So-called Floquet states can occur when physical systems are caused to vibrate rhythmically. These oscillations enable new physical properties that are not possible in a resting state. This usually requires a lot of energy, for example regular laser pulses for excitation. A study now shows that self-induced Floquet states can also form in tiny magnetic vortices without the need for external excitation or large amounts of energy. The results provide fundamental knowledge that could, in the long term, pave the way to universal adapters that connect electronic and quantum components.
At the end of the 19th century, the French mathematician Gaston Floquet discovered that physical systems can develop new states if they are regularly triggered. The periodic excitation creates additional oscillations and special electronic states that do not exist in the resting state. Until now, however, such Floquet states could usually only be created with strong laser pulses, which required a lot of energy.
La-Ola wave of impulses
A team led by Christopher Heins from the Helmholtz Center Dresden-Rossendorf has now discovered a way to create Floquet states with significantly less energy: through weak excitation with magnetic waves. In advance of the discovery, the researchers experimented with microscopic magnetic disks with a diameter of just a few hundred nanometers. Small magnetic vortices can arise in these disks, consisting of nickel and iron. The spins of the atoms arrange themselves in a circle like tiny compass needles.
A small impulse from outside causes each individual compass needle to tilt slightly and pass the impulse on to its neighbors. This creates waves that continue across the surface of the material like a La-Ola wave. “These waves are called magnons and can carry information through the magnet without any charge flowing,” explains Hein’s colleague Helmut Schultheiss. “This makes them interesting for research into novel computer technologies.”
Unexpected vibrations
When the researchers evaluated experimental data on magnons in the tiny magnetic disks, they found that, contrary to their expectations, there was not just a single resonance line, but a whole series of finely divided lines – an entire frequency comb. “At first we thought it was a measurement artifact or some kind of interference,” says Schultheiss. “But when we repeated the experiment, the effect appeared again. So it was clear: there was something new behind it!”
In fact, the additional frequencies could be explained by the fact that Floquet states had arisen in the magnetic vortices – without any strong laser pulses or other energy-intensive techniques. Apparently the excitation of the magnons alone is enough: if the waves become strong enough, they pass on part of their energy to the vortex core. It then begins to make a small circular movement around its center. This causes the entire magnetic state to enter into a rhythmic oscillation that produces additional frequencies. “We were amazed that such a small movement of the nucleus was enough to split the well-known spectrum of magnons into a whole series of new states,” says Schultheiss.
Low energy consumption
To create the Floquet states in the magnetic disks, power in the microwatt range is sufficient – a tiny fraction of what a cell phone needs in standby mode. This could open up new possibilities for linking electronics, spintronics and quantum devices. The frequency combs could help to better coordinate different systems. “We call this the universal adapter,” explains Schultheiss. “Just as a USB adapter can be used to connect devices with different ports, Floquet magnons could bring together frequencies that otherwise don’t fit together.”
In future studies, the team wants to test whether the principle can also be applied to other magnetic structures. The effect could also be important for the development of new computer technologies, as it makes it easier to couple magnon signals with electronic circuits or quantum systems. “On the one hand, our discovery opens up new ways to answer fundamental questions about magnetism,” says Schultheiss. “On the other hand, it could at some point help to better connect the world of electronics, spintronics and quantum information technology.”
Source: Christopher Heins (Helmholtz Center Dresden-Rossendorf) et al., Science, doi: 10.1126/science.adq9891