Memory effect of ball-milled and annealed nanosized hematite

Fine particles of hematite (mean size 55 nm) were produced by ball milling a mixture of hematite and pure Fe and annealing at 1000 °C. X-ray diffraction (XRD) and Mössbauer spectroscopy show that only α-Fe ₂O ₃ is present in the final product, with lattice and Mössbauer parameters that correspond to bulk hematite. ZFC and FC magnetization measurements were performed from 5 to 300 K, at different applied fields. Two magnetic regimes were observed: one at low temperatures (⩽100 K) that we ascribe to the magnetic moments in the outer shell of the particles that couple to the magnetic moments in the core, and another at higher temperature that corresponds to the Morin transition, finding that the Morin temperature is T_M=246 K. The memory effect is clearly observed in magnetic measurements that start from different remanence states and explained as dependent on the ordering of the magnetic moments within the particles.