EBSD investigation and magnetic properties of manganese ferrite crystallized in a sodium-silicate glass

Abstract

Glasses were prepared in the system (100 − x)(0.16Na₂O/0.10MnO/0.74SiO₂)/xFe₂O₃ (x = 15 and 25 in mol%) by the traditional melt-quenching technique. Some of them were subsequently crystallized at temperatures in the range from 520 to 700 °C for times from 10 min to 100 h. The valence states of Fe and Mn in selected glasses and glass-ceramics were investigated by X-ray photoelectron spectroscopy (XPS) and showed the presence mostly of Mn²⁺ while Fe occurs as both Fe²⁺ and Fe³⁺. X-ray diffraction (XRD) analyses of the 15 mol% Fe₂O₃ samples revealed that at lower temperatures and for short heat treatment times a solid solution with a spinel structure and a chemical composition between MnFe₂O₄ (jacobsite) and Fe₃O₄ (magnetite) crystallizes, while aegirine (NaFe(SiO₃)₂) additionally occurs at higher temperatures and for longer annealing times. Here the average crystallite size of the jacobsite-based phase varies between 4 and 10 nm, as determined by XRD. The XRD analysis of the 25 mol% Fe₂O₃ samples which spontaneously crystallize when quenching the melt only show the precipitation of the jacobsite–magnetite solid solution. The microstructure was investigated by scanning and transmission electron microscopy (SEM and TEM) as well as electron-backscatter diffraction (EBSD). The EBSD analyses show that the cubic crystals grow via the polygon as well as the dendritic growth mechanisms. The individual grains show independent orientations and inclusions of residual glass while a global crystallographic texture could not be detected. The obtained magnetization curves of the 25 mol% Fe₂O₃ samples show hysteresis and a Curie temperature of approximately 480 °C which supports the XRD results. The samples with 15 mol% Fe₂O₃ crystallized at 650 and 700 °C exhibit superparamagnetic behaviour and thin S-shaped magnetization curves with zero coercivity.