  One of the important issues of molecular spintronics is the control and
manipulation of charge transport and, in particular, its spin polarization
through single-molecule junctions. Using $ab$ $initio$ calculations, we explore
spin-polarized electron transport across single benzene derivatives attached
with six different anchoring groups (S, CH$_3$S, COOH, CNH$_2$NH, NC and
NO$_2$) to Ni(111) electrodes. We find that molecule-electrode coupling,
conductance and spin polarization (SP) of electric current can be modified
significantly by anchoring groups. In particular, a high spin polarization (SP
$>$ 80%) and a giant magnetoresistance (MR $>$ 140%) can be achieved for NO$_2$
terminations and, more interestingly, SP can be further enhanced (up to 90%) by
a small voltage. The S and CH$_3$S systems, on the contrary, exhibit rather low
SP while intermediate values are found for COOH and CNH$_2$NH groups. The
results are analyzed in detail and explained by orbital symmetry arguments,
hybridization and spatial localization of frontier molecular orbitals. We hope
that our comparative and systematic studies will provide valuable quantitative
information for future experimental measurements on that kind of systems and
will be useful for designing high-performance spintronics devices.
