  Magnetic skyrmions have been receiving growing attention as potential
information storage and magnetic logic devices since an increasing number of
materials have been identified that support skyrmion phases. Explorations of
artificial frustrated systems have led to new insights into controlling and
engineering new emergent frustration phenomena in frustrated and disordered
systems. Here, we propose a skyrmion spin ice, giving a unifying framework for
the study of geometric frustration of skyrmion crystals in a non-frustrated
artificial geometrical lattice as a consequence of the structural confinement
of skyrmions in magnetic potential wells. The emergent ice rules from the
geometrically frustrated skyrmion crystals highlight a novel phenomenon in this
skyrmion system: emergent geometrical frustration. We demonstrate how skyrmion
crystal topology transitions between a non-frustrated periodic configuration
and a frustrated ice-like ordering can also be realized reversibly. The
proposed artificial frustrated skyrmion systems can be annealed into different
ice phases with an applied current induced spin-transfer torque, including a
long range ordered ice rule obeying ground state, as-relaxed random state,
biased state and monopole state. The spin-torque reconfigurability of the
artificial skyrmion ice states, difficult to achieve in other artificial spin
ice systems, is compatible with standard spintronic device fabrication
technology, which makes the semiconductor industrial integration
straightforward.
