Cation inversion in bimagnetic CoO/Co1-xZnxFe2O4 core/shell nanoparticles: a XANES study

M.E. SALETA; G.C. LAVORATO; SANTIAGO J. A. FIGUEROA; JUNIOR C. MAURICIO; D. TOBIA; E. LIMA JR.; E.L. WINKLER; R.D. ZYSLER

Campinas

Workshop; LNLS 28th Annual Users? Meeting (RAU); 2018

LNLS

Aiming at controlling the exchange coupling at the AFM/FiM interface, we have fabricated a series of bimagnetic nanoparticles (NPs) by changing the shell composition. The system consists of 3 nm CoO NPs encapsulated in a 4 nm thick Co1-xZnxFe2O4 shell. A magnetic characterization shows that the coercive field decreases with Zn content while the exchange bias field presents a non-monotonous behavior, being maximum for intermediate concentrations. In order to elucidate the evolution of the magnetic properties with Zn incorporation, an investigation of the inversion degree of the spinel by XANES was performed. We have measured 5 different NPs and the bulk samples CoFe2O4 (CFO), ZnFe2O4 (ZFO) & Fe3O4 ([Fe]T[Fe]O2O4) at Fe, Co and Zn K-edges. In the region of pre-peak at Fe K-edge we have observed that their position and their shape do not vary in the NPs, the same behavior is presented by the sample CFO (partial inverted) and Fe3O4. This effect evidences some degree of inversion in the spinel structure. On theother hand, if we analyze the Co K-edge it is worth noting that the shape of the spectra of the NPs does not vary with the Zn content. This is a remarkable result because, a priori, the sample with 100% Zn only presents Co2+ from the CoO core. This is evidence that the Co of the core was partially dissolved and migrates to the shell producing a mixture of Co-Zn in the spinel with x=1. The analysis of the Fourier Transform brings information about the site occupancy of the different cations within the spinel structure (sites tetrahedral ?T? or octahedral ?O?). For the NPs and the bulk spinels both in Fe and Co K-edges data, peaks labeled as ?O? and ?T? are present. Besides, for Zn K-edge data only the ?T? peaks are present in the NPs and ZFO spectra. Therefore, the absence of the ?O? peak is a sufficient condition to exclude the occupancy of the octahedral site by the Zn atoms. These results reinforce the inversion between Fe and Co ions in the spinel (shell).