Temperature dependence of hyperfine magnetic fields in Fe-Ni

The ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ hyperfine magnetic field (HMF) distribution in bcc Fe-Ni alloys was calculated with a model of linear response of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ HMF to magnetic moments in the alloy. With the use of empirical parameters, the model largely accounts for the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ HMF distribution at low temperatures. As shown by experiments with Si solutes in Fe-Ni, the anomalously strong temperature dependence of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ HMF in Fe-Ni is not due to the temperature dependence of the HMF response parameters. By analyzing the shape of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ HMF distribution, we find that this anomalous temperature dependence results from a large thermal sensitivity of the magnetic moments at those Fe atoms with more Ni atoms as nearest neighbors. This correlated with a strong temperature dependence of the recoil-free fraction and the second-order Doppler shift in Fe-Ni. We suggest that the large mean-square thermal displacement of Fe atoms in Fe-Ni is the cause of the anomalously strong temperature dependence, and we offer two explanations for this effect. Additionally, we have found evidence for a pseudodipolar interaction in Fe-Ni, and we also discuss the problems of parametrizing the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ HMF solely in terms of the number of nearest neighbors of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ atom.