Neutron scattering has been used to reveal the nature of the magnetic ordering of the copper and the rare-earth (RE) ions in various high-T-c oxides. The magnetic interactions are found to be two-dimensional (2D) and antiferromagnetic in nature. The interaction strengths between the Cu ions are very strong, while that between the RE ions are usually very weak with the exceptions that Pr ions and Tb ions being strongly coupled. These basic properties leading to ordering temperatures that are typically above room temperature and less than a few degrees Kelvin for Cu and RE spins, respectively. Three-dimensional (3D) magnetic behavior is frequently being observed owing to the domain-enhancement effect. There are, however, several systems in which the crossover from 2D to 3D behaviors have not been observed.
