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In a thin film of cobalt and iron deposited on a silicon substrate, a tiny magnetic vortex twists into existence. Instead of flying apart, it holds its ground like a drop of oil on a hot skillet — except that instead of heat, it is held in place by an electric current flowing through a narrow constriction just a hundred atoms wide.

This vortex is called a magnetic droplet. Inside it, the magnetization points directly against the external field, like a spinning top that rotates the wrong way. Droplets usually don't live long, but in this device they are stable: supercomputer calculations show that the vortex can persist indefinitely without changing shape.

When the current is increased, the droplet begins to drift along the film. It crawls, deforms, and extra oscillations appear around its main frequency, like overtones on a guitar string. By tilting the external magnetic field, the researchers coaxed the droplet out of the narrow constriction and made it travel two microns — thousands of times its own size.

Such wandering vortices could become the basis for tiny generators of ultrahigh-frequency signals. They can already be placed on standard silicon chips, which means magnetic droplets might one day appear in radar systems, medical scanners, and communication devices.