Researchers from the University of Basel and ETH Zurich have developed a method to change the polarity of a specialized ferromagnet using a focused laser beam. This technology enables the use of light to design and reconfigure electronic circuits directly on a chip.

Principles of Ferromagnet Operation

Ferromagnets work thanks to the coordinated movement of countless tiny magnetic moments within a material. Each electron has a spin, which creates a small magnetic field. When the spins of electrons align in the same direction, their combined effect forms a strong and stable magnet, like those found in compasses or refrigerator doors. Such alignment is only possible when the interactions between spins are strong enough to overcome random thermal motion. Below a certain critical temperature, these interactions dominate, and the material becomes ferromagnetic. Typically, to change a magnet’s polarity, it must be heated above this temperature; as it cools, the spins settle into a new orientation, reversing the magnet’s direction.

Laser Switching Without Heating

In experiments, researchers managed to reorient the ferromagnetic state using only light, without raising the temperature. They used a material made of two atomically thin layers of molybdenum ditelluride, stacked with a slight twist relative to each other, which leads to unusual electronic behavior.

Topological States and Twisted Quantum Materials

In this twisted structure, electrons can organize into topological states that are fundamentally different and cannot be smoothly transformed into one another. The experiments allowed tuning the electrons between states that behave as insulators and those that conduct electricity like metals. In both cases, interactions between electrons led to parallel alignment of their spins, forming a ferromagnetic state.

Dynamic Control of Magnetic States

The laser pulse not only changes the magnet’s polarity but can also create new internal boundaries within the microscopic material, forming regions with topological ferromagnetic states. This process can be repeated, allowing dynamic control over both the magnetic and topological properties of the system.

Confirmation of Polarity Change

To confirm the change in polarity of a ferromagnet just a few micrometers in size, a second, weaker laser beam was used. Analysis of the reflected light made it possible to determine the orientation of the electron spins.

Application Prospects

The developed method can be used for optical recording of arbitrary and adaptable topological patterns on a chip. Such patterns could include miniature interferometers capable of detecting extremely weak electromagnetic fields, opening new possibilities for high-precision sensing technologies.