  Cd2Os2O7 shows a peculiar metal-insulator transition at 227 K with magnetic
ordering in a frustrated pyrochlore lattice, but its magnetic structure in the
ordered state and the transition origin are yet uncovered. We observed a
commensurate magnetic peak by resonant x-ray scattering in a high-quality
single crystal. X-ray diffraction and Raman scattering experiments confirmed
that the transition is not accompanied with any spatial symmetry breaking. We
propose a noncollinear all-in/all-out spin arrangement on the tetrahedral
network made of Os atoms. Based on this we suggest that the transition is not
caused by Slater mechanism as believed earlier but by an alternative mechanism
related to the formation of the specific tetrahedral magnetic order on the
pyrochlore lattice in the presence of strong spin-orbit interactions.
