NQR INVESTIGATION OF THE ISING-TYPE ANTIFERROMAGNETIC PHASE-TRANSITION OF THE GD-SUBLATTICE IN GDBA2CU3O6+X

The Cu-63,Cu-65 spin-lattice and spin-spin relaxation time (T-1,T-2) i n GdBa2Cu3O6+x, were investigated at low temperatures (1.3 - 4.2 K) in zero field. We studied the Gd phase transition near T-N = 2.2 K using the NQR of the Cu-chain site in order to look for an influence of the Cu magnetic moments in the CuO-planes on the Gd-spin dynamics. The co ntribution of the critical fluctuations of Gd to T-1 and T-2 at the Cu -chain vanishes due to the high symmetry. The temperature dependence o f the experimental T-1 follows closely T-1(-1) A + B(1 - T/T-N)ln\1 - T/T-N\ as is expected for an Ising-system for the non-critical modes. The same behaviour is observed for T-2, but with an additional critica l contribution in the non-superconducting samples with x < 0.4. While the dependence of T-N on the oxygen concentration is small (Delta T-N approximate to 80 mK), the relaxation rates increase by a factor of th ree with increasing x.