Magnetic Structures and Spin-Wave Excitations in Rare-Earth Iron Garnets Near the Compensation Temperature

We introduce a simple model for the ferrimagnetic non-collinear “magnetic umbrella” states of rare-earth iron garnets (REIGs), common when the rare-earth moments have non-zero orbital angular momentum. The spin-wave excitations are calculated within linear spin-wave theory and temperature effects via mean-field theory. This could be used to determine the magnetic polarization of each mode and thereby the spin currents generated by thermal excitations, including the effects of mixed chirality. The spectra reproduce essential features seen in more complete models, with hybridization between the rare-earth crystal field excitations and the propagating mode on the iron moments. By the symmetry of the model, only one rare-earth mode hybridizes, inducing a gap at zero wavenumber and level repulsion at finite frequency. At the compensation point, the hybridization gap closes, and finally, as we approach the Néel temperature, the hybridization gap appears to reopen. The chirality of the lowest mode changes its sign around the frequency at which the level repulsion occurs. This is important to estimate the spin current generation in REIGs.