Modification of the defect structure in MgB2 single crystals by carbon doping and neutron irradiation

Abstract

Carbon doping and neutron irradiation change the reversible parameters of pure MgB2 in a similar way, but influence the irreversible properties quite differently. In doped Mg(B1−xCx)2 the modified reversible parameters lead to decreased pinning at low fields compared to pure MgB2. On the other hand, the creation of defects results in the emergence of a second peak in Jc near Hc2. Contrary, in irradiated MgB2 an irreversible magnetization over most of the field range is observed. Irradiation of doped MgB2 leads to irreversible properties that are comparable to those of undoped irradiated MgB2 indicating, that the neutron induced defects dominate the irreversible behaviour. When discussing the fishtail in terms of an order–disorder transition of the vortex lattice, we obtain clearly different values for the density and the radius of the introduced defects in case of doping compared to irradiation. The results for the doped irradiated sample agree again with those from irradiation only. Thus, the defect matrix introduced by neutron irradiation is much more efficient in enhancing Jc due to the larger defect radius, which is comparable to the coherence length of the material.