Gd3+ ESR determination of the local spin susceptibility in Gd:YBa2Cu3Oy high-temperature superconductors

Gd3+ ESR at high fields in Gd-doped high-T-c superconductors is shown to be a precise probe of the conduction electron static spin susceptibility, chi. The Gd3+ ESR Knight shift at 245 GHz and the spin relaxation broadening at 245 and 9.2 GHz in Gd0.01Y0.99Ba2Cu3Oy (with various values of between 6.05 and 7.0) are in agreement with published Y-89 NMR data above 70 K and provide additional measurements at the rare earth site at much lower temperatures. In the superconducting compounds chi(T) differs strongly from the Yoshida function and at low T it follows a T or T-2 dependence. An anomalous ESR line shape is found at low T in the superconducting state; we provisionally attribute this to a small (10(-2)-10(-3) mu(B)) static magnetic moment on planar Cu sites.