Magnetic and Morphological Properties of Zn-Fe-Oxide-Based Core-Shell Nanoparticles

LOHR, JAVIER; DE ALMEIDA, ADRIELE APARECIDA; LUIZ C. C. M. NAGAMINE; M.S. MORENO; H.E. TROIANI; G.F. GOYA; T. TORRES MOLINA; FERNANDEZ-PACHECO, RODRIGO; E.L. WINKLER; M. VASQUEZ MANSILLA; R. COHEN; M.L. RODRIGUEZ; D. FREGENAL; R.D. ZYSLER; E. LIMA JR.

Uppsala

Congreso; Joint European Magnetic Symposia (JEMS 2019); 2019

JEMS

One-step synthesis by thermal decomposition is used to prepare three Zn-Fe-oxide-based core-shell bi-magnetic (Antiferro-/ferrimagnetic) nanoparticles. Zn/Fe ratios (0, 0.06 and 0.10) are obtained from the analyses of PIXE spectra. High resolution transmission electron microscopy images clearly show core-shell structures for the particle in all samples and an average particle size between 25 and 30 nm is inferred from size hystogram. X-ray diffraction profiles and structural analyses indicate that the core is composed by an antiferromagnetic (AFM) Wüstite (Fe1-yO) phase, whereas the shell is composed by a ferrimagnetic (FiM) ZnxFe3-xO4 spinel one. Electron-energy-loss spectroscopy analysis indicates that Zn is distributed almost homogeneously in the whole particle. This result indicates that Zn/Fe ratio of the first formed phase (Wüstite) is kept when the superficial oxidation occurs and results in the FiM ferrite phase. For this reason, the measured Zn/Fe ratio results always smaller than the initial value foreseen by the Fe and Zn acetylacetonate concentrations, due to the low solubility of the Zn in the Wüstite. Magnetization of the samples indicated that AFM phase is strongly coupled to the FiM structure of the ferrite shell resulting in a bias-field (HEB) appearing below the Néel temperature (T N) of FeO, with HEB values that depend on the core-shell relative proportion. The effective anisotropy of the shell decrease with an increase in the Zn content; at the same time, the AFM ordering of the core leads to an increment in the anisotropy of the whole system below T N. In-Field Mössbauer spectroscopy of the Zn richest sample reveals a strong magnetic frustration mainly to the site B of the ferrite, even at 5K.