Out-of-plane ferroelectricity, magnetoelectric coupling and persistent spin texture in two-dimensional multiferroics

Abstract: Two dimensional multiferroics with out of plane ferroelectricity hold significant promise for miniaturized magnetoelectric spin-orbit transistors, yet systems combining robust ferroelectricity and strong magnetoelectric coupling are exceedingly rare. Here, we demonstrate that epitaxial strain stabilizes out of plane ferroelectricity in exfoliated two dimensional Ruddlesden Popper derivatives. The hybrid improper ferroelectric Pc phase transitions to a competing P21 phase with purely in plane polarization upon switching, accompanied by a 90 degree rotation of weak ferromagnetism. Crucially, the Pc phase exhibits altermagnetism, while P21 displays full Brillouin zone band splitting, with persistent spin textures rotating 90 degree at the phase boundary. This work establishes a pathway to engineer two dimensional multiferroics that integrate vertical polarization, magnetoelectric coupling, and switchable spin textures, key features for next generation spintronic devices.