  We review our recent work on spin injection, transport and relaxation in
graphene. The spin injection and transport in single layer graphene (SLG) were
investigated using nonlocal magnetoresistance (MR) measurements. Spin injection
was performed using either transparent contacts (Co/SLG) or tunneling contacts
(Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a
factor of ~1000 and the spin injection/detection efficiency was greatly
enhanced from ~1% (transparent contacts) to ~30%. Spin relaxation was
investigated on graphene spin valves using nonlocal Hanle measurements. For
transparent contacts, the spin lifetime was in the range of 50-100 ps. The
effects of surface chemical doping showed that for spin lifetimes on the order
of 100 ps, impurity scattering (Au) was not the dominant mechanism for spin
relaxation. While using tunneling contacts to suppress the contact-induced spin
relaxation, we observed the spin lifetimes as long as 771 ps at room
temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene
(BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG
and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low
temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low
temperatures. Gate tunable spin transport was studied using the SLG property of
gate tunable conductivity and incorporating different types of contacts
(transparent and tunneling contacts). Consistent with theoretical predictions,
the nonlocal MR was proportional to the SLG conductivity for transparent
contacts and varied inversely with the SLG conductivity for tunneling contacts.
Finally, bipolar spin transport in SLG was studied and an electron-hole
asymmetry was observed for SLG spin valves with transparent contacts...
