Scientists from the University of Basel and ETH Zurich have found a way to flip a magnet using a short flash of light. In order to reverse magnetic polarity, it is typically required to subject the material to intense heating or expose it to an external magnetic field; however, the researchers have shown that a short laser pulse is sufficient in some cases.
To understand this discovery, one needs to imagine how a magnet works at the particle level. Every ferromagnet (even ordinary fridge magnets) contains billions of electrons. Each of these electrons has a quantum property called spin. It can be represented as a tiny magnetic needle. When a large number of these needles point in the same direction, their magnetic fields add up, and the entire material becomes a magnet.
However, it is not easy to reverse them. As a rule, this requires heating the magnet above the so-called critical temperature, the threshold at which the ordered arrangement of spins is destroyed. After being cooled, they can realign themselves in a different direction. This process requires energy and takes time.
The Swiss physicists have proven that this can be done without heating. In their experiment, they used an unusual material: molybdenum ditelluride. This compound of molybdenum and tellurium is classified among two-dimensional semiconductors: its crystalline layers are only a few atoms thick. The scientists took two of these atomically thin layers and placed them on top of each other, slightly rotating one layer relative to the other. This resulted in a so-called twisted structure, which alters the behavior of electrons significantly.
In this system, electrons start interacting with each other much more strongly than in conventional materials. This leads to the emergence of collective magnetic states, in which the spins of many electrons starts behaving in a coordinated manner, forming a miniature ferromagnet.
When this material get exposed to a short laser pulse, the light affects the electrons and causes them to change orientation at the same time. As a result, the spins of many electrons flip synchronously, and the entire microscopic magnet changes its polarity.
Furthermore, the laser makes it possible not only to flip the magnet, but also to create individual magnetic regions within the material, forming patterns of different magnetic states. It is possible to shape and reshape these structures over and over by controlling them with light.
In the future, this could pave the way for light-controlled electronics. The laser would be able to create and reconfigure magnetic structures on the chip so as to form logic elements, sensors and other components of electronic devices.
