Element cobalt exhibits surprising properties

The element cobalt is considered a typical ferromagnet with no further secrets. However, an international team led by HZB researcher Dr. Jaime Sánchez-Barriga has now uncovered complex topological features in its electronic structure. Spin-resolved measurements of the band structure (spin-ARPES) at BESSY II revealed entangled energy bands that cross each other along extended paths in specific crystallographic directions, even at room temperature. As a result, cobalt can be considered as a highly tunable and unexpectedly rich topological platform, opening new perspectives for exploiting magnetic topological states in future information technologies.

Cobalt is an elementary ferromagnet, and its properties and crystal structure have long been known. However, an international team has now discovered that cobalt hosts an unexpectedly rich topological electronic structure that remains robust at room temperature, revealing a surprising new level of quantum complexity in this material. '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 behaviour. This completely changes our current understanding of the fundamental properties of this elemental material.'

Spin-ARPES at BESSY II

Using spin- and angle-resolved photoemission spectroscopy (spin-ARPES) at the BESSY II synchrotron radiation source, the researchers uncovered a dense network of so-called magnetic nodal lines— topological band crossings in which two spin-polarized electronic states intersect continuously without opening an energy gap. These crossings form extended paths in momentum space inside the bulk of the crystal and give rise to fast, topologically robust charge carriers, which are essential for the development of new device functionalities in future information and spin-based technologies.

A key feature of the nodal lines in cobalt is that they are intrinsically spin-polarised. Because time-reversal symmetry is broken, the electronic states forming the nodal lines carry a net spin polarisation that can be fully reversed by switching the magnetisation direction. This provides direct magnetic control over the associated charge carriers—an essential ingredient for spintronic applications that is completely absent in non-magnetic nodal-line materials.

Cobalt as a model system

'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.'

Experimental data fit well to DFT

The experimental observations are supported by first-principles calculations based on density functional theory, carried out by a theory team headed by Dr. Maia G. Vergniory (Donostia International Physics Center and Université de Sherbrooke). The strong predictive power of these calculations lies in their ability to identify all nodal lines in the calculated bulk band structure at once. The calculations show excellent agreement with the measurements and confirm that the nodal lines in cobalt are protected by crystalline mirror symmetries combined with ferromagnetism. Importantly, the crossings remain gapless even in the presence of spin-orbit coupling.

Switching is possible

'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 behaviour 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.'

Beyond its technological implications, the authors suggest that similar topological features may exist in other elemental and transition-metal ferromagnets, opening new opportunities to discover exotic properties in these materials. They also propose ways to further control these properties, such as studying interfaces with materials that have high nuclear charge or exploring the effects of reduced dimensionality.

Big learnings

The discovery demonstrates that our current understanding of ferromagnetic metals was not complete. It shows that even the most familiar magnetic materials can still surprise us by hosting hidden, unusual quantum states, revealing exciting new directions for research in magnetism, topological states of matter and their excitations.

The results have been published 24 January 2026 in Communications Materials, an open-access, high-impact journal from the Nature Publishing Group.