Nonadiabatic, stochastic model for the classic relaxing paramagnet ferrichromeA: Theory and experiment

Mössbauer spectroscopy is an important tool for the study of systems showing ionic spin relaxation. Progress in stochastic theoretical models for such systems has made possible a detailed analysis of Mössbauer spectra showing relaxation effects in terms of meaningful physical parameters. In order to test the applicability of these methods, we have performed Mössbauer-effect measurements on the classic relaxing paramagnet ferrichrome $A$ down to 115 mK, and have analyzed our spectra using the nonadiabatic stochastic relaxation theory of Clauser and Blume. We have obtained good theoretical fits to our data over the temperature range from 4.2 K down to 115 mK holding fixed all parameters except sample temperature. From our results we obtain the values for the crystal-field spin-Hamiltonian parameters $D=-0.29\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $\frac{E}{D}=0.25\mathrm{}$. In addition, we determined that the hyperfine interaction is not isotropic or axial, and that the major component of the hyperfine interaction tensor has a value in the principal-axis system corresponding to a field at the nucleus of 215 kOe/unit spin. We find that from 4.2 K to 115 mK the spin-spin interaction is the dominant relaxation mechanism. A series of calculations and theoretical Mössbauer spectra are discussed in order to show the effects of various physical situations and their interpretations.