Optical Detection of Phase-Resolved Ferromagnetic Resonance in Epitaxial FeCo Thin Films

We report phased-resolved ferromagnetic resonance (FMR) measurements in an epitaxial Fe₅₀Co₅₀ thin film using a heterodyne, optical detection method. We track the evolution of the precessional phase across the saturated and unsaturated regimes along the FeCo hard axis. For the two regimes, opposite phase evolutions with frequency are observed, which coincide with the negative effective field dependence on the biasing field for the unsaturated modes. In addition, a nonzero phase advance between 0.2 and 0.4 rad is found for the saturated with respect to the unsaturated modes, which indicates an incomplete spatial overlap of the two modes in the film caused by surface pinning effect. The optical method has the advantage of resolving both the spatial and phase information in FMR measurements, which will be closely relevant and adaptable to epitaxial samples with multiple anisotropy axes, and for both flip-chip-based continuous films and spin-torque-based nano-devices.