Perfect inversion of complex structures

Perfect inversion of complex structures
Model of the perfect inversion of a magnetic or electric structure. The bottom layer contains information on the structure. The intermediate layer can be switched with the applied field. The reversal is depicted from left to right. The top layer shows the distribution of magnetization or polarization in the material. Credit: ETH Zurich

Perfectly inverting complex structures is of great technical importance. Researchers at ETH have now succeeded in turning the magnetic and electric structure of materials into their opposites using a single magnetic field pulse.

In unpleasantly loud environments, active noise reduction has been used in earphones and luxury cars in recent years. A microphone picks up the disturbing noise, from which a computer chip calculates the appropriate countermeasures: sound waves whose phases are exactly opposite to those of the ambient sound. The interference between those waves effectively erases the noise. Physicists and engineers seek to apply this principle of perfect inversion to other technologies—for instance, to the magnetic structure of materials. ETH professor Manfred Fiebig and his collaborators at the Department of Materials in Zurich have now succeeded in doing just that, with support from scientists in Europe, Japan, and Russia. Their results are published this week in the scientific journal Nature.

Fiebig’s team used so-called multiferroics for their experiments. Unlike many other materials that have either magnetic or electric order, multiferroics possess both: They are magnetically and, at the same time, electrically polarized and, as a consequence, align themselves both along magnetic and along electric fields. The physical mechanisms that bring about the magnetic and electric order inside the material are subtly coupled to each other. This makes it possible to influence the magnetization using electric fields rather than magnetic fields. “That’s much more efficient, as one needs an electric current to create magnetic fields, and that costs a lot of energy and creates annoying waste heat,” explains Naëmi Leo, a former Ph.D. student in Fiebig’s laboratory. In computers, for instance, where data is constantly written on magnetic hard drives, multiferroics could be key materials for significant energy savings.

Inspiration from Tangram shapes
At ETH, which has been an international leader in multiferroics research for quite some time, scientists took this idea one step further. “A material that allows one to control its magnetization using electric fields must necessarily have a rather complex structure,” says Fiebig.

He uses the Chinese Tangram puzzle to illustrate that principle: The more pieces available—triangles, squares, and parallelograms—the more elaborate shapes are possible. In the case of multiferroics, the shapes correspond to the symmetries of the material, which determine its physical properties. The more complex those symmetries, the more varied are the so-called order parameters. They describe the direction in which the magnetization points inside a multiferroic, and how the magnetization is coupled to the electric order.

Unexpected properties
If the atoms inside a material are arranged in such a complicated way, it is also very likely that it has other properties that are not obvious at first sight. “That’s why we didn’t want to limit ourselves to the well-known phenomena that have been studied for a long time, but rather try to see what other useful things multiferroics can do,” Fiebig says, and illustrates his research approach: “How can we recombine the pieces of the puzzle—that is, the order parameters—in different ways than those already known, and thus obtain new and useful properties?”

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