Critical current densities of doped MgB2 strands in low and high applied field ranges: The Jc(B) crossover effect

•We notice that when MgB2 is doped so as to increase its Birr and Bc2, the low-field Jc is decreased.•We propose that this phenomenon is intrinsic to superconductors with a large variability in Bc2.•Using the surface pinning function, we have shown that increases in Bc2 will decrease low-B Jc.•For an application where low-B Jc needs to be maximized, doping will be deleterious.•Rather, increases in connectivity should be explored.

Numerous classes of dopant have been added to MgB2 in order to raise the upper critical field, Bc2, and hence to increase the field range over which the pinned superconductor has the possibility of supporting supercurrent. Thus dopant additions to grain-boundary-pinned MgB2, for example, have the effect of raising the high field critical current density, Jc(B). However, at low fields, when B is relatively small compared to Bc2, Jc(B) decreases as Bc2 increases. This leads to a low field convergence, or even the intersection, of the Jc(B) curves of a family of variously doped MgB2 strands. Two important conclusions derive from this “crossover effect”: (i) Doping-induced increases of Bc2 should be applied only if improved high field properties are required. For low field applications of MgB2 such as: low field nuclear magnetic resonance imaging (MRI), synchrotron insertion devices, and current leads, doping should be avoided since not only would the increased Bc2 degrade Jc, but the possible chemical byproducts of doping may reduce connectivity; (ii) If an across-the-board increase in Jc(B) is desired there is no substitute for increased connectivity, in regard to which densification of the MgB2 layer (the subject of a separate report) is recommended.