Electrical conductivity in nanostructured magnetite–hematite composites produced by mechanical milling

Mechanical milling of Fe 3 O 4 powders for few hours produced nanometre-sized particles which exhibited a transition from cubic magnetite (Fe 3 O 4 ) phase to hexagonal hematite (α-Fe 2 O 3 ) phase. By extending the milling time to 10 h, a 95% transformation to the hematite phase could be achieved. These phases were characterized both by X-ray diffraction as well as by transmission electron microscopy. The latter showed the generation of interfaces in the nanoscale between the magnetite and hematite phases. Electrical resistivity was studied for the compacted samples in the temperature range 110-350 K. The data could be explained by a small polaron-hopping conduction mechanism. On the basis of electron microscopic data and the activation energy determined from the experimental results, it was concluded that the nano-interfaces created due to the above-mentioned phase transformation controlled this hopping conduction.