  Generating efficient and highly spin-polarized currents through nanoscale
junctions is essential in the field of nanoelectronics and spintronics. In this
paper, using $ab$ $initio$ electron transport calculations, we predict highly
conductive and perfect spin filtering of nickel atomic contacts in a nitrogen
environment, where a single N$_2$ molecule sits in parallel (energetically most
favorable) between two nickel electrodes. Such a particular performance is due
to the wave function orthogonality between majority spin $s$-like states of
ferromagnetic electrodes and the lowest unoccupied molecular orbital of the
N$_2$ molecule, and thus, majority spin electrons are completely blocked at the
interface. For the minority spin, on the contrary, two almost saturated
conducting channels were formed due to the effective coupling between
$d_{zx,zy}$ of the Ni atom and $p_{x,y}$ of the N atom, resulting in large
conductance of about 1$G_0$ ($=2e^2/h$). As a consequence, a single N$_2$
molecule acts as a highly conductive and half-metallic conductor. On the other
hand, the CO and NO incorporated molecular junctions exhibit rather low
conductance with a partially spin-polarized current.
