Abstract
The magnetic characteristics of LaCu${\mathrm{O}}_{2.5}$ comprising an antiferromagnetic two-chain spin-\textonehalf{} ladder have been investigated by $^{63}\mathrm{Cu}$ NMR measurements. The ground state of this compound was suggested from susceptibility measurements to be the singlet-spin liquid state. The present experiments have, however, revealed that LaCu${\mathrm{O}}_{2.5}$ is not in such a spin-liquid state, but rather a magnetic ordering takes place below ${T}_{N}\ensuremath{\sim}110$ K as evidenced by the large enhancement of the nuclear spin-lattice relaxation rate, ($\frac{1}{{T}_{1}}$) and of the spin-echo decay rate, $\frac{1}{{T}_{2}}$ leading to the disappearance of the Cu NMR signal. This magnetic ordered state has been found to be suppressed by substituting Sr for La by 5%, i.e., by hole doping.
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