Spintronics using C60 fullerenes: Interface and devices

T. Lan Ahn Tran

    Research output: ThesisPhD Thesis - Research UT, graduation UT

    54 Downloads (Pure)

    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.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • University of Twente
    Supervisors/Advisors
    • van der Wiel, Wilfred Gerard, Supervisor
    • de Jong, Machiel Pieter, Co-Supervisor
    Thesis sponsors
    Award date4 Dec 2013
    Place of PublicationEnschede
    Publisher
    Print ISBNs978-90-365-1225-1
    DOIs
    Publication statusPublished - 4 Dec 2013

    Fingerprint

    fullerenes
    molecules
    magnetic moments
    scanning tunneling microscopy
    x rays
    magnetic properties
    theses
    thin films
    diffusion length
    electronics
    diffraction
    dichroism
    crystallinity
    absorption spectroscopy
    charge transfer
    wave functions
    density functional theory
    moments
    orbitals
    life (durability)

    Keywords

    • EWI-24400
    • METIS-298996
    • IR-88046

    Cite this

    Tran, T. Lan Ahn. / Spintronics using C60 fullerenes: Interface and devices. Enschede : University of Twente, 2013. 118 p.
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    title = "Spintronics using C60 fullerenes: Interface and devices",
    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.",
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    year = "2013",
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    Tran, TLA 2013, 'Spintronics using C60 fullerenes: Interface and devices', Doctor of Philosophy, University of Twente, Enschede. https://doi.org/10.3990/1.9789036512251

    Spintronics using C60 fullerenes: Interface and devices. / Tran, T. Lan Ahn.

    Enschede : University of Twente, 2013. 118 p.

    Research output: ThesisPhD Thesis - Research UT, graduation UT

    TY - THES

    T1 - Spintronics using C60 fullerenes: Interface and devices

    AU - Tran, T. Lan Ahn

    N1 - http://eprints.ewi.utwente.nl/24400

    PY - 2013/12/4

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    N2 - 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.

    AB - 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.

    KW - EWI-24400

    KW - METIS-298996

    KW - IR-88046

    U2 - 10.3990/1.9789036512251

    DO - 10.3990/1.9789036512251

    M3 - PhD Thesis - Research UT, graduation UT

    SN - 978-90-365-1225-1

    PB - University of Twente

    CY - Enschede

    ER -