Proposing Altermagnetic‐Ferroelectric Type‐III Multiferroics with Robust Magnetoelectric Coupling

Abstract Multiferroic materials, characterized by the coexisting of ferroelectric polarization (breaking space‐ inversion symmetry, 𝒫) and magnetism (breaking time‐reversal symmetry, 𝒯), with strong magnetoelectric coupling, are highly sought after for advanced technological applications. Novel altermagnets, distinct from conventional magnets, have recently been revealed to exhibit unique spin polarization protected by crystal symmetry, which naturally overcomes the isolation of magnetism from ferroelectrics associated with spatial symmetry. In this study, a novel class of type‐III multiferroics is proposed, which leverages the unique symmetry of altermagnets to enforce spin‐ferroelectric locking, setting them apart from conventional multiferroics. Through first‐principles calculations, ferroelectric switching is shown to fully invert the spin polarization of altermagnets, equivalent to a 180° reversal of magnetic spin. This altermagnetic phase controlled by ferroelectrics can be effectively probed using the magneto‐optical Kerr effect, revealing a new class of multiferroics with intrinsic and deterministic magnetoelectric coupling. This theoretical advancement redefines the design principles of magnetoelectric materials and lays the foundation for the design of next‐generation spintronic devices leveraging altermagnetism.