Abstract
Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from an angularly-distorted tetragonal structure with c/a �� 1.04 to a bulk-like rhombohedral structure as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric properties barely change: the remanent polarization measured in the [001] direction slightly decreases from 69.2 ��1 ��C/cm2 in fully strained to 66.0 ��1 ��C/cm2 in fully relaxed films. Assuming a polarization direction along the long body diagonal of the respective distorted pseudo-cubic unit cells, this corresponds to a decrease of only 1.4% as c/a changes from 1.04 to 1.0. This illustrates the
difference between the present lone-pair driven mechanism of ferroelectricity and the one observed, for example, in BaTiO3.
