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, wh ose 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 trans ition we observed a giant negative magnetoresistance, falling off sharply a s the metal volume fraction decreases. For x < 0.4 we observed a large posi tive magnetoresistance of premagnetized samples, showing up in fields simil ar to 100 Oe and characterized by large response times. The field dependenc e 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 canno t be explained by known models of the anomalous Hall effect. (C) 1999 Ameri can Institute of Physics. [S1063-7834(99)00406-2].