Enhanced ferrimagnetism in auxetic NiFe2O4 in the crossover to the ultrathin film limit

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.