Flux pinning in nanoparticle doped MgB2/Cu tapes
The irreversibility fields Birr and critical current densities Jc of undoped and Si and SiC nanoparticle doped (5, 10 and 20 wt%) MgB2tapes were measured in the temperature (T) range 238 K and in magnetic fields B □ 16 T. Whereas Birr of undoped tapes varies smoothly with T, those of doped tapes show a change in slope around a crossover field Bcr which increases with nanoparticle content and also depends on their type. This indicates matching effect in vortex pinning, probably associated with Mg2Si nanoprecipitates formed during heat treatment. Indeed, Birr of doped tapes was enhanced in respect to that of undoped one with the highest enhancement for Birr ≈ Bcr, but the enhancement remained high ≈1.4 even for Birr >> Bcr (low temperatures). The variations of Jc and the pinning force density Fp = JcB with B and T support the above findings.