Magnetic correlations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ at superconducting concentrations

Neutron-scattering measurements have been carried out on three large single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ with superconducting transition temperatures ${\mathit{T}}_{\mathit{c}}$=25, 45, and 50 K. Strong two-dimensional magnetic correlations are found in the first two, and the magnetic cross sections at low temperatures are almost as strong as in compounds with smaller x. However, the magnetic intensities at low energies below 10 meV decrease considerably on heating to 300 K. A large decrease of the magnetic inelastic cross section was observed in the third sample (${\mathit{T}}_{\mathit{c}}$=50 K) relative to the first two at these low energies. The implications of these results are discussed.