The Suhl theory for ferromagnetic instability is extended to include the coupling between the magnetization and strain. The threshold for magnetoelastic instability is found to be nearly the same as that for subsidiary absorption in YIG. The experimental threshold is found to agree with the predicted curves; however, the threshold field is found to be about 10% higher than that obtained in the usual manner for subsidiary absorption threshold. This raises some questions about the extension of the linear region of the susceptibility plot to intersect the saturation value of susceptibility in order to obtain the critical field. Explanations are given for the ``beating'' of magnetoacoustic oscillations and for the necessity of long pulse lengths of microwave power in order to produce magnetoacoustic instabilities.

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