Effect of annealing on the giant Hall effect

We studied resistivity, magnetoresistivity, and Hall resistivity for a few percolating granular NiFe-Si${\mathrm{O}}_2$ films upon thermal annealing. It was found that both the resistivity and the Hall resistivity decrease by two orders of magnitude upon annealing at 520 °C, accompanied by a crossover from an approximately $-\ln T$ dependence of resistivity to a metallic behavior. On the other hand, transmission electron microscopy study showed that this pattern of resistivity and Hall resistivity change is not due to any new phases, but is associated with changes in particle size distribution. These results cannot be understood in the existing theoretical frame-work.