  Half-metals have been envisioned as active components in spintronic devices
by virtue of their completely spin-polarized electrical currents. Actual
materials hosting half-metallic phases, however, remain scarce. Here, we
predict that recently fabricated heterojunctions of zigzag nanoribbons embedded
in two-dimensional hexagonal boron nitride are half-semimetallic, featuring
fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity
originates from the transfer of charges from hexagonal boron nitride to the
embedded graphene nanoribbon. These charges give rise to opposite energy shifts
of the states residing at the two edges while preserving their intrinsic
antiferromagnetic exchange coupling. Upon doping, an
antiferromagnetic-to-ferrimagnetic phase transition occurs in these
heterojunctions, with the sign of the excess charge controlling the spatial
localization of the net magnetic moments. Our findings demonstrate that such
heterojunctions realize tunable one-dimensional conducting channels of
spin-polarized Dirac fermions that are seamlessly integrated into a
two-dimensional insulator, thus holding promise for the development of
carbon-based spintronics.
