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Mössbauer spectra of single-domain particles in a weak magnetic field

A three-level stochastic model taking into account the magnetic anisotropy, precession and diffusion of uniform magnetization of single-domain particles is developed in order to describe the Mössbauer absorption spectra of an ensemble of magnetic nanoparticles in a weak magnetic field. In contrast to conventional approaches searching for a distribution of the hyperfine field at nuclei, this model allows one to take into consideration physical mechanisms of formation of the magnetic hyperfine structure within the magnetic dynamics inherent to such materials. A number of qualitative effects observed in experimental Mössbauer spectra taken on small magnetic particles even in zero magnetic field can be self-consistently explained within the model in terms of the mean-field interparticle interaction. In particular, this model predicts the appearance of 57Fe magnetic sextets with a small hyperfine splitting slightly dependent on the particle size and temperature in a weak magnetic field and at high temperature, which look like effective ‘doublets’ of lines often observed in experimental spectra.