Are magnetite (Fe3O4) films on MgAl2O4 auxetic?

Magnetite (Fe3O4) films were fabricated on MgAl2O4 (001) single crystal substrates by pulsed laser deposition. In-plane and out-of-plane lattice constants were determined by X-ray diffraction. The apparent Poisson's ratio was determined as the negative ratio of the out-of-plane to in-plane strains. The results show that (i) the determination of Poisson's ratio by this method is only reliable for fully strained films and (ii) Poisson's ratio = 0.3 along the <100> direction is positive for this archetypal ferrite. Fe3O4 films grown on MgAl2O4 (001) are not auxetic.