Ezana Negusse

Brookhaven National Laboratory, New York City, New York, United States

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Publications (32)74.44 Total impact

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    ABSTRACT: Charge transfer effects which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La0.7Sr0.3MnO3 on Nb-doped SrTiO3(001) deposited via pulsed laser depositionootnotetextJ.-S. Lee et al., Phys. Rev. Lett. 105, 257204 (2010). We observe a similar effect for La0.7Sr0.3MnO3 grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO3 (STO) substrates. The reversed magnetic state occurrs in both samples, and a region of slightly increased charge density was revealed with x-ray reflectivity. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: ˜125K for the Nb:STO substrate and ˜240K for the undoped STO substrate. High resolution x-ray diffraction reveals a subtle relationship with the cubic-to-tetragonal structural transition of the STO substrate at ˜105 K. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide filmsootnotetextJ.-S. Lee et al., J. Phys. D: Appl. Phys. 44, 245002 (2011).
    02/2012;
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    ABSTRACT: The transition between Kondo and Coulomb blockade effects in discontinuous double magnetic tunnel junctions is explored as a function of the size of the CoPt magnetic clusters embedded between AlOx tunnel barriers. A gradual competition between cotunneling enhancement of the tunneling magnetoresistance (TMR) and the TMR suppression due to the Kondo effect has been found in these junctions, with both effects having been found to coexist even in the same sample. It is possible to tune between these two states with temperature (at a temperature far below the cluster blocking temperature). In addition, when further decreasing the size of the CoPt clusters, another gradual transition between the Kondo effect and direct tunneling between the electrodes takes place. This second transition shows that the spin-flip processes found in junctions with impurities in the barrier are in fact due to the Kondo effect. A simple theoretical model able to account for these experimental results is proposed.
    Physical review. B, Condensed matter 01/2012; 85(21). · 3.77 Impact Factor
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    ABSTRACT: The local atomic structure of graphene oxide has been probed using synchrotron radiations. Detailed investigations of recently proposed simplistic model of graphene oxide using x-ray absorption near edge spectroscopy have been performed. X-ray diffraction measurements and calculations indicate loss of coherence between graphene-like layers. However, larger in-plane structural coherence is understood to be present. Selected area electron diffraction measurements indicate the presence of graphitic regions in graphene oxide which is expected to produce interesting confinement effects in graphene oxide which could be important for the development of tunable electronic and photonic devices.
    Applied Physics Letters 07/2011; 99(1):013104-013104-3. · 3.79 Impact Factor
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    ABSTRACT: The polar discontinuity which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La0.7Sr0.3MnO3 on Nb-doped SrTiO3(0 0 1) deposited via pulsed laser deposition (Lee et al 2010 Phys. Rev. Lett. 105 257204). Here, we report on a similar effect for La0.7Sr0.3MnO3 grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO3 (STO) substrates. The reversed magnetic state occurred in both samples, while enriched Mn3+ regions (intermediate layer) are slightly different. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: ~125 K for the Nb : STO substrate and ~240 K for the undoped STO substrate. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide films.
    Journal of Physics D Applied Physics 06/2011; 44(24). · 2.53 Impact Factor
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    ABSTRACT: The polar discontinuity which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La{sub 0.7}Sr{sub 0.3}MnO on Nb-doped SrTiO(0 0 1) deposited via pulsed laser deposition (Lee et al 2010 Phys. Rev. Lett. 105 257204). Here, we report on a similar effect for La{sub 0.7}Sr{sub 0.3}MnO grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO (STO) substrates. The reversed magnetic state occurred in both samples, while enriched Mn{sup 3+} regions (intermediate layer) are slightly different. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: 125 K for the Nb : STO substrate and 240 K for the undoped STO substrate. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide films.
    Journal of Physics D Applied Physics 05/2011; 44(24). · 2.53 Impact Factor
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    ABSTRACT: The development of new materials with large room temperature spin polarizations and small conductivity mismatches with semiconductors is key for more complex spintronics devices. CoFe2O4 has a high Curie temperature (TC = 793 K), a large predicted spin polarization, and, when doped with iron, a conductivity similar to semiconductors; however, the magnetic properties of thin films are different from the bulk. To investigate the effect of strain, Co1-xFe2+xO4 thin films (0
