Bistability phenomena in electron paramagnetic resonance

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Abstract
A discussion of new bistability phenomena observed in electron-paramagnetic-resonance spectra is given. The effects depend on the number of paramagnetic centers and their positions in the cavity and occur also in the presence of a microwave linear response of the sample. Very accurate measurements with two different techniques show that the response of the resonator corresponds to a double well with two resonant frequencies when proper threshold conditions are exceeded. Theoretical results, based on circuit network theory combined with the Bloch equation response of a spin system at magnetic resonance, fully explain the measurements. The influence of different physical parameters on the mechanism driving the overall phenomenon is determined. The main features of the observed effects are discussed in terms of applications to the study of instabilities and noise effects.