Temperature Dependence of theFe57hfs in the Ordered Alloys FePd3and FePd near the Curie Temperature

The temperature dependence of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ hfs in ordered Fe${\mathrm{Pd}}_3$, FePd, and a disordered ${\mathrm{Fe}}_{0.222}$ ${\mathrm{Pd}}_{0.778}$ alloy has been measured, using the Mössbauer effect near the Curie temperature $T_c$. These measurements yield values for the critical exponent $\beta$ of 0.371±0.01, 0.377±0.01, and 0.394±0.02, respectively, in the temperature region $0.85<\mathrm{}\frac{T}{T_c}<0.99\mathrm{}$. The values of $\beta$ obtained for the ordered alloys are higher than the value $\beta =0.313\mathrm{}\pm 0.004$ predicted on the basis of a nearest-neighbor ising model but lie within the range of values $0.36\le \beta \le 0.41$ derived from a Heisenberg model. The values indicate that the long-range magnetic exchange interaction, known to exist in dilute Fe-Pd alloys, if it exists in these concentrated alloys, does not have an appreciable effect on the value of $\beta$. The fact that the present values of $\beta$ are lower than those previously obtained ($\beta \simeq 0.43$) for disordered Fe-Pd alloys containing ≃10% iron verifies the suggestion that the latter higher values are due to the presence of a magnetization distribution arising from the spatial distribution of iron atoms in disordered alloys.