Structural evolution-enabled BiFeO3 modulated by strontium doping with enhanced dielectric, optical and superparamagnetic properties by a modified sol-gel method

Multiferroic (BFO) nanoparticles doped with strontium with the general formula Bi1-x Sr x FeO3 (x = 0, 0.3, 0.5, 0.7) were synthesized using a modified sol-gel auto-combustion process. The structural, electrical, optical, and magnetic properties of the samples are discussed. The structural analysis, carried out using the x-ray powder diffraction technique, shows a structural transition from rhombohedral (R-3c) to cubic (Pm-3m) for the doping amount of strontium (Sr) equal to x = 0.3. Morphological analysis of the prepared samples were carried out using scanning electron microscopy (SEM). Frequency-dependent dielectric constant and ac conductivity were studied. The doped samples, with improved dielectric properties, can be used to fabricate different optoelectronic devices. Strong dielectric dispersion and broad relaxation were exhibited by all the samples. Cole-Cole plots were employed as an effective tool to study the dispersion parameters, namely, the optical dielectric constant, static dielectric constant, relaxation time, and spreading factor. The activation energy was calculated from the relaxation peaks and Cole-Cole plots, which were found to be compatible with each other. The bandgap of the samples was calculated using diffuse reflectance spectral (DRS) analysis. Sharp and strong photoluminescence in the IR region was observed in the samples, similar to ZnO, which was reported for the first time. Room-temperature and low-temperature magnetization studies point towards the superparamagnetic nature of the samples, with an improvement in magnetic properties with doping. The antiferromagnetic behavior of bulk bismuth ferrite transforms to superparamagnetic in nature for both pure and Sr-substituted bismuth ferrite nanoparticles due to the close dimensions of crystallite size with magnetic domains leading to the break-down of the frustrated spin cycloidal moment.