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We investigated microwave foldover and bistability responses for a microwave resonator structure containing a 100-μm-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 f0=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 f0. 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 fr changed with the absorbed power Pa according to fr=f0+BPa, with B=1.83 MHz/mW from the frequency shift data.
Date of Publication: March 2004