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The obtained electron backscattering diffraction data and high-resolution transmission electron microscopy have shown that the nanostructure of epitaxial HTS YBCO films evolves essentially with the film thickness. This happens due to a high deposition temperature and a high dislocation mobility that induce polygonization, dislocation rearrangement and a remarkable reduction of the out-of-plane dislocation density. The analysis of experimental thickness dependences of the critical current in the framework of existing pinning models leads to the conclusion that the c-axis-correlated pinning by out-of-plane edge dislocations plays a dominant role. Thus, the evolution of the dislocation nanostructure is responsible for the critical current density reduction. Embedding of nanoparticles into HTS films should be useful only if they are coherently coupled with the matrix and extra dislocations are being formed during the film growth. The nanoparticles just preserve a high density of dislocations. Another effect is self-assembling of nanoparticles within dislocation cores forming a 'bamboo structure'. This phenomenon may result in an enlargement of the dislocation normal core and an essential increase of the elementary pinning force.

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The dependence of the critical current density jc on the magnitude and direction of the magnetic field H is investigated in thin epitaxial films of YBa2Cu3O7−δ having a high value of jc in the absence of field (∼106 A/cm2 at 77 K) and a thickness d less than twice the magnetic field penetration depth λ. It is found that the jc(H) curves have a low-field plateau both for fields perpendicular and parallel to the film. In a magnetic field perpendicular to the film, the “effective pinning” plateau extends to a field corresponding to a density of Abrikosov vortices threading the film at which it is no longer favorable for them all to be pinned at edge dislocations in the interblock walls, and a fraction of them become unpinned. In contrast, in a field parallel to the film the end of the plateau is unrelated to depinning of threading vortices parallel to the film; instead, jc(H) decreases after the plateau region because the magnetic field parallel to the film weakens the pinning of vortices perpendicular to the film. The low-field plateau on jc(H) for an in-plane is wider than that for the case of normal magnetization. Therefore, the dependence of jc on the angle between H and the normal to the film has a single maximum at which the field is lying in the film plane. In films obtained by laser or electron-beam evaporation of YBa2Cu3O7−δ or its constituents, the measurements of jc on which were made by an inductive method, the decrease of jc with increasing field in the film plane begins at fields lower than the first critical field for penetration of the vortices into the film plane. For the magnetron-deposited film, where jc was measured by a transport method, this decrease of jc begins in a field much higher than that critical field. The possible relationships between the parameters of the jc(H) curves and the first critical field for penetration of the vortices into the film plane are discussed, including some which lead to different angular dependences of the critical current.

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