Structures and transport properties of polycrystalline Fe3O4 films

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. A transmission electron microscope image shows that the films consist of Fe3O4 grains well separated by grain boundaries with long-range and atomic scale disorders. The width of the long-range disordered grain boundaries observed by high resolution transmission electron microscopy is about similar to4 nm, which is consistent with the boundary dimension derived from the resistivity and magnetization measurements. The temperature dependence of resistivity indicates that the transport properties of the films are dominated by the mechanism of fluctuation-induced tunnelling of electrons across the grain boundaries. Ordinary and extraordinary Hall constants were measured to be -8.22 x 10(-11) and similar to-1.45 x 10(-8) Omega cm G(-1), which are two and three orders of magnitude larger than those of iron, respectively.