0.6 BaO-0.4 Fe₂O₃ glass was prepared by twin-roller quenching method and its heat treatment process was examined by ESR and Mössbauer spectroscopy. Fe³⁺ ions were found to occupy distorted tetrahedral sites in this glass. BaFe₂O₄ and Ba₂Fe₂O₅ were precipitated through the heat treatment at a temperature of 440°C. The former is an antiferromagnetic crystal having Néel temperature of 880K. Before the crystallization occurred, the linewidth of ESR spectrum decreased monotonously although neither isomer shift nor quadrupole splitting changed. It was considered that in this region Fe³⁺-O^2--Fe³⁺ bond angle dominantly changed as a result of thermal relaxation of structure. When the crystallization occurred, both isomer shift and quadrupole splitting decreased significantly. These facts indicated that Fe³⁺-O^2- distance was longer in the glass than in the precipitated crystals and that the coordination state of O^2- ions around Fe³⁺ was more distorted in the glass.
Hyperfine field increased with increasing heat treatment temperature. This phenomenon was explained by considering the size of crystallized BaFe₂O₄ particles.