Spin-polarized Interface States Research Articles

Edge-channel transport and spin-polarized interface states in folded graphene ribbons

The fabrication of devices that integrate two or more topological states simultaneously and achieve precise control over their edge state and interface states holds paramount importance. We report the quantized edge-channel transport and spin-polarized interface states in folded graphene ribbons under a magnetic field and predict that fully spin-polarized states can be generated at the interface. Our results also suggest that the system has the potential to exhibit the quantum spin Hall phase, the quantum Hall phase, and a coexistence of both phases. These phases can be manipulated by applying a bias potential and Zeeman splitting. Furthermore, the quantized conductance demonstrates a plateau at 3e2/h, which can be attributed to the presence of fully spin-polarized states. These outcomes provide a foundation for future explorations and manipulations of this novel form of edge-channel transport.

Hybridization-Induced Inversion of Spin Polarization at Rubrene/Ferromagnetic Cobalt Interface

The formation of hybrid interface states plays a crucial role in spin injection from a ferromagnetic metal into organic semiconductors. Here, we investigate the energy-level alignment and spin-polarized hybrid interface states between cobalt and rubrene using photoemission spectroscopy. We reveal that the presence of charge injection from cobalt to the rubrene molecular layer is determined by XPS and UPS measurements. We also find a spin-polarized interface state between cobalt and rubrene at 0.5 eV below the Fermi level, which can be attributed to chemical interaction between rubrene and Co(001). This hybridization state can induce spin polarization inversion at the rubrene/Co(001) interface as measured by spin-polarized UPS. The spin polarization inversion at the rubrene/Co(001) interface can be interpreted by a Zener exchange-type mechanism. On the basis of these observations, we obtain a deeper understanding of the electronic structure of the rubrene/Co(001) spinterface and show the importance for the quantitative description of rubrene/Co(001)-based organic spintronic devices.

Fluorinated Phthalocyanine Molecules on Ferromagnetic Cobalt: A Highly Polarized Spinterface

Spin-resolved photoemission spectroscopy experiments are performed on perfluorinated Co-phthalocyanine (F16CoPc) deposited onto ferromagnetic Co(001) to examine how doping (here with fluorine) of an organic semiconductor influences the interfacial electronic properties and whether the formation of highly spin-polarized interface states is possible. It is found that this latter property, initially reported for the nonfluorinated Pc, is also present for the fluorinated system Co/F16CoPc. This result shows that doping an organic semiconductor, which is an important and effective method to tailor the electronic transport properties of the molecules, does not inhibit the presence of a highly polarized spinterface.

Giant spin splitting induced by a symmetry-braking van der Waals interaction

It is shown, based on the density functional theory (DFT) calculations, that weak van der Waals interaction between monolayers of Pb and MoTe2 introduce a symmetry breaking that gives rise to spin polarized interface states. This is a departure from a conventional picture of band spin splitting in the adsorption systems as it is usually associated with the strong surface interactions e.g. covalent bonds, which results in electronic charge asymmetry in the vicinity of the adatom's nuclei. The analysis based on the effective Hamiltonian within the orbital-angular-momentum approach to the Rashba effect indicates that the observed spin splitting originates directly from the atomic spin-orbit coupling, with the orbital angular momentum unquenched due to broken inversion symmetry.

Unidirectional Spin Hall Magnetoresistance as a Tool for Probing the Interfacial Spin Polarization of Co2MnSi

Many Heusler compounds are predicted to be ferromagnetic half metals in the bulk, which makes them promising compounds for spintronics. However, for devices the transport spin polarization at specific interfaces requires optimization. We show that investigations of the unidirectional magnetoresistance provide an alternative approach to access this quantity. Based on a Wheatstone-bridge design we probed the unidirectional magnetoresistance of Co2MnSi/(Ag, Cu, or Cr)(0.5 nm)/Pt (or Ta) multilayers and separate the spin-dependent unidirectional spin Hall magnetoresistance from other contributions. We demonstrated that by the insertion of a thin epitaxial Ag layer the spin-dependent contribution is doubled corresponding to a significant increase of the transport spin polarization, which is discussed in the framework of highly spin polarized interface states.

Influence of thermal annealing on the spin injection and spin detection through Fe/GaAs interfaces

A strong bias asymmetry of the spin-injection efficiency through an epitaxial Fe/GaAs Schottky tunnel contact is observed. Low-temperature post-growth thermal annealing is shown to strongly affect the spin-injection efficiency. The annealing leads either to a reduction or an enhancement. The spin accumulation is addressed electrically in a lateral spin-valve geometry using a non-local spin-valve setup at liquid helium temperatures. A spin-injection efficiency of up to 5.5% is estimated from experimental results. The electrical properties of the Schottky tunnel diode do not reflect the bias asymmetry and the changes in the spin-injection efficiency during annealing. Formation of spin-polarized interface states (IS) close to the Fermi-level is a possible explanation. The IS will not only radically affect the spin-injection but also the spin-detection process.

Tunable Electronic and Magnetic Properties of Graphene Flake-Doped Boron Nitride Nanotubes

Carbon-doped boron nitride nanostructures including nanosheets, nanoribbons, and nanotubes have drawn enormous research attention because of their tunable electronic properties and widespread applications. In this work, we explore the electronic and magnetic properties of graphene flake-doped single-walled boron nitride nanotubes (BNNTs) on the basis of first-principles calculations. Theoretical results reveal that the band structures of these doped BNNTs can be effectively engineered by embedding graphene flakes with different sizes and shapes. Moreover, the Lieb theorem works for the triangle graphene flake-doped BNNTs, and the corresponding doped systems are ferromagnetic, originating from the spin-polarized interface states. All BNNTs embedded with the triangular graphene flakes with relatively small sizes are typical bipolar magnetic semiconductors, which can be easily tuned into half-metals by carrier doping, opening the door to their promising applications in spintronic devices.

