Magnetic irreversibility in ultrafine ZnFe2O4 partices

Abstract: 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 (ND) data showed that the resulting cubic spinel structure is oxygen-deficient, with ~7% of Fe3+ ions occupying the tetrahedral A sites. Magnetization curves taken at 4.2 K showed absence of saturation up to fields H = 9 Tesla, associated to a spin-canted 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 to spin disorder. Annealing samples at 300 {\deg}C yields an average grain size <d> = 50(6) nm, and ~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, assigned to partial recovery of the collinear spin structure. Complex M\"ossbauer spectra were observed at room temperature and 80 K, with broad hyperfine field distributions spanning from ~10 T 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.