Neupane's group started with a higher-resolution version that ignores spin. Those scans revealed a telltale splitting in the material's electronic bands: two energy levels separated by a small gap.

To check whether the pattern matched altermagnetism, they turned to spin-resolved ARPES. The two split states carried opposite spin polarizations, the signature they were seeking.

Detecting those features was not easy. Photoemission experiments are extremely surface-sensitive; even small amounts of contamination can drown out the signals. Collaborators grew high-quality samples of Co1/4TaSe2, and Neupane's group screened them for clean surfaces before mapping the electronic structure.

"The significance became clear once the experimental measurements consistently matched our theoretical predictions," Neupane said. "Seeing those independent pieces of evidence converge gave us confidence that we had identified a genuine layered altermagnet."

Why this material?

Co1/4TaSe2 belongs to a family of compounds called transition-metal dichalcogenides, or TMDs. Their layers bond only weakly, so researchers can peel them apart, restack them, or insert atoms between them to build thin films and other electronic structures.

Cobalt atoms sandwiched between the layers give the material its magnetic character. Because the layered structure is highly tunable, scientists can modify the compound and observe how those changes reshape its electronic and magnetic behavior.

"Evidence for altermagnetism in a versatile materials platform opens a lot of new possibilities," said Milo Sprague, the study's lead graduate student. "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."

The team also found that the altermagnetic signals originated inside the bulk of the material, not just at its surface. Researchers had previously debated which region of a layered compound would carry the most relevant electronic state.

What remains open?

Researchers are pursuing altermagnets for spintronics, a field that tries to encode and process information in electron spin. Spintronic devices could in principle operate faster and consume less energy than today's charge-based electronics, provided engineers can control the underlying materials.

"If this approach proves viable, then layered altermagnets will be at the forefront of electronics development," Neupane said.

Several theoretical questions remain. Scientists still do not know precisely why altermagnetism appears in some materials rather than slipping into ferromagnetism or other magnetic orders, nor how it responds to temperature changes, strain, or chemical substitution.

"There are many details to the theory of how altermagnets work that haven't been explored or verified yet," Neupane said. "Now that we have identified several platforms for answering these questions, more advanced studies into these materials are underway."

The work was supported by the U.S. Department of Energy, Office of Science, under Award Number DE-SC0024304.

Journal reference: Sprague M., Mondal M.I., Sakhya A.P., Regmi R.B., Sadhukhan S., Kumay A.K., Sheokand H., Mazin I.I., Ghimire N.J., Neupane M. "Observation of Altermagnetic Spin-Splitting in an Intercalated Transition Metal Dichalcogenide." Nature Communications, 2026; 17 (1). DOI: 10.1038/s41467-026-76784-x