Unconventional Magnetism, Sliding Ferroelectricity, and Magneto-Optical Kerr Effects in a Multiferroic Bilayer

Abstract

Antiferromagnetic (AFM) materials offer a promising platform for exploring novel couplings between altermagnetic (AM) spin-splitting and magneto-optical Kerr effect (MOKE), with potential applications in next-generation quantum technologies. In this work, first-principles calculations, symmetry analysis, and kp modeling are employed to demonstrate how interlayer sliding in AFM multiferroic bilayers enables engineering of the electronic, magnetic, and magneto-optical properties. This study reveals an unprecedented dimension-driven AM crossover, where the 2D paraelectric (PE) bilayer exhibits spin-degenerate bands protected by the [C2||Mc] spin-space symmetry, while the 3D counterpart manifests AM spin-splitting along kz not equal to 0 paths. Furthermore, interlayer sliding breaks the Mc symmetry and stabilizes a ferroelectric (FE) state characterized by compensated ferrimagnetism and a Zeeman effect, which produces non-relativistic spin-split bands. In the FE phase, the inclusion of spin-orbit coupling (SOC) lifts accidental degeneracies, creating `alternating' spin-polarized bands due to the interplay of Zeeman and Rashba effects. Crucially, the spin polarization, ferro-valley polarization, and Kerr angle are simultaneously reversible by switching either interlayer sliding or the Neel vector. These findings highlight the rich coupling between electronic, magnetic, and optical orders in sliding multiferroics, thereby paving the way for ultra-low-power spintronics and optoelectronic devices.