The world of physics and materials science was recently stirred by a discovery that opens the door to a completely new era of magnetic technologies. Scientists have successfully uncovered previously hidden magnetic properties and the fundamental mechanisms behind a completely new type of magnet, using advanced optical techniques. At the center of their research, the details of which were published last month, specifically on July 7, 2025, in the prestigious scientific journal Physical Review Research, is an organic crystal believed to be one of the most promising candidates for a so-called "altermagnet." This newly discovered, third class of magnetic materials promises revolutionary changes in the development of electronics, data storage, and quantum computing. Unlike conventional ferromagnets, which strongly attract metals, and antiferromagnets, whose magnetic fields cancel each other out, altermagnets exhibit a unique and hitherto unseen magnetic behavior that combines the characteristics of both classical groups in a fascinating way.

Understanding Altermagnetism: The Third Magnetic Force

To understand the importance of this discovery, it is necessary to delve into the basics of magnetism. Ferromagnets, such as iron or nickel, are the materials we most commonly encounter; their internal magnetic moments, or electron spins, are aligned in the same direction, creating a strong external magnetic field. On the other hand, in antiferromagnets, adjacent spins are oriented in opposite directions, causing their magnetic fields to cancel each other out, and the material as a whole exhibits no external magnetization. Altermagnets represent a fundamentally different approach. Although, like antiferromagnets, they have no net external magnetization because their spins also cancel out, their internal crystal structure and symmetry lead to unique electronic behavior. Atoms with opposite magnetic moments in altermagnets are connected by crystal rotation or mirror symmetry, which causes the material's electronic structure to exhibit spin polarization dependent on the direction of electron motion. In practice, this means that altermagnets can conduct a spin-polarized current, a property that was previously reserved almost exclusively for ferromagnets. This hybrid nature makes them extremely attractive for applications in spintronics, a technological branch that aims to use the spin of electrons, not just their charge, for transmitting and processing information.

Japanese Scientific Breakthrough in Materials Research

A research team from Tohoku University in Japan, led by Associate Professor Satoshi Iguchi of the Institute for Materials Research, faced a major challenge. "Unlike typical magnets that attract each other, altermagnets show no net magnetization, but can nevertheless affect the polarization of reflected light," Iguchi points out. "This makes them extremely difficult to study using conventional optical techniques." It was this obstacle that prompted his team to develop a completely new approach. In collaboration with colleagues from the Japan Synchrotron Radiation Research Institute and the physics departments of Tohoku University and Kwansei Gakuin University, Iguchi applied a new, general formula for light reflection derived directly from Maxwell's equations. This formula is applicable to a wide range of materials, including those with low crystal symmetry, such as the organic compound that was the subject of their research. By successfully applying this theoretical innovation, the team was able to precisely clarify the magnetic properties and the origin of altermagnetism in the observed crystal.

Innovative Optical Method for Detecting Hidden Properties

The new theoretical framework enabled the scientists to develop a precise optical measurement method, which they then applied to the organic crystal κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. A key element of their experiment was the measurement of the magneto-optical Kerr effect (MOKE). MOKE is a phenomenon in which the polarization of light changes upon reflection from the surface of a magnetized material. Although the effect itself is known, its application to materials without net magnetization posed a major challenge. Using their new method, the team successfully measured MOKE and from this data extracted the so-called off-diagonal spectrum of optical conductivity. This spectrum provides extremely detailed information about the magnetic and electronic properties of the material, revealing interactions that are otherwise invisible to standard methods. This success not only confirmed their theoretical assumptions but also demonstrated the power of the newly developed optical technique as a key tool for future research in the field of altermagnetism and related quantum materials.

Organic Crystal as the Key to the Future

The choice of material for this pioneering research was not accidental. The organic crystal κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl belongs to the family of so-called organic conductors, materials that owe their conductivity to complex organic molecules rather than metal atoms. These materials are extremely interesting due to their adaptability, light weight, and the fact that their properties can be finely tuned through chemical modifications. The specific crystal is known for being on the border between an insulator and a metal and exhibits antiferromagnetic ordering at low temperatures. Its complex, layered structure and low symmetry made it an ideal candidate for testing theories about altermagnetism. The confirmation of the altermagnetic nature in such an organic compound is particularly significant because it opens up the possibility of creating magnetic devices that are lightweight, flexible, and potentially biocompatible, which was previously unimaginable with traditional magnetic materials based on heavy metals.

The Spectrum of Discovery: What Does the Data Say?

Analysis of the obtained optical conductivity spectrum revealed three key features that unequivocally confirm the altermagnetic nature of the material. First, pronounced peaks were observed at the edges of the spectrum, indicating spin-dependent energy band splitting (spin band splitting). This is a fundamental property that allows for the existence of spin-polarized currents and is direct evidence that the material, despite zero magnetization, possesses an internal magnetic structure similar to ferromagnets. Second, the real component of the measured spectrum is related to the distortion of the crystal lattice and the so-called piezomagnetic effects, where mechanical stress can induce magnetization. Third, the imaginary component of the spectrum is related to rotational currents within the material, which is another subtle signature of unusual magnetic ordering. These findings together not only firmly classify κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl in the class of altermagnets but also provide deep insight into the fundamental physical mechanisms that govern their exotic properties.

Implications for Future Technology and Spintronics

The implications of this research extend far beyond fundamental physics. "This research opens the door to the investigation of magnetism in a broader class of materials, including organic compounds, and lays the foundation for the future development of high-performance magnetic devices based on lightweight, flexible materials," concludes Iguchi. The potential applications are enormous, especially in the field of spintronics. Since altermagnets can carry a spin-polarized current without creating external magnetic fields, they would allow for the creation of much denser memory chips (MRAM) that would not suffer from mutual interference. This would lead to faster, smaller, and more energy-efficient computers and mobile devices. Their resistance to external magnetic fields makes them ideal for secure data storage. Also, the speed at which the magnetic state in altermagnets can be changed, measured in terahertz, is thousands of times faster than in ferromagnets, which opens the way towards ultra-fast data processing. The discovery of altermagnetic properties in an organic crystal could even lead to the development of new medical sensors or flexible electronics that can be integrated into clothing or even the human body.