Spin-flop transitions in Co-and Ni-doped single crystals of the spin-Peierls cuprate CuGeO3

The recent discoveries of a spin-Peierls transition in two inorganic oxide compounds permit new studies of this novel magnetic transition through subs titutional doping. We report the investigations of nickel and cobalt substi tutions on the copper site in copper germanate, CuGeO3. The magnetization a nd specific heat of single crystal samples are used to explore the effects of doping that give rise to a spin-flop transition. Specifically, we find t hat the Neel state exhibits a spin-flop transition for either dopant, with a critical field of approximately 1 tesla. Surprisingly, the easy axis in t he ordered state for nickel doping is the a-axis. This differs from the c-a xis easy axis observed for other dopants, such as Zn, Co and Mn.