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Flux pinning and critical currents in polycrystalline MgB_2

The results of accurate electrical resistivity and ac susceptibility measurements for a well-characterized, single phase MgB2 sample are reported. The onset of electrical resistivity in magnetic fields B less than or equal to I T is thermally activated R exp(-U(T, B)/k(B)T) with U-0(B) = U(T = 0, B) infinity 1/rootB for B greater than or equal to 0.2T. The pinning potential U-0(B less than or equal to I T) is similar to that for YBa2Cu3O7 compound, but the region of flux liquid is quite narrow and the uppercritical field Bc(2) fairly low. High-resolution ac susceptibility measurements show homogeneous superconducting behavior (no Josephson coupled grains) and yield true zerofield critical cur-rent density J(c) similar to (1 - T/T-c)(n) with n approximate to 1.2. The comparison of literature results for J(c) with our ones shows pronounced mass density dependence of J(c) in bulk untextured MgB2 samples.

flux pinning; critical currents; MgB_2

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