Magnetostriction of the spin-Peierls cuprate CuGeO₃

Abstract

The isothermal, longitudinal magnetostriction of the spin-Peierls cuprate CuGeO₃ has been measured along the three orthorhombic directions up to 14 Tesla by means of a high resolution capacitance dilatometer. For all three axes we observe anomalies at both the dimerized/incommensurate (D/I) and the dimerized/uniform (D/U) transition whose sizes and signs differ. A precise H-T phase diagram is determined from the field and temperature dependence of the lattice constants, which roughly agrees with theoretical predictions. At the D/I transition the magnetostriction shows a jumplike behavior and a hysteresis indicating a first order transition. From the jumps of the magnetostriction at the D/I transition we estimate considerable, uniaxial stress dependences of the critical magnetic field H _c, which correlate with those of the spin-Peierls transition temperature. A finite magnetostriction is also resolved 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 pressure dependences of the magnetic susceptibility. Based on our findings the relevance of different structural parameters for the magnetic exchange interaction is discussed.