Surface impedance of GdBa sub(2)Cu sub(3)O sub(7- delta )

The complex surface impedance of a number of GdBa sub(2)Cu sub(3)O sub(7- delta ) single crystals has been measured at 10, 15 and 21 GHz using a cavity perturbation technique. At low temperatures a marked increase in the London penetration depth and surface resistance is observed associated with the paramagnetic and antiferromagnetic alignment of the Gd spins. The penetration depth has a sharp change in slope at the Neel temperature, T sub(N) approx. 2.2K, and the real part of the conductivity peaks at a frequency dependent temperature slightly above T sub(N). The observed temperature and frequency dependence can be described by a model which assumes a negligibly small interaction between the Gd spins and the electrons in the superconducting state, with a frequency dependent magnetic susceptibility and a Gd spin relaxation time tau sub(s) being a strong function of temperature. Above T sub(N), tau sub(s) has a component varying as 1/(T - T sub(N)), while below T sub(N) it increases approx. T super(-5).