A Two-Dimensional Unconventional Multiferroic Material FeSnSe ₃ with Complex Magnetoelectric Switching

Two-dimensional multiferroics with strong magnetoelectric coupling are highly desirable for electrically controlled spintronic applications but remain exceedingly rare. Here, using first-principles calculations, we predict monolayer FeSnSe 3 as a stable two-dimensional multiferroic that hosts coupled ferroelectricity and altermagnetism. The ground state adopts an orthorhombic antiferroelectric structure with Néel-type antiferromagnetic order and can reversibly switch to a ferroelectric phase through a low energy barrier of 0.20 eV per formula unit. Phonon spectra and formation-energy calculations confirm the dynamic and thermodynamic stability of all competing phases. While the antiferroelectric ground state remains spin-degenerate, the ferroelectric phase becomes a genuine altermagnet characterized by symmetry-protected nonrelativistic spin splitting along non-high-symmetry directions. Ferroelectric switching preserves the magnetic order but modifies the magnetic space group, thereby enabling electrical control of altermagnetic responses. Furthermore, the ferroelectric phase exhibits a finite in-plane anomalous Hall conductivity with the Néel vector aligned parallel to the Hall current, providing a direct link between electric polarization, magnetic symmetry, and transport behavior. These results establish FeSnSe 3 as a promising platform for symmetry-engineered two-dimensional multiferroics and electrically programmable antiferromagnetic spintronic devices.