Topological correlated electronic states and magnetic domains in a van der Waals ferromagnet Ni_1/4TaSe₂

Layered ferromagnets with strongly correlated electrons offer a rich platform for exploring emergent phenomena such as itinerant magnetism, unconventional superconductivity, and many-body states. Ni_1/4TaSe₂ is one such system that undergoes a ferromagnetic transition below 58 K and exhibits superconductivity upon depletion of Ni ions. Using spin- and angle-resolved photoemission spectroscopy (SARPES), we confirmed the spin polarization of electronic states at the Fermi level and observed the breaking of time-reversal symmetry at both the Γ and M points. Circular dichroic SARPES measurements enabled us to isolate the Berry curvature, revealing the material's non-trivial topological properties. Polarization-dependent x-ray absorption spectroscopy (XAS) and photoemission electron microscopy (PEEM) mapping of magnetic domains provided direct evidence of ferromagnetism. Together, these results experimentally validate theoretical predictions of spin-polarized states and non-trivial topology in Ni_1/4TaSe₂, offering new insights into the interplay between topology, ferromagnetism, and superconductivity.