A Magnetic Vortex That Conducts Its Own Orchestra express
In a tiny magnetic dot, smaller than a micron, lives a vortex — magnetization swirls like water in a drain. At the center of this swirl is a core that itself orbits in a small circle, like a spinning top that's been given a nudge. If you aim a microwave signal at such a dot, the vortex responds with oscillations at a single frequency, like a string plucked by a bow.
But if you shift the vortex core from its usual spot with a magnetic field, the behavior changes. Now, in response to the same signal, not one frequency but a whole comb is born: a multitude of lines spaced at equal intervals, like the teeth of a comb. This is the Floquet mode — oscillations that multiply themselves, generating ever new frequencies.
The scientists built a model that explains this effect. It turned out that the feedback between the core's rotation and the magnetization oscillations creates several stable orbits. The vortex can choose one of them, and that determines whether it will sing a single note or a whole chord of frequency comb.
Such a controllable comb is not just a curiosity. Frequency combs are already used in optics for ultra-precise clocks and spectroscopy. Now a similar tool appears in the world of magnetism: perhaps it will become the basis for new devices where information is encoded not in zeros and ones, but in the character of a magnetic vortex's oscillations.
Frequency combs in optics have made it possible to measure the frequency of light with an accuracy of 18 decimal places — enough to notice your clock losing a second over billions of years.
In Greg Egan's novel 'Schild's Ladder', physics is based on topological defects — vortices that store information in their twist; here, too, a vortex remembers its state.