Out-of-plane magnetoresistance in ferromagnet/graphene/ferromagnet spin-valve junctions

Abstract

Out-of-plane spin-injection and detection through naturally stacked graphene layers were investigated in ferromagnet/graphene/ferromagnet (FGF) junctions. We obtained a maximum magnetoresistance (MR) of 4.6% at $T=4.2$ K in the junction of a four-layer graphene insertion, having a very small area$\ensuremath{-}$junction-resistance product of 0.2 $\ensuremath{\Omega}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}$m${}^{2}$. According to resistance-temperature and current-voltage characteristics, the graphene layer in the FGF junction acted as a metal-like insertion rather than as an insulating barrier. A lower value for the interfacial spin asymmetry coefficient ($\ensuremath{\gamma}=0.25\ifmmode\pm\else\textpm\fi{}0.05$) obtained from the fitting of variations with interfacial resistance implies that the spin-injection efficiency along the out-of-plane direction was reduced by spin-flip scattering at graphene/ferromagnet interfaces. Our results showed that highly transparent graphene/ferromagnet interfaces with crystalline ferromagnet (FM) electrodes are required to achieve higher spin-injection efficiency through the graphene layer in a FGF junction along the out-of-plane direction.