The effect of hydrogen on composition, microstructure and magnetic properties of strontium hexaferrite

Strontium hexaferrite powder, synthesised conventionally, has been treated in a hydrogen atmosphere and subsequently calcined in air. The phase identi fication studies showed decomposition of strontium hexaferrite into Fe2O3 a nd Sr7Fe10O22 and the reduction of the resultant Fe2O3 during reaction with hydrogen. The magnetic measurements indicated a substantial increase in th e saturation magnetisation and a marked decrease in the remanence and intri nsic coercivity during hydrogenation due to the formation of the alpha-Fe p hase. After re-calcination, the hexaferrite phase re-formed and the remanen ce and saturation magnetisation exhibited values close to the initial ones but there was a significant improvement in the intrinsic coercivity. This w as attributed mainly to very fine grains formed as a result of hydrogenatio n and subsequent re-calcination. The highest intrinsic coercivity obtained in this work was 400 kA/m (5 kOe) which occurred after hydrogenation at 700 degrees C for 1 h under an initial pressure of 1.3 bar followed by recalci nation at 1000 degrees C for 1 h, These studies show that strontium hexafer rite powder with an enhanced Value of intrinsic coercivity can be produced from the conventionally synthesised powder. (C) 2000 Elsevier Science Ltd a nd Techna S.r.l. All rights reserved.