Magnetic transitions in the system YBa2Cu2.8Co0.2O6+y.

We have studied the oxygen dependence of the two magnetic transitions (antiferromagnetic ordering of chains and planes) in YBa/sub 2/Cu/sub 2.8/Co/sub 0.2/O/sub 6+/ital y// using neutron scattering. It is found that both transition temperatures increase with decreasing oxygen concentration. At /ital y/approx.0.37 (/equivalent to//ital y//sub 0/) the two transition temperatures are equal, so that chains and planes order at a single transition temperature for /ital y/less than or equal to/ital y//sub 0/. For /ital y/=1 the compound is superconducting at 60 K. Therefore, this system qualitatively exhibits the magnetic and superconducting properties of pure YBa/sub 2/Cu/sub 3/O/sub 6+/ital y// while providing important insight on the oxygen dependence of chain site magnetic ordering. A discussion is presented which also includes results on Ni and Al substitutions.