Emergent Geometric Frustration of Artificial Magnetic Skyrmion Crystals
  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. 
Fusheng Ma, C. Reichhardt, Weiliang Gan, C. J. Olson Reichhardt, and   Wen Siang Lew
cond-mat.str-el