Role of MgO impurity on the superconducting properties of MgB2
  We address the effect of MgO impurity on the superconducting properties of MgB2. The synthesis of MgB2 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 ac applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB2 having more amount of MgO (with Tc = 37.1K) whereas the pure sample MgB2 having minor quantity of MgO shows diamagnetic Meissner effect with Tc = 38.8K. M-H measurements at 10K 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 R(T)H using RN = 90%, 50% and 10% criterion on pure sample of MgB2 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. 
Dharmendra Kumar Singh, Brajesh Tiwari, Rajveer Jha, H. Kishan and V.   P. S. Awana
cond-mat.supr-con