Nonstoichiometric Ba Hexaferrites, Prepared by Partial or Complete Ion Exchange

Abstract
Non-stoichiometric hexaferrite particles (M type structure) in the form of single crystals or fine particles have been prepared by complete or partial ion exchange from β"-ferrite precursor phase. Complete exchange yields directly the hexaferrite phase. Partial exchange maintains the β" structure and heating at T > 600°C is necessary for the M type structure to be formed. The crystal structure of non-stoichiometric hexaferrites is described as an M-type hexaferrite with β-type intergrown cells. The Curie temperature is that of stoichiometric BaFe₁₂O₁₉, T_c=460°C. Saturation magnetization in a field of 20 kOe at room température is 35-65 emu/g and the coercive field H_c ≈ 1.5-2.2 kOe. The higher values are obtained after annealing, which also increases grain size from 50 to 200 nm.