Hysteresis in Superconductors. II. Experimental Tests for Critical States

Abstract

Hysteretic states associated with minor and major hysteresis cycles were investigated to determine their degree of criticality and the extent to which they exist in the volume of the superconductor. A variety of experiments were performed which can distinguish between hysteresis resulting from surface currents and hysteresis due to flux pinning. The measurements include (1) comparison of the temperature dependence of the critical current with the temperature dependence of the remnant magnetization, (2) changes in the remnant magnetization with applied transport currents, (3) comparison of the temperature dependence of remnant magnetization from minor and major cycles, (4) changes of the magnetization in external fields during and after application of transport currents, (5) changes in magnetization in solid and hollow cylinders, and (6) the temperature dependence of remnant magnetization in hollow cyclinders of type-I and type-II superconductors, and (7) the variation of critical current with diameter. In the systems studied, states were found to be (a) entirely subcritical, (b) partially critical and partially subcritical, and (c) entirely critical in degree. Only a part, and under many conditions none, of the volume was found to exist in the critical state. The experiments show that, in the systems studied, hysteresis is predominantly due to surface currents and not to flux pinning. The pertinence of these experiments to various models for hysteresis and the origin of the surface currents is discussed.