Co-sputtered thin films consisting of cobalt nano-grains embedded in graphite-like carbon and their magnetic properties

The microstructure and the magnetic properties of co-sputtered cobalt-carbon thin films were analyzed. Cobalt-carbon thin films consisting of nano-crystalline cobalt grains separated by graphite-like carbon were obtained after subsequent annealing. The phase of the cobalt grains depended on the carbon concentration; a hexagonal close-packed phase and a heavily faulted close-packed phase were observed. The grain size ranged from 4 to 20 nm. The in-plane magnetic hysteresis loops measured at room temperature showed that the coercivity and the squareness were drastically changed with the carbon concentration. These changes can be related to a change in the magnetocrystalline anisotropy of the cobalt grains due to a difference in the phase and/or size effect that can be explained by the presence of superparamagnetic particles. The highest coercivity (660 oersteds) was measured in a him with a large grain size as well as a hexagonal close-packed cobalt phase. The annealing temperature was found to have an optimum value; the magnetic properties were improved upon annealing due to carbide decomposition, but at high annealing temperatures, coalescence of the grains resulted in sample degradation.