Effect of vortex annihilation lines on magnetic relaxation in high-temperature superconductors

We show that in superconductors of type II, where Abrikosov vortices with different polarities are present, the areas where they meet and annihilate each other (B=0 lines) "attract"a significant part of the magnetization current j. This leads to redistribution of j over the sample, and as a result, the rate of magnetic relaxation is reduced. This effect is significant in the case of weak dependence of the activation energy U on j, particularly for the flux flow (U=0) and at early stages of flux creep (U≤kT). The slowdown of the relaxation is mostly pronounced in the remanent state, where the B=0 lines are located at the edges of the sample. In the case of flux flow, the remanent magnetization decays as m∝1/t instead of the usual "field-on"exponential dependence m∝exp(-t/τ). The effect is important and observable in the magnetization measurements, for instance, in La2-xSrxCuO4 crystals and other novel superconducting materials.