Crystallites of magnetic charges in artificial spin ice

Artificial spin-ice systems are lithographically fabricated arrays of interacting ferromagnetic nanometre-scale islands; a procedure to thermalize two types of artificial spin ice with different geometries has now been developed, resulting in unprecedentedly large ground-state domains in square lattices and crystallites of ordered magnetic charges in kagome lattices. Disorder in artificial spin ice Artificial spin-ice systems, first reported in 2006, are lithographically fabricated arrays of interacting ferromagnetic nanoislands. The magnetic moments of the islands, or 'spins', attempt to align with each other but not all succeed, creating a 'frustrated' system. Frustration prevents complete order and gives rise to interesting dynamic and magnetic properties. A limitation of artificial spin-ice systems has been that they are usually found in an 'athermal', frozen state, preventing the experimental investigation of novel phases that can emerge from thermal fluctuations of frustrated structures. Zhang et al. have now developed a procedure to thermalize two types of artificial spin ice with different geometries. They observe the formation of unprecedented large ground-state domains for square lattices, and the crystallization of magnetic (monopole-like) charges in kagome spin ice. The work opens the possibility of studying a new landscape of magnetic phases and behaviour. Artificial spin ice.sup.1 is a class of lithographically created arrays of interacting ferromagnetic nanometre-scale islands. It was introduced to investigate many-body phenomena related to frustration and disorder in a material that could be tailored to precise specifications and imaged directly. Because of the large magnetic energy scales of these nanoscale islands, it has so far been impossible to thermally anneal artificial spin ice into desired thermodynamic ensembles; nearly all studies of artificial spin ice have either treated it as a granular material activated by alternating fields.sup.2 or focused on the as-grown state of the arrays.sup.3. This limitation has prevented experimental investigation of novel phases that can emerge from the nominal ground states of frustrated lattices. For example, artificial kagome spin ice, in which the islands are arranged on the edges of a hexagonal net, is predicted to support states with monopolar charge order at entropies below that of the previously observed pseudo-ice manifold.sup.4. Here we demonstrate a method for thermalizing artificial spin ices with square and kagome lattices by heating above the Curie temperature of the constituent material. In this manner, artificial square spin ice achieves unprecedented thermal ordering of the moments. In artificial kagome spin ice, we observe incipient crystallization of the magnetic charges embedded in pseudo-ice, with crystallites of magnetic charges whose size can be controlled by tuning the lattice constant. We find excellent agreement between experimental data and Monte Carlo simulations of emergent charge-charge interactions.
Author(s): Sheng Zhang [sup.1] , Ian Gilbert [sup.2] , Cristiano Nisoli [sup.3] , Gia-Wei Chern [sup.3] , Michael J. Erickson [sup.4] , Liam O'Brien [sup.4] [sup.5] , Chris Leighton [sup.4] [...]