Spontaneous spin-valley polarization in NbSe₂ at a van der Waals interface

A proximity effect at a van der Waals (vdW) interface enables creation of an emergent quantum electronic ground state. Here we demonstrate that an originally superconducting two-dimensional (2D) NbSe₂ forms a ferromagnetic ground state with spontaneous spin polarization at a vdW interface with a 2D ferromagnet V₅Se₈. We investigated the anomalous Hall effect (AHE) of the NbSe₂/V₅Se₈ magnetic vdW heterostructures, and found that the sign of the AHE was reversed as the number of the V₅Se₈ layer was thinned down to the monolayer limit. Interestingly, the AHE signal of those samples was enhanced with the in-plane magnetic fields, suggesting an additional contribution to the AHE signal other than magnetization. This unusual behavior is well reproduced by band structure calculations, where the emergence of the Berry curvature along the spin-degenerate nodal lines in 2D NbSe₂ by the in-plane magnetization plays a key role, unveiling a unique interplay between magnetism and Zeeman-type spin-orbit interaction in a non-centrosymmetric 2D quantum material.