Dynamics of an ensemble of single-domain magnetic particles

The dynamics of an ensemble of noninteracting single-domain magnetic particles is investigated both on the basis of analytic solution of the Fokker–Planck equation and in the framework of the reduced-description method. It is shown that in the general case the shape of the resonance and relaxation curves is not Lorentzian. In the isotropic case, the deviations from Lorentzian form reach $7\%$. In the presence of anisotropy, the main source of broadening of a resonance is thermal spread of the precession frequencies of the magnetic moments. An exact expression is obtained for the integral time of longitudinal relaxation of magnetic. particles with axial anisotropy; it is valid for any value of the potential barrier. It is shown that for isotropic particles the description based on one and two lowest moments of the distribution function is in good agreement with the obtained exact results. In the first approximation of the moment method a generalized equation of Landau-Lifshitz-Bloch type is obtained; it gives a reduced description of the dynamics of the ensemble of magnetic particles in the general nonlinear case.