We have measured the magnetic susceptibilities of single crystals of ${\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{GeO}}_3$ with extremely low Zn concentration $\mathrm{}\left(x\right)\mathrm{}$ lower than $x=5\mathrm{}\times {10}^{-3}$ at very low temperatures to investigate the spin-Peierls and antiferromagnetic transitions. The results show that the undoped ${\mathrm{CuGeO}}_3$ has no antiferromagnetic phase down to 12 mK and there exists an antiferromagnetic long-range order with the easy axis along the $c$ axis for $x$ down to $1.12\left(2\right)\mathrm{}\times {10}^{-3}.$ The minimum observed Néel temperature was 0.0285 K for the $x=1.12\mathrm{}\left(2\right)\mathrm{}\times {10}^{-3}$ sample. From the concentration dependence of the Néel temperature it is concluded that there is no critical concentration for the occurrence of the antiferromagnetic long-range order. This indicates that the dimerization sustains the coherence of the antiferromagnetic phase of the spin polarization in impurity-doped systems and is consistent with the theory of the impurity-doped spin-Peierls system. The temperature dependence of the susceptibilities at $T>\mathrm{}{T}_N$ of all samples indicates that the magnetic correlations between localized spins are enhanced by a relatively large interchain interaction of ${\mathrm{CuGeO}}_3.$

• Received 24 March 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R575