First-principles study of multiferroic Aurivillius-type interfaces in ferroelectric PbTiO<sub>3</sub>

Multiferroic materials have attracted considerable attention due to their novel quantum phenomena, including magnetoelectric coupling and topological domains, which are derived from the cross-coupling mechanism between ferroelectric order and magnetic order. However, the discovery of intrinsic multiferroic materials exhibiting magnetoelectric coupling remains limited, as ferroelectricity typically originates from the d<sup>0</sup> electronic configuration, while ferromagnetism relies on partially filled d<sup><i>n</i></sup> state. Based on first principles calculations, this work demonstrates that electronic structure of PbTiO<sub>3</sub> perovskite can be engineered by introducing an Aurivillius-type interface layer, which induces localized magnetic moments at the interface. The results reveal that when the system maintains strong electric polarization (up to 116.88 μC/cm<sup>2</sup>), the interfacial charge changes the electron occupancy of oxygen atoms, thereby resulting in interface magnetism and magnetoelectric coupling in PbTiO<sub>3</sub>. Notably, this multiferroic state exhibits pronounced interface localization, with the magnetic moment decaying rapidly as the layer thickness increases. Importantly, the emergent magnetism is asymmetric, resulting in a net positive spontaneous magnetization of 2.0<i>μ</i><sub>B</sub>. This observation indicates the emergence of ferrimagnetism at the interface. Furthermore, the interfacial region displays p-type conductivity behavior, exhibiting characteristics of two-dimensional hole gas (2DHG), and the density of holes and the density of charge carriers at the interface are several times higher than those in typical heterostructures. Overall, our work proposes a novel mechanism for designing multiferroic and providing a promising strategy for developing magnetoelectric-coupled multiferroic devices.