Linear magnetoresistance of graphene in contact with inhomogeneous disordered graphitic carbon

We synthesized graphene via chemical vapor deposition(CVD) on platinum foils and transferred graphene to Si$_{\mathrm{3}}$N$_{\mathrm{4}}$ membranes for inspection by transmission electron microscope (TEM), or to SiO$_{\mathrm{2}}$/Si for fabricating field-effect transistors. Dark-field TEM shows that the graphene is decorated with disordered (nanocrystalline) graphitic carbon which is spatially inhomogeneous. The impurity layer can easily be mistaken for a second graphene layer in optical microscopy. Atomic force microscopy shows that impurities form between graphene and Pt, supporting a ``growth-from-below'' model. The impurity-decorated graphene exhibits linear magnetoresistance (LMR) which is carrier-density-dependent and nonsaturating up to 8 Tesla. No LMR is observed with graphene samples with little impurities, or in exfoliated graphene. We understand the LMR as due to an effective inhomogeneous random-resistor network arising from the spatially inhomogeneous nature of the graphene/impurity system. The results may shed light on the previously-observed LMR in graphene on Si-face SiC.