<title>Non-equilibrium magnetism of nanoparticles revealed in static and radiofrequency measurements</title>

Abstract

Magnetic relaxation effects revealed in Moessbauer spectra and magnetization measurements of nanoparticles are discussed in the framework of a general model for magnetic dynamics of ensemble of single-domain particles. The phenomenological model is based on a generalization of the well-known Stoner-Wohlfarth model within more accurate description of relaxation processes and corresponding time-dependent hyperfine interactions in the magnetic system. This model allows one to treat numerically both Moessbauer spectra in radiofrequency magnetic field and magnetization curves in alternative low-frequency magnetic field as well as temperature demagnetization FC and ZFC curves in a self-consistent way within the same set of physical parameters inherent to the system studied. Besides that, a number of qualitative effects can be explained or predicted within the approach, which include interaction effects, relaxation-stimulated resonances in Moessbauer spectra under radiofrequency field excitation, specific shapes of Moessbauer spectra within precession of particle's uniform magnetization, and asymptotic high-temperature magnetization and susceptibility behavior different from the classical Langevin's high-temperature limit for ideal superparamagnetic particles. Corrections to the above-mentioned effects within more general models based on the Landau-Lifshitz-Gilbert or Braun kinetic equations are also discussed.