Flux-pinning mechanism in silicone-oil-doped MgB2: Evidence for charge-carrier mean free path fluctuation pinning

Flux-pinning mechanism of MgB2 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).