Evolution of magnetoresistance mechanisms in granular Co/C films with different conduction regimes

Co/C granular films with 3–21% Co content are fabricated by facing-target magnetron sputtering. As the Co content increases, the isolated Co particles in the a-C matrix gradually connect with each other, and consequently the electrical transport of the Co/C films changes from insulating to metallic conduction. The insulating samples show spin-dependent negative magnetoresistance (MR), which is related to the alignment of the spins at the interface between Co particles and the carbon matrix. Interfacial spin polarization P0 is related to the amount of interfacial hybridization, which is influenced by the size of sp2 carbon clusters in the a-C matrix. Metallic samples show the competition of negative MR and positive linear MR. Negative MR observed at low temperatures is caused by a weak localization effect and is suppressed when the magnetic field is above 15 kOe. Positive linear MR is observed at high fields and high temperatures, and could be explained by quantum electron–electron interaction theory. The magnetotransport properties in the Co/C films are significantly influenced by the configuration of Co particles and, consequently, the conduction regime rather than the size of sp2 carbon clusters in the a-C matrix.