[en] Magnetic relaxation effects revealed in Moessbauer spectra and magnetization measurements of nanoparticles are discussed in the framework of specific models for magnetic dynamics of an ensemble of single-domain particles, which allow one to treat both the temperature and magnetic field-dependent magnetization curves and Moessbauer spectra in a self-consistent way within the same set of physical parameters inherent to the system studied. Based on the stochastic approaches, a simplified 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 Moessbauer absorption spectra of nanoparticles in a weak magnetic field.