Ferromagnetism And Orbital Order In A Topological Ferroelectric

We explore via density functional calculations the magnetic doping of a topological ferroelectric as an unconventional route to multiferroicity. Vanadium doping of the layered perovskite La2Ti2O7 largely preserves electric polarization and produces robust ferromagnetic order and, hence, proper multiferroicity. The marked tendency of dopants to cluster into chains results in an insulating character at generic doping. Ferromagnetism stems from the symmetry breaking of the multiorbital V system via an unusual "antiferro''-orbital order, and from the host's low-symmetry layered structure.