Artificial "spin ice" in a geometrically frustrated lattice of nanoscale   ferromagnetic islands
  We report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the lowtemperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized. 
R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J.   Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Cresp...