Spintronics using C60 fullerenes

Abstract

The research described in this thesis focuses on spin polarized transport through thin films of C60 fullerene molecules in vertical spin valves, spin polarized hybridization effects at C60/bcc-Fe(001) interfaces, and the ordering of C60 molecules on epitaxial bcc-Fe(001) thin films. The spin polarized transport in C60-based spin valves (vertical Co/Al2O3/C60/NiFe junctions), were studied by both experiments and modelling. The experimental results have been interpreted using a model based on a superposition of direct- and multi-step tunnelling via a Gaussian density of intermediate states in the C60 layer. Analogous to conductivity mismatch in the diffusive regime, the junction magnetoresistance drops continuously as the amount of intermediate tunnelling steps increases, regardless of the spin lifetime and spin diffusion length. The electronic and magnetic properties of the interface between C60 molecules and a Fe(001) surface. The C K-edge X-ray absorption spectroscopy show charge transfer from the Fe substrate to C60 molecules, and strong interfacial bonding between C60 and Fe. The hybridization between C60 Π (Π *) orbitals and Fe 3d wave functions leads to a distinct, oscillatory magnetic moment (changing sign for different states) of C60-derived interfacial electronic states, as is evident from C K-edge x-ray magnetic circular dichroism (XMCD) spectra. A combined computational (density functional theory) and experimental study on the magnetic properties of monolayers bcc- C60/Fe(001)interfaces is presented. The calculations and experimental results show that the hybrid interface states lead to magnetic moments on the C60 molecules that are coupled antiparallel to the Fe moments, and that the adsorption of C60 on Fe(001) reduces the magnetic moment of the top Fe layer by ∼6%. The crystallinity and molecular ordering of C60 films on epitaxial Fe/MgO(001) surfaces, using X-ray diffraction (XRD) and scanning tunnelling microscopy (STM) has been studied. XRD analysis of 100 nm thick C60 molecular films shows that a strongly (111)-textured layer is obtained when growth is carried out at elevated temperature (100 °C). STM measurements show that C60 forms a highly ordered monolayer on Fe(001).The molecules are arranged in a quasi-hexagonal pattern that superficially resembles the (111) plane of bulk fcc C60 but shows a considerable lattice mismatch with that structure.