Magnetic Frustration Induced Formation of the Spin-Peierls Phase in CuGeO3: Experimental Evidence

Measurements of the magnetostriction of single crystalline CuGe${\mathrm{O}}_3$ at temperatures between 2 and 60 K and for magnetic fields up to 14 T are presented. At low temperatures the magnetostriction is dominated by changes of the spontaneous strains at the phase transitions characteristic for spin-Peierls systems. At higher temperatures a magnetoelastic coupling is found which is unexpectedly large and strongly anisotropic. A comparison of the magnetostriction well above the spin-Peierls transition temperature $T_{\mathrm{SP}}$ and the thermal expansion anomalies at $T_{\mathrm{SP}}$ yields a striking correlation between the uniaxial pressure dependencies of the spin-susceptibility and $T_{\mathrm{SP}}$. It is argued that this strongly supports electronic models of the spin-Peierls transition in CuGe${\mathrm{O}}_3$ which are based on competing antiferromagnetic intrachain interactions.