Ultrasonic study of the magnetic phase diagram of the spin-Peierls systemCuGeO3

The elastic constants ${\mathit{C}}_{11}$, ${\mathit{C}}_{22}$, and ${\mathit{C}}_{33}$ of a ${\mathrm{CuGeO}}_3$ single crystal were measured between 2 and 20 K and in magnetic fields up to 14 T by an ultrasonic propagation technique. Below the spin-Peierls temperature in zero field a pronounced anomaly which is a hardening showing the typical temperature dependence of an order parameter was observed in the three constants; only ${\mathit{C}}_{33}$ showed precursor effects above the transition temperature where a substantial softening is obtained. The magnetic phase diagram is mapped out by following these anomalies in a field. The diagram is in excellent qualitative agreement with the experimental data gathered on other spin-Peierls systems and with the predictions of theoretical models. However, a new phase boundary line can be deduced within the dimerized phase from a supplementary elastic anomaly observed in a field near 8 T in ${\mathit{C}}_{33}$ only.