Flux-pinning mechanism in silicone-oil-dopedMgB2: Evidence for charge-carrier mean free path fluctuation pinning

Flux-pinning mechanism of MgB(2) doped with 10 wt % silicone-oil sintered at low and high temperatures has been investigated by magnetic measurements. The field dependence of the critical current density, j(c)(B), was analyzed within the collective pinning model. A crossover field, B(sb), from the single vortex to the small vortex bundle-pinning regime was observed. For both types of sintered samples, the temperature dependence of B(sb)(T) at low temperature is in good agreement with the delta l pinning mechanism, i.e., pinning associated with charge-carrier mean free path fluctuation. At temperatures close to the critical temperature, however, there is evidence for delta T(c) pinning, which is associated with spatial fluctuations of the transition temperature. These results provide strong evidence that the liquid precursor, silicone oil, produces very small pinning centers and enhances the j(c)(B).