  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.
