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

Small monodomain magnetic particles of uniaxial magnetic anisotropy are considered. The superparamagnetic relaxation is assumed to happen by coherent rotation of the spins. The results of existing models for the calculation of superparamagnetic relaxation times are shortly reviewed. A different model based on phonon interaction with the total spin of the particle under the presence of an external magnetic field is presented and corresponding relaxation times are calculated and compared with the existing theories. The calculation of M\"ossbauer superparamagnetic relaxation spectra in the low-temperature limit and for higher temperatures is discussed.