Novel magnetic state in d4 Mott insulators

We show that the interplay of strong Hubbard interaction U and spin-orbit coupling lambda in systems with d(4) electronic configuration leads to several unusual magnetic phases. Most notably, we find that competition between superexchange and spin-orbit coupling leads to a phase transition from a nonmagnetic state predicted by atomic physics to a novel magnetic state in the large-U limit. We show that the local moment changes dramatically across this phase transition, challenging the conventional wisdom that local moments are robust against small perturbations in a Mott insulator. The Hund's coupling plays an important role in determining the nature of the magnetism. We identify candidate materials and present predictions for resonant x-ray scattering signatures of the unusual magnetism in d(4) Mott insulators.