Chromium triiodide (CrI3) bilayers are magnetic. The interlayer coupling determines whether adjacent layers align ferromagnetically or antiferromagnetically. This coupling is uniform across the crystal — every point on the interface experiences the same exchange interaction. A small twist changes everything.
When two CrI3 bilayers are twisted relative to each other by about one degree, a moiré superlattice forms — a periodic pattern of regions where the layers are locally aligned, locally shifted, and locally rotated. The magnetic exchange varies across this pattern. Researchers observing twisted double-bilayer CrI3 found spontaneous magnetic textures extending up to 300 nanometers — an order of magnitude larger than the moiré wavelength itself. At small twist angles, the texture size increases with twist angle, opposite to the underlying moiré periodicity. The system contains Néel-type skyrmions — topological magnetic structures that had never been experimentally observed in any twisted two-dimensional magnet.
The untwisted system has two magnetic options: all up or all down. The twisted system has a continuum. The moiré pattern doesn't create new magnetic interactions — it spatially modulates existing ones, introducing frustration where different regions prefer different alignments. The skyrmions and extended textures emerge from this frustration. The twist is less than one degree. The magnetic restructuring spans hundreds of nanometers.
The general principle: a small geometric perturbation can produce structures much larger than itself when it introduces spatial competition between locally preferred states. The perturbation doesn't add energy or complexity — it redistributes the existing interactions so they can no longer be satisfied simultaneously. What emerges is not the perturbation scaled up. It is a qualitatively new pattern that the unperturbed system could not access.