Transformation in structural phase on annealing and its impact on optical and dielectric properties of manganese ferrite nanoparticles

Manganese ferrite (MFO) nanoparticles (NPs) were synthesized via the sol–gel auto-combustion method and annealed at 400, 500, and 600 °C. The as-prepared and annealed MFO NPs were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, and vibrating sample magnetometer (VSM). XRD patterns showed that as-prepared and annealed NPs at 400 °C exhibited pure cubic spinel phase of MFO NPs which get decomposed into α -Fe 2 O 3 and a -Mn 2 O 3 for NPs sintered at 500 and 600 °C The crystallite size was found to be increased with an increase in annealing temperature. The structural phase transformation was confirmed by FTIR and Raman studies. The optical band gap ( E g ) was decreased (1.70 to 1.43 eV) with increase in annealing temperature which lies in visible range of electromagnetic spectrum. So, these NPs could be used as photocatalyst for the degradation of dye under sun light irradiation. The magnetic hysteresis loop for as-prepared and annealed NPs at 400 °C suggested the formation of soft magnetic material. The saturation magnetization was dropped (61.4–2.2 emu/g) drastically for MFO NPs with increase in sintering temperature. The real and imaginary dielectric constant ( εʹ and εʹʹ ) and dielectric losses (tan δ ) were observed to decrease with increasing the annealing temperature and frequency. Furthermore, the AC (alternating current) conductivity was found to be increased with increasing applied frequency. On the other hand, tangent loss falls considerably with rise in applied frequency. Variations that occurred in magnetic, dielectric, and optical properties of MFO NPs are attributed to the transformation of structural phase of MFO NPs with increase in annealing temperature.