Study of ac-susceptibility for co-doped MgB2 superconductor with nano-Al2O3 and nano-C at low magnetic field

摘要

The ac-susceptibility as a function of temperature is studied for undoped and nano-carbon in co-doped MgB2 with different doping level under the presence of varying ac-applied field (H-ac= 0.5, 1.0, 2.5, 5.0, 10, 15, 25, 35, 50 Oe). The peak of the imaginary part of ac-susceptibility is increasing with the increase of ac-field and shifted to the lower temperature region. The transition temperature, T, drops than increases slightly for C-3 wt% and C-4 wt% samples with the increase of the nano-carbon substitution for co-doped MgB(2 )sample. The FWHM of all the sharp XRD peaks confirms that the average grain size becomes smaller with the co-doping. The collective pinning model was taken into account to calculate the activation energy and current density. In addition, the activation energy decreases with the increase of the nano-C concentration in co-doped MgB2 sample while the activation energy enhances as the current density improves with varying ac-field.