Defect Dynamics in Artificial Colloidal Ice: Real-Time Observation,   Manipulation and Logic Gate
  We study the defect dynamics in a colloidal spin ice system realized by filling a square lattice of topographic double well islands with repulsively interacting magnetic colloids. We focus on the contraction of defects in the ground state, and contraction/expansion in a metastable biased state. Combining real-time experiments with simulations, we prove that these defects behave like emergent topological monopoles obeying a Coulomb law with an additional line tension. We further show how to realize a completely resettable "NOR" gate, which provides guidelines for fabrication of nanoscale logic devices based on the motion of topological magnetic monopoles. 
Johannes Loehr, Antonio Ortiz-Ambriz, Pietro Tierno
cond-mat.soft