A new way to couple magnetism and electrical polarization has been discovered in CuFeO2

Magnetism and electricity are two of the fundamental forces of nature. Combining them into a single multiferroic material in which one controls the other is not only of fundamental interest, but also relevant for practical applications. “Multiferroic materials can be used as magnetic sensors that change the sign of their electrical polarization with a small magnetic field,” says Yoshikazu Tanaka of the RIKEN SPring-8 Center in Harima. After studying the properties of multiferroic CuFeO₂Tanaka and his colleagues were able to verify a new mechanism by which magnetism and electricity can be coupled in a single material.

There are different ways in which magnetism and electrical polarization, called ferroelectricity, are coupled in multiferroics. An understanding of these effects is important because not only are multiferroics quite rare, but a better understanding of their properties could also help develop materials where these effects are strong enough for applications.

Researchers have studied the magnetic properties of CuFeO₂ using an X-ray beam from the SPring-8 center’s synchrotron facility. X-rays specifically probe the electronic states of iron ions in the crystal that are related to its magnetic properties, and experiments reveal that these electronic states extend through the material periodically. This arrangement is directly responsible for the multiferroic properties, as it breaks crystal symmetry and leads to displacement of electrically charged atoms in the crystal.

At room temperature, each of the iron atoms is surrounded by a symmetrical arrangement of oxygen atoms and the magnetic moments of the iron atoms are disordered. However, at low temperature, the magnetic moments take the form of a screw (Fig. 1). Each magnetic moment slightly changes the energy of the chemical bonds in the crystal, depending on the relative orientation between the direction of the chemical bond and the magnetic moment. The resulting force then distorts the crystal structure and leads to electrical polarization.

Although such a coupling model between magnetism and ferroelectricity has been proposed theoretically, this work represents the first experimental evidence for this particular mechanism. Moreover, although the practical applications of CuFeO₂ themselves are limited due to the low temperatures at which this coupling occurs, the finding could also guide the demonstration of similar materials that have practical applications, Tanaka says. “In the future, this could lead to the discovery of other materials based on the same mechanism that work at room temperature.”