Influence of Grain Boundaries and Sn Distribution on the H-T Phase Diagram of Nb3Sn

We measured the 1st and 3rd harmonics of the AC magnetic susceptibility as a function of the temperature and the DC magnetic field, on several Nb₃Sn samples (single crystal, polycrystalline samples and multifilamentary wires), with different pinning properties. From these measurements, the Bragg phase, the disordered one and the irreversibility line have been identified in the H-T phase diagram of analysed samples. We observed that the presence of the pinning due to grain boundaries increases the region of the disordered phase in respect to the pinning due to point defects, and gives the possibility to detect an irreversibility line. Nevertheless it is possible to observe a phase transition between the Bragg and the disordered phase if the sample has an almost spatially uniform tin content. On the contrary, in a sample characterized by similar grain boundaries properties but where a tin gradient is present, the Bragg glass phase tends to disappear.