REVERSIBLE MAGNETIZATION AND TORQUES IN ANISOTROPIC HIGH-KAPPA TYPE-II SUPERCONDUCTORS

The reversible magnetization of anisotropic high-kappa type-II superconductors in an applied field H of arbitrary orientation with respect to the principal axes of the sample is considered in the framework of the Ginzburg-Landau theory with an anisotropic effective mass. We examine the procedure of obtaining the free-energy density F from its corresponding expression in the isotropic case by simply replacing the Ginzburg-Landau parameter kappa by a kappa-approximately that depends on the orientation of H relative to the principal axes. This procedure is valid when H is along one of the principal axes for arbitrary values of H between H(c1) and H(c2) and is also valid to a good approximation when H is not along one of the principal axes, but only when H >> H(c1). Because of the dependence of F on the orientation of H, when H is not parallel to one of the principal axes, the average magnetic-flux density B is not parallel to H, and a torque associated with the transverse magnetization exists, tending to orient the sample so that the value of kappa-approximately is the largest. Expressions for the magnetization and the torque are obtained from a variational model that permits the analytic calculation of F in the Ginzburg-Landau regime including, in addition to the supercurrent kinetic energy and the magnetic-field energy, the kinetic-energy and the condensation-energy terms arising from suppression of the order parameter in the vortex core. It is also pointed out that a comparison of the present theory with torque measurements can provide a way to estimate the upper critical field H(c2)(thet,phi), the thermodynamic field H(c), and the ratio m1:m2:m3 (m(i), i = 1,2,3, are the principal values of the effective-mass tensor m(ij)) in the temperature region where the Ginzburg-Landau theory is appropriate.