Dielectric response of epitaxially strained CoFe$_2$O$_4$ spinel thin films

Aiming to explore strain effects on the dielectric permittivity of ultrathin films of oxides with spinel structure, we report here on the thickness (4–160 nm) dependence of the dielectric response of CoFe2O4 (CFO) epitaxial films grown on La2/3Sr1/3MnO3 buffered SrTiO3(001) substrates. It is found that films thicker than ∼30 nm display bulklike permittivity values (ɛr≈14); however, a pronounced and gradual ɛr reduction is observed for thinner films when the in-plane compressive strain induced by the substrate increases. First-principle calculations are used to simulate the variation of the permittivity of CFO spinel thin films under epitaxial strain; in agreement with simple bond-length considerations, the out-of-plane permittivity is predicted to increase under in-plane compressive strain due to the resulting out-of-plane lattice expansion, but this enhancement can be overcompensated if this expansion is suppressed, resulting in an effective reduction of permittivity. However, the predicted reduction is substantially smaller than observed experimentally. We discuss possible mechanisms to account for this observation.