Multi-level relaxation model for describing the Mössbauer spectra of single-domain particles in the presence of quadrupolar hyperfine interaction.

A multi-level stochastic model taking into account the magnetic anisotropy, precession and diffusion of the uniform magnetization of single-domain particles is developed in order to describe the Mössbauer absorption spectra of an ensemble of magnetic nanoparticles in the presence of quadrupolar hyperfine interaction with an arbitrary orientation of its principal axes. This model allows one to take into account physical mechanisms for forming the hyperfine structure in a real situation and can be easily realized even on a personal computer. In particular, now one can numerically describe qualitative features of temperature evolution of the Mössbauer spectral shape from a 'symmetric' magnetic sextet to a quadrupolar doublet of lines, which has been observed in a large number of experimental spectra of (57)Fe nuclei in magnetic nanoparticles for almost half a century.