Pinning and irreversibility in superconducting bulkMgB2 with added nanodiamonds

Abstract

Resistance, R(T), andmagnetization, M(B), studieson superconducting bulk MgB2 samples containing nanodiamonds (ND) as additives (wt% of ND:x = 0%, 1%, 3%, 5%, 7% and 10%) were recently published in two articles (Vajpayee et al 2007Supercond. Sci. Technol. 20 S155, Vajpayee et al 2008 J. Appl. Phys. 103 07C708). Themain observations reported were significant improvements in the critical current densityJc(B), irreversibilityline Birr(T) and uppercritical field Bc2(T) with ND addition. However, a closer look shows that as regards the potential of thistechnologically important material at higher magnetic fields and temperatures,there is still a lot of room for improvement. With that in mind we revisit theR(T) andM(B) data and analyze them, in the present work. We show that, despite ND addition,Jc dependsstrongly on B in the high field region and tends to vanish at irreversibility lines that lie deep, i.e. at around 0.3Bc2(T), inthe B–T phase diagram. The irreversibility lines, determined by in the presence of B, arefound to lie at around 0.5 Bc2(T) in the phase diagram. These results for pinning and irreversibility lines arediscussed in the light of various models such as those of surface sheathsuperconductivity, magnetically introduced percolation in polycrystallineMgB2, thermally assisted flux motion (TAFM) and a modified flux line shear mechanism. Ouranalysis hints at TAFM and weak pinning channels with distributed superconductingproperties percolating in our samples determining the irreversibility and pinningproperties.