MAGNETOSTRICTION OF THE SPIN-PEIERLS CUPRATE CUGEO3

The isothermal, longitudinal magnetostriction of the spin-Peierls cupr ate CuGeO3 has been measured along the three orthorhombic directions u p to 14 Tesla by means of a high resolution capacitance dilatometer. F or all three axes we observe anomalies at both the dimerized/incommens urate (D/I) and the dimerized/uniform (D/U) transition whose sizes and signs differ. A precise H-T phase diagram is determined from the fiel d and temperature dependence of the lattice constants, which roughly a grees with theoretical predictions. At the D/I transition the magnetos triction shows a jumplike behavior and a hysteresis indicating a first order transition. From the jumps of the magnetostriction at the D/I t ransition we estimate considerable, uniaxial stress dependences of the critical magnetic field H-c, which correlate with those of the spin-P eierls transition temperature. A finite magnetostriction is also resol ved within the high temperature uniform phase of CuGeO3 for all three lattice directions. These data show a pronounced, strongly anisotropic spin-lattice coupling in CuGeO3 and allow to derive the uniaxial pres sure dependences of the magnetic susceptibility. Based on our findings the relevance of different structural parameters for the magnetic exc hange interaction is discussed.