Similarity and Difference between Magnetic- and Nonmagnetic-Impurity Effects in Spin-Peierls Cuprate CuGeO₃

We have measured the magnetic susceptibility of single-crystal Cu_{1-x}Ni_xGeO_3 to study doping effects of the magnetic impurity (Ni^{2+} has S=1 spin) on the inorganic spin-Peierls (SP) material CuGeO_3. We observed the disappearance of the SP transition and the abrupt increase of the Neel temperature from 2.5 to 3.4 K at x nearly equal to 0.020, which indicates the first-order phase transition between the dimerized-antiferromagnetic (D-AF) and the uniform-antiferromagnetic (U-AF) phases, as was discovered in Mg-doped (Mg^{2+} is nonmagnetic) system [T. Masuda et al., Phys. Rev. Lett. 80, 4566 (1998)]. This indicates that this transition is universal for the doping to Cu site. We also found another phase with the easy axis in the a-c plane in low x region.