  We demonstrate the use of an external field to stabilize and control defect
lines connecting topological monopoles in spin ice. For definiteness we perform
Brownian dynamics simulations with realistic units mimicking experimentally
realized artificial colloidal spin ice systems, and show how defect lines can
grow, shrink or move under the action of direct and alternating fields.
Asymmetric alternating biasing fields can cause the defect line to ratchet in
either direction, making it possible to precisely position the line at a
desired location. Such manipulation could be employed to achieve fast, dense,
and mobile information storage in these metamaterials.
