Magnetotransport Properties Of Carbon Based Magnetic Composite Films

Amorphous carbon-based magnetic composite films have attracted much attentiondue to several advantages, including long spin diffusion length and relaxation time,tunable electrical properties, and interesting interface phenomena. Thanks to the uniqueelectronic structure and rich physical properties, the electrical transport properties andmagnetoresistance effects of the carbon-based magnetic composite films are abundant,but sometimes are too complicated to be understood. Systematic investigation is thusneeded to provide further insights into the underlying physics for the carbon-basedmagnetic composite systems.In this thesis, Co/C granular films, Cr/C granular films and Fe3O4/a-C/n-Siheterojunctions were fabricated by facing-target magnetron sputtering. Themicrostructure, magnetic properties, electrical transport properties andmagnetoresistance effects were studied systematically.In Co/C films, it was found that as Co content increases, the isolated Co particlesin the a-C matrix gradually connect with each other, and consequently the electricaltransport of the Co/C films changes from insulating to metallic conduction. Theinsulating samples show spin-dependent negative magnetoresistance which is related tothe alignment of the spins at the interface between Co particles and carbon matrix.Besides cotunneling effect, higher interfacial spin polarization, which is probablyinduced by the hybridization between Co3d and C2p electrons, also contributes to thelarge negative magnetoresistance. Metallic samples show the competition between thenegative MR caused by weak localization effect and positive linear MR goverened byquantum electron-electron interaction theory.Cr/C composite films show the inhomogeneous granular structure with Cr-richnanoparticles embedded in amorphous carbon matrix. The low-temperature negativespin-dependent magnetoresistance up to19.5%at90kOe and2K was observed in thesputtered Cr/C films, which strongly supports the suggestion that the interfacialhybridization between metal and carbon atoms rather than metallic nanoparticlescontributes to the efficient spin polarization in the Cr/C films. The calculated spinpolarization of the Cr/C films with7.9%Cr content was21%. Besides, the Cr/C filmswere found to exhibit ferromagnetism from2to300K. The ferromagnetism wasconsidered to be induced by exchange interactions of bound magnetic polarons formed around Cr atom impurities. The high Curie temperature may originate from the p dhybridization between Cr impurities and C atoms.Fe3O4/a-C/n-Si junctions exhibit nonlinear and slightly asymmetric current–voltagebehaviors. For the junctions with thinner a-C layers, the current transport is primarydominated by thermionic field emission. The tunneling was found as the dominantmechanism for the junctions with thicker a-C layers. Compared to Fe3O4/n-Si junctions,the positive magnetoresistance in the Fe3O4/a-C/n-Si junctions was enhancedsignificantly. A possible mechanism for the enhancement of magnetoresistance is theheavier intermixing between Fe3O4and a-C layer with the thicker a-C layer, which leadsto a stronger scattering at the interface.