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

TL;DR

This study reveals that ultrathin NiFe2O4 films exhibit enhanced ferrimagnetism linked to auxetic behavior and reduced unit cell volume, driven by strain effects rather than cationic inversion.

Contribution

It demonstrates that reduced dimensionality induces auxetic behavior in NiFe2O4, leading to increased magnetic exchange without cationic inversion, a novel insight into ultrathin ferrimagnetic oxides.

Findings

Enhanced saturation magnetization in ultrathin films

Reduced out-of-plane lattice constant under strain

Auxetic behavior linked to magnetic properties

Abstract

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.