Negative and positive magnetoresistance of graphenes at room-temperature

Graphenes were prepared via micromechanical cleavage of natural graphite flakes and transferred onto a silicon wafers with a 285 nm SiO2 thick film on the surface. Nickel was used as the electrodes to study the magnetoresistance of graphenes devices at room temperature. We find that the magnetoresistances of graphenes shows positive sign while the magnetoresistances of pure nickel have negative sign at the same external magnetic field when a Z-direction magnetic field (0.24 T, graphenes on the X-Y plane) is applied. The magnitude of magnetoresistance of graphenes is about +0.20% and 5 times larger than that of the same thickenss pure nickel film. When the magnetic field is along the X-direction (-0.26 T, perpendicular to Y-direction current), the magnetoresistance of graphenes and nickel film both have negative sign and value of -0.04% and -0.03%, respectively. The magnetoresistance of graphenes and nickel film both have positive sign and value of +0.09% and +0.04%, when the magnetic field is along the Y-direction (-0.24 T, parallel to Y-direction current), respectively. The mechanism of these observations is attributed to the edge ferromagnetism of graphenes. Our work shows that grapheses may play an important role in spin device operated at room temperature.