CHARACTERISATION OF THE NÉEL STATE IN Zn AND Ni DOPED CuGeO₃ S. Coad¹, J.-G. Lussier², D. McK. Paul¹ and D. F. McMorrow²

¹Dept. of Physics, Univ. of Warwick, Coventry, CV4 7AL, UK ² Dept. of Solid State Physics, Riso National Lab, DK-4000 Roskilde, Denmark.

Single crystal derivatives of the spin-Peierls (S-P) system, CuGeO₃, doped with concentrations of Zn (0.5% to 2.4%) and Ni (1.7% to 6%) have been studied using neutron scattering and SQUID magnetometry. Our study confirms that the presence of impurities generally suppresses the S-P state and leads to the onset of a Néel state at low temperatures. Although the effect of doping is fundamentally similar for Zn and Ni doped crystals, neutron scattering measurements of several magnetic reflections in the b*-c* plane reveal that the preferred spin orientation of the Cu²⁺ moments is along the c* axis for the Zn doped crystals and along the a* axis in the Ni doped material. In contrast to a recent report about the lack of saturation in the ordered moment in a Zn doped crystal, our measurements show the expected saturation of the magnetic intensity at low temperatures.