Ferromagnetism in multiferroic BiFeO3 films: A first-principles-based study

An ab initio scheme is developed, and first-principles calculations are performed, to investigate ferromagnetism in BiFeO3 (BFO) thick and ultrathin films. These systems all possess a weak magnetization that results from a spin canting (that is induced by the tilting of the oxygen octahedra) and that increases from 0 to similar or equal to 0.027 mu(B) as the temperature is decreased below the Neel temperature. Such findings contradict a suggestion that the coupling between magnetic dipoles and mismatch strain leads to the previously reported large values for the magnetization in BFO films. This spin canting is also found to be essential for the linear magnetoelectric effect to occur.