SIMPLE THEORY OF SUPERPARAMAGNETISM AND SPIN-TUNNELING IN MOSSBAUER-SPECTROSCOPY

The magnetic relaxation of isolated small (< 100 Angstrom) monodomain magnetic particles is due to superparamagnetic relaxation (predominant at high temperatures) and eventually quantum tunneling of the magneti c moment (at low temperatures). The superparamagnetic relaxation proce ss can be formally described by an (multiple phonon absorption and emi ssion) Orbach process with an anisotropy Hamiltonian due to crystallin e or form anisotropy (H) over cap(Ion) = S-z(2) and a usual dynamical spin-Hamiltonian for the spin-phonon interaction. From this Mossbauer spectra can be calculated using ab-initio or stochastic methods. Phono n-assisted tunneling and its influence on Mossbauer spectra are discus sed.