Hysteretic characteristics of low-field microwave absorption of a Co thin film

We have investigated the spin dynamics of a sputtered Co thin film using our broadband ferromagnetic resonance (FMR) spectrometer. From FMR spectra taken at frequencies of 4–20 GHz, we found that our Co film has a $g$-factor of 2.25 and a Gilbert damping factor of $4.5×108s−1$⁠, indicating an enhanced spin-orbit interaction compared to bulk material or epitaxial films. Besides the normal FMR mode in the saturated state, we also observed the evolution of the low-field hysteretic behavior in the unsaturated state, which affects the FMR mode as the excitation frequency is lowered from 5.000 to 1.636 GHz. We found that the microwave absorption process persists in the unsaturated state for frequencies higher than 1.868 GHz such that the absorption minima occur at −12 Oe on a down-field sweep and at $+12Oe$ on a up-field sweep, respectively.