Ba$_{\mathrm{2}}$NiOsO$_{\mathrm{6}}$: a Dirac-Mott insulator with ferromagnetism near 100 K

The ferromagnetic semiconductor Ba$_{\mathrm{2}}$NiOsO$_{\mathrm{6}}$ ($T_{\mathrm{mag}}$\textasciitilde 100 K) was synthesized at 6 GPa and 1500 $^{\circ}$ C. It crystallizes into a double perovskite structure [\textit{Fm}-3$m$; $a \quad =$ 8.0428(1) Å], where the Ni$^{\mathrm{2+}}$ and Os$^{\mathrm{6+}}$ ions are perfectly ordered at the perovskite B-site. We show that the spin-orbit coupling of Os$^{\mathrm{6+}}$ 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 \textit{\textgreater }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 ($T_{\mathrm{mag}}$\textless 180 K), the spin-gapless semiconductor Mn$_{\mathrm{2}}$CoAl ($T_{\mathrm{mag}}$\textasciitilde 720 K), and the ferromagnetic insulators EuO ($T_{\mathrm{mag}}$\textasciitilde 70 K) and Bi$_{\mathrm{3}}$Cr$_{\mathrm{3}}$O$_{\mathrm{11}}$ ($T_{\mathrm{mag}}$\textasciitilde 220 K). It is also qualitatively different from known ferrimagnetic insulator/semiconductors, which are characterized by an antiparallel spin arrangement. Our report of cubic Ba$_{\mathrm{2}}$NiOsO$_{\mathrm{6}}$ heralds a new class of FM insulator oxides, which may be useful in developing a practical magnetic semiconductor that can be employed in spintronic and quantum magnetic devices.