Multiferroic properties induced by defect dipoles in thin Ca3Mn2O7 films at room temperature

Improper ferroelectrics are a class of materials with the potential to design multiferroic properties. In this paper, we report on the effect of defect dipoles on the ferromagnetic and ferroelectric properties of Ca3Mn2O7 improper ferroelectric thin films. Two kinds of defects dipoles, Mn3+-oxygen vacancy (Mn3+-VO∙∙) and Li+-Al3+ doping, were studied. The experimental results show that both Mn3+-VO∙∙ and Li+-Al3+ defect dipoles can introduce ferromagnetism in the films, and Li+-Al3+ defect dipole can also improve ferroelectricity with self-polarization. The first-principle calculations based on local density approximation demonstrate the local lattice distortion, including title MnO6 octahedron and off-center displacement of Mn ion, plays crucial role in multiferroic properties of the film. This work gives an effective method to design the ferroelectricity and ferromagnetic properties in improper ferroelectric oxides with layered structure through constructing defect dipoles.