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Pure ultrafine ZnFe2O4 particles have been obtained from mechanosynthesis ofthe ZnO and Fe2O3 oxides. The average grain diameter was estimated from x-raydiffraction to be <d> = 36(6) nm. Refinement of neutron diffraction (ND) datashowed that the resulting cubic spinel structure is oxygen-deficient, with ~7%of Fe3+ ions occupying the tetrahedral A sites. Magnetization curves taken at4.2 K showed absence of saturation up to fields H = 9 Tesla, associated to aspin-canted produced by the milling process. Field-cooled (FC) andzero-field-cooled (ZFC) curves showed irreversible behavior extending wellabove room temperature, which is associated to spin disorder. Annealing samplesat 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 datarefinement in annealed samples. Concurrently, decrease of magneticirreversibility is noticed, assigned to partial recovery of the collinear spinstructure. Complex M\"ossbauer spectra were observed at room temperature and 80K, 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 localenvironments. The above data suggest the existence of Fe-rich clusters,yielding strong superexchange interactions between Fe ions at A and B sites ofthe spinel structure.

Magnetic irreversibility in ultrafine ZnFe2O4 partices