Structural, Chemical, and Magnetic Investigations of Core-Shell Zinc Ferrite Nanoparticles

We investigate here the internal structure of zinc ferrite nanoparticles designed and prepared by a soft chemistry method to elaborate magnetic nanocolloids. The strategy used to avoid acid dissolution modifies the chemical composition of the surface of the nanoparticles, which are described as a core of stoichiometric zinc ferrite surrounded by a maghemite shell. Measurements of X-ray absorption near-edge spectroscopy, extended X-ray absorption fine structure, and X-ray diffraction are undertaken to investigate the local structure of nontreated nanocrystals and of surface-treated ones as a function of their sizes. The qualitative analysis of X-ray absorption results indicates a nonequilibrium cation distribution among the interstitial sites of the zinc ferrite nanocrystals core. Ab-initio calculations of theoretical photoelectron backscattering phases and amplitudes give, by fitting Fourier transformed EXAFS data at both Zn and Fe K-edges, an average inversion degree of 0.34. This value well matches the result of Rietveld refinement of X-ray diffraction data. Magnetization measurements performed on dilute aqueous nanocrystal dispersions, liquid at room temperature and frozen at low temperatures, are carried out in order to test the obtained results.