Flux-pinning properties of single crystalline and dense polycrystalline MgB2

Magnetic hysteresis loops (MHLs) have been measured on single-crystalline and dense polycrystalline MgB2 prepared under high pressure by micro-Hall probe and superconducting quantum interference device magnetometer. Heating effect due to the viscous force on vortices has been found by changing the field-sweep rate. Magnetic critical current densities obtained from MHLs by Bean model show different field dependence for single and polycrystalline samples. The scaling behaviors of flux-pinning force are also different. There is only one pinning force peak for polycrystalline sample, whereas there are three pinning force peaks for single-crystalline sample. The irreversibility field has been determined by different methods. The anisotropy of flux pinning and the pinning mechanism in MgB2 are discussed.