Imagine throwing a stone into the water. Waves spread out. Something similar happens in tiny magnetic disks, barely thicker than a hair. Researchers at the Helmholtz-Zentrum Dresden-Rossendorf have now discovered something astonishing there. They found vibrational states that no one had ever seen before in magnetic vortices.
The discovery began with a puzzle. The team led by Dr. Helmut Schultheiß tested extremely small magnetic disks made of nickel-iron. Each disk is only a few hundred nanometres in size. Magnetic vortices form in these disks. The researchers excited these vortices with magnetic waves. But instead of the expected simple oscillation, they suddenly saw a whole bundle of signals. "At first we thought it was a measurement artifact or some kind of interference pulse," recalls Schultheiss. However, the effect occurred again and again.
Information transmission like a La-Ola wave
What happens in these tiny discs? In a magnet, the smallest components behave like mini compass needles. They arrange themselves in a circle. When they are pushed, a wave is created. Each needle tilts a little and passes on the impulse. Similar to a La-Ola wave in a stadium. Experts call these waves magnons. "These magnons can carry information through the magnet without any charge flowing," explains Schultheiß. This makes them interesting for new computer technologies.
The Dresden researchers discovered that the core of the magnetic vortex begins to rotate when strongly excited. This tiny rotational movement is enough to rhythmically change the magnetic state. This creates the new oscillation states. The theory for this comes from the 19th century French mathematician Gaston Floquet. However, such Floquet states were previously only known from experiments with strong laser pulses.
An adapter for the computer world
The special thing about the discovery is that instead of a single oscillation, many different frequencies are generated simultaneously. The researchers refer to this as a frequency comb. Like a comb, the frequencies are neatly lined up next to each other. It is precisely this variety that makes the effect so interesting. "We call this the universal adapter," says Schultheiß. Electronics, spintronics and quantum computers work with different frequencies. The Floquet magnons could bring them together.
There is another advantage. Where other experiments need powerful lasers, microwatts are enough here. That is a tiny fraction of what a cell phone consumes in standby mode. The low energy requirement makes the discovery particularly practical.
The team is already planning further experiments. They want to test whether the effect also works in other magnetic structures. Schultheiß emphasizes the twofold significance of the work. "On the one hand, our discovery opens up new ways of answering fundamental questions about magnetism. Secondly, it could one day help to better connect the worlds of electronics, spintronics and quantum information technology."
