Influence of Spin Waves upon Harmonic Generation in Ferrites

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Abstract
Near the Suhl second‐order threshold for spin‐wave excitation in ferrimagnetic resonance the spin waves have, in general, important influences on harmonic generation which can be predicted from the theory of spin waves in ferrites. These influences are potentially useful for experimental studies of the properties of spin waves, and their understanding is important for the operation of practical frequency multipliers that use ferrites. The second‐harmonic generation from the uniform mode itself is described by a simplified theory in which the only influence of spin waves is to alter the loss parameter of the resonant component. Two further effects are then predicted which are of interest: (1) A strong enhancement of the third and fourth harmonics of the uniform mode occurs above threshold, resulting from interaction of a nonresonant uniform mode component with the spin‐wave dipolar field. (2) Random inhomogeneities or crystalline anisotropy can lead to a direct radiation of spin‐wave energy into an external second‐harmonic circuit, since the spin waves are elliptically polarized and the proper coupling mechanisms to produce coherence exist. Experimental results in certain instances give quantitative agreement with the theory of direct spin‐wave contribution to second‐harmonic generation, and a study of harmonic generation can lead to conclusions as to the nature of spin waves excited in resonance experiments.