PHYSICAL REVIEW B VOLUME 48, NUMBER 10 1 SEPTEMBER 1993-II Magnetic behavior of metastable fcc Fe-Cu after thermal treatments P. Crespo and A. Hernando Instituto de Magnetismo Aplicado, UCM-REBUFF, P. O. Box 155, Las Rozas, 28230 Madrid, Spain R. Yavari and O. Drbohlav St. Martin d'Heres 38402, France A. Garcia Escorial CENIM, Consej o Superior de Inuestigaciones Cientificas CSIC, Auda G. de. l Amo, 8, 28040 Madrid, Spain J. M. Barandiaran and I. Orue Uniuersidad del Pass Vasco, P. O. Box 644, 48080 Bilbao, Spain (Received 29 March 1993; revised manuscript received 30 April 1993) A ferromagnetic and supersaturated fcc Fe5lCu49 solid solution has been obtained by mechanical al- loying. 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 car- ried out on samples at diferent 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 increas- ing temperatures of the thermal treatment. The Mossbauer 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 T 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 tempera- ture. I. INTRODUCTION Although iron and copper are almost immiscible in the equilibrium state, Uenishi et a/. , Yavari, Desre, and Benameur and Eckert et a/. have recently produced fcc Fe-Cu solid solutions by mechanical alloying. Thin films were previously obtained by vapor deposition (Chien et al. ). Several authors have reported magnetic measurements on as-milled samples in a broad compositional range. Uenishi et a/. ' have shown that the magnetic moment for the Fe atom is around 2. 2pz for Fe contents higher than 50%%uo Fe, but it falls to zero for less than 20% Fe. Chien et a/. have shown that the Curie temperature is very sen- sitive to the composition. For Fe contents higher than 70% the solid solution is bcc, while for contents lower than 60% it is fcc. For intermediate compositions both phases coexist. Yavari, Desre, and Benameur show a near equality of the magnetic moment at room temperature for both the fcc phase and its decomposed state. By considering the difference in Curie temperatures between the fcc Fe-Cu phase and a iron, which are, respectively, about 500 and 1040 K, the reported equality of magnetic moments at 300 K suggested to us the existence of crossover in the magnetization curves at lower temperatures. In order to analyze a possible leakage of iron magnetic moment at in- termediate states of the fcc Fe-Cu decomposition process, a study of the magnetization behavior of fcc Fe-Cu after thermal treatment is performed in this work. II. EXPERIMENTAL TECHNIQUES Samples were prepared by mechanical alloying of pure Cu in thin foil form and Fe powder in a Fritsch vibrating ball mill modified to allow vacuum levels of 10 torr in the vial before introducing an operating pressure of 1. 5 atm of argon. Milling was done in a stainless-steel vial with a 6-cm steel ball. The milling process was moni- tored by x-ray diffraction (XRD). After milling for 360 h, only Bragg peaks of the fcc solution remain. In order to avoid overlapping of peaks, high-resolution x-ray-diffraction patterns were obtained at very low scale velocity using Cu Ka radiation. Additional information is obtained from the peak widths. Grain size was es- timated using Scherrer's formula, from the Cu (111) and Fe (110) peaks. The thermal stability was examined by differential scanning calorimetry (DSC), at a scanning rate of 20 K/min up to 923 K, under pure argon flow, treatments at different temperatures were carried out in the DSC in the same conditions. The heat treatments of samples for magnetic measurements were performed in the same calorimeter by heating up at 20 K/min to 423, 523, 623, 723, and 923 K and cooling down to 300 K. Low-temperature magnetic measurements and hys- 0163-1829/93/48(10)/7134(6)/$06. 00 48 7134 1993 The American Physical Society