Ic hysteresis in polycrystalline high Tc superconductors : Double-valued Ic curves with identically magnetized grains, final fields and temperature: an effect of edge return fields

Abstract

In the analysis of I c hysteresis phenomena, many researchers have assumed that the return field H r( H a, T) obeys a linear relationship, H r =C M g (H a ,T) where C is a parameter of the specimen which may depend on T, and M g is the magnetization of the grains. Kwasnitza and Widmer [IEEE Trans. Magn. 27 (1991) 1202] measured both I c and M g at a chosen temperature after various magnetic field cycles terminating at the same chosen final field H f. They found I c to be an intricate double-valued function of M g and that the two different curves traced by I c versus M g changed their structure as a function of H f. We show that these results indicate unambiguously that the configurations of the induced currents circulating along the periphery of the grains exert the main influence on H r. We apply simple models to calculate the return field along the edges of grains in the form of thin ribbons and thin disks containing two concentric zones of countercirculating induced persistent currents of density J cg independent of H, and the magnetic moments they generate. Then exploiting critical state expressions for I c and the Evetts–Glowacki superposition concept we reproduce all the observations of Kwasnitza and Widmer, and make predictions on the evolution of the double-valued curves of I c( H f) versus M g( H f) as a function of H f over a wide range extending from zero to high fields.