Role of the Ytterbium Spins in the Spin Reorientation in YbFeO3

The spin-reorientation transition in YbFeO3 is explained by including the rare earth-iron exchange interaction in a spin-wave model, without resorting to a temperature-dependent iron anisotropy. In a four-sublattice model (2 Fe and 2 RE) the iron spins are coupled to each other and to the RE spins by both symmetric and antisymmetric exchange. There are four modes at k=0. Two of these modes reduce to those of the iron-only model in the absence of the RE spins. Another mode has the frequency of a single RE spin in the exchange field of the iron sublattices. The fourth mode goes to zero frequency when the Fe-RE interaction cancels the iron anisotropy, resulting in a spin-reorientation transition. Measurements of far-infrared absorption spectra of YbFeO3 have been made and confirm many aspects of the model. The similarities between the spin-reorientation transitions in different orthoferrites suggest that a similar model may be valid for all.