Living islands of driven two-dimensional magnetic colloids on the disordered substrate

We investigate, by Langevin simulations, the depinning of driven two-dimensional magnetic colloids on a substrate with randomly distributed pinning centers. The magnetic colloids are modeled as particles interacting with each other through repulsive magnetic dipole and attractive Lennard–Jones potentials. We find living islands above the depinning when the attraction between colloidal particles dominates. The critical pinning force increases and the living islands disperse gradually with an increasing strength of the pinning centers. But with an increasing strength of the repulsion between colloidal particles, the critical pinning force increases first and then decreases, and the living islands appearing at the low repulsion strength disappear when the repulsion strength is increased above certain value at which the peak effect takes place. When the repulsion strength is further increased, moving smectic and moving crystal structures arise above the depinning where the repulsion between colloidal particles dominates. On increasing the attraction strength, we find that the critical pinning force, which remains unchanged basically at the low attraction strength, decreases and the living islands form gradually in the plastic flows. When the attraction strength is increased to be large enough, the critical pinning force becomes unchanged again and the living islands and phase separation between different islands occur manifestly. The thermal fluctuations at low temperatures compete with the pinning potential, conducive to the formation of living islands, but they smooth the pinning potential at high temperatures, leading to the rapid decrease in the critical pinning force and the dispersion of the living islands. (paper)