Ferromagnetism in the Mott insulator Ba2NaOsO6
  Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d^1 ferromagnetic Mott insulator with an ordered moment of ~0.2 Bohr magnetons per formula unit and TC = 6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet groundstate anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering. 
A. S. Erickson, S. Misra, G. J. Miller, R. R. Gupta, Z. Schlesinger,   W. A. Harrison, J. M. Kim, I. R. Fisher