Critical state model and the magnetic behaviour of high T_c superconductors

Abstract

Intragrain critical current densities (J_c) are usually inferred from magnetization measurements using Bean's critical state model. Using this model results are presented for the case of J_c decaying exponentially with field and explanations are given for the observed changes in the shape of the magnetization hysteresis curve as (1) the sample temperature is changed; (2) the sample dimension is changed; (3) the maximum field to which the sample is cycled is varied; and (4) the field direction is changed for a single crystal sample. Some of the observed thermomagnetic history effects in the magnetization of the high temperature superconductors are also explained. Finally, d.c. and low-frequency a.c. magnetization data on various Rga₂Cu₃O₇ superconductors are presented where features originating from intergranular and intragrain regions are identified and explained.