Antiferromagnetic long-range order in Cu_{1-x}Zn_xGeO_3 with extremely   low Zn concentration
  We have measured the magnetic susceptibilities of single crystals of Cu_{1-x}Zn_xGeO_3 with extremely low Zn concentration (x) lower than x=5x10^{-3} at very low temperatures to investigate the spin-Peierls and antiferromagnetic transitions. The results show that the undoped 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(2)x10^{-3}. The minimum observed Neel temperature was 0.0285 K for x=1.12(2)x10^{-3} sample. From the concentration dependence of the Neel temperature it is concluded that there is no critical concentration for the occurrence of the antiferromagnetic long-range order. 
K. Manabe, H. Ishimoto, N. Koide, Y. Sasago and K. Uchinokura