Researchers at the University of Basel and ETH Zurich have demonstrated a method to reverse the polarity of a ferromagnet using only a laser beam, without applying heat. Their findings, published in the journal Nature, open a potential path toward reconfigurable electronic circuits built directly on a chip using light.
Flipping a magnet’s polarity has traditionally required heating the material above its critical temperature, disrupting the orderly alignment of electron spins, and allowing them to resettle in a new direction as the material cools. The Basel-ETH team bypassed that process entirely.
What the Experiment Actually Did
The researchers worked with a material composed of two atomically thin layers of the organic semiconductor molybdenum ditelluride, stacked with a slight twist between them. That twist produces unusual electronic behavior, allowing electrons to organize into what physicists call topological states.
In those states, electrons’ spins align in parallel, generating a ferromagnetic effect. The team, led by Prof. Dr. Tomasz Smoleński at Basel and Prof. Dr. Ataç Imamoğlu at ETH Zurich, used a focused laser pulse to flip the collective orientation of all those spins at once. The switch was permanent.
“Our main result is that we can use a laser pulse to change the collective orientation of the spins,” said Olivier Huber, a PhD student at ETH who carried out the measurements alongside Kilian Kuhlbrodt and Smoleński. “This switching was permanent and, moreover, the topology influences the switching dynamics,” Smoleński added.
Three Fields, One Experiment
What makes the result notable is not just the magnetic switching. The experiment merges three distinct areas of condensed matter physics simultaneously: strong electron-electron interactions, topology, and dynamical control using light.
“What’s exciting about our work is that we combine the three big topics in modern condensed matter physics in a single experiment,” Imamoğlu said.
Topological states are structurally distinct in a way that resists gradual transformation, similar to how a sphere and a doughnut differ fundamentally in shape. No continuous deformation converts one into the other. The researchers could tune electrons between topological states that behave as insulators and those that conduct electricity like metals, with ferromagnetism present in both cases.
More Than a Simple Flip
The laser does not only reverse the ferromagnet’s polarity. It can define new internal boundaries within the microscopic material, carving out regions where the topological ferromagnetic state exists. Because the process is repeatable, the team can dynamically control both the magnetic and topological character of the system.
The ferromagnet itself measures only a few micrometers across. To verify that polarity had genuinely reversed, the researchers directed a second, weaker laser beam onto the material as a probe.
Earlier research had shown that individual electron spins could be manipulated with light. This study moves beyond that, demonstrating simultaneous switching of an entire ferromagnetic state. The distinction matters for practical applications: reconfiguring whole magnetic domains, rather than single spins, is what would be needed to write and rewrite circuit-level logic using light.
Photo by Logan Voss on Unsplash
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