Effects of the sintering temperature on the flux-pinning mechanism and the activation energy of malic-acid doped MgB2

The flux-pinning mechanism and activation energy of 10 wt % malic acid-doped MgB2 were investigated by measuring of the critical current density and resistivity as a function of magnetic field and temperature. A crossover field, Bsb, was observed from the single vortex to the small vortex bundle pinning regime. For the sintered sample, the temperature dependence of Bsb(T) at low temperature is in good agreement with the δ pinning mechanism, i.e., pinning associated with charge-carrier mean free path fluctuation. The activation energy was decreased by increasing the magnetic field and increased by increasing sintering temperature.