Researchers from the University of Basel and ETH Zurich have demonstrated how to reverse the polarity of special ferromagnetic materials using a focused laser beam. This advance points to a future where we can use light to design and reconfigure electronic circuits directly on chips.
Ferromagnetic materials work because vast numbers of small magnetic moments within the material move in unison. Each electron has a property called spin that generates a very small magnetic field. When many of these spins align in the same direction, their combined effect creates a strong, stable magnet, like the one found in a compass or refrigerator door.
This alignment can only occur if the interactions between the spins are strong enough to overcome random thermal motion. Below a certain critical temperature, these cooperative interactions become dominant and the material becomes ferromagnetic.
Reversing the polarity of a magnet usually requires heating it above its critical temperature. As the temperature increases, the ordered arrangement breaks down, allowing the spins to rearrange. When the material cools again, the spins settle into a new collective direction and the magnet points in a different direction.
Laser switching without heat
A team led by Professor Tomasz Smoleński from the University of Basel and Professor Ataç Imamoğlu from ETH Zurich achieved this reorientation using only light, without increasing the temperature. Their findings were published in the journal Nature.
“What’s interesting about our work is that it combines three big topics of modern condensed matter physics in one experiment: strong interactions between electrons, topology, and dynamical control,” says Imamoğlu.
To achieve this, the researchers used a carefully engineered material consisting of two atomically thin layers of the organic semiconductor molybdenum ditelluride. The layers are stacked with a slight twist between them, which causes unusual electronic behavior.
Topological states and twisted quantum materials
In this twisted structure, the electrons are organized into states known as topological states. These conditions can be understood with a simple analogy. Balls don’t have holes, but donuts do. No matter how much you transform a ball, you can’t turn it into a donut without cutting or tearing it. Similarly, topological states are fundamentally different and cannot be smoothly converted into each other.
In experiments directed by Smolensky and Imamoğlu, the researchers were able to tune electrons between topological states where they behave as insulators and conduct electricity like metals. In both cases, interactions between the electrons aligned their spins in parallel, creating a ferromagnetic state.
“Our main result is that we can change the collective direction of the spins using laser pulses,” says Olivier Huber, a PhD student at ETH who carried out the measurements with Kilian Courbroth and Tomasz Smolenski. Previous work had shown that individual electron spins could be manipulated with light, but this work demonstrates switching the polarity of an entire ferromagnetic material at once. “This switching is permanent, and furthermore, the topology influences the dynamics of the switching,” says Smoleński.
Dynamic control of magnetic state
Lasers do more than just flip magnets. It is also possible to define new internal boundaries within fine-scale materials, creating regions where topological ferromagnetic states exist. This process can be repeated, allowing researchers to dynamically control both the magnetic and topological properties of the system.
To confirm that the tiny ferromagnetic material, just a few micrometers in diameter, had indeed reversed polarity, the researchers shined a second, weaker laser beam on it. By analyzing the reflected light, they were able to determine the direction of the electron spin.
“In the future, we will be able to use our method to optically write arbitrary adaptable topology circuits on chips,” Smoleński says. Such circuits could include miniature interferometers that can detect extremely small electromagnetic fields, opening new possibilities for precision sensing technology.
