Three-dimensional magnetic vortices have been discovered by scientists from the Helmholtz-Zentrum Dresden-Rossendorf in Germany together with colleagues from the Paul Scherrer Institute in Switzerland. The results were published in Physical Review Letters . Vortex states are potential antennas for fast wireless data transmission.
“So far, magnetic vortex states have been observed only in two dimensions; in other words, within a plane,” explains Sebastian Wintz, physicist at the Helmholtz-Zentrum Dresden-Rossendorf. These magnetic vortices typically occur in nanometer-scale magnetic disks. Wintz and colleagues have now investigated three-dimensional magnetic layer systems. The researchers stacked two magnetic disks, separated by a thin nonmagnetic metal layer. Due to this special design, all magnets surrounding the intermediate layer arrange themselves in equally oriented three-dimensional vortices — an entirely new observation.
Static, three-dimensional magnetic vortices are formed between two magnetic layers around a nonmagnetic intermediate layer.
Magnetic vortices help researchers improve their fundamental understanding of magnetic materials and offer very promising applications in communication technology. “Three-dimensional magnetic vortices could permit stable high performance antennas for ultrafast, wireless transmission in, for example, mobile communications or Wi-Fi,” says Wintz. The reason is revealed by a more detailed look into a single magnetic disk, as well as the magnetic layer system.
In a magnetic disk, all magnets are arranged in a circle — just as if individual bar magnets are placed one behind another. Even though the magnets do not move, scientists refer to them as magnetic vortices. In the center of the magnetic disks, the vortex core, the magnets can no longer arrange themselves in a circle; instead, they point out of the circle, either upward or downward. When connected to direct current, the vortex core starts moving in a circle. In doing so, it emits characteristic electromagnetic waves. If the speed is too high, though, the system will become unstable, the magnetization direction will switch, and the radio wave is interrupted. The magnets in the vortex core, now align in the opposite direction, once again start spinning and emit new waves — until the speed gets too high again. Thus, continuous data transmission isn't possible.
This is different if two magnetic disks, separated by a thin nonmagnetic metal layer, are stacked on top of each other. Each magnetic disk is only 10 nm thick and has a diameter of about 500 nm. The intermediate layer may cause the magnets in each magnetic disk to not align precisely along the circle and cause them to slightly incline either toward the vortex core or to the outside. The closer the magnets get to the metal layer, the more they incline in such a way that all magnets — both above and below the intermediate layer — point in the same direction: The magnets form a static, three dimensional vortex around the metal layer between the core and the outer edge.
In this configuration it is no longer possible for the magnets to simply switch their direction. “The three-dimensional magnetic vortices stabilize the magnetization in the vortex core,” said Wintz. The magnetic direction inside the vortex core is, thus, retained even at high rotation speeds. “It's conceivable to obtain frequencies of more than 1 GHz,” he added.
In order to produce magnetic disks with wafer-thin metallic intermediate layers, Wintz used HZDR's electron beam lithography. “We've used the rare metal rhodium and we've finally obtained the desired properties by modifying the thickness and roughness of the layers,” he said. The magnetic vortices were revealed at the Swiss Light Source (SLS), the synchrotron light facility of the Paul Scherrer Institute. Next, the researchers will examine the behavior of such magnetic disk pairs as high-frequency vortex antennas.
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