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Electricity and magnetism are closely related: Power lines generate a magnetic field, rotating magnets in a generator produce electricity. However, the phenomenon is much more complicated: electrical and magnetic properties of certain materials are also coupled with each other. Electrical properties of some crystals can be influenced by magnetic fields—and vice versa. In this case one speaks of a "magnetoelectric effect." It plays an important technological role, for example in certain types of sensors or in the search for new concepts of data storage.

A special material was investigated for which, at first glance, no magnetoelectric effect would be expected at all. But careful experiments have now shown that the effect can be observed in this material, it only works completely differently than usual. It can be controlled in a highly sensitive way: Even small changes in the direction of the magnetic field can switch the electrical properties of the material to a completely different state.

"Whether the electrical and magnetic properties of a crystal are coupled or not depends on the crystal's internal symmetry," says Prof. Andrei Pimenov from the Institute of Solid State Physics at TU Wien. "If the crystal has a high degree of symmetry, for example, if one side of the crystal is exactly the mirror image of the other side, then for theoretical reasons there can be no magnetoelectric effect."

This applies to the crystal, which has now been examined in detail—a so-called langasite made of lanthanum, gallium, silicon and oxygen, doped with holmium atoms. "The crystal structure is so symmetrical that it should actually not allow any magnetoelectric effect. And in the case of weak magnetic fields there is indeed no coupling whatsoever with the electrical properties of the crystal," says Andrei Pimenov. "But if we increase the strength of the magnetic field, something remarkable happens: The holmium atoms change their quantum state and gain a magnetic moment. This breaks the internal symmetry of the crystal."