We report elastic neutron diffraction and muon spin relaxation ( $\mu \mathrm{SR}$) measurements of the quasi-one-dimensional antiferromagnets ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ and ${\mathrm{Ca}}_2{\mathrm{CuO}}_3$, which have extraordinarily reduced $T_N/J$ ratios. We observe almost resolution-limited antiferromagnetic Bragg reflections in ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ and obtain a reduced ordered moment size of $\sim 0.06{\mu }_{\mathrm{B}}$. We find that the ratio of ordered moment size $\mu \left({\mathrm{Ca}}_2{\mathrm{CuO}}_3\right)/\mu \left({\mathrm{Sr}}_2{\mathrm{CuO}}_3\right)=1.5\mathrm{}\left(1\right)$ roughly scales with their Néel temperatures, which suggests that the ordered moment size of quasi-one-dimensional antiferromagnets decreases continuously in the limit of vanishing interchain interactions.

• Received 10 October 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.1787