First diode for mag­netic fields

Innsbruck quantum physicists have constructed a diode for magnetic fields and then tested it in the laboratory. The device, developed by the research groups led by the theorist Oriol Romero-Isart and the experimental physicist Gerhard Kirchmair, could open up a number of new applications.’

Above Image: Sketch of the unidirectional magnetic coupling between two coils placed near a moving conductor. When the left coil is energized, the magnetic field reaches the right coil (top). When the right coil is energized, the magnetic field does not reach the left one (bottom). (Credit: Luis Veloso)

Electric diodes are essential electronic components that conduct electricity in one direction but prevent conduction in the opposite one. They are found at the core of any electronic component, being one of the most essential building blocks. Until now there were no such components for magnetic fields. Physicists at the University of Innsbruck and the ÖAW Institute of Quantum Optics and Quantum Information (IQOQI) are now changing that. They designed the first diode for magnetic fields and demonstrated its functionality in the laboratory.

“Our device makes possible to transfer the magnetic field from a first magnetic element – for example a magnet or a coil – to a second one. When roles are inverted and one tries to send magnetic field from the second to the first, no magnetic field is transferred,” explains first author Jordi Prat Camps, who is now a researcher at the University of Sussex in England. Technically speaking, this means that the mutual inductances between the two sources, which are routinely assumed to be symmetric, can be made extremely asymmetric. The key element to obtain this result is the use of an electrical conductor that moves with constant velocity. “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”, says Jordi Prat Camps.

Coupled magnetic elements are found in many key technologies like electric motors, transformers, magnetic memories or MRI machines. In all of them, magnetic elements are symmetrically coupled. “The availability of a new magnetic tool like a diode might thus open a bunch of new possibilities”, Gerhard Kirchmair points to the future. For example, the efficiency of wireless charging devices could be improved, since the energy can only flow from the charging station to the device and not in the other direction.

For the first time they demonstrated that the coupling between two magnetic elements can be made extremely asymmetrical.

Physical Review Letters –

ABSTRACT
Lorentz reciprocity establishes a stringent relation between electromagnetic fields and their sources. For static magnetic fields, a relation between magnetic sources and fields can be drawn in analogy to the Green’s reciprocity principle for electrostatics. So far, the magnetostatic reciprocity principle remains unchallenged and the magnetostatic interaction is assumed to be symmetric (reciprocal). Here, we theoretically and experimentally show that a linear and isotropic electrically conductive material moving with constant velocity is able to circumvent the magnetostatic reciprocity principle and realize a diode for magnetic fields. This result is demonstrated by measuring an extremely asymmetric magnetic coupling between two coils that are located near a moving conductor. The possibility to generate controlled unidirectional magnetic couplings implies that the mutual inductances between magnetic elements or circuits can be made extremely asymmetric. We anticipate that this result will provide novel possibilities for applications and technologies based on magnetically coupled elements and might open fundamentally new avenues in artificial magnetic spin systems.

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