In-plane ferroelectricity and enhanced Curie temperature in perovskite BaTiO3 epitaxial thin films

In-plane ferroelectric polarization in BaTiO3 (BTO) epitaxial thin films on MgAl2O4(001) (MAO) is reported. The directional dependence of both in-plane polarization curves and Raman spectroscopy shows that the films have an orthorhombic structure at room temperature, in contrast to the tetragonal structure of the corresponding bulk. The largest in-plane polarization value among BTO-based tensile-strained films is obtained. The temperature dependence of the lattice constants shows that the Curie temperature of the thin films is as high as 220 degrees C, which is higher than that of the bulk by 100 degrees C. The significant enhancement of the Curie temperature is attributed to high-quality coherent epitaxial growth due to perfect matching between the lattice parameter of the c-axis of BTO and that of MAO.