Effect of anisotropic superconductivity on flux pinning in polycrystalline MgB2

Magnetization curves of dense polycrystalline MgB2 have been measured by SQUID magnetometer and calculated by solving the flux creep equation numerically. Considering the anisotropic superconductivity in MgB2 and the grain orientation in polycrystalline samples as well as the flux creep, the experimental data can be fitted very well by a two-exponential model. According to our fitting results, the nonscaling behavior of flux pinning force and the rapid decrease of critical current density of polycrystalline MgB2 at medium and higher magnetic fields originate from the anisotropic superconductivity in MgB2, the dispersion of grain orientations and the flux creep. Through this way, the real dependence of critical current density on temperature and magnetic field can be extracted, and the flux pinning mechanism is discussed. A universal method is therefore proposed for studying flux pinning in polycrystalline samples with intrinsic anisotropic superconductivity.