Cobalt contains a rich network of topological electronic states that remain stable even at room temperature.

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. Advanced Measurements Reveal Hidden Quantum Features 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." Magnetic Control of Quantum States 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. "Magnetic nodal-line materials are rare in nature, and in most known cases such crossings are extremely difficult to stabilize or control," explains Sánchez-Barriga. "The observation of multiple symmetry-protected nodal lines in a simple elemental ferromagnet is therefore highly unexpected and establishes cobalt as a model system for studying the interplay between topology and magnetism." Theory Confirms Experimental Results The experimental findings were supported by first-principles calculations based on density functional theory, performed by a theoretical team led by Dr. Maia G. Vergniory of the Donostia International Physics Center and Université de Sherbrooke. These calculations successfully identified all of the nodal lines present in cobalt's bulk electronic structure and showed excellent agreement with the experimental measurements. The analysis confirmed that the nodal lines are protected by crystalline mirror symmetries working together with ferromagnetism. The crossings also remain gapless even when spin-orbit coupling is taken into account. Electrons Behave Like Massless Particles "In certain directions inside the crystal, the nodal lines intersect and cross the Fermi energy where electrons can move freely," explains Sánchez-Barriga. "Near these crossings, electrons in the material behave like massless, relativistic-like particles, similar to how light behaves, and can travel extremely fast. This is an exceptional behavior that has never been observed in any elemental ferromagnet before. Moreover, by changing the direction of the magnetic field, it is possible either to open a gap at the crossing or to fully control the spin texture of the nodal lines while retaining the unique properties of the gapless state. This is exactly the kind of switch on-off functionality sought for practical applications." The ability to manipulate these electronic states using magnetic fields could make cobalt a valuable platform for developing future devices that rely on controlling both charge and spin. New Possibilities for Magnetism and Quantum Materials Beyond potential technological applications, the researchers believe the discovery may point to similar hidden topological features in other elemental and transition-metal ferromagnets. If confirmed, this could open the door to finding a wide range of previously unknown quantum phenomena in materials that have been studied for decades. The team also proposed several ways to further tune these properties, including investigating interfaces with materials that contain heavy elements with high nuclear charge and exploring how the behavior changes in reduced dimensions. The results highlight how even some of the most familiar materials can still produce major scientific surprises. The discovery suggests that our understanding of ferromagnetic metals remains incomplete and reveals new opportunities for research into magnetism, topological matter, and the unusual excitations that emerge from these quantum states. The study was published in Communications Materials, an open-access journal from Nature Portfolio. The study involved researchers from HZB, Diamond Light Source, Donostia International Physics Center, the University of the Basque Country, the Leibniz Institute for Solid State and Materials Research Dresden, TU Dresden, IMDEA Nanoscience (Madrid), and Université de Sherbrooke (Canada).

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