MAGNETIC AND STRUCTURAL-PROPERTIES OF AS-MILLED AND HEAT-TREATED BCC-FE70CU30 ALLOY

A ferromagnetic solid solution with a nominal atomic composition Fe70Cu30 and a body-centered structure has been obtained by high-energy ball milling. The decomposition of the system is monitored by X-ray diffraction (XRD), magnetic measurements and Mossbauer spectroscopy. According to XRD, for heating temperatures below 723 K there is only a bcc phase in the material, while for heating temperatures above 723 K a new phase, with a fee structure, appears, suggesting that the solid solution has decomposed into bcc-Fe and fcc-Cu. However, the magnetic behavior observed during the decomposition process indicates that this evolution is more complex than the simple decomposition into the equilibrium phases. This behavior can be summarized in two points: (1) a decrease in the magnetization at 5 K, and (2) drastic changes in the coercive field with the thermal treatment, soft magnetic behavior for the material in the as-milled state, superparamagnetism for low heating temperatures and a hardening of the material heated to above 723 K, for which the values of the coercive field at room temperature are several times higher than those for the as-milled sample. The Mossbauer spectroscopy performed at room temperature shows that for the heat-treated samples the Fe atoms are in two different phases: a ferromagnetic phase, which evolves to bcc-Fe, and a paramagnetic phase.