The magnetic susceptibility of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ for different oxygen concentrations is measured up to temperatures of 1000 K. In the semiconducting concentration range $\delta >0.5\mathrm{}$, the susceptibility exhibits the typical behavior of a two-dimensional Heisenberg antiferromagnet. For the exchange interactions Cu(2)-Cu(2) and Cu(1)-Cu(1) we estimate $J=1000\mathrm{} \mathrm{and} 3000$ K, respectively. The concentration dependence of the paramagnetic susceptibility at the semiconductor-superconductor phase boundary provides indications for the existence of antiferromagnetic fluctuations in the superconductors.

• Received 18 July 1988

DOI:https://doi.org/10.1103/PhysRevB.39.858