Researchers Confirm Exotic Altermagnetism

Scientists have revealed the hidden magnetic behavior of a newly discovered type of magnet by applying advanced optical methods. Their investigation centered on an organic crystal thought to be a strong candidate for an “altermagnet,” a third category of magnetic material that has only recently been proposed. Altermagnets behave in ways that set them apart from the more familiar ferromagnets and antiferromagnets. The findings were reported in Physical Review Research.

“Unlike typical magnets that attract each other, altermagnets do not exhibit net magnetization, yet they can still influence the polarization of reflected light,” points out Satoshi Iguchi, associate professor at Tohoku University’s Institute for Materials Research. “This makes them difficult to study using conventional optical techniques.”

To address this challenge, Iguchi and his team applied a newly developed general formula for light reflection to the organic crystal. This approach enabled them to successfully identify the crystal’s magnetic characteristics and trace their origin.

The group also comprised Yuka Ikemoto and Taro Moriwaki from the Japan Synchrotron Radiation Research Institute; Hirotake Itoh from the Department of Physics and Astronomy at Kwansei Gakuin University; Shinichiro Iwai from the Department of Physics at Tohoku University; and Tetsuya Furukawa and Takahiko Sasaki, also from the Institute for Materials Research.

The team’s newly derived general formula for light reflection was based on Maxwell’s equations and is applicable to a wide range of materials, including those with low crystal symmetry, such as the organic compound studied here.

This new theoretical framework also allowed the team to develop a precise optical measurement method and apply it to the organic crystal κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. They successfully measured the magneto-optical Kerr effect (MOKE) and extracted the off-diagonal optical conductivity spectrum, which provides detailed information about the material’s magnetic and electronic properties.

The results revealed three key features in the spectrum: (1) edge peaks indicating spin band splitting, (2) a real component associated with crystal distortion and piezomagnetic effects, and (3) an imaginary component linked to rotational currents. These findings not only confirm the altermagnetic nature of the material but also demonstrate the power of the newly developed optical method.

“This research opens the door to exploring magnetism in a broader class of materials, including organic compounds, and lays the groundwork for future development of high-performance magnetic devices based on lightweight, flexible materials,” adds Iguchi.

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