Temperature and magnetic field dependence of rare-earth ⇆ iron exchange resonance mode in a magnetic oxide studied with femtosecond magneto-optical Kerr effect

In magnetic materials, the exchange is the strongest quantum interaction due to the Pauli exclusion principle. For that reason it can induce high-frequency modes f(exch) of the magnetization precession. In this work we investigate these modes over a wide range of temperatures (50-300 K) and magnetic fields up to 10T in a bismuth-doped garnet with perpendicular magnetic anisotropy by performing femtosecond magneto-optical pump-probe experiments. Near the compensation temperature T-M the divergence of 1/f(exch)(T) allows identifying unambiguously f(exch) with the rare-earth <-> iron exchange mode. In addition, at low temperature f(exch) is independent of the field as usually observed. In contrast, we find that near T-M, f(exch) decreases linearly with an increasing magnetic field. This behavior is explained in the context of the ferromagnetic resonance theory by including the perturbation term linear in the external applied field H-ext.