Long range magnetic ordering in Na₂IrO₃

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We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb lattice magnet Na₂IrO₃, a strong candidate for a realization of a gapless spin-liquid. Using resonant x-ray magnetic scattering at the Ir L₃-edge, we find 3D long range antiferromagnetic order below Tₙ=13.3 K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the a-axis. Combining the experimental data with first principles calculations, we propose that the most likely spin structure is a novel "zig-zag" structure.