Role of MgO impurity on the superconducting properties of MgB_2

摘要

We address the effect of MgO impurity on the superconducting properties of MgB_2. The synthesis of MgB_2 is very crucial because of sensitivity of Mg to oxidation which may lead to MgO as a secondary phase. Rietveld refinement was performed to determine the quantitative volume fraction of MgO in the samples synthesized by two different techniques. Both the samples were subjected to magnetization measurements under dc and a.c. applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB_2 having more amount of MgO (with T_c = 37.1 K) whereas the pure sample MgB_2 having minor quantity of MgO shows diamagnetic Meissner effect with T_c = 38.8 K. M-H measurements at 10 K reveal a slight difference in irreversibility field which is due to MgO impurity along with wide transition observed from ac magnetic susceptibility measurements. The magnetotransport measurements ρ(T) using ρN = 90%, 50% and 10% criterion on pure sample of MgB_2 has been used to determine the upper critical field whereas the sample having large quantity of MgO does not allow these measurements due to its high resistance.