Download Nature of magnetism in double perovskite Ba2NaOsO6

Nature of magnetism in double perovskite Ba2NaOsO6
K.-W. Lee1 and W. E. Pickett2
1
Department of Display and Semiconductor Physics, Korea University, Jochiwon, Chungnam
339-700, Korea
2
Department of Physics, University of California, Davis, CA. 95616, USA
Heavy atoms with large relativistic effects (spin-orbit coupling) and open atomic
shells may resist breaking of lattice symmetry in cases where their lighter counterparts would
succumb.
Ba2NaOsO6 is a rare example of a Os7+ compound (d1: single occupation of the t2g
orbits), also being ferromagnetic insulating (Tc ~ 7 K). Nevertheless, this system has the ideal
cubic structure, indicating no orbital ordering. Our analysis based on a first principles tightbinding dispersion model, spin-orbit coupling, Hund's coupling, and strong Coulomb
repulsion shows that the net magnetic moment of the OsO6 cluster nearly vanishes due to spin
and orbital cancellation. Quenching (hybridization) leads to magnetization with a small net
moment. This degree of compensation is unprecedented in transition metals.
Including the large spin-orbit coupling ξ=0.30 eV and the Hund's rule exchange
coupling (0.35 eV) leaves, surprisingly, a nearly pure-spin orbital occupied. An intra-atomic
Coulomb repulsion U=1.2 eV is necessary to produce the observed (Mott) insulating phase.
The small ordered moment (0.2 μB) reflects large quantum fluctuations, which prevent
orbital ordering to a broken-symmetry state.