Novel Multiferroic Phases and Phenomena in Epitaxial (111) BiFeO3 Films

Multiferroics are attracting much interest because they simultaneously possess ordered electric and magnetic dipoles. In particular, numerous recent studies are devoted to find novel multiferroic phases, as, for example evidenced by the flurry of activities that accompanied the discovery of the so-called T-phase in BiFeO3 systems, when these latter are made in forms of (001) epitaxial films and subject to high-enough compressive strains. Here it is predicted, via the combined use of a genetic algorithm and first-principles calculations, that novel multiferroic phases, as well as new phenomena, can also occur in epitaxial BiFeO3 films, but when grown along the less conventional [111] direction and when experiencing large enough tensile strains. One example includes the hexagonal YMnO3-type P6(3)cm phase, that exhibits an anomalous behavior for its out-of-plane electric polarization and that can also undergo magnetic transitions when varying the tensile strain. Another striking example is the emergence of an unusual crystal structure of triclinic symmetry, that possesses controllable polarization's direction as well as magnetic spiral structures whose characteristics (e.g., periodicity and propagation direction) can be altered by epitaxial strain. Such findings may open new ways to design multiferroics and novel devices exploiting their cross-coupling between electric and magnetic properties.