Researchers from Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Donostia International Physics Center, Leibniz Institute for Solid State and Materials Research Dresden and IMDEA Nanoscience have identified ferromagnetic hexagonal close-packed (hcp) cobalt as a prototypical magnetic nodal-line semimetal that remains robust at room temperature, turning a classic ferromagnet into a highly tunable topological platform for spintronics.
Cobalt has long been regarded as a textbook elemental ferromagnet with a supposedly well-understood band structure, examined in detail for over 40 years. Using spin- and angle-resolved photoemission spectroscopy (spin-ARPES) at the BESSY II synchrotron, the team now observes entangled, spin-polarized bands that cross along extended paths in momentum space without opening an energy gap, even at room temperature. These measurements reveal a dense network of magnetic nodal lines - topological band crossings between two spin-polarized states - that give rise to fast, robust charge carriers central to future information and spin-based technologies.
First-principles calculations based on density functional theory (DFT) classify hcp cobalt as hosting two main families of nodal features, depending on the symmetry of the underlying momentum plane. In the kz=0 (Γ) plane, manifolds of twofold-degenerate, mirror-protected nodal rings enclose the Γ and K points, while in the kz=π (A) plane, spin-polarized nodal lines span the entire A–L–A path. Importantly, these crossings remain gapless even in the presence of spin–orbit coupling, and several appear at the Fermi level, meaning they directly control low-energy transport. All observed nodal features at the Fermi level belong to the minority-spin channel, with majority-spin partners quenched by spin-dependent electron–electron correlations, yielding strongly spin-polarized carriers.
The DFT results show excellent agreement with spin-ARPES data and confirm that the nodal lines are stabilized by crystalline mirror symmetries combined with ferromagnetism. Because time-reversal symmetry is broken, the electronic states forming the nodal lines carry a net spin polarization that can be fully reversed by switching the magnetization direction. Near the nodal crossings, electrons behave like effectively massless, relativistic-like quasiparticles that can travel very fast, providing highly mobile, topologically robust carriers. By changing the magnetization direction, it becomes possible either to open a gap at selected crossings or to continuously tune the spin texture of the nodal lines while preserving their gapless character—exactly the kind of magnetic “on–off” functionality desired for spintronic devices.
The magnetic space group of hcp cobalt matches that of Fe3GeTe2, implying that small perturbations of the magnetization axis could generate finite topological charges, such as Weyl points, and further enrich the spectrum of accessible topological states. At the same time, cobalt’s elemental nature circumvents many fabrication challenges of complex compounds, and its high Curie temperature ensures that the magnetic nodal lines and associated surface states are stable at and above room temperature. These findings not only reframe cobalt as a topological ferromagnet with transport dominated by spin-polarised Weyl-like carriers but also suggest that other elemental and transition-metal ferromagnets may host similarly hidden topological manifolds, opening new directions for spintronic materials design and for exploring the interplay between magnetism and topology in simple metals.