Magnetoresistance of Fe-SiO2 granular films

A new type of magnetoresistance (MR) was observed in Fe-SiO2 granular films. The samples were prepared on glass substrates using ion-beam sputtering technique. MR ratios of the order of 2.7% at room temperature and 4.5% at 210 K were obtained when a magnetic field of 1.2 T was applied parallel to the film plane. We have measured MR as a function of Fe volume fraction (f(V)) of the films. It was found that MR appears only in samples with f(V) less than the percolation threshold (f(C)) and has a peak value at f(V) approximate to 0.28. We also found that the enhancement of MR always appears together with the increase of resistance (R) of samples in the region 0.28 less than or equal to f(V) < 0.5. Our results indicate that MR in these materials arises from the spin-dependent tunneling effect and imply that the size of Fe particles and the thickness of SiO2 tunnel barriers may play an important role in the MR effect of Fe-SiO2 granular films. (C) 1997 Published by Elsevier Science Ltd.