  The recently suggested mechanism [Y. Song and H. Dery, Phys. Rev. Lett. 113,
047205 (2014)] of the three-terminal spin transport is based on the resonant
tunneling of electrons between ferromagnetic and normal electrodes via an
impurity. The sensitivity of current to a weak external magnetic field stems
from a spin blockade, which, in turn, is enabled by strong on-site repulsion.
We demonstrate that this sensitivity exists even in the absence of repulsion
when a single-particle description applies. Within this description, we
calculate exactly the resonant-tunneling current between the electrodes. The
mechanism of magnetoresistance, completely different from the spin blocking,
has its origin in the interference of virtual tunneling amplitudes. Spin
imbalance in ferromagnetic electrode is responsible for this interference and
the resulting coupling of the Zeeman levels. This coupling also affects the
current in the correlated regime.
