The Temperature Dependent Line Shape of Mössbauer Hyperfine Spectra in the Paramagnetic and the Ordered Region

A general, microscopic and quantum mechanical theory of the temperature dependent line shape of Mössbauer hyperfine spectra is presented. This theory covers the slow as well as the fast relaxation region and spectra of both paramagnetic and magnetically ordered samples. On a phenomenological level the theory results in generalized Wickman equations but with explicit expressions for the relaxation parameters in terms of the interaction Hamiltonians. The connection between this fully quantum mechanical theory and the stochastic models is also discussed. In the second part relaxation times are calculated for special interaction Hamiltonians. The relation between the relaxation times obtained by Mössbauer measurements and the results of other relaxation time measurements, especially those of the longitudinal susceptibility χ|, is also considered. In the third part the critical slowing down of the relaxation time above a transition point is briefly discussed. Finally the superparamagnetism hypothesis, which is able to explain the so‐called anomalous spectra measured below transition points, is outlined.