as a function of magnetic field and temperature revealed via resonant and non-resonant x-ray scattering*

We studied correlation of lattice symmetry and magnetism in Ba₂NaOsO₆ (BNOO) showing that it was tetragonal (T) even at 300K, with a transition into an orthorhombic (O) phase near a "ferromagnetic" transition at T_c~6.9K . Fluctuations of O-phase seem to appear well above T_c, and calorimetric data, concurrently recorded with scattering, indicates a T-to-O transition at T>T_c. At T<T_c, a commensurate order at q=(1,0,0) (a is the shortest axis of O phase) is observed. In a magnetic field normal to q, this staggered order saturates, indicating a coherent rotation of ordered magnetic domains to align with field direction as well as revealing some qualitative differences with bulk M(H) data. Due to structural twins, bulk measurements are susceptible to effects of field applied along all three directions simultaneously, while scattering uniquely probes magnetic order within individual twins. This field evolution of q-satellite along with its angular, polarization, and azimuthal properties of resonant scattering is indicative of a novel orbital-ordered phase with quantization (or principal) axis confined to the orthorhombic bc-plane.