What is ferromagnetic resonance

It is pointed out that two distinct resonance effects take place in ferrite samples at microwave frequencies, ferromagnetic resonance and an effect due to the shape of the specimen. The latter effect is called Kittel resonance in this paper. It is commonly supposed that the two effects are the same, but it is shown that this is not the case. In ferromagnetic resonance, the diagonal and off-diagonal elements of the permeability tensor (μ and α respectively) exhibit anomalous dispersion, and microwave fields are excluded from the specimen. In Kittel resonance, nothing spectacular is happening to μ and α, but the fields in the specimen become very large, giving rise to a large loss; it is this effect that is observed in resonant cavity experiments. The behaviour of cavities and wave-guides containing ferrites is discussed in the light of these facts, and a new interpretation is arrived at of certain effects in waveguides when μ < α.