Locally frozen magnetic field in HTSC ceramics

TL;DR

This study investigates how a locally frozen magnetic field in YBa2Cu3O7-x ceramics varies with excitation field, cooling mode, and demagnetizing field, revealing insights into the behavior of weak links and current structures at 77 K.

Contribution

It provides a detailed analysis of the dependence of frozen magnetic fields on excitation parameters and cooling modes in HTSC ceramics, highlighting the role of inhomogeneous critical currents and phase transitions.

Findings

Dependence on excitation field explained by inhomogeneous critical currents.

Transition from frozen current structure to ZFC mode observed.

Stable coexistence of microscopic current loops with opposite directions.

Abstract

The value of a locally frozen magnetic field in a region with a diameter of 0.5 mm in a 0.5-mm-thick YBa2Cu3O7-x plate was investigated as a function of the excitation field (to 2x10^4 A/m), plate cooling mode (in the absence or presence of a field; i.e., zero-field cooling (ZFC) or field coupling (FC)), and local demagnetizing field. Analysis of the measurement results in the noted range of excitation fields showed the following: (i) the dependence on the excitation field for the ZFC mode is explained by the local inhomogeneity of critical currents of weak links in the ceramic Josephson medium and is limited by their maximum value at the temperature of the experiment (77 K); (ii) the dependence on the excitation field for the FC mode contains a portion of the magnetic phase transition from the frozen current structure, typical of the initial portion of the dependence, to the current structure characteristic of the ZFC freezing mode, and is limited by this transition; and (iii) the dependence on the demagnetizing field for the ZFC mode can be explained by the stable coexistence (without annihilation) of microscopic current loops with opposite current directions in the ceramics.