OBSERVATION OF THE NEEL STATE IN DOPED CUGEO3

The effect of doping on the spin-Peierls (SP) transition in single crystals of CuGeO3 has been investigated using neutron scattering techniques and susceptibility measurements. When the Cu2+ site is partly substituted by either Zn2+ (4%) or Ni2+ (1.5%) the SP is suppressed, and a long-range Neel state develops below T-N = 2.8 K (Zn) and T-N = 2.2 K (Ni) with a wave vector of (0, 1, 1/2). This observation contrasts with earlier reports of a spin glass transition in the doped compounds in the same temperature range. Our results also indicate that the reported Neel state in the nominally pure compound results from impurities. Preliminary measurements of the Zn-doped sample show that the gap that opens in the magnetic excitations below T-SP collapses in the doped compound