Nature Materials, Published online: 03 March 2026; doi:10.1038/s41563-026-02515-8Faraday microscopy and magnetic force microscopy with circularly polarized picosecond optical pulses are used to investigate domain switching in a ferromagnetic Pt/Co/Pt thin film. A stochastic model explains the nucleation and growth of magnetic domains, taking into account light helicity and the relative magnetization of neighbouring domains.