Domain Structure Effects in an Anomalous Ferrimagnetic Resonance of Ferrites

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Abstract
Measurements of the intrinsic tensor permeability [μ] of unsaturated Ni ferrite at 9300 mc, using cavity perturbation techniques, have revealed an anomalous resonance for negative (anti‐Larmor) circularly polarized fields. The resonance has a line width of 70 oe and occurs very near the coercive force which is 13.2 oe. The usual resonance for positive (Larmor) circularly polarized fields occurs at 2750 oe. An interesting consequence of the low‐field resonance is that below saturation K″ is negative, where K=K′−iK″ is the off‐diagonal component of [μ]. Above saturation K″ is positive. The change in sign of K″ is shown not to violate energy considerations. These effects are explained by a theory involving rf demagnetizing poles on Bloch walls based on a model with a physically probable domain structure. The theory extends the Polder‐Smit mechanism in unmagnetized media to include applied dc fields and circularly polarized rf fields. It is applicable to all ferrites, both mono‐ and polycrystalline, having domain walls free to move in dc fields of the order of the coercive force. Since the anomalous resonance depends upon the domain structure and appears to occur generally, it is a probe of considerable usefulness for the study of magnetization processes and high‐frequency phenomena in ferrites.