Magneto-Optical Studies of Static and Dynamic Spin Alignment in Ferrimagnetic Thin Films*

We investigate a series of ferrimagnetic thin film samples consisting of (FeCo)_1-xGd_x and Fe_1-xGd_x where the Gd content was varied to modify magnetic parameters. The ferrimagnetism arises from the anti-ferromagnetic alignment of the Gd and transition metal (TM) moments. A magnetic compensation occurs at room temperature (RT). Low energy magneto-optical Kerr effect (MOKE) probes only the TM states near the Fermi level. For Fe-rich samples, the hysteresis loop is aligned with the external field while Gd-rich samples display a reversed loop. The coercivity of the films diverges when the magnetic sub-lattices are compensated. We use pump-probe time-resolved MOKE (tr-MOKE) to examine the role of compensation on spin dynamics. All samples exhibit a sharp change in TR-MOKE signal at short (~1 ps) timescales, confirming the demagnetization. The sign of the magnetization reverses across compensation, as observed in the static MOKE. Longer timescales reveal precessional oscillation signals that increase in frequency with increasing external magnetic field. The data are analyzed to extract magnetization and damping values, as well as the gyromagnetic ratio.