Ising-Like Spin Anisotropy and Competing Antiferromagnetic-Ferromagnetic Orders in GdBaCo₂O_5.5 Single Crystals

In RBaCo₂O_5+x compounds (R is rare earth), a ferromagnetic-antiferromagnetic competition is accompanied by a giant magnetoresistance. We study the magnetization of detwinned GdBaCo₂O_5.5 single crystals and find a remarkable uniaxial anisotropy of Co³⁺ spins which is tightly linked with the chain oxygen ordering in GdO_0.5 planes. Reflecting the underlying oxygen order, CoO₂ planes also develop a spin-state order consisting of Co³⁺ 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.