In RBaCo_{ 2}O_{ 5+x} compounds (R is rare earth), a ferromagnetic-antiferromagnetic competition is accompanied by a giant magnetoresistance. We study the magnetization of detwinned GdBaCo_{ 2}O_{ 5.5} single crystals, and find a remarkable uniaxial anisotropy of Co^{ 3+} spins which is tightly linked with the chain oxygen ordering in GdO_{ 0.5} planes. Reflecting the underlying oxygen order, CoO_2 planes also develop a spin-state order consisting of Co^{ 3+} ions in alternating rows of S=1 and S=0 states. The magnetic structure appears to be composed of weakly coupled ferromagnetic ladders with Ising-like moments, which gives a simple picture for magnetotransport phenomena.
Categories: cond-mat.str-el cond-mat.mtrl-sci