Ba₂NiOsO₆: A Dirac-Mott insulator with ferromagnetism near 100 K

The ferromagnetic semiconductor Ba2NiOsO6 (T-mag similar to 100 K) was synthesized at 6 GPa and 1500 degrees C. It crystallizes into a double perovskite structure [Fm-3m; a = 8.0428(1) angstrom], where the Ni2+ and Os6+ ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga, Mn) As and (Cd, Mn) Te (T-mag < 180 K), the spin-gapless semiconductor Mn2CoAl (T-mag similar to 720 K), and the ferromagnetic insulators EuO (T-mag similar to 70 K) and Bi3Cr3O11 (T-mag similar to 220 K). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba2NiOsO6 should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices