Longitudinal Susceptibility of Ferromagnets in Strong rf Fields

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Abstract
The mechanism of spin‐wave instability in an rf magnetic field applied parallel to the dc field is explained by simple, physical arguments. These spin waves give rise to rf and dc magnetic moments parallel to the dc field. The major part of the rf magnetic moment lags a quarter‐period behind the rf magnetic field so that the susceptibility is almost purely imaginary. The real part of the susceptibility vanishes unless the frequency of the unstable spin waves differs from half the pump frequency. The dc magnetic moment induced by the rf field is always larger than the amplitude of the rf magnetic moment. The theoretically expected variation of the susceptibility with the rf magnetic field is discussed and compared with experimental data.