In $R{\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{o}}_2{\mathrm{O}}_{5+x}$ compounds ($R$ is rare earth), a ferromagnetic-antiferromagnetic competition is accompanied by a giant magnetoresistance. We study the magnetization of detwinned ${\mathrm{G}\mathrm{d}\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{o}}_2{\mathrm{O}}_{5.5}$ single crystals and find a remarkable uniaxial anisotropy of ${\mathrm{C}\mathrm{o}}^{3+}$ spins which is tightly linked with the chain oxygen ordering in ${\mathrm{G}\mathrm{d}\mathrm{O}}_{0.5}$ planes. Reflecting the underlying oxygen order, ${\mathrm{C}\mathrm{o}\mathrm{O}}_2$ planes also develop a spin-state order consisting of ${\mathrm{C}\mathrm{o}}^{3+}$ 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.

• Received 19 December 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.227201