Synthesis and magnetic properties of Ni-Al₂O₃ thin films

Ni-Al₂O₃ nanocomposite thin films have been produced on sapphire, silicon, and silica substrates by a combination of sol-gel processing and partial reduction reactions. Transmission electron microscopy shows Ni particles, ∼20 nm in diameter, embedded in slightly larger diameter alumina grains. X-ray diffraction lattice parameter measurements suggest that the Ni is in a state of nonhydrostatic strain. Magneto-optical Kerr effect measurements indicate that the Ni particles in the films on the silicon and silica substrates support perpendicular magnetization. The saturation Kerr rotation increases linearly with film thickness to values above pure Ni and independent of reflectivity, indicating that the material is behaving as a Faraday rotator. The enhanced magnetic properties of the composite films are related to the nonhydrostatic strain developed in the Ni particles during fabrication. It is argued that the strains originate from the coefficient of thermal expansion mismatch between the film and substrate, and likely the volume shrinkage associated with the reduction reaction.