▎ 摘　　要

We fabricated spin valves of NiFe/Al2O3/single layer graphene (SLG)/Co, NiFe/SLG/Co, and NiFe/Al2O3/Co. In the fabrication, lithography processes using e-beam resist or photoresist were avoided to obtain residue-free clean junctions in all devices. While all three types of magnetic junctions showed a distinctly clear spin valve effect from 10 to 300 K, the polarity of magnetoresistance (MR) revealed different signs. A negative MR value (-1.6% at 10 K) was observed in the spin valve effect for the junction with the Al2O3/SLG interlayer, however the other types of junctions showed conventional positive MR values. The positive or negative MR signs are explained by the Julliere model of spin-dependent tunneling.