Magnetic, X-ray and Mossbauer studies on magnetite/maghemite core-shell nanostructures fabricated through an aqueous route

Uniform 6-13 nm sized 0D superparamagnetic Fe3O4 nanocrystals were synthesized by an aqueous `co-precipitation method' under a N-2 atmosphere as a function of temperature to understand the growth kinetics. The crystal phases, surface charge, size, morphology and magnetic characteristics of as-synthesized nanocrystals were characterized by XRD, Raman spectroscopy, FTIR, TG-DTA, BET surface area, dynamic light scattering along with zeta potential, HR-TEM, EDAX, vibrating sample magnetometry and Mossbauer spectroscopy. TEM investigation revealed highly crystalline spherical magnetite particles in the 8.2-12.5 nm size range. The kinetically controlled as-grown nanoparticles were found to possess a preferential (311) orientation of the cubic phase, with a highest magnetic susceptibility of similar to 57 emu g(-1). The Williamson-Hall technique was employed to evaluate the mean crystallite size and microstrain involved in the as-synthesized nanocrystals from the X-ray peak broadening. In addition to FTIR and Raman spectra, Rietveld structural refinement of XRD confirms the magnetite phase with 5-20% maghemite in the sample. VSM and Mossbauer spectral data allowed us to fit the magnetite/maghemite content to a core-shell model where the shell is 0.2-0.3 nm thick maghemite over a magnetite core. The activation energy of