COMPETITION BETWEEN THE SINGLET-SPIN LIQUID-STATE AND THE MAGNETIC GROUND-STATE IN A 2-CHAIN SPIN-1 2 ANTIFERROMAGNETIC LADDER COMPOUND LACUO2.5 - A CU-63 NMR-STUDY/

The magnetic characteristics of LaCuO2.5 comprising an antiferromagnet ic two-chain spin-1/2 ladder have been investigated by Cu-63 NMR measu rements. The ground state of this compound was suggested from suscepti bility measurements to be the singlet-spin liquid state. The present e xperiments have, however, revealed that LaCuO2.5 is not in such a spin -liquid state, but rather a magnetic ordering takes place below T-N si milar to 110 K as evidenced by the large enhancement of the nuclear sp in-lattice relaxation rate, (1/T-1) and of the spin-echo decay rate, 1 /T-2 leading to the disappearance of the Cu NMR signal. This magnetic ordered state has been found to be suppressed by substituting Sr for L a by 5%, i.e., by hole doping.