H(T) Phase Diagram in Nb3Sn: A Different Behavior in Single Crystals,   Polycristalline Samples and Multifilamentary Wires
  A comparative study of magnetic behavior in a DC magnetic field up to 19 Tesla has been undertaken on different Nb3Sn samples, i.e. single crystals, polycrystal samples and multifilamentary wires. From the 1st and 3rd harmonics of the AC magnetic susceptibility a peak effect was experimentally observed in the single crystal and in the polycrystal, whereas this phenomenon is absent in the measured wires. The corresponding H vs T phase diagram reveals a different behavior between wires on one hand and polycrystalline and single crystal samples on the other. In particular, from the 3rd harmonics it has been observed that vortex thermally activated flux creep phenomena are relevant in the wires, whereas the static critical state models are more appropriate to describe single crystals. 
M. G. Adesso, D. Uglietti, R. Fluekiger, M. Polichetti and S. Pace