Magnetoresistance effect in Ag–Fe3O4 and Al–Fe3O4 composite films

Abstract

The Agx–(Fe3O4)1−x and Agx–(Fe3O4)1−x composite films were prepared by dc sputtering on Si(100) substrates. The x-ray diffraction results show that the films contain essentially only the cubic inverse spinal phase from Fe3O4 and face-centered cubic phase from Ag or Al. The transmission electron microscopy images indicate that the metal granules are randomly distributed with Fe3O4 grains. The resistivity determined from the four-probe method decreases rapidly with increasing metal content. At x≒0.5, a percolation occurs. The conducting path is formed from metal granules in series with Fe3O4 grains. The magnetoresistance (MR) is defined to be {R(H=0.8 T)−R(H=0)}/R(H=0). It has been found that MR is isotropic and the appearance of Ag granules has significant impact on the MR effect. Furthermore, a positive MR region appears with 0.011<x<0.1 in Agx–(Fe3O4)1−x. On the contrary, the incorporation of Al granules does not have the same effect on MR as in Agx–(Fe3O4)1−x. A slow increase of MR with Al content might be due to Coulomb blockade. The extra contribution to MR in Agx–(Fe3O4)1−x can be attributed to spin injection from Fe3O4 into Ag granules so that spin accumulation in Ag granules impedes the current causing a larger resistance under a field.