Dramatically enhanced polarization has been found for (001), (101), and (111) films, relative to that of BiFeO3 crystals. The easy axis of spontaneous polarization lies close to (111), for the various oriented films. BiFeO3 films grown on (111) have a rhombohedral structure, identical to that of single crystals; whereas films grown on (101) or (001) are monoclinically distorted from the rhombohedral structure, due to the epitaxial constraint.

In BiFeO3 films, it has been found that epitaxial constraint results in the destruction of a space modulated spin structure. For (111)c films, relative to corresponding bulk crystals, it is shown (i) that the induced magnetization is enhanced at low applied fields; (ii) that the polarization is dramatically enhanced; whereas, (iii) the lattice structure for (111)c films and crystals is nearly identical. Our results evidence that eptiaxial constraint induces a transition between cycloidal and homogeneous antiferromagnetic spin states, releasing a latent antiferromagnetic component locked within the cycloid.