  Recent experimental discoveries of two-dimensional (2D) magnets have
triggered intense research activities to search for atomically thin magnetic
systems. Using first-principles calculations, we predict the emergence of 2D
magnetism in the monolayers (MLs), few layers, and surfaces of nonmagnetic
layered electrides Hf2X (X = S, Se, Te) consisting of three-atom-thick Hf-X-Hf
stacks. It is revealed that each bulk Hf2X hosts a novel quantum state of Dirac
nodal lines with a high density of states arising from Hf-5d cationic and
interlayer anionic electrons around -0.9 eV below the Fermi level EF. However,
for the MLs, few layers, and surfaces of Hf2X, such hybridized states are
shifted toward EF to generate van Hove singularities, leading to a Stoner
instability. The resulting surface ferromagnetism gives rise to strongly
spin-polarized topological surface states at Hf2X(001), demonstrating that
anionic electrons, 2D magnetism, and band topology are entangled with each
other. Our findings will open new perspectives for the discovery of 2D magnets
via exploiting surface effects in nonmagnetic layered electrides.
