Long Range Magnetic Ordering in Na2IrO3

We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb-lattice magnet Na{sub 2}IrO{sub 3}, a candidate for a realization of a gapless spin liquid. Using resonant x-ray magnetic scattering at the Ir L{sub 3} edge, we find three-dimensional long-range antiferromagnetic order below T{sub N} = 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 zig-zag structure.