The surface impedance of GdBa2Cu3O7-delta

The complex surface impedance of a number of GdBa2Cu3O7-delta Single crysta ls has been measured at 10, 15 and 21 GHz using a cavity perturbation techn ique. At low temperatures a marked increase in the London penetration depth and surface resistance is observed associated with the paramagnetic and an tiferromagnetic alignment of the Gd spins. The penetration depth has a shar p change in slope at the Neel temperature, T-N similar to 2.2K, and the rea l part of the conductivity peaks at a frequency dependent temperature sligh tly above T-N. The observed temperature and frequency dependence can be des cribed by a model which assumes a negligibly small interaction between the Gd spins and the electrons in the superconducting state, with a frequency d ependent magnetic susceptibility and a Gd spin relaxation time tau (s) bein g a strong function of temperature. Above T-N, tau (s) has a component vary ing as 1/(T - T-N), while below T-N it increases similar to T-5.