Crystallite size induced crossover from paramagnetism to superparamagnetism in zinc ferrite nanoparticles

• Synthesis of paramagnetic and superparamagnetic zinc ferrite.
• Fe3+ ions along with the concomitant incomplete crystallization are responsible for superparamagnetic behavior.
• Increase of Fe–O bonds with crystallization.

Present work investigates the crossover from paramagnetism to superparamagnetism as a function of crystallite size in zinc ferrite nanoparticles using near edge X-ray absorption spectroscopy. Synthesized paramagnetic and superparamagnetic nanoparticles exhibit presence of Fe2+ and Fe3+ ions with dominant concentration of Fe3+ ions. Fe L- and O K-edges spectra of paramagnetic nanoparticles consist of more intense spectral features compared to that of superparamagnetic nanoparticles. This reflects enhanced t2g and eg symmetry states of Fe–O hybridized states in paramagnetic nanoparticles induced by increased degree of crystallization.