Low-field magnetic torque of a single crystal MgB2

We have investigated the magnetic torque of MgB2 single crystal synthesized by the vapor transport method. We use a torque magnetometer consisting of a 4-K closed cycle refrigerator and a variable field permanent magnet up to 10 kG. The torque can be measured as an off-balance signal of the Wheatstone bridge of the four piezoresistors on a Si cantilever. The torque curves are analyzed by the Kogan model. The superconducting anisotropy gamma is rather independent of temperature, but is dependent on field. As expected from the BCS theory, the prefactor of the Kogan formula, which corresponds to the lower critical field H-cl, changes smoothly as a function of temperature. We consider that the field dependence of gamma comes from the multiple superconducting gaps and their different upper critical fields H-c2. (C) 2003 Elsevier B.V. All rights reserved.