A study of dielectric, optical and magnetic characteristics of maghemite nanocrystallites

• Synthesis of maghemite nanoparticles using coprecipitation pyrolysis method.
• Spherical shaped 25 nm particle size achieved with homogeneous and uniform size distribution.
• Study of low and high frequency dielectric response of maghemite nanoparticles.
• A UV/Vis optical band gap of 2.3 eV was obtained which is larger than the bulk sample value.
• Room temperature (300 K) and 77 K magnetic studies of nanomaghemites.

The maghemite (γ-Fe2O3) nanocrystallites have been synthesized by a co-precipitation pyrolsis method through 3 h annealing at 275 °C. Structural characterization measurement (X-ray diffraction) confirmed nanoparticles to be in pure maghemite phase with a particle size (as estimated through Scherrer formula) to be 27 nm. Scanning electron microscope (SEM) image confirmed the average particle size to be almost 25 nm with an almost spherical morphology and uniform distribution. In the dielectric measurements (which is the key novelty of this work), we observe a steep decrease in the dielectric constant at the low frequencies which can be attributed as a Maxwell–Wagner type effect. Whereas, at higher frequencies dielectric constant becomes almost frequency independent which can be regarded as a relaxation time phenomena. In the optical characteristics, the band gap of nano-maghemites has been calculated from the absorption spectra in the ultraviolet–visible (UV–Vis) range, which turns out to be 2.3 eV. Finally, in the magnetic measurements we performed a magnetization versus applied field (M–H) loop at the room temperature as well as 77 K. We notice an increasing trend in the values of saturation magnetization (Ms) and coercive field (Hc) as a function of applied temperatures. At room temperature (77 K) Ms and Hc values are found to be 50.4 (56.5) emu/g and 202.4 (248.6) Oe respectively, which manifests a suppression of thermal randomization of magnetic moments of maghemite nanoparticles with decreasing temperatures.