Scientists at the University of Basel and ETH Zurich have reversed the polarity of a specialized ferromagnet with a focused laser beam, without heating the material. This achievement, detailed in Nature, combines electron interactions, topology, and dynamical control in a single experiment. The method hints at future light-based electronic circuits on chips.
Ferromagnets rely on aligned electron spins to create stable magnetic fields, a process that typically requires heating above a critical temperature to reverse polarity. However, a team led by Prof. Dr. Tomasz Smoleński at the University of Basel and Prof. Dr. Ataç Imamoğlu at ETH Zurich has demonstrated a heat-free alternative using laser light.
The researchers employed a material consisting of two atomically thin layers of molybdenum ditelluride, stacked with a slight twist to induce unusual electronic properties. This structure allows electrons to form topological states, which resist smooth transformation like the difference between a ball and a doughnut. In these states, whether insulating or metallic, electron interactions align spins into a ferromagnetic configuration.
By applying a laser pulse, the team changed the collective spin orientation, achieving a permanent switch. "Our main result is that we can use a laser pulse to change the collective orientation of the spins," noted Olivier Huber, a PhD student at ETH Zurich who conducted measurements alongside Kilian Kuhlbrodt and Tomasz Smoleński. The topology affected the switching dynamics, and the laser also enabled creation of internal boundaries for topological ferromagnetic regions.
Polarity reversal was verified by analyzing light reflected from a second, weaker laser beam, confirming the spin reorientation in the micrometer-scale ferromagnet. "What's exciting about our work is that we combine the three big topics in modern condensed matter physics in a single experiment: strong interactions between the electrons, topology and dynamical control," Imamoğlu explained.
The findings appear in Nature under the title "Optical control over topological Chern number in moiré materials," with authors including O. Huber, K. Kuhlbrodt, and others (DOI: 10.1038/s41586-025-09851-w). Smoleński envisions using this to optically write adaptable topological circuits on chips, potentially for precision sensing like miniature interferometers detecting small electromagnetic fields.
