Magnetic phase transitions in Fe2O3-Bi 2O3-B2O3 glasses

Magnetic and structural properties of iron-containing bismuth borate glasses, whose composition is denoted as xFe₂O₃·80. 0-x)Bi₂O₃·20.0B₂O₃, in mol% (18.2≤x≤40.0), have been explored. The glasses manifest intriguing magnetic behaviors explainable in terms of the coexistence of a spin glass phase and magnetic clusters. The x = 18.2 glass shows a spin glass transition at 3.5 K, while the contribution of magnetic clusters to the magnetic properties becomes more significant as the content of Fe₂O₃, x, is increased. We have performed detailed experiments on the x = 32.0 glass for which two different magnetic transitions are observed, as demonstrated by the temperature and frequency of the ac magnetic field dependences of the dc and ac susceptibilities. It is revealed from the measurements of magnetic ageing and memory effects that the magnetic clusters are frozen to form a superspin glass-like state with strong inter-cluster interactions at low temperatures. Transmission electron microscopy clarifies that the magnetic clusters are ascribable not to nanocrystals but to some phases possessing amorphous structures. Also, we have found from the observation of exchange bias effects that an interplay of the clusters with the spin glass phase brings about an exchange anisotropy field after cooling in the presence of magnetic field.