Pinning and irreversibility in superconducting bulk MgB2 with added nanodiamonds

Resistance, R(T), and magnetization, M(B), studies on 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 2007 Supercond. Sci. Technol. 20 S155, Vajpayee et al 2008 J. Appl. Phys. 103 07C708). The main observations reported were significant improvements in the critical current density Jc(B), irreversibility line Birr(T) and upper critical field Bc2(T) with ND addition. However, a closer look shows that as regards the potential of this technologically important material at higher magnetic fields and temperatures, there is still a lot of room for improvement. With that in mind we revisit the R(T) and M(B) data and analyze them, in the present work. We show that, despite ND addition, Jc depends strongly on B in the high field region and tends to vanish at irreversibility lines that lie deep, i.e. at around 0.3 Bc2(T), in the B-T phase diagram. The irreversibility lines, determined by R(T)→0 in the presence of B, are found to lie at around 0.5 Bc2(T) in the phase diagram. These results for pinning and irreversibility lines are discussed in the light of various models such as those of surface sheath superconductivity, magnetically introduced percolation in polycrystalline MgB2, thermally assisted flux motion (TAFM) and a modified flux line shear mechanism. Our analysis hints at TAFM and weak pinning channels with distributed superconducting properties percolating in our samples determining the irreversibility and pinning properties.