Absence of auxeticity in CoFe 2 O 4 epitaxial films

Abstract We report the absence of auxeticity in CoFe 2 O 4 , a magnetic spinel oxide. A CoFe 2 O 4 (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 CoFe 2 O 4 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 ∼0.32 was obtained. Moreover, the Poisson ratio derived from the compliance coefficients agrees well with our experimental observation.