Structural, electrical and magnetic properties of carbon-nickel composite thin films

Carbon-nickel composite thin films (600 nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25-800 degrees C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni3C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the aspect ratio as high as 1:60 in the sample prepared at 200 degrees C). The carbon matrix was amorphous, or graphite-like depending on deposition temperature. At low deposition temperatures T-S (25-400 degrees C) the Ni3C nanoparticles were of hcp phase. Samples prepared at T-S >= 600 degrees C contained ferromagnetic fee Ni nanoparticles. A correlation was found between the structural, electrical and magnetic properties of the composites. To characterise the films, dependences, such as resistivity vs. temperature, current vs. voltage, differential conductivity vs. bias voltage, and magnetoresistivity, were determined. For example, the tunneling effect was found in samples in which the metallic nanoparticles were separated by 2-3 nm thick amorphous carbon. When the metallic nanoparticles were connected by graphite-like carbon regions (having a metallic conductivity, in contrast to a-C), the temperature coefficient of the resistivity became slightly positive. An anisotropic magneto resistivity of similar to 0.1% was found in the sample that contained ferromagnetic columnar fee Ni. Zero magneto resistivity was found in the sample in which the metallic nanoparticles were of non-magnetic hcp phase. (C) 2005 Elsevier Ltd. All rights reserved.