are interpreted using an analytical formalism which indicates the presence of strong magnetic anisotropy. This was further confirmed using micromagnetic modeling, which reveals the field-dependent magnetization alignment in the microstructures. Our results demonstrate a simple and efficient method for determining magnetic anisotropies in individual microstructures by means of on-chip rf spectroscopy.