Thermal release of hidden magnetic moments in low and high T(c) type II superconductors.

Measurements of the evolution of the magnetic moment of nonideal (i.e. pinning rich) type II superconductors in stationary magnetic fields H a as a function of time at constant temperatures T < Tc or as the temperature is varied to or from Tc provide insight into the dynamics of flux lines and their interactions with the pinning sites. Three concentric magnetic moments coexist in these materials when the isothermal sweep of Ha causes the locus of the magnetization ⟨M⟩ to migrate along any path joining the upper with the lower envelopes of the major hysteresis curves. The "outermost" magnetic moment is always diamagnetic and is generated by the field opposing Meissner surface current. The two other magnetic moments fill the bulk of the specimen, currents circulating in one direction in the outer annular volume embrace an inner volume occupied by counter-rotating persistent currents. In semi-reversible type II superconductors the magnitude of the Meissner magnetic moment is important compared to that arising from the currents in the bulk whereas in very hysteretic specimens it is negligible. In both types of material it is a fairly straightforward procedure to establish configurations in various static Ha where the opposing magnetic moments exactly cancel each other so that the macroscopic net magnetization is zero. However the internal pattern of persistent currents and magnetic flux density profiles which prevail in these special ⟨M⟩ = 0 states is complicated, fascinating and provides a full crucible for the investigation of flux line behaviour. In this thesis I present the results of our experimental study of the evolution of the magnetization of these initial ⟨M⟩ = 0 states during warming to Tc as a function of the static applied field H a. The phenomena encountered are seen to depend dramatically on the direction of the change of Ha causing ⟨M⟩ to migrate from one envelope of the major hysteresis loop to the opposite. Further we find major differences in the behaviour exhibited by the semi-reversible type II superconductors in comparison with that manifested by the highly hysteretic samples. We also see that the low Tc and high Tc semi-reversible specimens display very similar but highly intricate behaviour. Finally we show that a simple model which exploits the critical state concept and applies the principle of conservation of flux lines can account for the major features of all our observations