Enhanced magnetoresistance effects in bulk polycrystalline Ag-added magnetite

A series of bulk polycrystalline Ag-added Fe3O4 with a nominal composition, (Fe3O4)1−xAgx (x is molar fraction) with x=0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by conventional solid-state reaction. X-ray diffraction patterns show that the pure Fe3O4 sample (x=0) has a single-phase inverse spinel structure, while the Ag-added samples are two-phase composites consisting of a ferrimagnetic Fe3O4 phase and a non-magnetic metal Ag phase. The bright-field transmission electron microscopy images exhibit that the samples are typical granular solids with a porosity of about 22%. The addition of Ag slightly increases the average grain size of the Fe3O4 phase and significantly enhances the MR effect of bulk polycrystalline Fe3O4 samples. Of all the samples the x=0.3 sample has a maximal MR of −5.1% at 300 K in a magnetic field of 1 T, and −6.8% in 5 T, which are approximately three times greater than the corresponding MR values (−1.8% at 1 T and −2.4% at 5 T) of the Fe3O4 sample. This enhancement of the MR can be attributed to the combination effect from the spin-dependent scattering at the interfaces between the Fe3O4 grains and the Ag granules and the spin-polarized tunneling at grain boundaries of Fe3O4 phase of the spin-polarized electrons.