Fine Structure in the Decline of the Ferromagnetic Resonance Absorption with Increasing Power Level

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Abstract
A theory of ferromagnetic resonance at high signal powers is developed. The stationary response at high power levels is investigated for the case in which the unstable spin waves have the same frequency as the applied signal. It is found that a fine structure should be superimposed on the general decline of the resonance absorption with increasing power level. This fine structure arises from the discrete nature of the spin-wave spectrum. It should be observable even if the frequency separation of adjacent spin-wave modes is much smaller than the inverse of their relaxation times. The fine structure appears as a series of kinks superimposed on the general decline of the resonance absorption with increasing power level. The separation of subsequent kinks increases with decreasing sample volume and increasing exchange field. An interpretation of experimental data along the lines suggested in this paper should yield information about the strength of the exchange coupling.