A new way in which magnetism and electric polarization are coupled has been discovered in CuFeO2

A new way in which magnetism and electric polarization are coupled has been discovered in CuFeO2
Structure of CuFeO2. The magnetism of the iron atoms (brown, with green arrows showing magnetism) influences the chemical bonds with the other elements (blue: copper, red: oxygen) to create a ferroelectric polarization (purple arrow P). Credit: 2012 The American Physical Society

Magnetism and electricity are two of the fundamental forces of nature. Combining them in a single multiferroic material in which one controls the other is not only of basic interest, but also relevant for practical applications. "Multiferroic materials can be used as magnetic sensors that change the sign of their electric polarization with a small magnetic field," says Yoshikazu Tanaka from the RIKEN SPring-8 Center in Harima. After studying the properties of the multiferroic CuFeO2, Tanaka and his colleagues have been 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 electric polarization, so-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 might also help to develop materials where these effects are suitably strong for applications.

The researchers studied the magnetic properties of CuFeO2 using a beam of x-rays from the synchrotron facility at the SPring-8 Center. The x-rays specifically probe the electronic states of the in the crystal that are related to its magnetic properties, and the experiments reveal that these electronic states extend throughout the material in a periodic manner. This arrangement is directly responsible for the multiferroic properties, as it breaks the crystal symmetry and leads to a shift of the electrically charged atoms in the crystal.

At room temperature, each of the iron atoms is surrounded by a symmetric arrangement of and the magnetic moments of the are in disorder. However, at low temperatures the magnetic moments assume the shape of a screw (Fig. 1). Each magnetic moment slightly alters the energy of the in the crystal, depending on the relative orientation between the chemical bond direction and the magnetic moment. The resulting force then distorts the crystal structure and leads to an .

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 for CuFeO2 itself are limited owing to the low temperatures at which this coupling occurs, the discovery could also guide the demonstration of similar materials which do have practical applications, explains Tanaka. "In future, this may lead to the discovery of other materials based on the same mechanism that work at room temperature."

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More information: Tanaka, Y., et al. Incommensurate orbital modulation behind ferroelectricity in CuFeO2. Physical Review Letters 109, 127205 (2012). prl.aps.org/abstract/PRL/v109/i12/e127205
Journal information: Physical Review Letters

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Feb 22, 2013
This is another example of Art Winfree's theory of coupled oscillators, which he articulated circa 1967. Per the article, "the electronic states extend throughout the material in a periodic manner." That's a collective periodic oscillation. Then, "at low temperatures the magnetic moments assume the shape of a screw." That is also a collective periodic oscillation. (Winfree's usual phrase was "limit cycle oscillator.")

Art Winfree's coupled oscillator law is a general mathematical theory, which he applied more or less exclusively to biological synchronization. See Sync, the book by Steve Strogatz. But it also applies to multiferroics, Cooper pairs, and all other coherent behavior in physics, including phases of matter and their transitions. IMO.

Feb 23, 2013
The picture is a little busy, and thus hard to follow. One useful detail is the thin black line-four of them--which run from about 1 on the clock to about 7 on the clock. The black line is several inches long, placed there as a guide (though not called out in the text). Notice that the green arrow at the top of the thin black line is exactly antisynchronous to the partner at the bottom of the same line. This is an instance of coupled oscillators, following a standard, precise pattern in Winfree's model (0 and 180 degrees). Simple, but powerful.

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