Team member Lukas Weymann in the lab at TU Wien. Photo: TU Wien.Electricity and magnetism are closely related: power lines generate a magnetic field and rotating magnets in a generator produce electricity. But the phenomenon is actually much more complicated, because the electrical and magnetic properties of certain materials are also coupled with each other. The electrical properties of some crystals can be influenced by magnetic fields, and vice versa. This is known as the 'magnetoelectric effect', and it plays an important technological role, for example in certain types of sensors or in the search for new concepts of data storage.
Now, a team of researchers from Austria, Russia and the Netherlands has discovered that the relationship between electricity and magnetism is even more complicated. They were investigating a special material for which, at first glance, no magnetoelectric effect would be expected at all. But through careful experiments, they showed that the effect could be observed in this material after all, although it works in completely different way than usual. As they report in a paper in npj Quantum Materials, even small changes in the direction of the magnetic field can switch the electrical properties of this 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," states Andrei Pimenov from the Institute of Solid State Physics at Vienna University of Technology (TU Wien) in Austria. "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 is the case with the crystal that the team investigated – a so-called langasite made of lanthanum, gallium, silicon and oxygen, and 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 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."
From a purely geometrical point of view, the crystal is still symmetrical, but the magnetism of the atoms has to be taken into account as well, and this is what breaks the symmetry. As a consequence, the electrical polarization of the crystal can be changed with a magnetic field.
"Polarization is when the positive and negative charges in the crystal are displaced a little bit, with respect to each other," explains Pimenov. "This would be easy to achieve with an electric field – but due to the magnetoelectric effect, this is also possible using a magnetic field."
The stronger the magnetic field, the stronger its effect on the electrical polarization. "The relationship between polarization and magnetic field strength is approximately linear, which is nothing unusual," says Pimenov. "What is remarkable, however, is that the relationship between polarization and the direction of the magnetic field is strongly non-linear. If you change the direction of the magnetic field a little bit, the polarization can completely tip over. This is a new form of the magnetoelectric effect, which was not known before." A small rotation can determine whether the magnetic field can change the electrical polarization of the crystal or not.
"The magnetoelectric effect will play an increasingly important role for various technological applications. In a next step, we will try to change magnetic properties with an electric field instead of changing electrical properties with a magnetic field. In principle, this should be possible in exactly the same way."
If they succeed, this presents a promising new way to store data in solids. "In magnetic memories such as computer hard disks, magnetic fields are needed today," Pimenov says. "They are generated with magnetic coils, which requires a relatively large amount of energy and time. If there were a direct way to switch the magnetic properties of a solid-state memory with an electric field, this would be a breakthrough."
This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.