COLLAPSE IN THE STONER-WOHLFARTH NONINTERACTING-PARTICLE MODEL

A theory of Mossbauer spectra of noninteracting Stoner-Wohlfarth (SW) particles interacting with rf magnetic fields is developed. The theory makes it possible to calculate the absorption spectra for arbitrary f requency and amplitude of the rf field. The main features of the Stone r-Wohlfarth model are discussed. The Liouville superoperator formalism is used to generalize the results to the case of arbitrarily time-var ying hyperfine fields at a nucleus. To understand the qualitative feat ures of the collapse effect that are observed in the Mossbauer spectra of SW particles the particular case of a circularly polarized hyperfi ne field is studied, and an analytical expression is obtained describi ng the Mossbauer spectra for this case. An analysis is alsb made for w eak rf magnetic fields and in this case the resonance behavior of the Mossbauer lines is traced as a function of the frequency of the rf fie ld. (C) 1998 American Institute of Physics. [S1063-7761(98)01705-3].