Physicists have traditionally divided magnetic materials into ferromagnets and antiferromagnets. Now, researchers led by the U.S. Department of Energy’s SLAC National Accelerator Laboratory say they have found strong experimental evidence for a third class: alternating-current magnetism, also known as altermagnetism. The work, published in Science, identifies a distinctive chiral magnetic structure in manganese telluride (MnTe), a material long classified as an antiferromagnet. The discovery could eventually support advances in spintronics, computer memory, quantum devices and energy-efficient data centers.
Beyond ferromagnets and antiferromagnets
“In recent years, scientists have observed phenomena that don’t fit neatly into the ferromagnetism/antiferromagnetism dichotomy,” said lead author Daniel Jost, an associate research scientist at SLAC and the Stanford Institute for Materials and Energy Sciences. “Researchers have compiled evidence for a third type of magnetism that bridges these two classes of properties. We believe our results provide solid experimental confirmation that this third step, alternating magnetism, is real.”
Magnetism is familiar from everyday objects such as refrigerator magnets and compasses, but its microscopic origin is rooted in quantum mechanics. Electrons possess a quantum property called spin. An electron is not literally spinning like a small ball, but its spin behaves in some ways like angular momentum. Scientists commonly describe the two possible spin orientations as “up” and “down.”
When electron spins form an ordered pattern, a material can develop magnetic properties. That order does not necessarily produce a magnetic field that can be detected outside the material.
In ferromagnets, neighboring electron spins align in the same direction, such as up-up-up or down-down-down. Their combined effects create the external magnetic field associated with ordinary permanent magnets.
Antiferromagnetism, identified as a distinct category in 1933, has the opposite arrangement. Neighboring spins point in alternating directions—up-down-up-down—so their magnetic contributions cancel and the material has no overall magnetization.
A proposed third form of magnetic order
For nearly 90 years, these two categories provided the basic framework for understanding magnetic order in materials. More recent experiments, however, have revealed behaviors that do not fit completely into either group.
Some materials have alternating spins and no net magnetization, like antiferromagnets, while also supporting useful spin currents, a property associated with ferromagnetic behavior. Spin currents transfer information through the spin of electrons rather than relying solely on their electrical charge.
This combination led researchers to propose a third magnetic class. In 2022, scientists introduced the term “alternating current magnets” for materials with these characteristics.
The key difference is found in the relationship between the magnetic atoms and their surrounding crystal structures. In these materials, the spins alternate in opposite directions, but the crystal environments around those spins rotate relative to one another.
That rotation breaks a symmetry present in conventional antiferromagnets. Although the structural difference is subtle, it can significantly alter electron behavior, allowing these materials to combine properties that were previously considered incompatible.
Chirality provides missing evidence
Experimental evidence for altermagnetism had been accumulating, but researchers still needed a definitive signature showing that it represented a distinct magnetic phase.
“While there were several factors that strongly supported the realization of alternative geomagnetism by 2024, there were still missing pieces of the puzzle,” Jost said.
One missing piece was chirality, sometimes described as handedness. An object is chiral when its mirror image cannot be perfectly superimposed on the original. Human hands provide a familiar example: the left and right hands are mirror images, but a left hand does not fit a right-handed glove.
In alternating-current magnets, the arrangement of atoms and electron spins can produce chirality in the material’s electronic and magnetic structure. Conventional antiferromagnets do not display the same characteristic chiral behavior. Detecting it therefore provides a way to distinguish genuine altermagnets from materials that only resemble them.
The researchers selected manganese telluride as their test material. For decades, MnTe was regarded as an ordinary antiferromagnet, but more recent studies had raised the possibility that it belonged to the proposed third category.
Polarized X-rays expose the magnetic structure
The team carried out experiments at the European Synchrotron Radiation Facility (ESRF) in France. Its ID32 beamline produces precisely controlled polarized X-rays, enabling researchers to examine subtle magnetic structures that are otherwise difficult to detect.
The researchers directed circularly polarized X-rays at manganese telluride. In one beam, the electric field rotated counterclockwise as the light traveled; in the other, it rotated clockwise. By comparing how the material scattered the two types of light, the researchers looked for evidence of chirality.
“Chiral materials scatter left-polarized X-ray beams differently than right-polarized beams,” Jost explained. “Non-chiral materials scatter each ray of light the same way.”
The experiment found clear differences in how manganese telluride interacted with the two X-ray polarizations. This chiral response was consistent with the signal expected from altermagnetism and was not expected in conventional antiferromagnets.
“This chiral feature gave us a solid experimental basis to say that alternative magnetism is in a class of its own,” Jost said.
The researchers conducted their first experiment in 2024. They returned in early 2025 to repeat the measurements and reproduced the results, strengthening the evidence for a distinct magnetic phase.
“This technique helped confirm that manganese telluride indeed exhibits variable magnetism,” said co-author Rachel Nickell, a postdoctoral fellow at ESRF who helped design and conduct the experiment at the ID32 beamline. “Although AC magnetism was only proposed a few years ago, the rapid progress in confirming these models has been incredible.”
Potential applications in spintronics
The findings could have implications for spintronics, a field that uses electron spin to develop electronic devices with potentially greater performance and lower energy consumption.
Conventional electronics primarily store and process information by controlling electron charge. Spintronic devices add another method by manipulating electron spin.
That additional control could allow devices to operate faster, resist interference more effectively and use less power. These advantages could be particularly valuable in computer memory, large data centers and other systems that process vast amounts of information.
Both established forms of magnetism have limitations for spintronic applications.
“Ferromagnetic materials are sensitive to magnetic fields,” Jost explained. “If you had a permanent magnet in your pocket and you walked by a spintronic device made of ferromagnets, the bits could start flipping and the information stored there could be scrambled. Antiferromagnets solve that problem, but they’re actually invisible, so the data stored there can’t be easily read or used to drive an efficient spin current.”
Altermagnets could offer a combination of these desirable characteristics. Like antiferromagnets, their opposing spins produce no net magnetization, helping protect them from unwanted magnetic interference. At the same time, their unusual electronic structure can generate spin-polarized currents that may be used to retrieve information.
“Altermagnet combines the best of both worlds,” Jost said. “They are stable because they have zero net magnetization, but they also have the ability to generate the spin-polarized currents needed to easily extract information. This gives us even more freedom in our work.”
Future directions
Confirming altermagnetism gives scientists a foundation for studying new classes of materials and tailoring them for technological applications. Combining magnetic stability with the ability to manipulate spin-based information could help address challenges that have limited traditional spintronic devices.
The next step is to determine how to control these alternating magnetic states using external influences such as light or electricity. The researchers hope that this control will eventually enable practical devices that exploit the materials’ unusual magnetic properties.
Significant development remains before altermagnetic materials can be widely used. However, the discovery provides an experimental basis for further research into quantum technology, computer memory and more energy-efficient data centers. The research received partial funding from the DOE Office of Science.
Source: www.sciencedaily.com