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We investigated microwave foldover and bistability responses for a microwave resonator structure containing a 100-/spl mu/m-thick yttrium iron garnet single-crystal film. We applied a static field of 3100 Oe perpendicular to the film plane, with continuous-wave and pulse microwave excitation at 4 to 5 GHz, and powers ranging from below 0 dBm up to 25 dBm. For input powers below 0 dBm, the main ferromagnetic resonance line was Lorentzian, centered at f/sub 0/=4540 MHz, and had a frequency width at half power of 17.6 MHz. Input powers above 0 dBm yielded nonlinear effects. For powers from 0 to 7 dBm, frequency sweeps gave distorted lines but no foldover. From 7 to 20 dBm, a hysteretic foldover response developed, with a gradual rise and a steep drop in absorbed power for up sweep and a cusp response on down sweep. We observed a corresponding nonlinear and bistability response when power sweep measurements were made at fixed frequency. The threshold frequency for bistability was about 15 MHz above f/sub 0/. The frequency shifts with power and the effect of changes in the sweep rate or pulse duty cycle indicate a thermal origin of these power-dependent nonlinear responses. All of the results could be accurately modeled though a resonance analysis in which the mode frequency f/sub r/ changed with the absorbed power P/sub a/ according to f/sub r/=f/sub 0/+BP/sub a/, with B=1.83 MHz/mW from the frequency shift data.