Topological kinetic crossover in a nanomagnet array

Abstract

Ergodic kinetics, which are critical to equilibrium thermodynamics, can be constrained by a system’s topology. We studied a model nanomagnetic array in which such constraints visibly affect the behavior of the magnetic moments. In this system, magnetic excitations connect into thermally active one-dimensional strings whose motion can be imaged in real time. At high temperatures, our data showed the merging, breaking, and reconnecting of strings, resulting in the system transitioning between topologically distinct configurations. Below a crossover temperature, the string motion is dominated by simple changes in length and shape. In this low-temperature regime, the system is energetically stable because of its inability to explore all possible topological configurations. This kinetic crossover suggests a generalizable conception of topologically broken ergodicity and limited equilibration.

Real-space observation of ergodicity transitions in artificial spin ice

“…1a ). The total system comprises over 28000 nanomagnets which, though not a thermodynamic limit in that it does not approach Avogadro’s number, is consistent with system sizes of nanomagnets that avoid significant edge and boundary effects, as previously reported in literature 3 , 8 , 9 , 12 , 21 – 23 . Following sample fabrication, the sample is kept at room temperature for several weeks before being transferred into the photoemission electron microscope (PEEM) 24 for magnetic imaging.…”