Electronic and magnetic properties of the interface between metal-quinoline molecules and cobalt

It was recently established that spin injection from a ferromagnetic metal into an organic semiconductor depends largely on the formation of hybrid interface states. Here we investigate whether the magnetic properties of the interface between cobalt and tris(8-hydroxyquinolinato)-Al(III) (${\text{Alq}}_{3}$), the most prominent molecular candidate for organic spin-valve devices, can be modified by substituting the aluminum atom with either gallium or indium. The electronic structure of ${\text{Alq}}_{3}$, ${\text{Gaq}}_{3}$, and ${\text{Inq}}_{3}$ and the properties of their interfaces with ferromagnetic cobalt are probed experimentally, by using different photoemission spectroscopy methods, and theoretically, through density functional theory calculations. For all cases, the results highlight the presence of spin-polarized interface states. However no striking difference between the properties of the various molecules and interfaces is observed. This is a consequence of the fact that the molecules frontier orbitals are mainly localized on the ligands and they show only a negligible contribution coming from the metal ion.

Interlayer exchange coupling in iron/silicon nanostructures

A superexchange mechanism is considered for the interlayer exchange coupling in Fe/Si nanostructures. It is assumed that the most important role in this mechanism is played by iron-silicon interfaces, in whose immediate vicinity the contact-induced ferromagnetic phase of body-centered iron monosilicide forms and spin-polarized interface states arise. The magnetic coupling between iron layers is accomplished by means of the interaction of interface states through the normal-semiconductor spacer layer and involves both intraband and interband processes. The nonmonotonic character of the dependence of the interband exchange coupling on the spacer thickness and composition (the occurrence of a maximu, the change in shape, the possible sign reversal of the bilinear component) is caused by competition between the ferro-and antiferromagnetic super-exchange components and by the complex character of the spacer electronic spectrum (in particular, by the occurrence of several equivalent extrema in the semiconductor bands). This model qualitatively reproduces the main features of the mechanism of interlayer exchange coupling in real Fe/Si nanostructures.

Spin properties of quantum wells with magnetic barriers. II. Inverted band ordering and spin polarized interface states

The electronic band-edge spectrum of the interface states in the magnetic semiconductor quantum wells based on narrow-gap semiconductors with mutually inverted band arrangement is studied within the envelope-function formalism. Interface states are shown to appear in these structures in the case of overlapping bulk bands of the constituents. The hybridization between the bare $\mathrm{sp}$-electron states and the d states of the Mn atoms leads to spin splitting. The spin-splitting effect of the interface states as a function of external magnetic field, well width, band offsets, and fraction of the magnetic atoms, is studied. One essential result is that one can design a structure where the states localized at the interfaces only have one spin direction. The results give evidence of the perspective for using the magnetic semiconductor structures in spin electronics.

Two-dimensional spin-polarized states of Ag on Fe(100).

We present an experimental and theoretical investigation of the spin-dependent electronic structure of a Ag monolayer on Fe(100). By spin- and angle-resolved photoemission we identify several spin-polarized states of two-dimensional (2D) character and determine their band dispersion. The experimental results are compared with ab initio band-structure calculations. Three kinds of spin-polarized 2D states could be identified along the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}-X\ifmmode\bar\else\textasciimacron\fi{} high-symmetry direction of the surface Brillouin zone: (i) 3d-derived Fe surface states which are only weakly modified by the Ag overlayer; (ii) Fe surface states with a substantial sp character which, upon adsorption of a Ag overlayer, hybridize with the Ag atoms and form interface states; (iii) Ag-induced spin-polarized interface states which are shared by both of the constituents of the interface atoms and do not have any counterpart at the clean Fe surface.

Spin-polarized interface states at the Pd(111)/Fe(110), Pd(111)/Co(0001), and Pt(111)/Co(0001) interfaces.

The spin-resolved electronic structure of epitaxial Pd overlayers on Fe(110) and on Co(0001) and of Pt overlayers on Co(0001) has been investigated by means of spin-resolved as well as spin-integrated, angle-resolved photoemission spectroscopy. Valence-band energy-distribution curves (EDC's) for thin overlayer films show at the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} point of the surface Brillouin zone overlayer-induced peaks at -1.5 eV binding energy for Pd/Fe, -1.3 eV for Pd/Co, and -1.2 eV for Pt/Co. These states saturate in intensity for overlayer thicknesses between 1 and 1.5 atomic layers (AL) and do not disperse with $_{\mathrm{\ensuremath{\perp}}}$, the component of the photoelectron wave vector normal to the film surface. This behavior indicates interface state character for these states, which have mainly Pd 4d or Pt 5d character. The spin-resolved EDC's of these interface states reveal spin components with a peak separation of about 200\ifmmode\pm\else\textpm\fi{}50 meV, whereby the minority-spin peak is more tightly bound than the majority-spin peak in all cases. For overlayer thicknesses of about 2--3 AL the development of a second interface state near -0.6 eV binding energy with no measurable peak separation of its spin components has been observed in all systems. Above about 5 AL thickness bulk Pd(111) and Pt(111) states are found for which no spin polarization is evident.

Interface magnetization: Cu films on Ni(100)

Results of a self-consistent tight-binding calculation for one and two atomic layers of Cu on ferromagnetic Ni(100) show a Cu-induced moment reduction of about 0.33 and 0.13..mu../sub B/ per atom in the first and second Ni layers, respectively. The Cu d orbitals play almost no role. Even a single layer of Cu gives an almost idea (surface-insensitive) interface behavior in the Ni, so spin-polarized interface states should be easily observable by photoemission.