The effects of neutron irradiation upon superconducting single crystals were studied. Transmission electron microscopy investigations revealed large radiation-induced defects with dimensions comparable to the superconducting coherence length. Accordingly, measurements of the magnetic moment revealed pronounced modifications of the reversible and irreversible properties after irradiation. In particular, attention was focused upon the emergence of a second peak (fish-tail), situated near to Hc2 at low neutron fluences, but extending over a large part of the superconducting phase diagram at high doses. The history dependence of the critical current density on the low-field side of the fishtail region was studied by measuring minor hysteresis loops and interpreted in terms of an order-disorder transition of the vortex lattice.

Fishtail Effect in Neutron-Irradiated Superconducting MgB2 Single Crystals. M.Zehetmayer, M.Eisterer, J.Jun, S.M.Kazakov, J.Karpinski, B.Birajdar, O.Eibl, H.W.Weber: Physical Review B, 2004, 69[5], 054510 (7pp)