Investigation of temperature dependence of surface anisotropy field and constant in the system of BaFe12O19 nanocrystals

We show a possibility to determine the parameters characterizing surface magnetic anisotropy of platelet BaFe12O19 nanocrystals with thickness comparable to the magnetoperturbed surface layer of high-anisotropic hexagonal crystal. Taking into account the above-mentioned specific character of the particles, we introduce the constant KS as the surface anisotropy energy per unit volume of near-surface layer. Temperature dependence of the surface anisotropy field as one of the components of effective anisotropy was corrected taking into account the thermal fluctuations. Discovered anomaly initiated calculation of the surface anisotropy constant with different approaches for two different temperature ranges. Analyzing the KS sign changes with temperature, we conclude about the transformation of surface anisotropy from plane to axis type.