Effect of annealing temperature on the structural, photoluminescence and magnetic properties of sol–gel derived Magnetoplumbite-type (M-type) hexagonal strontium ferrite

Magnetoplumbite-type (M-type) hexagonal strontium ferrite particles were synthesized via sol–gel technique employing ethylene glycol as the gel precursor at two different calcination temperatures (800 and 1000 °C). Structural properties were systematically investigated via X-ray diffraction (XRD), field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), photoluminescence spectrophotometry and superconducting quantum interference device magnetometer. XRD results showed that the sample synthesized at 1000 °C was of single-phase with a space group of P63/mmc and lattice cell parameter values of a=5.882 Å and c=23.048 Å. EDS confirmed the composition of strontium ferrite calcined at 1000 °C being mainly of M-type SrFe12O19 with HRTEM micrographs confirming the ferrites exhibiting M-type long range ordering along the c-axis of the crystal structure. The photoluminescence (PL) property of strontium ferrite was examined at excitation wavelengths of 260 and 270 nm with significant PL emission peaks centered at 350 nm being detected. Strontium ferrite annealed at higher temperature (1000 °C) was found to have grown into larger particle size, having higher content of oxygen vacancies and exhibited 83–85% more intense PL. Both the as-prepared strontium ferrites exhibited significant oxygen vacancies defect structures, which were verified via TGA. Higher calcination temperature turned strontium ferrite into a softer ferrite.

► High annealing temperature produced M-type ferrite with higher oxygen vacancies.
► Photoluminescence intensity is proportional to the existence of oxygen vacancies.
► XRD data showed cell contraction well suited to the change of oxygen vacancies.
► Shift in hysteresis loop due to defect-induced exchange bias was observed.