Magnetite crystallization in a sodium-calcium-silicate glass with high iron oxide concentration-Effect on the magnetic properties

Glasses with mol% composition 16 Na2O center dot 10 CaO center dot 54 SiO2 center dot 20 Fe2O3 were prepared by the melt-quenching technique. Heat treatment at 580 degrees C for 1 and 3 h resulted in the crystallization solely of magnetite and additionally, of hematite for longer times. Transmission electron microscopy shows growth mainly of cubic magnetite with crystallite sizes <= 16 nm. X-ray photoelectron spectroscopy reveals the presence of Fe3+ and Fe2+ ions in both the glass and the glass-ceramics. Mossbauer spectroscopy of the glass detects octahedrally and tetrahedrally coordinated Fe3+, while Fe2+ solely occurs in octahedral coordination. In the glass-ceramics, magnetite was detected as well as Fe3+ in both tetrahedral and octahedral coordination. Magnetization curves of the heattreated samples recorded at room temperature and below show ferrimagnetic behaviour. From the thermomagnetic curves recorded in the temperature range from 300 to 900 K, the Curie and superparamagnetic blocking temperatures are determined. Thermomagnetic measurements in the range from 5 to 300 K reveal the nanoparticle size dependence of the observed Verwey transition.