Identification of a High Temperature Magnetic Phase Transition in Ball-milled Nanocrystalline Fe-cu Alloys

Fe -Cu alloys (40≤x≤90) prepared by ball milling nominally pure (99%) Fe and Cu powders and warm x 100-x compacted (at ∼300 °C) were investigated by differential scanning calorimetry, scanning electron microscopy, x-ray diffraction, and magnetic susceptibility measurements. Both FCC and BCC diffraction peaks (indicative of pure Fe and Cu) showed that the mixtures were still two phase even after milling for 400 h, and that they were comprised of 6 nm to 10 nm diameter grains. Surprisingly, however, calorimetric measurements indicate the presence of a large endothermic peak for these nanocrystalline composites on heating near 600 °C and an exothermic peak near 400 °C on cooling. Magnetic measurements show that these materials are ferromagnetic at room temperature and remain so (with decreasing saturation magnetization) up to near the Curie Point of -Fe, 770 °C. However, near 600 °C on heating (and also near 400 °C on cooling) the magnetic susceptibility indicates the existence of a magnetic phase change. High temperature x-ray diffraction data show these effects are due to the oxidation of Fe to form magnetite which subsequently transforms into wustite. The thermal hysteresis observed in the magnetic and thermal data is due to the sluggishness of the latter transformation. Furthermore, heating to temperatures in excess of 600 °C results in the dissolution of Cu into the ironoxides which does not reprecipitate on cooling. PACS: 61.66.Dk, 64.70.Kb, 75.30.Cr, 75.60.Jp