Controlling phase transition in monolayer metal diiodides XI₂ (X: Fe, Co, and Ni) by carrier doping

We applied the generalized Bloch theorem to verify the ground state (most stable state) in monolayer metal diiodides 1T-XI₂ (X: Fe, Co, and Ni), a family of metal dihalides, using the first-principles calculations. The ground state, which can be ferromagnetic, antiferromagnetic, or spiral state, was specified by a wavevector in the primitive unit cell. While the ground state of FeI₂ is ferromagnetic, the spiral state becomes the ground state for CoI₂ and NiI₂. Since the multiferroic behavior in the metal dihalide can be preserved by the spiral structure, we believe that CoI₂ and NiI₂ are promising multiferroic materials in the most stable state. When the lattice parameter increases, we also show that the ground state of NiI₂ changes to a ferromagnetic state while others still keep their initial ground states. For the last discussion, we revealed the phase transition manipulated by the hole–electron doping due to the spin–spin competition between the ferromagnetic superexchange and the antiferromagnetic direct exchange. These results convince us that metal diiodides have many benefits for future spintronic devices. (paper)