Ferromagnetic resonance has been observed in a single-crystal sample of barium ferrite, BaFe12O19, under both single-domain and multidomain conditions, for orientations of the static field from 0°–90° with the easy axis of magnetization. The results verify in detail the predictions of the theory of Smit and Beljers for single-domain conditions, and show qualitative agreement with the multidomain theory.

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Mössbauer and neutron diffraction studies of barium ferrite show that the iron ion in the 2b (trigonal bipyramid) site oscillates or hops between two equilibrium positions on either side of the mirror plane of the bipyramid, for temperatures above 80 K. Since this motion is uncorrelated from one (2b) site to another, it can lead to two-magnon scattering of spin waves due to the time-dependent fluctuation in the anisotropy energy. The contribution of this mechanism to the ferromagnetic resonance linewidth of barium ferrite is calculated, using time-dependent perturbation theory. For reasonable values of the parameters, this linewidth is of the order of 10 Oe at 300 K.