Topological kinetic crossover in a nanomagnet array

Common explanations of equilibrium thermodynamics depend critically on ergodicity, i.e., the ability to explore all available states, but ergodic kinetics can be constrained by a system’s topology. We report an experimental study of a model nanomagnetic array, in which such constraints visibly affect the behavior. In this system, magnetic excitations are connected in thermally active one-dimensional strings whose motion can be imaged in real-time. At high temperatures, the kinetics include the merging, breaking, and reconnecting of strings, resulting in the system transitioning between topologically distinct configurations. Below a crossover temperature, however, the string motion is dominated by simple changes in length and shape. In this low-temperature regime, the system is energetically stable because of the inability to explore the full set of possible topological configurations. This kinetic crossover suggests a generalizable conception of topologically broken ergodicity and limit..., To prepare the samples, we use electron-beam lithography to write patterns on Si/SiO x substrates with spin-coated bilayer resists. A layer of ~ 2.5 nm thick permalloy (Ni80Fe20) was then deposited into the patterns via ultrahigh vacuum electron beam evaporation, followed by aluminum capping layers of 2 nm thick to prevent oxidation of the underlying permalloy.
The moment configuration in our samples was measured using x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM), which yields real-space images of the island magnetic moment,  conducted at the PEEM-3 endstation at beamline 11.0.1.1 of the Advanced Light Source, Lawrence Berkeley National Lab. We heated the sample from 250 K to 360 K in 5–10 K steps and took 100 PEEM images at the Fe L 3 absorption edge at each temperature point. The 100 PEEM images consisted of ten exposures with a left-circularly polarized X-ray beam followed by ten exposures with a right-circularly polarized beam, repeated five times....,