Research: Fractal-hopping monopoles

Spin ices have crystal lattices that consist of tetrahedra of magnetic ions. In a ground state, two of the four spins on each tetrahedron point in and two point out. When an excitation called the magnetic monopole is created, this rule is violated as the monopole moves through the crystal. Monopole dynamics are reflected in quantities such as magnetic noise, the measurements of which have shown a different frequency dependence from the one that the simplest model predicts. Hallén et al. solved this puzzle by realizing that the monopole motion is more restricted than previously thought and is limited to a cluster with a fractal structure.

J. N. Hallén, S. A. Grigera, D. A. Tennant, C. Castelnovo, R. Moessner,
Dynamical fractal and anomalous noise in a clean magnetic crystal,
Science 378, 1218 (2022)