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

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Abstract
The structure and magnetic properties of a series of granular Fe1−xCox/SiO2 and Ni1−xCox/SiO2 films have been studied. Both Fe1−xCox and Ni1−xCox 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 Fe1−xCox and Ni1−xCox 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.