Unusual temperature dependence of the magnetic anisotropy constant in barium ferrite BaFe12O19

We report magnetic hysteresis loops in a wide temperature range (4-700 K) for silica-coated barium ferrite BaFe12O19 nanoparticles. The saturation magnetization M-s and the first magnetic anisotropy constant K are determined simultaneously from the magnetic hysteresis loop using the law of approach to saturation. It is remarkable that K is linearly proportional to M-s and varies precisely with temperature as K(T) K(0)[1 (T/T-C)(1.58)] in the whole temperature range below the Curie temperature T-C (740 K). The unusual temperature dependence of the anisotropy constant and its linear relation with the saturation magnetization in BaFe12O19 are not predicted from the existing theoretical models. (C) 2011 American Institute of Physics. [doi:10.1063/1.3657851]