Onset temperature of intrinsic pinning in a REBCO coated conductor from critical current anisotropy

Abstract

• A detailed analysis is given of the evolution with temperature of the angle dependence of critical current in a REBCO coated conductor. • Particular attention is given to the development of the ubiquitous ab -plane peak for field parallel to the plane of the tape. We highlight that this peak appears as the result of two distinct mechanisms, stacking faults at higher temperatures and intrinsic pinning at lower temperatures. • This peak is unusually weak at 77K, due to process conditions favouring a low density of stacking fault defects. • The peak grows rapidly with decreasing temperature relative to the c-axis critical current starting from 60K, signalling the onset of intrinsic pinning by the layered structure of REBCO. • This growth is quantified through fitting with maximum entropy functions. • The angle dependence of n -values is also shown. With decreasing temperature, the ab -plane feature appears first as a dip at 65K, then a sharper peak starting from 45K. Both of these features broaden with decreasing temperature. The field-angle dependence of the critical current in a REBa 2 Cu 3 O 7 coated conductor has been measured over a fine range of temperatures from 15 K to 80 K. The particular sample is demonstrated to have a very low fraction of extrinsic planar defects by its near-isotropic critical current at 77 K. At a representative magnetic field of 3 T we are able to track the emergence of the ab-plane peak usually associated with intrinsic pinning and quantify its evolution with temperature by fitting to a maximum-entropy function. We are also able to observe the evolution of dip and peak features in the angle dependence of the power-law index n with decreasing temperature and show that a dip appearing at 65 K is supplanted by a sharper positive peak from 50 K. The combination of critical current and power-law index temperature dependences shows the onset of intrinsic pinning at 50 K.