A study of the evolution of the constituent phases and magnetic propertiesof hydrogen-treated Sr-hexaferrite during calcination

A hydrogen treatment followed by calcination, has been developed in order t o enhance the intrinsic coercivity of Sr-hexaferrite (SrFe12O19). Fully hyd rogen-treated Sr-hexaferrite consists of a mixture of 73%, by weight, of al pha Fe and 27% of Sr7Fe10O22 phases. Calcination of this material to reform the SrFe12O19 phase occurs in two stages. Between room temperature and 600 degrees C, oxygen was absorbed resulting in a large increase in weight wit h the formation of a mixture of SrFeO3-x, and Fe2O3 (alpha and gamma). Duri ng the second stage, the intermediate phases reacted to form SrFe12O19 at a temperature of between 700 and 800 degrees C. A partial desorption of oxyg en occurred until calcination reached completion at 1000 degrees C. The mag netization at 1100 kA m(-1) and the remanence were similar to those of the untreated material, but, because of a much refined grain size, the intrinsi c coercivity was considerably larger, with values around 400 kA m(-1). Grai n growth occurs at temperatures >1000 degrees C, resulting in a decrease in the intrinsic coercivity. (C) 1999 Kluwer Academic Publishers.