Enhancement of intrinsic pinning in the high-temperature superconductor TmBa2Cu3Oy: Manifestation of the interaction between vortices and a magnetic rare-earth ion

The magnetic-field dependences of the low-frequency complex permeability have been measured for thin-plate single crystals of the high-temperature superconductor TmBa2Cu3Oy in a static external magnetic field H and an alternating magnetic field with the amplitude h (h << H) both directed along the plate plane (crystallographic ab plane). Two different contributions to the permeability have been revealed. The first contribution (mu(v)) is independent of the amplitude and is associated with the vibrational motion of vortices along the c axis. The second contribution (mu(c)) linearly depends on the amplitude and is due to the occurrence of a critical state along the structure layers. The field dependences of mu(v) and the critical current exhibit features indicating the appearance of an additional intrinsic pinning mechanism, which is initiated by the magnetic field and is caused by the interaction of the vortex system with a magnetic rare-earth ion layer. This effect is enhanced as the temperature increases and approaches the critical value T-c. A comparison of the results for two single crystals shows that the interaction of the vortex and magnetic systems weakly depends on the quality of the crystals. The interaction decreases only slightly for the sample with the smaller T-c value. A method has been proposed to increase pinning by the partial substitution of magnetic transition metal ions, primarily iron ions for ions at the yttrium position. Such a substitution is possible up to 20%, keeping a single phase and without a decrease in T-c.