Magnetotransport properties of mesoscopic graphite spin valves

We report the magnetoresistance (MR) properties of quasi-two-dimensional mesoscopic graphite (MG) spin valve devices consisting of MG flakes contacted by ferromagnetic (FM) electrodes. For devices in which an ultrathin magnesium oxide (MgO) tunnel barrier is inserted at the FM/MG interface, the spin valve effect has been observed, with MR magnitudes up to 12% at 7 K and signals persisting up to temperatures as high as 60 K. In contrast, the spin valve effect has not been seen in devices without MgO, suggesting the importance of spin-dependent interfacial resistance for spin injection into MG. In addition, an investigation of the voltage bias dependence and gate voltage dependence of MR has been performed.