Molecular Auxetic Behavior of Epitaxial Co-Ferrite Spinel Thin Film

In functional oxide materials so-called molecular auxetic behavior is extremely rare. Here, it is reported in the CoFe 2 0 4 spinel structure. A CoFe 2 0 4 epitaxial thin film under compressive axial strain also reduces its cell dimensions in the transverse direction with a Poisson's ratio of -0.85. A hinge-like honeycomb network in the spinel structure is identified as being responsible for the negative Poisson's ratio. This phenomenon has a substantial effect on the functional properties of CoFe 2 O 4 and enables the construction of a new class of nano-devices.