  We investigate the sensitive interplay between magnetic, electronic and
structural properties in the ferrimagnetic oxide NiFe2O4. Emphasis is placed on
the impact of reduced dimensionality in the crossover from bulk-like to
ultrathin films. We observed an enhanced saturation magnetization $M_S$ for
ultrathin NiFe2O4 films on Nb-SrTiO3 (001) substrates that co-occurs with a
reduced out-of-plane lattice constant under compressive in-plane epitaxial
strain. We found a bulk-like cationic coordination of the inverse spinel
lattice independent of the NiFe2O4 film thickness -- thus ruling out a cationic
inversion that nominally could account for an enhanced $M_S$. Our study instead
uncovers a reduction of the unit cell volume, i.e. an auxetic behavior in
ultrathin NiFe2O4 films, which may result in an enhanced magnetic exchange
caused by an increased interatomic electronic localization.
