Anisotropy of magnetization discontinuity at vortex-lattice melting in untwinned

Abstract

We measured the magnetic torque τ experienced by an untwinned (Formula presented) single crystal in external magnetic fields up to (Formula presented) below the critical temperature (Formula presented) as a function of Θ, the angle between H and the c axis of the crystal. At the vortex-lattice melting transition we observe discontinuities in both τ and (Formula presented) that are related to changes in the transverse components of the magnetization vector M and (Formula presented) respectively. We use thermodynamic relationships to determine the direction of the vector (Formula presented) in space, and show that (Formula presented) is always directed parallel to M. The discontinuities in magnetization (Formula presented) and in entropy (Formula presented) vanish slightly below the temperature where the melting field (Formula presented) extrapolates to zero, which could indicate the existence of a lower critical point of the vortex-lattice melting line. From our (Formula presented) data we are able to extract the differences in the reduced specific heat (Formula presented) between the vortex-fluid and the vortex-solid phases, and we compare these results with corresponding thermal data. We finally examine the validity of standard angular scaling rules for anisotropic superconductors for the melting fields (Formula presented) at temperatures as high as (Formula presented). © 2000 The American Physical Society.