Internal stress influence on the coercivity of FeCuNbSiB thin films

Thin films of Finemet-type alloy with thickness varying from 50 to 1000 nm have been deposited by RF sputtering and annealed at temperature ranging from 150 to 450 °C. Their magnetic and structural properties have been characterized using alternating gradient field magnetometry and X-ray diffraction. In addition, the stress in the films has been measured as a function of temperature from the curvature of the wafers using a laser scanning technique.The coercive field of the films first decreases with annealing temperature due to stress relaxation, and then increases again when crystallisation begins. The optimal annealing conditions comprises between the glass transition and the crystallisation temperature.Its is observed that the coercivity of the as-deposited material is continuously decreasing as the thickness increases, following an inverse square root dependence, in relation with the stress-induced magneto-elastic contribution to the total anisotropy. By opposition, it has been found that the coercive field of devitrified and totally relaxed films is inversely proportional to film thickness. In order to explain this evolution, a model is proposed, based on random anisotropy considerations applied to thin films in which the anisotropy was considered localised in the dimension of thickness.