Temperature Dependent Magneto-Optical Studies of Dynamic Spin Alignment in FeGd Thin Films

We examine temperature dependent spin dynamics in ferrimagnetic Fe_1-xGd_x thin films using pump-probe, ultrafast time-resolved magneto-optic Kerr effect (tr-MOKE). The Gd content (x) was varied to alter the magnetic compensation temperature (T_MC).  Room temperature scans reveal a rapid demagnetization which occurs on the sub-ps timescale, followed by a return to equilibrium that is accompanied by coherent oscillatory motions of the Fe/Gd spins with differing timescales.  The demagnetization signal reverses sign as a function of Gd content, indicating that we move through the Fe/Gd ratio that produces T_MC at room temperature.  The GHz-scale ferromagnetic resonance oscillations damp out relatively quickly (~200 ps) and exhibit a non-monotonic relationship with Gd content. In a sample which has T_MC ~ 225 K, we observe a similar variation of the GHz range FMR oscillations as a function of temperature and we also track much faster dynamics closer to time zero.  These quickly-damped oscillations occur up closer to ~600 GHz, and the frequency of this mode also varies with temperature, having a peak close to T_MC.  We attribute this  faster, near-THz scale mode to an exchange mode oscillation where the angle between the normally anti-ferromagnetically aligned Fe and Gd moments deviates from 180 deg.