DOPING EFFECTS ON THE MAGNETIC PHASE-DIAGRAM OF THE SPIN-PEIERLS SYSTEM CUGE1-XSIXO3

The magnetic phase diagram of Si-doped (0.7 and 1.0%) CuGeO3 single cr ystals was obtained from an ultrasonic velocity measurement. The magne tic phase diagram is similar to what is generally found in pure spin-P eierls (SP) systems: a SP instability is identified although an additi onal antiferromagnetic phase is observed at the lowest temperatures. T he diagram is weakly anisotropic: a spin-flop (SF) phase is observed f or the magnetic field aligned along the c axis and a hard axis along a is deduced from the angular dependence of the SF field. These results are complemented by superconducting quantum interference device magne tization measurements.