Surface anisotropy in giant magnetic coercivity effect of cubic granular FeCo/SiO₂ and NiCo/SiO₂ films: A comparison with Néel's theory

The structure and magnetic properties of a series of granular Fei_1-xCo_x/SiO₂ and Ni_1-xCo_x/SiO₂ films have been studied. Both Fe_1-xCo_x and Ni_1-xCo_x alloy fine grains posses cubic structure within the cobalt composition x<0.7. The grains grow to about 150-200 Å in diameter when the annealing temperature is between 570 and 600 °C, where the samples show a giant magnetic coercivity effect. The experimental results can be successfully accounted for by the thermal relaxation theory, assuming the surface anisotropy dominates in the giant magnetic coercivity effect. By fitting the theory into the experimental results, the surface anisotropy constants of Fe_1-xCo_x and Ni_1-xCO_x fine grains can be obtained in the range x<0.7. The surface anisotropy from Néel's theory, on the other hand, manifests qualitative similarity to the fitting results as a function of Co contents.