Structure and magnetic properties of ultrafine lithium ferrite crystallized from a borate glass.

A glass with the mol. % compositions 51.7 B₂O₃/9.3 K₂O/1 P₂O₅/27.6 Li2O/10.4 Fe₂O₃ was crystallized at temperatures in the range from 400 to 540 °C for different periods of time (2-12 h). X-ray diffraction showed cubic Li ferrite with a spinel structure, LiFe₅O₈, with small crystallites with sizes in the range from 3 to 31 nm. While the samples crystallized at 440, 480, and 500 °C reveal a disordered phase, crystallization at 540 °C indicates a phase transformation to the ordered state. Magnetization curves showed that the samples crystallized at 440 °C are superparamagnetic with very low maximum magnetization, while the samples crystallized at 480 and 500 °C show thin clear S-shaped magnetization curves with zero coercivity and hence are also superparamagnetic. By contrast, the sample crystallized at 540 °C shows a coercive field of 40 Oe and thus is ferrimagnetic. The magnetic properties can be tailored by the size of the ferrite crystals and hence by the crystallization conditions. [ABSTRACT FROM AUTHOR]