At room temperature, polycrystalline Fe3O4 films with thicknesses ranging from 5 to 1120 nm have been prepared by reactive sputtering. Transmission electron microscopy image shows that uniform Fe3O4 grains are well separated by grain boundaries and their size decreases with the film thickness. The changes of resistivity as a function of temperature reveal a grain boundary dominated electron tunneling transport mechanism. The magnetoresistance MR=ρH-ρo=ρ0 measured at room temperature for the films with thickness larger than 200nm is～7.4% in 46 kOe magnetic field, which is one of the largest value have ever been reported for Fe3O4 films under the same conditions. As the thickness reduced from 80 to 5nm, MR decreases from～6.5% to～1.1% due to magnetization reduction and the enhanced spin-flip scattering at the film surface.

low 400℃, the upward shift of D and G Lines in Raman spectra indicates that the amorphous carbon layers are changing from ones with bond-angle disorder and fourfold-bonding only to ones containing threefold-bonding. In the crystalline stage, the amorphous carbon layers in the as-deposited multilayers crystallize to graphite crystallites in the annealing temperature range of 500-600℃. The rapid increase in the intensity ratio of D line to G line and dramatic decrease in linewidth further confirm this substantial structural change. In the grain growth stage, the specimens are annealed at temperatures higher than 700℃. The decrease in the intensity ratio implies a growth in the graphite crystallite dimensions, which is consistent with the XRD and TEM results.

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. 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～4nm, which is consistent with the boundary dimension derived from the resistivity and magnetization measurements. Temperature dependence of resistivity indicates that the transport properties of the films are dominated by the mechanism of fluctuation-induced tunneling of electrons across the grain boundaries. Ordinary and extraordinary Hall constants were measured to be-8.22×10-11 and～-1.45×10-8Ωcm/G, which are two and three orders of magnitude larger than those of iron, respectively.

(Fe3O4)1-x-(SiO2)x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar+O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe2/1log−∝Tρ3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance (MR){ρ(H)-ρ(o)/ρ(0)} of about-4.7% for Fe3O4 films, and-1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under 46-kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at. 4.0≥x

We studied the structure and magnetic properties of as-deposited and subsequent annealed CoxC100-x granular films fabricated by a DC facing-target magnetron sputtering system at room temperature using atomic force microscopy, X-ray diffraction, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy and vibrating sample magnetometer. The average roughness, Ra, of the as-deposited CoxC100-x granular films is smaller than that of Si (100) substrates. X-ray diffraction and transmission electron microscopy analyses indicate that the as-deposited CoxC100-x granular films are composed of～2 nm amorphous cobalt grains dispersed in amorphous carbon matrix, and their morphology is composition independent. The high resolution TEM image of the as-deposited Co30C70 film shows that cobalt and carbon have already separated during the deposition, even if the aggregation of cobalt is not complete. Annealing at 300-450℃ causes the crystallization of amorphous cobalt followed by an increase of grain size and the graphitisation of the amorphous carbon matrix. The constrictions arising from the structural environment result in the coexistence of the hcp and fcc Co phases at temperatures higher than the phase transformation point of 425℃. Magnetic measurements reveal that the coercivity of the as-deposited CoxC100-x granular films decreases with the increasing cobalt composition, and increases with the decrease in film thickness. The enhanced coercivity can be attributed to the weakened intergrain interaction due to the increased percolation threshold and/or the destruction of long-range domain structures caused by the film thickness reduction.