Magneto-optical properties of cobalt and Co₃O₄ thin films: Influence of microstructural properties

Two kinds of cobalt thin films, labeled smooth and nanostructured according to their surface properties, were deposited on a glass substrate by varying the deposition conditions during magnetron sputtering. Complementary characterization techniques were first used to investigate the internal microstructure of these two films, which give evidence that the smooth film structure is dense and close to bulk cobalt while the nanostructured film contains columnar-like structures of few nanometers’ width surrounded by an oxide layer. The smooth film shows a linear magnetic behavior, while a hysteresis loop has been observed for the nanostructured film, due to the presence of pinning-type domain wall motion. The nanostructured film is more porous to oxygen diffusion than the smooth one, which explains that its oxidation starts at lower annealing temperature than that of the smooth one. The spectral magneto-optical properties of the oxidized films were studied, revealing for the first time, three diamagnetic transitions at around 700, 1330, and 1450 nm, involving Co²⁺ and/or Co³⁺ ions. The blue shift observed for the nanostructured film highlights the impact of the initial morphology of the films on the cation's crystal coordination.