ac susceptibility of the ferromagnetic phase of theAuFesystem

The ac susceptibility of a Au-22-at.%-Fe alloy has been studied as a function of both applied field and thermal history in the vicinity of the Curie temperature $T_C$. The principal maximum in the ac susceptibility becomes weaker, broader, and moves to lower temperatures relative to the zero-field maximum, as the external field is increased. A second narrower peak is consequently observable, and is identified as the true critical peak. Analysis of the critical behavior reveals that the exponent $\gamma =1.51\mathrm{}$ while $\delta =3.6\mathrm{}$ for the as quenched sample, and its effective Fe moment is estimated at about $6{\mu }_B$. The effects of thermal history are striking; $T_C$ is found to increase from 213 to 278°K on aging the rapidly quenched specimen for 1500 h at 24°C, but subsequent aging between 100 and 300°C causes $T_C$ to fall to about 228°K. Both $\gamma$ and ${\mu }_{\mathrm{eff}}$ are unchanged by such treatment, but the apparent value for $\delta$ increases to 3.9. These results are not consistent with the formation of large magnetic clusters (or precipitates) during heat treatment, but may be explained by the occurrence of short-range order which varies the arrangement of magnetic neighbors.