Enhanced ferrimagnetism in auxeticNiFe2O4in the crossover to the ultrathin-film limit

We investigate the sensitive interplay between magnetic, electronic, and structural properties in the ferrimagnetic oxide ${\text{NiFe}}_2{\text{O}}_4$. 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 ${\text{NiFe}}_2{\text{O}}_4$ films on ${\text{Nb-SrTiO}}_3$ (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 ${\text{NiFe}}_2{\text{O}}_4$ film thickness, thus ruling out a cationic inversion that nominally could account for an enhanced $M_S$. Moreover, our study instead uncovers a reduction of the unit cell volume, i.e., an auxetic behavior in ultrathin ${\text{NiFe}}_2{\text{O}}_4$ films.