Orbital-quenching–induced magnetism in Ba2NaOsO6

The double perovskite Ba2NaOsO6 with heptavalent Os (d ) is observed to remain in the ideal cubic structure (i.e. without orbital ordering) despite single occupation of the t2g orbitals, even in the ferromagnetically ordered phase below 6.8K. Analysis based on the ab initio dispersion expressed in terms of an Os t2g-based Wannier function picture, spin-orbit coupling, Hund’s coupling, and strong Coulomb repulsion shows that the magnetic OsO6 cluster is near a momentless condition due to spin and orbital compensation. Quenching (hybridization) then drives the emergence of the small moment. This compensation, unprecedented in transition metals, arises in a unified picture that accounts for the observed Mott insulating behavior. Copyright c © EPLA, 2007 Orbital physics in transition metal oxides has attracted a good deal of attention for three decades, with much of the activity focused on coupling to spin, charge, and lattice degrees of freedom in d systems. The d configuration is typically found in the early 3d transition metals (TMs), i.e. titanates and vanadates, and the associated phenomena —often revolving around the tendency to orbitally order— are remarkably rich. The d configuration can also occur in the midto late-5d TM ions, which are distinguished also by unusually high formal valence states. Unless fully itinerant, partially filled d shells lead to non-spherical ions that often are accommodated by orbital ordering (OO), that is, ordered alignment of the filled orbitals [1,2] in a manner that minimizes strain, electronic, and magnetic energies [3]. An outcome is that OO has been identified as the driving mechanism in symmetrybreaking structural and magnetic transitions [2], and may also be coupled to magnetism and charge order, the Mott insulating d perovskite YTiO3 being a well-studied example [4]. Very different behavior is shown by the d system LaTiO3 which has a structural distortion although evidence of OO has been difficult to obtain. A prominent explanation is that orbital fluctuations dominate, leading to a disordered orbital liquid ground state [5,6]. The double-perovskite structure Ba2NaOsO6 (BNOO) is a rare example of a heptavalent osmium compound, (a)Current address: Department of Display and Semiconductor Physics, Korea University Jochiwon, Chungnam 339-700, Korea. also rare because it is a ferromagnetic insulator [7,8] (TC = 6.8K), and it shows other perplexing behavior. Although it has a single 5d electron in the t2g complex that orders magnetically, it shows no evidence of orbital order that would destroy its cubic symmetry. On the other hand, the sister compound La2NaOsO6 with high-spin d 3 Os configuration with nominally cubic symmetry is observed to be highly distorted [9], which is ascribed to misfit arising from the small cation radius. The question of spin ordering surely is a delicate one, since isostructural and isovalent Ba2LiOsO6 (BLOO) orders anti ferromagnetically in spite of a very similar Curie-Weiss susceptibility [7] and nearly identical volume. The question of formal valence, and identification of several material constants, can be identified from first principles local density approximation (LDA) calculations using two all-electron full-potential electronic methods of FPLO and Wien2k [10–12]. The Fermi level EF lies within the t2g bands, confirming the heptavalent nature of the Os ion. A crystal (ligand) field splitting of almost 5 eV separates the centroids of the eg and t2g bands, reflecting the unusually strong 5d-2p hybridization, and a gap of roughly 1.5 eV separates the t2g symmetry 1Erickson et al. [8] measure no hysteresis, suggesting BNOO is not a simple ferromagnet (such as having non-collinearity). 2In FPLO, basis orbitals were chosen such as Ba (4d5s5p)6s6p5d, Na (2s2p)3s3p3d, Os (4d4f5s5p)6s6p5d, and O 2s2p3d. (The orbitals in parentheses indicates semicore orbitals.) In Wien2k, the basis size was determined by RmtKmax = 7.0 and APW sphere radii (2.5 for Ba, 2.14 for Na, 1.86 for Os, 1.65 for O).