The computers of the future may rely on more than the electrical charge carried by electrons. Researchers are increasingly interested in another fundamental electron property called spin, which could provide a new way to move and process information inside electronic devices.
Conventional computers use the movement of electrical charge to handle data. If scientists can reliably control electron spin as well, they may be able to develop entirely new approaches to computing that operate faster and consume less energy.
A research team led by UCF Professor of Physics Madhab Neupane has now identified a material that could help make that possible. Neupane and his collaborators found experimental evidence of altermagnetism, a recently recognized form of magnetism that combines useful features associated with two better known magnetic states: ferromagnetism and antiferromagnetism.
A Different Kind of Magnetism
Ferromagnetism is the type of magnetic behavior familiar from ordinary magnets. In a ferromagnetic material, magnetic moments point in the same direction, producing an overall magnetic field.
That behavior can be valuable in electronic technologies, but it comes with a drawback. Ferromagnets can generate stray magnetic fields that interfere with surrounding components, a growing concern as electronic devices become smaller and more densely packed.
Antiferromagnets work differently. Their magnetic moments point in opposite directions, effectively canceling one another and greatly reducing stray magnetic fields. However, these materials do not possess some of the electronic characteristics that make ferromagnets attractive for technological applications.
Altermagnets could offer advantages from both categories.
Like antiferromagnets, they can operate without creating unwanted stray magnetic fields. At the same time, they can generate and detect spin currents, meaning the movement of electron spins through a material. Researchers are investigating whether these spin currents could eventually carry information through future electronic systems.
Neupane and his collaborators found experimental signatures of altermagnetism in Co1/4TaSe2, a layered material that contains magnetic cobalt atoms. The material could now serve as a flexible experimental platform for studying this unusual magnetic state while potentially helping advance electronic and spintronic technologies.
“These materials are distinguished from more conventional antiferromagnets by their ability to generate and detect spin currents without the negative effect of producing stray fields,” Neupane says. “This new property makes them very well positioned for use in many different applications — including spintronics, ultrafast memory devices, terahertz networks and energy-efficient electronics.”
Detecting Altermagnetism in Electron Behavior
To determine whether Co1/4TaSe2 really displayed altermagnetism, the researchers examined how electrons behaved inside the material.
They used angle-resolved photoemission spectroscopy, or ARPES, a technique that allows researchers to measure the energy and motion of electrons and reconstruct a material’s electronic structure.
“Our approach was to use higher-resolution methods that were insensitive to the electron’s spin to measure the splitting in the energy levels,” Neupane says. “Then, we complemented this measurement with spin-resolved ARPES to conclusively tell if this looks like altermagnetism.”
The first measurements revealed a distinctive splitting within the material’s electronic bands. The researchers then turned to spin-resolved ARPES for a more detailed examination.
Those measurements showed that the separated electronic states had opposite spin polarizations, an important signature expected from altermagnetism.
Observing that effect clearly was not simple. Photoemission techniques are extremely sensitive to surface conditions, meaning even small amounts of contamination could obscure the electronic signals the scientists wanted to measure.
Collaborators produced high-quality samples of Co1/4TaSe2, while Neupane’s group carefully checked them for exceptionally clean surfaces before mapping their electronic properties.
“The significance became clear once the experimental measurements consistently matched our theoretical predictions,” Neupane says. “Seeing those independent pieces of evidence converge gave us confidence that we had identified a genuine layered altermagnet.”
An Ultra-Thin and Highly Tunable Material
The evidence for altermagnetism was not the only feature that made Co1/4TaSe2 appealing to the researchers.
The material consists of extremely thin sheets stacked together. Because the individual layers are only weakly connected, researchers can separate them and combine them into very thin structures. That flexibility makes layered materials attractive for thin-film devices and other emerging electronic technologies.
This class of layered materials is known as transition-metal dichalcogenides, or TMDs.
In Co1/4TaSe2, magnetic cobalt atoms positioned between the layers contribute to its unusual magnetic characteristics. The layered structure also gives scientists considerable control over the material, allowing them to modify it and then observe how those changes influence both its electronic and magnetic properties.
Researchers also wanted to determine where the electronic signatures associated with altermagnetism were coming from.
Before these experiments, scientists did not know whether the important altermagnetic features in layered materials would primarily originate at the surface or deeper inside the material.
The measurements indicated that the relevant electronic state came mainly from within the material itself and showed clear evidence of altermagnetic order.
“Evidence for altermagnetism in a versatile materials platform opens a lot of new possibilities,” says Milo Sprague, the study’s lead graduate student researcher. “There’s currently a lot of debate in altermagnetic theory about how the spin-polarized electronic states interact with other magnetic phenomena. Now we have a material that we can easily modify to explore these new questions.”
Using Electron Spin for Future Electronics
Nearly all conventional electronics depend on electron charge to transmit and process information. But electrons also possess spin, another intrinsic property that researchers hope can be harnessed for computing.
The field devoted to using electron spin in electronic devices is called spintronics.
Altermagnets are especially interesting for spintronics because they can create and detect spin currents without generating the troublesome stray magnetic fields associated with conventional ferromagnets.
“As electronic devices continue to shrink, researchers need new materials that can operate faster while consuming less energy,” Neupane says.
Layered materials are already being studied for applications ranging from extremely small transistors to optical technologies and other advanced electronics. Separately, researchers are investigating ways to use spin currents to transport digital information.
Layered altermagnets could potentially connect those two areas. Their extremely thin and adjustable structures could make it possible to control electron spin while avoiding the magnetic interference produced by ordinary magnets.
“If this approach proves viable, then layered altermagnets will be at the forefront of electronics development,” Neupane says.
Major Questions About Altermagnetism Remain
The discovery also provides researchers with something important for basic science: a material that can be modified and studied as scientists investigate the many unresolved questions surrounding altermagnetism in Co1/4TaSe2.
Researchers still do not completely understand why this unusual magnetic state forms, why it can sometimes be favored over competing magnetic arrangements, including ferromagnetism and other forms of antiferromagnetism, or exactly how it behaves under different conditions.
Theoretical work suggests that competing interactions among electrons may influence which type of magnetic order develops. Scientists are now trying to determine how accurately those theories describe what actually occurs in real materials.
“There are many details to the theory of how altermagnets work that haven’t been explored or verified yet,” Neupane says. “Now that we have identified several platforms for answering these questions, more advanced studies into these materials are underway.”
Because researchers can alter Co1/4TaSe2 and track how its properties respond, the material offers a valuable new testing ground for exploring altermagnetism and determining how it interacts with other magnetic and electronic effects.
This material is based upon work supported by the U.S. Department of Energy, Office of Science under Award Number DE-SC0024304.