Thermal stability of Co/C multilayers prepared by a dual-facing-target sputtering system was studied. A picture of the thermally induced changes in the microstructure was obtained using complementary measurement techniques including low-angle and high-angle X ray diffraction, transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. It was found that the period expansion, reflectivity change and compound formation that were observed after annealing are caused by structural changes both in the sublayers and at the interfaces. Below 400℃, the period expansion is mainly caused by the graphitization of the amorphous carbon layers, and a significant increase in the reflectivity at grazing incidence was observed. By 500℃, the crystallization and agglomeration of Co layers induce an enormous period expansion and a serious decrease in reflectivity. A small amount of carbide is found to form at this temperature. Our results imply that new multilayer structures, e.g., compound multilayers will have to be developed for use at high temperatures or under high X-ray incident flux.

The structural stability of CoN/CN soft X-ray multilayers has been investigated by using complementary measurement techniques. Annealing in the low-temperature range of 473-523 K, we find three interdiffusion features, different from Co/C multilayers [H.L. Bai et al.: Thin Solid Films 286, 176 (1996)]. (1) The interdiffusion critical wavelengths were calculated as 2.00-2.04nm at temperatures ranging from 473 to 523 K, which is equal to those of Co/C multilayers within the experimental error, indicating that the interdiffusion behaviours in the CoN/CN multilayers are still decided by the thermodynamic properties of the Co-C system. (2) The effective interdiffusivities and macroscopic diffusion coefficients are smaller. (3) The activation energy for diffusion is larger. The features imply that it is possible to improve the thermal stability of Co/C multilayers by doping with N atoms.

Period expansion of Co/C and CoN/CN soft x-ray multilayers has been investigated by x-ray diffraction and Raman spectroscopy. Below the anneal temperature of 400℃, the period expansion (<12%) of Co/C multilayers is mainly caused by the graphitization of the amorphous carbon layers. By 500 oC, the crystallization and agglomeration of Co layers induce an enormous period expansion (～40%). The period expansion of CoN/CN multilayers is only 4% at 400℃, which is much smaller than that of Co/C multilayers. The interface patterns of the CoN/CN multilayers still exist even if they were annealed at 700℃ The Raman spectroscopy analyses indicate that the formation of the sp3 bonding can be suppressed effectively by doping N atoms, and thus the period expansion is decreased considerably at annealing temperatures below 600℃. The significant suppression of grain growth above 600℃ is believed to be attributed to the coexistence of hcp and fcc Co structures induced by interstitial N atoms, which cause the high-temperature period expansion decrease. The results also imply that the structural stability of Co/C soft x-ray multilayers can be significantly improved through doping N atoms.

Carbon nitride films have been fabricated by a dc facing-target reactive sputtering system for various N2 fractions (PN) in the gas mixture. Complementary measurement techniques, including atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy (HRTEM), were used to systematically study the morphology and microstructure of the films. AFM images show that the average surface roughness increases with increasing P-N. XPS analyses indicate that the concentration of N is not directly proportional to PN, and it rises quickly to a saturated value of similar to33 at% at a PN of 20%, which can be attributed to the chemical sputtering effect. The ratio N-C(sp2)/N-C(sp3) increases with increase in PN from 0% to 20%, and then decreases with further increase in P-N. However, the number of sp2-hybridized C atoms continues to increase over the whole range of PN, as evidenced by Raman and FTIR measurements. The growth of a disordered sp2 C structure at P-N below and above 20% can be attributed to the incorporation of N and the compressive stress relaxing, respectively. Raman scattering and HRTEM analyses reveal an incomplete ordering process of the sp2 C structure with increase in PN.

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy (TEM) images show that the films consist of quite uniform Fe3O4 grains well separated by grain boundaries. It was found that the tunneling of spin polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance MR=ρ(H)-ρ(o)=ρ(0) shows linear and quadratic magnetic field dependence in the low field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial Fe3O4 films consisting of a large fraction of antiferromagnetic antiphase domain boundaries (APBs). At 300 K, the size of MR reaches-7.4% under a 50 kOe magnetic field, which is, by far, the largest MR observed in polycrystalline Fe3O4 films.