Altermagnetism-driven type-III multiferroicity in CrSiTe₃-based van der Waals structures

Altermagnets exhibit unique nonrelativistic spin splitting and zero net magnetization, but achieving active electrical control over their magnetic states remains a primary challenge for spintronic applications. In this work, using first-principles calculations, we demonstrate that interlayer sliding in CrSiTe_3-based van der Waals structures provides an effective route to engineer type-III multiferroicity driven by the intrinsic coupling of altermagnetism with sliding ferroelectricity. In a CrSiTe_3/SnS_2 heterostructure, breaking the inversion symmetry that connects the magnetic sublattices drives a transition from a conventional antiferromagnetic state to an altermagnetic state, accompanied by the emergence of out-of-plane polarization and nonrelativistic spin splitting. However, the absence of an inversion symmetry restoration pathway prevents reversible control of both polarization and spin orientation. To overcome this limitation, we design a SnS_2/CrSiTe_3/SnS_2 sandwich structure, where interlayer sliding enables switching between inversion-symmetric and inversion-broken states. This process simultaneously reverses electric polarization and spin orientation, realizing spin-polarization locking in an altermagnetic system. Furthermore, type-III multiferroicity is achieved in bilayer CrSiTe_3 without external substrates. These results establish a general strategy for integrating altermagnetism with sliding ferroelectricity and provide a viable route toward electrically controllable altermagnetic spintronic applications.