For decades, cobalt has been considered one of the best understood magnetic metals. Its crystal structure and fundamental properties have been extensively studied, and scientists believe there are few surprises left to uncover. But new research reveals that this familiar element hides an unexpectedly complex quantum environment within its electronic structure.
An international team led by Dr. Jaime Sánchez Barriga from Helmholtzzentrum Berlin (HZB) has discovered that cobalt contains a rich network of topological electronic states that are stable even at room temperature. The discovery challenges long-held assumptions about metals and suggests that they may play an important role in future electronic and spin-based technologies.
Advanced measurements reveal hidden quantum features
Researchers used spin and angle-resolved photoemission spectroscopy (spin ARPES) at the BESSY II synchrotron radiation facility to examine the electronic structure of cobalt in unprecedented detail. Their measurements revealed a dense network of magnetic nodal lines. This is a special type of topological band crossing in which two spin-polarized electronic states intersect continuously without forming an energy gap.
These intersections do not occur at isolated points, but are spread out along paths in momentum space throughout the crystal. The resulting electronic states can support very fast and topologically robust charge carriers, making them particularly attractive for future information technology and spintronics applications.
“Cobalt has been one of the most well-known and extensively studied ferromagnetic elements over the past 40 years, and its electronic structure was thought to be well understood,” said HZB physicist Dr. Jaime Sánchez Barriga, who led the study. “What we have discovered, however, is a topologically interesting band structure with numerous crossings and nodes that govern its low-energy electronic behavior. This completely changes our current understanding of the fundamental properties of this elemental material.”
Magnetic control of quantum states
One of the most important aspects of the newly discovered nodal lines is that they are inherently spin-polarized. Since cobalt is ferromagnetic and breaks time reversal symmetry, the electronic states associated with these nodal lines carry a net spin polarization.
Importantly, the spin polarization can be completely reversed by changing the direction of the material’s magnetization. This provides direct magnetic control over the charge carriers bound to the nodal lines. This is a feature not present in non-magnetic nodal materials and is highly desirable for spintronics technology.
“Magnetic nodal materials are rare in nature, and in most known cases such crossings are very difficult to stabilize or control,” explains Sánchez-Barriga. “The observation of symmetry-protected multiple nodal lines in simple elemental ferromagnets is therefore highly unexpected and establishes cobalt as a model system for studying the interaction between topology and magnetism.”
Theory supports experimental results
The experimental results were supported by first-principles calculations based on density functional theory, conducted by a theoretical team led by Dr. Maia G. Bernioly of the Donostia International Physics Center and the University of Sherbrooke.
These calculations successfully identified all nodal lines present in the bulk electronic structure of cobalt and showed excellent agreement with experimental measurements. Analysis confirms that the nodal lines are protected by crystal mirror symmetry that cooperates with ferromagnetism. Even when considering spin-orbit coupling, there is no gap in the intersection.
Electrons behave like massless particles
“In certain directions inside the crystal, the nodal lines intersect and intersect the Fermi energy, where electrons can move freely,” Sánchez-Barriga explains. “Near these intersections, electrons in the material behave like massless relativistic particles, similar to the behavior of light, and can move very quickly. This is an exceptional behavior that has never been observed in any elemental ferromagnetic material before. Furthermore, By changing the direction of the magnetic field, it is possible to open gaps at intersections or to completely control the spin organization of the nodal lines, while maintaining the unique properties of the gap-free state. ”
The ability to manipulate these electronic states using magnetic fields could make cobalt a valuable platform for developing future devices that rely on control of both charge and spin.
New possibilities for magnetism and quantum materials
Beyond potential technological applications, the researchers believe this finding may indicate similar hidden topological features in other elemental and transition metal ferromagnets. If confirmed, this could open the way to discovering a wide range of previously unknown quantum phenomena in materials that have been studied for decades.
The research team also proposed several ways to further tune these properties. This includes investigating interfaces with materials containing heavy elements with high nuclear charge and investigating how behavior changes in reduced dimensions.
The results highlight how even the most well-known materials can yield big scientific surprises. This discovery suggests that our understanding of ferromagnetic metals is still incomplete and reveals new opportunities for research into magnetism, topological materials, and the anomalous excitations arising from these quantum states.
This research Communication materialsthe open access journal of Nature Portfolio.
The study involved researchers from HZB, Diamond Light Source, Donostia International Physics Center, University of the Basque Country, Leibniz Institute for Solid State Materials Dresden, Dresden University of Technology, IMDEA Nanoscience (Madrid), and the University of Sherbrooke (Canada).
