Conductivity, magnetoresistance, and the Hall effect in granular Fe/SiO2 films

We have investigated the conductance, magnetoresistance, and Hall effect in granular Fe/SiO2 films with size of the iron grains around 40 Angstrom, whose volume fraction x lies in the range 0.3-0.7. The conduction activation regime has been established for x < 0.6. On the insulator side of the transition we observed a giant negative magnetoresistance, falling off sharply as the metal volume fraction decreases. For x < 0.4 we observed a large positive magnetoresistance of premagnetized samples, showing up in fields similar to 100 Oe and characterized by large response times. The field dependence of the Hall effect in the dielectric samples, as in the metallic samples, correlates with their magnetization. We found that the Hall resistance is proportional to the square root of the longitudinal resistance, which cannot be explained by known models of the anomalous Hall effect. (C) 1999 American Institute of Physics. [S1063-7834(99)00406-2].