Magnetic properties and flux pinning in single crystalline and dense polycrystalline MgB 2

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Magnetic properties of single crystalline and dense polycrystalline MgB 2 prepared under high pressure have been investigated in a wide field range through local and global magnetization measurements using micro-Hall probe and SQUID magnetometer. The lower critical field, H c1, measured by Hall probe shows a BCS-like temperature dependence and can be fitted by two-gap model. Field dependence of magnetic critical current density is quite different for single and polycrystalline samples. There is only one pinning force peak for bulk sample, whereas there are two pinning force peaks for single crystalline sample. The upper critical field, H c2, and the irreversibility field, H irr, have been measured with magnetic field parallel and perpendicular to the boron sheets and the anisotropy ratio of H c2 is estimated to be about 3.