Scientists discover a hidden quantum world inside cobalt

For decades, cobalt has been considered one of the best understood magnetic metals. Its crystal structure and basic properties have been studied extensively, leading scientists to believe there were few surprises left to uncover. But new research has revealed that this familiar element harbors an unexpectedly complex quantum landscape hidden within its electronic structure.

An international team led by Dr. Jaime Sánchez-Barriga of Helmholtz-Zentrum Berlin (HZB) discovered that cobalt contains a rich network of topological electronic states that remain stable even at room temperature. The findings challenge long-held assumptions about the metal and suggest it could play an important role in future electronic and spin-based technologies.

The researchers used spin- and angle-resolved photoemission spectroscopy (spin-ARPES) at the BESSY II synchrotron radiation facility to examine cobalt's electronic structure in unprecedented detail. Their measurements uncovered a dense network of magnetic nodal lines, which are special topological band crossings where two spin-polarized electronic states intersect continuously without forming an energy gap.

Rather than occurring at isolated points, these crossings extend along paths in momentum space throughout the crystal. The resulting electronic states can support extremely fast and topologically robust charge carriers, making them particularly attractive for future information technologies and spintronics applications.

"Cobalt is one of the most familiar and extensively studied ferromagnetic elements over the last 40 years, and its electronic structure was thought to be well understood," says HZB physicist Dr. Jaime Sánchez-Barriga, who led the study. "However, what we find is a topologically interesting band structure with numerous crossings and nodes that dominate its low-energy electronic behavior. This completely changes our current understanding of the fundamental properties of this elemental material."

One of the most significant aspects of the newly discovered nodal lines is that they are inherently spin-polarized. Because cobalt is ferromagnetic and breaks time-reversal symmetry, the electronic states associated with these nodal lines carry a net spin polarization.

Importantly, that spin polarization can be completely reversed by changing the direction of the material's magnetization. This provides direct magnetic control over the charge carriers linked to the nodal lines, a capability that does not exist in non-magnetic nodal-line materials and is highly desirable for spintronic technologies.

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