Origin of the Large Polarization in Multiferroic YMnO₃ Thin Films

Multiferroic materials have attracted much interest because of their ability to control magnetism by the application of an electric field. This ability is expected to reduce the power required by electronic devices and to increase their speed. However, the number of multiferroic materials discovered so far has been small, and ferromagnetism and ferroelectricity in the known materials are often much weaker than required for practical applications. Hence, it is important to find novel multiferroic materials. In 2011, we succeeded in fabricating a YMnO₃ multiferroic film having a dielectric polarization exceeding that of previous multiferroic thin films, and performed x-ray diffraction measurements to investigate the magnetic structure and lattice strain of this film. We discovered that the spin of Mn ions has two coexisting magnetic structures, namely the cycloidal and E-type antiferromagntic (AF) orderings, and that the lattice strain due to E-type AF orderings is the origin of the large electric polarization. The ability to produce multiferroic YMnO₃ thin films and the understanding of the mechanism inducing the large electric polarization is expected to support the design of multiferroic materials for practical applications in the future.