Microstructures and Magnetic Properties of Crystallized BaO-Fe₂O₃-B₂O₃ Glasses with Fine BaO·6Fe₂O₃ Precipitates

Abstract

Crystalline phases and amorphous formation regions for the pseudo-ternary BaO-Fe₂O₃-B₂O₃ system were investigated using melt spinning and water quenching techniques. Specimens were heat treated, examined to determine the composition and temperature ranges where the BaO·6Fe₂O₃ precipitated, and the relationship between crystalline phases and magnetic properties was discussed.
All as-quenched specimens produced by melt spinning with a composition range of less than 70 mol%Fe₂O₃ and less than 70 mol%BaO were in an amorphous state, and amorphous formation region in the case of water quenching was restricted to the composition range of less than 40 mol%Fe₂O₃ and more than 20 mol%B₂O₃.
In the pseudo-ternary BaO-Fe₂O₃-B₂O₃ system, the composition regions were classified according to the kind of precipitated phases. In each region, the relationship between the heat treatment temperature and the relative amount of the precipitated phases was clarified. The amount of BaO·6Fe₂O₃ in the composition region between lines P (the tie-line between BaO·6Fe₂O₃ and BaO·B₂O₃) and Q (the tie-line between BaO·6Fe₂O₃-BaO·Fe₂O₃ and BaO·B₂O₃-3BaO·B₂O₃ eutectics) increased as the BaO/Fe₂O₃ ratio in a sample became closer to 1/6. In this region, the composition ranges were determined where fine BaO·6Fe₂O₃ particles of high coercive force (higher than 240 kA/m (3 kOe)) precipitated from the amorphous phases after heating. In the composition region between lines Q and R (the tie-line between BaO·Fe₂O₃ and 3BaO·B₂O₃), some ribbon specimens showed the maximum coercive force of 488 kA/m (6.1 kOe), but they contained very fine BaO·6Fe₂O₃ particles which indicated superparamagnetism.