  Here, we systematically investigate the structure, phase stability,
half-metallicity, and topological electronic structure for a new topological
spintronic material NaV2O4. The material has a tetragonal structure with
excellent dynamical and thermal stabilities. It shows a half-metallic ground
state, where only the spin-up bands present near the Fermi level. These bands
form a Weyl nodal line close to the Fermi level, locating in the kz = 0 plane.
The nodal line is robust against SOC, under the protection of the mirror
symmetry. The nodal line band structure is very clean, thus the drumhead
surface states can be clearly identified. Remarkably, the nodal line and
drumhead surface states have the 100% spin polarization, which are highly
desirable for spintronics applications. In addition, by shifting the magnetic
field in-plane, we find that the Weyl nodal line can transform into single pair
of Weyl nodes. The Weyl-line and Weyl-node fermions in the bulk, as well as the
drumhead fermions on the surface are all fully spin-polarized, which may
generate new physical properties and promising applications.
