Ferromagnetic transitions have been observed in dilute random solutions of Co in Pd having Co concentrations between 0.07 and 4.5 at.%. The transition regions near $T_c$ have been explored by using the Mössbauer effect of ${\mathrm{Fe}}^{57}$ present as a very dilute impurity in the alloys. The hyperfine fields have been measured by the standard Doppler technique at low temperatures and by a thermal scanning method near the transition temperatures. Data have been interpreted as showing the effects of statistical fluctuations in the concentration of the alloys, and have been analyzed to yield values for the mean Curie temperature and the mean-square deviation in Curie temperature as a function of concentration. These values have been related to the spatial dependence of the exchange interaction through a molecular-field calculation. It is found that if one assumes the interaction is given by a Gaussian $J\left(r\right)=\exp (-\frac{r^2}{4{\sigma }^2})$, in correspondence with neutron diffraction results in Co-Pd alloys, then the values deduced from this study are $\sigma =11.5\mathrm{}\pm 1.2$ Å and $\frac{J_0}{k}=0.02\mathrm{}$ °K. However, comparison of the effective range obtained in the present study with the neutron data and the total localized moment suggest that the Gaussian form factor is incorrect at large distances and that the interaction has a more slowly varying and weak tail.

• Received 6 July 1966

DOI:https://doi.org/10.1103/PhysRev.155.460