Absence of auxeticity in CoFe2O4 epitaxial films

We report the absence of auxeticity in CoFe2O4, a magnetic spinel oxide. A CoFe2O4(111) epilayer was grown on the ZnO sacrificial layer, and the in-plane and out-of-plane lattice parameters were precisely determined by X-ray reciprocal space maps. To block the influence from stoichiometry variations, the same CoFe2O4 epilayer was lifted off from the substrate and was used as the reference sample. No auxetic behavior was found, and a positive Poisson ratio of similar to 0.32 was obtained. Moreover, the Poisson ratio derived from the compliance coefficients agrees well with our experimental observation.