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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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The room temperature ferromagnetic resonance (FMR) absorption at 8.9–11.2 GHz in two planar hexagonal ferrites, Zn2Y and MnZnY, has been found to exhibit strong nonlinear behavior at low power levels corresponding to microwave field amplitudes on the order of 10 mOe. For somewhat larger amplitudes, 20–50 mOe, the asymmetric resonance curves show pronounced foldover effects. With increasing static field, the absorption increases gradually to a steep cutoff at a field several hundred Oe above the position of the low power FMR. With decreasing field, a sharp spike occurs coincident with the high field tail of the low power FMR line, followed by a gradual decrease which tracks the upsweep absorption. The detailed shapes of these curves depend strongly on sweep rate. Steady-state absorption curves at the highest powers employed, with 350 mW incident power and a microwave field amplitude of 58 mOe, result only for very slow sweep rates of 20 Oe/min or less.