The critical state in YBa2Cu3O7-x high temperature superconductor

A practical method was devised for measurements of the diamagnetic response of high temperature superconductor specimens. The method is useful for the study of the temperature and field dependencies of the local magnetic induction and its two-dimensional visualisation B(T,H(a)x,y) with a resolution of 0.1 gauss and spatial resolution of 0.1 mm in the plane of the specimen surface. The proposed method facilitates quite simple and quick measurements at liquid nitrogen temperature and in applied H-a magnetic field in excess of 1 T. Cross-shaped miniature Hall probes (HPs) of thin film n-lnSb on semi-insulating substrate i-GaAs with a small magnetically sensitive zone of 0.1 x 0.1 mm(2) were used as measuring transducers. The high degree of doping of n-lnSb layers (n similar to 10(18) cm(-3)) resulted in a sensitivity of 5-7 mu V mT(-1) (with nominal current up to 10 mA through HPs) and thermosensitivity of less than 0.03% K-1, and made it possible to operate without the need for thermal stabilization of transducers. This method enabled us to study anisotropy of the critical current densities (J(c)) of melt-grown YBa2Cu3O7-x pellets. Also, we have been able to determine local cracks and grain connectivity in the YBa2Cu3O7-x pellet samples. (C) 1997 Published by Elsevier Science Ltd