Anisotropy of the superconducting state properties and phase diagram of MgB2 by torque magnetometry on single crystals

The angular and temperature dependence of the upper critical field H-c2 in MgB2, was determined from torque magnetometry measurements on single crystals. The H-c2 anisotropy gamma(H) was found to decrease with increasing temperature, in disagreement with the anisotropic Ginzburg-Landau theory, which predicts that gamma(H) is temperature independent. This behaviour can be explained by the two band nature of superconductivity in MgB2. An analysis of measurements of the reversible torque in the mixed state yields a field dependent effective anisotropy gamma(eff), which can be at least partially explained by different anisotropies of the penetration depth and the upper critical field. It is shown that a peak effect in fields of about 0.85H(c2) is a manifestation of an order-disorder phase transition of vortex matter. The H-T phase diagram of MgB2 for H\\c correlates with the intermediate strength of thermal fluctuations in MgB2, as compared to those in high and low T-c superconductors. (C) 2002 Elsevier Science B.V. All rights reserved.