Magnetic irreversibility in ultrafine ZnFe2O4 particles

Pure ultrafine ZnFe2O4 particles have been obtained from mechanosynthesis of the ZnO and Fe2O3 oxides. The average grain diameter was estimated from x-ray diffraction to be < d >=36(6) nm. Refinement of neutron diffraction data showed that the resulting cubic spinel structure is oxygen deficient, with similar to 7% of Fe3+ ions occupying the tetrahedral A sites. Magnetization curves taken at 4.2 K showed the absence of saturation in fields up to H=9 T, associated with a spin-canting produced by the milling process. Field-cooled (FC) and zero-field cooled (ZFC) curves showed irreversible behavior extending well above room temperature, which is associated with spin disorder. Annealing samples at 300 degrees C yields an average grain size < d >=50(6) nm, and similar to 16% of Fe3+ ions at A sites. Partial oxygen recovery is also deduced from neutron data refinement in annealed samples. Concurrently, decrease of magnetic irreversibility is noticed, and assigned to partial recovery of the collinear spin structure. Complex Mossbauer spectra were observed at room temperature and 80 K, with broad hyperfine field distributions spanning from similar to 10 to similar to 40 T. At T=4.2 K, hyperfine field distributions indicate high disorder in Fe local environments. The above data suggest the existence of Fe-rich clusters, yielding strong superexchange interactions between Fe ions at A and B sites of the spinel structure. (C) 2000 American Institute of Physics. [S0021-8979(00)05611-5].