Flux Pinning in Neutron Irradiated MgB2 Single Crystals.

We report on the effects of neutron irradiation on the irreversible magnetic properties of MgB2 single crystals. The size of the newly created defects is comparable to the superconducting coherence length of MgB2, which makes these defects particularly suitable for pinning flux lines. Indeed, we observe significant quantitative and qualitative modifications of the critical current density. In particular, a second peak (fishtail effects) emerges in the field dependence and is accompanied by history effects. The fishtail effect is thoroughly studied with respect to different neutron fluences and good agreement with a recent theoretical explanation by an order-disorder transition is obtained.