Some fundamentals of Mössbauer spectroscopy and of fluctuating magnetic hyperfine interactions are reviewed. An expression for calculation of spin-phonon interaction transition probabilities on the ground of a Debye model is given. Basic ideas of superparamagnetism and classical results for low-temperature relaxation rates are presented. A theory of superparamagnetism based on spin-phonon interaction is introduced. It is shown how to calculate from this Mössbauer spectra taking all spin-levels into account. Experimental spectra of ferrofluids are compared with simulated spectra and found in good accordance.
