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Vortex pinning by correlated disorder in nanoparticle doped MgB_2

The magnetoresistivity and critical current density of five Si-nanoparticle doped ($c\leq 10$ wt\%) and undoped Cu-sheathed MgB$_{;2};$ tapes and wires have been measured for temperatures $T\geq 28$ K in magnetic fields $B\leq 0.95$ T. The irreversibility lines $B_{;irr}; (T)$ for doped tapes show a change in a slope of the temperature variation with a crossover field depending on Si-content. Such $B_{;irr}; (T)$ variation is typical for high-temperature superconductors (HTS) containing columnar defects (a crossover occurs around the matching field $B_{;\phi };$) and is very different from a smooth $B_{;irr}; (T)$ variation in undoped MgB$_{;2}; $ samples. However, in nanoparticle doped MgB$_{;2}; $, the enhancement of $B_{;irr}; $ with respect to that of the undoped one, persists at all field scales (including $% B\gg B_{;\phi }; $), which is not the case in HTS containing columnar defects. The microstructure studies of nanoparticle doped MgB$_{;2}; $ samples show uniformly dispersed Mg$_{;2}; $Si nanopercipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped samples support the above findings. The impact of these results for a further enhancement of flux pinning and critical currents in MgB$_{;2}; $ is briefly discussed.

vortex pinning; MgB_2

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