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 ($U\ensuremath{\gtrsim}kT$). 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 $m\ensuremath{\propto}1/t$ instead of the usual ``field-on'' exponential dependence $m\ensuremath{\propto}exp(\ensuremath{-}t/\ensuremath{\tau})$. The effect is important and observable in the magnetization measurements, for instance, in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ crystals and other novel superconducting materials.