Ferromagnets operate through the synchronized movement of countless tiny magnetic moments within a material. Each electron possesses a property known as spin, generating a minuscule magnetic field. When numerous spins align, they produce a robust and stable magnet, akin to those found in compasses or refrigerator doors.
This alignment is contingent upon the strength of interactions between spins, which must surpass random thermal motion. Below a specific critical temperature, these interactions prevail, transforming the material into a ferromagnetic state.
Traditionally, altering a magnet's polarity necessitates heating it beyond this critical temperature. At elevated temperatures, the orderly arrangement of spins disintegrates, allowing for a rearrangement. Upon cooling, the spins settle into a new orientation, leading the magnet to point in a different direction.
Innovative Laser Switching
A collaborative team led by Prof. Dr. Tomasz Smoleński from the University of Basel and Prof. Dr. Ataç Imamoğlu from ETH Zurich has achieved this reorientation solely through light, eliminating the need for temperature elevation. Their groundbreaking research is featured in the journal Nature.
"What excites us about our research is the integration of three pivotal themes in modern condensed matter physics: strong electron interactions, topology, and dynamic control," notes Imamoğlu.
To realize this breakthrough, the researchers utilized a precisely engineered material composed of two atomically thin layers of the organic semiconductor molybdenum ditelluride. These layers are slightly twisted, resulting in unique electronic behavior.
Understanding Topological States
This twisted configuration allows electrons to form topological states. To illustrate, consider a ball, which has no holes, versus a doughnut, which has one. No matter how one reshapes a ball, it cannot become a doughnut without cutting it. Similarly, topological states are inherently distinct and cannot morph into one another smoothly.
During their experiments, Smoleński and Imamoğlu's team successfully tuned electrons between topological states that act as insulators and those that conduct electricity like metals. In both scenarios, electron interactions resulted in aligned spins, yielding a ferromagnetic state.
"Our key finding is that we can utilize a laser pulse to modify the collective orientation of the spins," explains Olivier Huber, a PhD candidate at ETH involved in the measurements. While previous studies indicated that individual electron spins could be manipulated using light, this research demonstrates the ability to switch the polarity of an entire ferromagnet simultaneously. "This switching is permanent, and notably, the topology affects the switching dynamics," adds Smoleński.
Dynamic Control of Magnetic Properties
The laser's function extends beyond merely flipping the magnet; it can also establish new internal boundaries within the material, generating areas where the topological ferromagnetic state exists. This process is repeatable, enabling researchers to dynamically manage both the magnetic and topological characteristics of the system.
To verify the polarity reversal of the minuscule ferromagnet, measuring just a few micrometers across, the team employed a second, less intense laser beam. By analyzing the reflected light, they could ascertain the orientation of the electron spins.
"In the future, our method could facilitate the optical writing of adaptable topological circuits on a chip," concludes Smoleński. Such advancements could lead to miniature interferometers capable of detecting minute electromagnetic fields, paving the way for innovative precision sensing technologies.