A three-order-parameter bistable magnetoelectric multiferroic metal

Using first-principles calculations we predict that the layered-perovskite metal Bi₅Mn₅O₁₇ is a ferromagnet, ferroelectric, and ferrotoroid which may realize the long sought-after goal of a room-temperature ferromagnetic single-phase multiferroic with large, strongly coupled, primary-order polarization and magnetization. Bi₅Mn₅O₁₇ has two nearly energy-degenerate ground states with mutually orthogonal vector order parameters (polarization, magnetization, ferrotoroidicity), which can be rotated globally by switching between ground states. Giant cross-coupling magnetoelectric and magnetotoroidic effects, as well as optical non-reciprocity, are thus expected. Importantly, Bi₅Mn₅O₁₇ should be thermodynamically stable in O-rich growth conditions, and hence experimentally accessible.