Claimed to be the first-ever “magnetic field diode”, a device in which one wire coil can transfer its magnetic field to a second coil, but not the other way around has been created by Jordi Prat-Camps and colleagues at the University of Innsbruck in Austria. The team believes that further improvements to the diode could allow it to be used in a wide range of applications in electrical devices.

Electrical diodes, which allow currents to flow in one direction but not the other, are a fundamental component of electronics. Engineers have long sought a comparable device that would direct magnetic fields in only one direction – something that would be incredibly useful in many technologies. Until now, however, the Lorentz reciprocity principle has been a formidable barrier to creating a practical magnetic field diode.

The principle states that when a magnetic field source, such as a conducting wire coil, induces a field in a second source, the second will be able to induce a field in the first. Therefore, a magnetic field diode in which the field can only be transferred coil-to-coil in one direction should not be possible.

U-shaped cross-section

Through theoretical calculations, Prat-Camps and colleagues identified a special situation where the symmetry dictated by the reciprocity principle can be broken. They found that this could be achieved if the two coils were placed within the walls of a hollow, conducting cylinder rotating at a constant velocity. The team then built a cylinder with walls that have a U-shaped groove containing the coils (see figure) and demonstrated its practicality in the lab.

“Our device makes possible to transfer the magnetic field from a first magnetic element. When roles are inverted, and one tries to send magnetic field from the second to the first, no magnetic field is transferred,” says Prat-Camps, who is now at the University of Sussex in the UK. “When the conductor is properly placed near to the magnetic elements and is moved at the right speed, the coupling between them becomes unidirectional, and a diode for magnetic fields is realized.”

If a commercially-viable magnetic field diode can be made, it could transform the capabilities of electrical components. It could, for example, enable wireless charging where energy only flows in one direction. It could also be used to improve devices that currently use symmetrically-coupled magnetic elements, including electric motors, transformers and MRI machines. While the device created by the team is currently bulky and not yet practical for use in everyday electrical components, the researchers believe it could be greatly improved with further research.

The diode is described in Physical Review Letters.

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