A ferromagnetic and supersaturated fcc ${\mathrm{Fe}}_{51}$ ${\mathrm{Cu}}_{49}$ solid solution has been obtained by mechanical alloying. After subsequent thermal treatments the fcc phase undergoes a spinodal decomposition which finally, at 780 K, yields a mixture of fcc and bcc phases. In this work, a systematic magnetic study is carried out on samples at different decomposition states in order to determine the process of transformation into the stable phases. We observe a 20% maximum diminution on the magnetic moment with increasing temperatures of the thermal treatment. The Mo¨ssbauer spectrum taken at 8 K shows that 20% of the Fe atoms are in a nonferromagnetic state. On the other hand, upon heating up to 723 K the room-temperature coercive field increases dramatically to 640 Oe, and after cooling down to 10 K it decreases to 270 Oe. Deviations from the ${\mathit{T}}^{3/2}$ law in the temperature dependence of the magnetization have been observed. This behavior is explained by fluctuations in composition due to the spinodal decomposition, which lead to fluctuations of the magnetic order parameters, i.e., magnetic moment and Curie temperature.

A Comment on the Letter by A. R. Yavari et al., Phys. Rev. Lett. 68, 2235 (1992).

Experimental results corresponding to the saturation magnetization and coercive field during the decomposition, upon annealing, of bcc and fcc ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$, obtained by mechanical alloying are reported. The overall behavior points out that the decomposition takes place in two steps: (i) at low temperatures a decrease of the saturation magnetic moment as well as an anomalous thermal dependence of coercive field are observed, however, no phase transformation is detected, and (ii) for further annealing temperatures a new phase appears; the magnetization tends to increase and the coercive field abruptly increases. The analysis of the results leads us to conclude that the first step corresponds to a spinodal decomposition. Fluctuations in the local composition give rise to coexistence of adjacent regions with Curie temperature varying continuously in a range of 1000 K across distances of a few nanometers, thus allowing the tailoring of the magnetic nanostructures.