Long range magnetic ordering in Na$_2$IrO$_3$
  We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb lattice magnet Na$_2$IrO$_3$, a strong candidate for a realization of a gapless spin-liquid. Using resonant x-ray magnetic scattering at the Ir L$_3$-edge, we find 3D long range antiferromagnetic order below T$_N$=13.3 K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the {\it a}-axis. Combining the experimental data with first principles calculations, we propose that the most likely spin structure is a novel "zig-zag" structure. 
X. Liu, T. Berlijn, W.-G. Yin, W. Ku, A. Tsvelik, Young-June Kim, H.   Gretarsson, Yogesh Singh, P. Gegenwart and J. P. Hill
cond-mat.mtrl-sci