Vortex pinning

The irreversibility field (Hm), which has to be measured experimentally, is an important parameter since it determines the size of the final M developed. As expected Hm depends upon temperature, and irreversibility field v. temperature curves are shown in Fig. 4.57. It is apparent that at liquid nitrogen temperature YBCO is suited to permanent magnet applications whereas Bi-2223 is not. Because of the energy dissipation accompanying vortex motion and referred to earlier, vortex pinning also raises /c. 

Understanding of vortex pinning mechanisms will steadily develop and with it the technology of optimising compositions and structures of materials for trapped field magnets, particularly the melt-textured, large grain varieties. 

The vortex pinning constant k can roughly be estimated by the energy gained as the condensation energy x has to be supplied over the volume of the vortex... 

A. Diaz, L. Mechin, P. Berghuis, J. E. Evetts Evidence 2.131 for vortex pinning by dislocations in YBa2Cu307 8... 

Chapter 3. Grain Connectivity and Vortex Pinning in High-Temperature Superconductors J. Horvat... 

The symmetry changes of the vortex lattice in borocarbide superconductors affect their pinning properties as was shown for YNi2B2C (Silhanek et al. 2001). For the field orientation // c, the reorientation transition of the vortex lattice mentioned above was found to be associated with a significant kink in the volume pinning force Fp, whereas in the basal plane (for H c) the signature of nonlocal effects is a fourfold periodicity of Fp. 

In fig.4 (lower panel) it is schematically presented the structure of the SC vortex in the SDW/CDW + SC state. Since arising of the CDW results in the lattice modulation so that wave of dislocation walls is formed (fig.4 (middle panel)). As known, such dislocation walls are effective centers for pinning of SC vortices. Note that in such a structure every fifth wall is equivalent to first one ( cn+4 - c ). In this a model a vortex core has AF SDW structure which is also outside a core too. Because of equivalence of c and c +4 dislocation walls vortex core becames to be two part in form fluctuating in space (cf. with [14]). 

Defects in crystals are known to have a potentially major influence on phase behavior. For instance, dislocation unbinding is believed to be central to the 2D melting transition, while in 3D there is evidence to suggest that defects can act as nucleation centers for the liquid phase [105]. In superconductors, defects can pin vortices and influence vortex melting [106]. 

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