A microscopic model for the spin-spin relaxation of Fe3+ in an amorphous Mössbauer sample

A model for the spin-spin relaxation of glassy paramagnetic substances in external magnetic fields is developed and applied to frozen aqueous solutions of Fe(NO3)3. In these the ferric ions are surrounded by weakly distorted octahedral complexes of water molecules. The relaxation process is characterized by several relaxation frequencies which replace the single flip frequency of earlier approaches. In contrast to the latter, they can be calculated from the microscopic properties of the sample. The only model parameter is the ligand field coupling constant D determining the relaxational behavior. A set of Mossbauer spectra, measured under various experimental conditions, is well described by a single value of the ligand field parameter D = 2.2gμB kG. Some characteristic features of the model become obvious in the spectra, e.g. the fading of certain absorption lines with increasing Fe3+ concentration.