  Magnetic structures and the relationship between spin and charge-orbital
orderings of an A-site ordered double-perovskite manganite SmBaMn2O6, an
anticipated multiferroic material, were investigated by means of neutron
diffraction. The spin arrangement in MnO2 planes perpendicular to the c axis is
revealed to be the same as that in the A-site disordered half-doped manganites
CE-type but the stacking pattern is found to be different displaying a unique
twofold period. The temperature dependence of the superlattice magnetic and
nuclear reflections clarifies that the antiferromagnetic spin ordering occurs
at a temperature slightly lower than the temperature at which a rearrangement
of the charge-orbital orderings occurs. The result evidences that the
rearrangement leads the spin ordering. The intensities of the magnetic
reflections are found to change across Tf = 10 K, suggesting a spin-flop by 90
[deg.] while keeping the Mn spin ordering pattern unchanged.
