Tunable magnetic transition and thermoelectric performance in single-layer metal dibromides MBr2 (M: Fe and Co)

Abstract

Investigation of magnetic transition and thermoelectric characteristics in the single-layer metal dibromides 1T-MBr₂ (M: Fe and Co) has been performed by generalized Bloch theorem within density functional theory. Using the lowest total energy, we identified the ferromagnetic, spiral, or antiferromagnetic state formed by a flat spiral configuration in the primitive unit cell. For the non-doped case, FeBr₂ and CoBr₂ possessed the ferromagnetic and spiral ground states, respectively. As the lattice parameter increased, the initial ground state changed to a new ground state, thus creating a magnetic transition. Also, incorporating hole-electron doping led to a magnetic transition, which was sensitive to some lattice parameters. For the optimized lattice parameters, we investigated thermoelectric performance using the initial ground state for some temperatures within the Boltzmann transport theory. We found a low (high) figure of merit in FeBr₂ (CoBr₂) at the Fermi level only for the paramagnetic state, where the temperature was above the Curie temperature. The emergence of a spiral state and a high figure of merit might lead to multiferroic and thermoelectric materials.