Magnetic susceptibility and phase diagram of CuGe_1-xSi_xO₃ single crystals

Magnetic-susceptibility measurements have been performed on single crystals of the quasi-one-dimensional system CuGe_1-xSi_xO₃, for a large number of x values (0<~x<~0.1) and down to very low temperatures (70 mK<~T<~20 K). Si doping has three major effects: The drastic destruction of the spin-Peierls phase, the appearance at low temperature of a three-dimensional antiferromagnetic order, and the freeing of some S=1 / 2 spins. We have studied in detail the temperature dependence of the susceptibility in order to determine, in the low-temperature range, the proportion of free spins (i.e., paramagnetic) and the proportion of spins being in the spin-Peierls state. A quite complete (T,x) phase diagram and its qualitative interpretation are given. We have also carried out field-dependent susceptibility measurements in the antiferromagnetic phase to study the spin-flop transition. The spin-flop field varies between 1 and 15 kOe depending on the Néel temperature of the sample. A (H,T) phase diagram is given for several Si-doping levels.