Temperature and magnetic field dependence ofrare−earth↔ironexchange 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 ${f}_{\mathrm{exch}}$ of the magnetization precession. In this work we investigate these modes over a wide range of temperatures ($50--300\phantom{\rule{0.16em}{0ex}}\text{K}$) 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 $1/{f}_{\mathrm{exch}}(T)$ allows identifying unambiguously ${f}_{\mathrm{exch}}$ with the rare-earth $\ensuremath{\leftrightarrow}$ iron exchange mode. In addition, at low temperature ${f}_{\mathrm{exch}}$ is independent of the field as usually observed. In contrast, we find that near ${T}_M,\phantom{\rule{0.28em}{0ex}}{f}_{\mathrm{exch}}$ 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 ${H}_{\mathrm{ext}}$.