Spin-lattice-relaxation-like model for superparamagnetic particles under an external magnetic field

Small monodomain magnetic particles of uniaxial magnetic anisotropy are con sidered. The superparamagnetic relaxation is assumed to happen by coherent rotation of the spins. The results of existing models for the calculation o f superparamagnetic relaxation times are shortly reviewed. A different mode l based on phonon interaction with the total spin of the particle under the presence of an external magnetic field is presented and corresponding rela xation times are calculated and compared with the existing theories. The ca lculation of Mossbauer superparamagnetic relaxation spectra in the low-temp erature limit and for higher temperatures is discussed.