Dual-transition-metal strategy for a two-dimensional metallic FeCoO₂ system with above-room-temperature...

The realization of two-dimensional systems with coexisting magnetic order and polar structures is fundamentally constrained by the competing requirements for robust magnetism and stable inversion-symmetry-breaking distortions. In this work, we theoretically propose a dual-transition-metal design strategy to address this challenge, exemplified by a hexagonal FeCoO₂ monolayer. Owing to the electronic asymmetry between Fe and Co cations, a strong superexchange interaction network is established, giving rise to a high Néel temperature of 515 K and a sizable magnetic anisotropy energy of 40 µeV per atom. Simultaneously, the ordered occupancy of nonequivalent transition-metal ions breaks inversion symmetry, resulting in an intrinsic out-of-plane electric polarization of 3.97 pC m^-1. The coexistence of metallic antiferromagnetism and intrinsic polar behavior emerges directly from the cation ordering, without reliance on external fields or interfacial engineering. Importantly, both the magnetic and polar characteristics remain robust under moderate biaxial strain. These results demonstrate the feasibility of dual-transition-metal engineering as a general route toward high-temperature two-dimensional systems with coupled spin and charge degrees of freedom.