Thermomagnetic instability and critical current density in MgB2 monofilamentary tapes

Thermomagnetic instability in MgB2 monofilamentary tapes with paramagnetic nickel alloy and superconducting niobium matrix, has been investigated by measuring hysteresis loops and magnetic relaxations. The analysis has been performed on monofilamentary tapes where titanium, tungsten and silicon carbide impurities have been added to the superconducting phase. We show that the addition of impurities results in an increase of the thermomagnetic instability at low field without considerably increasing the critical current density. The analysis of the magnetic relaxation measurements shows a very strong pinning in MgB2, corresponding to an intrinsic critical current density larger than the experimental measured value. The interplay between thermal and magnetic diffusion limits the critical current density in MgB2 in spite of impurities addition.