Boundaries of critical state stability of Nb3Al hard superconductor in H-T plane

Stability of the critical state of type-II hard superconductors with respect to small fluctuations of temperature T or magnetic field He is strongly dependent on material properties. For this purpose, heat capacity C(T,He), magnetization M(T,He) and magnetostriction ΔL(T,He) of the superconductor were measured in a strong (up to 12 T) magnetic field. The experimental data on heat capacity in a wide temperature range and in magnetic fields Hc1 <= He <= Hc2, were used to determine the gap width and some other parameters such as the coefficient of the linear term defining the electron contribution to the heat capacity and the Debye temperature. The experiments on magnetization allowed the critical current dependences of the superconductor Jc(T, He) to be recovered. The experimental data on temperature and field dependences of thermal and conducting properties were used to calculate magnetization and magnetostriction hysteresis loops.