Pinning by twin boundaries and peak effect in YBaCuO high-T c superconductors

Measurements of the imaginary part of the ac magnetic susceptibility of single crystals and melt-textured samples of YBa₂Cu₃O_x (YBCO) at T=77 K in a magnetic field ranging between 1 and 20 kOe are reported. If the dc magnetic field H_dc is rotated in the ab plane of the sample, the magnetic susceptibility and critical current density j_c have peaks corresponding to the magnetic field aligned with twin boundaries. Peaks in the curve of j_c versus magnetic field are observed at angles corresponding to these peaks, where \(j_c \propto \sqrt {H_{dc} } \)AH_dc in a wide range of magnetic fields. The results have been interpreted in terms of the theory describing twin boundaries as a system of quasi-planar pinning sites. The pinning is strong if the elastic displacements of flux lines are of the order of the vortex lattice constant d_f. These displacements decrease with the magnetic field because of the decrease in d_f, and the contribution of the elastic energy to the Gibbs potential is reduced accordingly, which is the cause of the peak effect.