Orbital Order Triggered Out-of-Plane Ferroelectricity in Magnetic Transition Metal Dihalide Monolayers.

Abstract

Despite decades of multiferroic research, orbital-order-driven ferroelectricity remains exceptionally rare. Here, we demonstrate spontaneous out-of-plane ferroelectric polarization in monolayer magnetic transition-metal dihalides through first-principles calculations. Partially occupied d-orbitals in edge-sharing octahedra stabilize two-dimensional spatial orbital order, breaking inversion symmetry to induce coupled electronic and ionic polarization perpendicular to the plane. Distinct from previously reported metallic orbital-ordered systems, this mechanism operates in insulating states with noncollinear orbital interactions driving a transition between distinct insulating phases. Accompanying asymmetric Jahn-Teller distortions amplify polarization through lattice contributions. Crucially, this phenomenon emerges as a universal feature across a family of monolayer magnetic dihalides rather than being material-specific. Our work establishes orbital-ordering as a robust pathway to engineer intrinsic two-dimensional multiferroicity, expanding the design principles for multifunctional quantum materials. The interplay between orbital physics and ferroelectricity revealed here opens unexplored avenues for manipulating coupled electronic and structural orders in atomically thin systems.