The surface impedance of GdBa₂Cu₃O₇−δ

The complex surface impedance of a number of GdBa2Cu3O7−δ single crystals has been measured at 10, 15 and 21 GHz using a cavity perturbation technique. At low temperature 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 Néel temperature, TN ∼ 2.2K, and the real part of the conductivity peaks at a frequency dependent temperature slightly above TN. 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 τs being a strong function of temperature. Above TN, τs has a component varying as 1/(T − TN), while below TN it increases ∼ T−5.