M. Vucinic-Vasic

Magnetization enhancement in nanostructured random type MgFe2O4 spinel prepared by soft mechanochemical route

In this paper we report results of structural, spectroscopic, and magnetic investigations of MgFe2O4 nanoparticles prepared by soft mechanochemical synthesis. MgFe2O4 nanoparticles crystallize in Fd3¯m space group with mixed cation distribution and reduced percentage of Fe3+ at tetrahedral (8a) sites. Discrepancy in the cation distribution compared to that in the bulk Mg–ferrite is one of the highest known. X-ray line broadening analysis reveals crystallite size and strain anisotropy. The saturation magnetization, Msat=62 emu/g measured at 5 K is twice higher than that found in the bulk counterparts. Such high value of Msat is attributed to the low value of cation inversion parameter (δ=0.69), to the core/shell structure of the nanoparticles and to the surface/volume ratio. Mossbauer spectrum collected at room temperature reveals ferrimagnetic ordering between Fe3+ ions in 8a and 16d sites, while zero-field-cooled (ZFC) and field-cooled (FC) M(T) measurements were shown superparamagnetic state above 350 K.

Temperature-induced structure and microstructure evolution of nanostructured Ni0.9Zn0.1O

The crystal structure and microstructure of as-prepared and annealed Ni0.9Zn0.1O were refined at room temperature in both the Fm{\overline 3}m and R{\overline 3}m space groups. It is shown that below the Neel point (458 K), where magnetic ordering triggers the presence of a trigonal strain, the common usage of a higher-symmetry non-admissible space group for crystal structure and microstructure analysis via the Rietveld method may result in both an incorrect structure description and incorrect microstructure parameters (size and strain). More realistic microstructure data can be obtained by whole powder pattern modelling of the powder diffraction data. Increasing the annealing temperature causes a reduction of the trigonal distortion as well as an increase in domain size. Simultaneously, the Raman spectra become less resolved, a clear indication of domain growth and structural evolution of the structure towards cubic symmetry (R{\overline 3}m → Fm{\overline 3}m).