Temperature dependence of hyperfine magnetic fields in Fe-Ni

The 57Fe hyperfine magnetic field (HMF) distribution in bcc Fe-Ni alloys was calculated with a model of linear response of the 57Fe HMF to magnetic moments in the alloy. With the use of empirical parameters, the model largely accounts for the 57Fe HMF distribution at low temperatures. As shown by experiments with Si solutes in Fe-Ni, the anomalously strong temperature dependence of the 57Fe HMF in Fe-Ni is not due to the temperature dependence of the HMF response parameters. By analyzing the shape of the 57Fe 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 57Fe HMF solely in terms of the number of nearest neighbors of the 57Fe atom.