  By first principle calculations, a dual spin filter effect under finite bias
voltages is demonstrated in an antiferromagnetic junction of symmetric zigzag
graphene nanoribbon (ZGNR). Unlike conventional spin filter devices using half
metallic materials, the up- and down-spin electrons are unidirectionally
filtered in the counter direction of the bias voltage, making the junction a
dual spin filter. On the contrary, asymmetric ZGNRs do not exhibit such a spin
filter effect. By analyzing Wannier functions and a tight-binding model, we
clarify that an interplay between the spin polarized band structure of $\pi$
and $\pi^*$ states near the Fermi level and decoupling of the interband hopping
of the two states, arising from the symmetry of the wave functions, plays a
crucial role in the effect.
