Magnetic graphene nanocomposites: electron conduction, giant magnetoresistance and tunable negative permittivity

Magnetic graphene nanocomposites (MGNCs) with surface-adhered magnetic nanoparticles (NPs) are synthesized by a facile thermal-decomposition method. Two different sized graphenes (Gra-10 and Gra-40) are used. The stacking of a few layers of NPs is revealed by the AFM observation in the nanocomposites, especially with a higher particle loading. The TEM observations show that the average particle size increases from 12.1 to 17.4 nm with increasing particle loading from 2 to 10% on Gra-10 substrate. The NPs exhibit a core@shell structure with an iron core and iron oxide shell, confirmed by high resolution TEM, selected area electron diffraction and X-ray diffraction analysis. The graphene size and particle loading dependent behavior such as dielectric permittivity, electrical conductivity, magnetization and giant magnetoresistance (GMR) are observed. The electrical conductivity has been significantly changed in the different sized graphenes after coating with NPs (conductivity: Gra-10 > NPs/Gra-10; Gra-40 < NPs/Gra-40). The MR is observed to vary from 38 to 64% at 130 K, and even higher MR of about 46-72% is observed at 290 K. More interestingly, the dielectric permittivity can be tuned from negative to positive at high frequency with increasing particle loading. All the results indicate that graphene with smaller size obtains superior properties than the one with larger size.