Emergence of ferromagnetism at a vacancy on a non-magnetic ferroelectric PbTiO3 surface: A first-principles study

We investigated the origin of ferromagnetism due to a vacancy on a non-magnetic ferroelectric PbTiO3 (0 0 1) surface using first-principles spin-density-functional theory calculations, and successfully provided electronic-level insights into this phenomenon. The results demonstrate that an oxygen vacancy intrinsically induces ferromagnetism, which is remarkably enhanced and stabilized on the TiO2-terminated surface favorably rather than inside PbTiO3. Detailed electronic structure analysis revealed that the ferromagnetism originates from the spin-polarized dzx and dx2-y2dx2-y2 states of the two Ti atoms nearest to the oxygen vacancy on the surface. The origin of ferromagnetism for an oxygen vacancy inside PbTiO3 differs from that an oxygen vacancy on the surface; it is due to the eg (dx2-y2dx2-y2 and dz2dz2) state of the single nearest Ti atom. These findings provide significant fundamental insights for designing multiferroics in conventional ferroelectrics.