Magnetic transitions in the systemYBa2Cu2.8Co0.2O6+y

We have studied the oxygen dependence on the two magnetic transitions (antiferromagnetic ordering of chains and planes) in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_{2.8}{\mathrm{Co}}_{0.2}{\mathrm{O}}_{6+y}$ using neutron scattering. It is found that both transition temperatures increase with decreasing oxygen concentration. At $y\approx 0.37$ ($\equiv y_0$) the two transition temperatures are equal, so that chains and planes order at a single transition temperature for $y\le y_0$. For $y=1\mathrm{}$ the compound is superconducting at 60 K. Therefore, this system qualitatively exhibits the magnetic and superconducting properties of pure $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+y}$ 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.