Critical current properties at high magnetic fields in polycrystalline MgB2 superconductors

Critical current properties at high magnetic fields are analyzed using the percolation theory for polycrystalline MgB2 superconductors. In the theory the effect of anisotropic irreversibility field of each grain due to random orientation is considered. Relatively low irreversibility field of this superconductor in spite of fairly strong flux pinning strength is attributed to the percolating nature for superconductor with low electrical connectivity. This theoretical approach explains the experimental result showing higher irreversibility field for samples with higher packing factor. It is concluded that the high packing factor is inevitable for high critical current density at high magnetic fields.