Ferromagnetic resonance (FMR) absorption was observed in a spherical single‐crystal sample of Y3Fe5O12(YIG) at 9.5 GHz and room temperature. Resonance absorption curves were obtained as a function of microwave power level and duty cycle for different static field directions relative to the crystal axes. Foldover effects and other nonlinear behavior were seen at microwave field amplitudes (hrf) between 9 and 50 mOe with the applied static field in the (110) plane. When FMR absorption is observed with a high power cw signal (hrf=50 mOe) with the biasing field along either the 100 or the 110 directions and decreasing in magnitude, an absorption curve is obtained that has a long broad shoulder which is shifted below the low power FMR biasing field position. With increasing field in these directions, the absorption curve develops a cusp below the low power biasing field position. With the bias field in the 111 direction the absorption curve shows cusps for both increasing and decreasing fields at this power level. Field shifts in the observed resonance spectra do not converge to the zero anisotropy resonance field value, but to a field value below the low power 110 direction FMR field position. These effects disappear when the experiments are carried out using pulsed microwaves with a pulse width of 100 mgr;sec and duty cycles below 1. These phenomena are therefore attributed to the effects of heating on the magnetocrystalline anisotropy properties of single‐crystal YIG. The quantitative results, however, are not in accord with simple heating considerations.
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