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 identification studies showed decomposition of strontium hexaferrite into Fe2O3 and Sr7Fe10O22 and the reduction of the resultant Fe2O3 during reaction with hydrogen. The magnetic measurements indicated a substantial increase in the saturation magnetisation and a marked decrease in the remanence and intrinsic coercivity during hydrogenation due to the formation of the α-Fe phase. After re-calcination, the hexaferrite phase re-formed and the remanence and saturation magnetisation exhibited values close to the initial ones but there was a significant improvement in the intrinsic coercivity. This was attributed mainly to very fine grains formed as a result of hydrogenation and subsequent re-calcination. The highest intrinsic coercivity obtained in this work was 400 kA/m (5 kOe) which occurred after hydrogenation at 700°C for 1 h under an initial pressure of 1.3 bar followed by re-calcination at 1000°C for 1 h. These studies show that strontium hexaferrite powder with an enhanced value of intrinsic coercivity can be produced from the conventionally synthesised powder.