The initial decay rate R = d M /d(j/j 0 ) along the envelope of the major hysteresis curves and along bridges between these envelopes is examined in the framework that the current density of the relaxing field profiles initially exists in a critical state and can be described by a product function where j c = j 0 (T , t)f [H (x ), T , t)] with dj 0 /dt assumed independent of H . This simple model is shown to reproduce the pertinent observations of Beasley et al and Kwasnitza and Widmer. The model also predicts interesting structure in the curves of R versus H along bridges where H changes sign during its swing between the magnetization envelopes when j c descends steeply with H .

Several workers have measured the rates of decay of remanent magnetic moments in granular high- superconductors where the temperature-field histories generated trapped flux profiles comprising two concentric regions of countercirculating persistent currents. Exploiting a simple model based on the critical state for bulk specimens and the conservation of flux for the redistribution of vortices, and neglecting , we account for the major features of these observations although in some cases the number of trapped vortices is small. The model makes readily testable predictions of the relative magnitudes of the initial rates of magnetic relaxation for various configurations of flux density profiles.

We report on measurements of the evolution of the magnetization M upon warming from T 0 (4.2 or 77 K) to T c for type II superconductors with the locus of M initially situated along bridges between the envelopes of the major hysteresis curves. Strongly hysteretic and semi-reversible specimens of low and high T c type II superconductors have been studied (Nb 3 Zr, VTi, YBCO, Nb and BiSCCO). Concentric, oppositely directed magnetic moments coexist in the specimens when M lies along the bridges. We focused on the cases where M = 0, hence, before warming, the constituent moments cancel each other exactly. Since they diminish in magnitude at different rates as T is increased to T c , their sequential 'release' leads to a rich variety of phenomena. A simple model exploiting the critical state, an equilibrium Meissner current and conservation of flux accounts for all the intricate behaviour encountered.

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