Defect engineering for nontrivial multiferroic orders in SrTiO3

Although nanoscale multiferroics with coupled ferroelectricity and magnetism are of great technological and fundamental importance, the ferroic orders are inevitably destroyed in atomic-scale dimensions due to the intrinsic size effect or depolarization field. Here, we propose a strategy for atomic-size multiferroics by engineering the otherwise detrimental point defects in nonmagnetic paraelectric SrTiO3. Systematic exploration using first-principles and molecular dynamics simulations demonstrates that the Ti antisite (Ti-sr) defects in SrTiO3 have a tendency toward clustering with compact closure, which contributes to complex electrical polarization due to the large off-centering displacements of Ti-sr defects and the resulting separation between negative and positive charges. The Ti-sr defects also give rise to highly localized magnetic moments attributed to their unpaired spin-polarized electrons. The defect clustering and the associated multiferroic properties thus lead to atomic-size multiferroics with unusual ferroic orders.