Magnetoresistance in Vertical Metal-Graphene-Metal Junctions

Predictions of spin filtering at graphene interfaces have prompted various experimental efforts to measure magnetoresistance and spin polarization of currents at graphene-metal interfaces. However, different experimental geometries and fabrication methods have resulted in different observed behaviors, from metallic transport to tunneling and from tunnel magnetoresistance to spin splitting. We review these results and present measurements and modeling of our own high-quality graphene junctions. We fabricate graphene junctions on highly crystalline, lattice-matched substrates by chemical vapor deposition and in-situ electron-beam evaporation of the top contact metal, followed by wafer-scale device patterning methods. We compare the behavior of graphene junctions with three different ferromagnetic bottom-top electrode combinations: NiFe-Fe, NiFe-Co and NiFe-Ni and elucidate the top interface transport mechanism by inserting a copper spacer layer between the graphene and ferromagnetic film.