Emergent Strings in Santa Fe Artificial Spin Ice*

Artificial spin ice systems are two-dimensional arrays of interacting ferromagnetic nanoislands with uniaxial single-domain magnetization. By carefully designing the geometry of these arrays, some systems show collective phenomena associated with emergent higher-order frustration. Here we report a study on the Santa Fe Ice (SFI) system, which has an intrinsic population of excited vertices built into the geometry. Its magnetic configuration can be represented via emergent “strings” that connect all excited vertices, and thus moment fluctuations can be understood through the strings’ kinetics. SFI is predicted to have either a disordered ground state or a long-range ordered ground state associated with the distribution of excited vertices, depending on the local interactions. We have performed a magnetic force microscopy study on SFI after annealing near the Curie temperature and photoemission electron microscopy (PEEM) measurements on thermally active SFI. Our results indicate that SFI represents an unusual instance of emergent topological complexity.