Theory of auto-oscillations in high-power ferromagnetic resonance

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Abstract
An account is given of the frequently observed next notable event as the microwave signal power is increased well beyond the threshold of the old spin-wave instability: relatively-low-frequency auto-oscillations. These are attributed to a Hopf bifurcation. Taking into account excitation of the entire degenerate spin-wave manifold in the three standard cases (main resonance, subsidiary absorption, parallel pumping), we use center-manifold theory to show that a small number of new modes grow out of the excited steady state of that manifold, yielding a limit cycle which we identify with the observed auto-oscillations with characteristics qualitatively in agreement with experiment.