Effect of magnetic field on the flux pinning mechanisms in Al and SiC co-doped MgB2 superconductor
MgB 2 superconductor samples co-doped with 0.02 wt. Al 2 O 3 and 0-0.05 wt. SiC were studied by magnetization - magnetic field (M-H) loop measurements at different temperatures. The critical current density has been calculated by the Bean model, and the irreversibility field, H irr , has been obtained by the Kramer method. The pinning mechanism of the co-doped sample with 2% Al and 5% SiC was investigated in particular due to its having the highest H irr . The normalized volume pinning force f = F/F max as a function of reduced magnetic field h = H/H irr has been obtained, and the pinning mechanism was studied by the Dew-Houghes model. It was found that the normal point pinning (NPP), the normal surface pinning (NSP), and the normal volume pinning (NVP) mechanisms play the main roles. The magnetic field and temperature dependence of contributions of the NPP, NSP, and NVP pinning mechanisms were obtained. The results show that the contributions of the pinning mechanisms depend on the temperature and magnetic field. From the temperature dependence of the critical current density within the collective pinning theory, it was found that both the δl pinning due to spatial fluctuations of the charge-carrier mean free path and the δT c pinning due to randomly distributed spatial variations in the transition temperature coexist at zero magnetic field in co-doped samples. Yet, the charge-carrier mean-free-path fluctuation pinning (δl) is the only important pinning mechanism at non-zero magnetic fields.