Internal Field ofFe57in Nickel from 77°K to the Curie Point

The internal field of very dilute ${\mathrm{Fe}}^{57}$ impurities in nickel has been measured from 77°K to 0.99 $T_c$. The temperature dependence of the field is not the same as that of the magnetization, the discrepancy reaching 15% at intermediate temperatures. Correction of both magnetization and internal field to constant volume increases the divergence. Over the entire experimental temperature range, the internal field can be represented by a Brillouin function of the form proposed by Jaccarino, Walker, and Wertheim for ${\mathrm{Mn}}^{55}$ in iron, indicating quasi-independent localized impurity moments. The empirical exchange field between Fe impurities and the Ni host is larger than the field between Ni ions, in contrast to the relatively weaker coupling of Mn in Fe. Therefore, the conditions for quasi-independent localized moments seem to be fulfilled for the case of the substantially stronger, as well as the weaker, couplings, in accord with a prediction of Callen, Hone, and Heeger.