Thickness dependence of magnetic and magneto-transport properties of polycrystalline Fe₃O₄ films prepared by reactive sputtering at room temperature

At room temperature, polycrystalline Fe₃O₄ films with thickness in the range 5-1120 nm have been prepared by reactive sputtering. Transmission electron microscopy imaging shows that uniform Fe₃O₄ grains are well separated by grain boundaries and their size decreases with film thickness. The change of resistivity as a function of temperature reveals a grain boundary dominated electron tunnelling mechanism. The magnetoresistance MR = (ρ_H - ρ₀)/ρ₀ measured at room temperature for the films thicker than 200 nm is ∼-7.4% under a magnetic field of 46 kOe, which is among the largest values ever reported for Fe₃O₄ films under the same measuring conditions. As the thickness reduces from 80 to 5 nm, MR decreases from -6.5% to -1.1% due to the enhanced spin-flip scattering at film and grain surfaces.