IRREVERSIBILITY TEMPERATURES OF NB3SN AND NB-TI

For fine multifilamentary Nb3Sn and Nb-Ti wires, it was found that there exists a surprisingly large temperature region (e.g., approximately 1 and 0.5 K at 2 T for a Nb3Sn and Nb-Ti wire, respectively) below the mean-field critical-magnetic-field line H(c2)(T) where the magnetization is reversible during a cycle of warming and cooling. The magnetic-field dependence of the lower boundary of this temperature region, i.e., the irreversibility temperature T(r)(H), was shown to follow theoretical predictions for flux-lattice melting based on an analysis of thermally induced flux-line fluctuations calculated with nonlocal lattice elasticity and the Lindemann melting criterion.