    03/2011;
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    ABSTRACT: The local atomic structure of graphene oxide has been probed using synchrotron radiations. Detailed investigations of recently proposed simplistic model of graphene oxide using x-ray absorption near edge spectroscopy have been performed. X-ray diffraction measurements and calculations indicate loss of coherence between graphene-like layers. However, larger in-plane structural coherence is understood to be present. Selected area electron diffraction measurements indicate the presence of graphitic regions in graphene oxide which is expected to produce interesting confinement effects in graphene oxide which could be important for the development of tunable electronic and photonic devices.
    Applied Physics Letters 01/2011; 99(1). · 3.79 Impact Factor
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    ABSTRACT: The electronic, magnetic and transport properties of iron-doped cobalt ferrite (Co1-xFe2+xO4) thin films grown epitaxially on MgO (001) substrates are investigated by soft x-ray absorption and photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, superconducting quantum interference device magnetometry, and resistivity measurements. The crystal structure for Co1-xFe2+xO4 is determined to be nearly inverse spinel, with the degree of inversion increasing for increased doping until it becomes fully inverse spinel for Fe3O4. The doped iron cations have a valency of 2+ and reside solely on octahedral sites, which allows for conduction owing to hopping between Fe2+ and Fe3+ octahedral cations. The addition of Fe2+ cations increases the electron density of states near the Fermi energy, shifting the Fermi level from 0.75 to 0 eV with respect to the top of the valence band, as the doping increases from x = 0.01 to 1. This change in electronic structure results in a change in resistivity by over two orders of magnitude. In contrast, the magnetic properties of CoFe2O4 thin films, characterized by a significantly reduced saturation magnetization compared to the bulk and large magnetic anisotropies, are affected less significantly by doping in the range from 0 to 0.63. These results show that Co1-xFe2+xO4 has tunable electronic properties while maintaining magnetic properties similar to CoFe2O4.
    Physical Review B 01/2011; 83(3):35121-10. · 3.66 Impact Factor
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    ABSTRACT: The magnetic properties of iron-doped cobalt ferrite (Co1-xFe2+xO4) (001) thin films grown epitaxially on MgO (001) substrates are investigated by superconducting quantum interference device magnetometry and soft x-ray magnetic linear and circular dichroisms. All Co1-xFe2+xO4 (0.01
    Physical review. B, Condensed matter 01/2011; 84. · 3.77 Impact Factor
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    ABSTRACT: Chromium-doped gallium sesquiselenide, Cr:GaSe, is a member of a new class of dilute magnetic semiconductors exploiting intrinsic vacancies in the host material. The correlation among room-temperature ferromagnetism, surface morphology, electronic structure, chromium concentration, and local chemical and structural environments in Cr:GaSe films grown epitaxially on silicon is investigated with magnetometry, scanning tunneling microscopy, photoemission spectroscopy, and x-ray absorption spectroscopy. Inclusion of a few percent chromium in GaSe results in laminar, semiconducting films that are ferromagnetic at room temperature with a magnetic moment 4{sub B}/Cr. The intrinsic-vacancy structure of defected-zinc-blende -GaSe enables Cr incorporation in a locally octahedral site without disrupting long-range order, determined by x-ray absorption spectroscopy, as well as strong overlap between Cr 3d states and the Se 4p states lining the intrinsic-vacancy rows, observed with photoemission. The highest magnetic moment per Cr is observed near the solubility limit of roughly one Cr per three vacancies. At higher Cr concentrations, islanded, metallic films result, with a magnetic moment that depends strongly on surface morphology. The effective valence is Cr{sup 3+} in laminar films, with introduction of Cr° upon islanding. A mechanism is proposed for laminar films whereby ordered intrinsic vacancies mediate ferromagnetism.
    Physical review. B, Condensed matter 01/2011; 83(4):45203-9. · 3.77 Impact Factor
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    ABSTRACT: It is common to find a thin ( ∼ 0.5 nm) layer of Mg deposited prior to the MgO tunnel barrier in crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions, due to the improved device performance that results. However, despite their common usage, the reasons why such layers are effective are unclear. We use structures that model the lower electrode of such devices to show that a suitably thick Mg insert layer enhances the crystal quality of both MgO and CoFe(B), permits interfacial oxides to reduce back to a metallic ferromagnetic state, and hence improves magnetic switching of the CoFe(B) electrode, properties which are inextricably linked to device performance.
    Applied Physics Letters 12/2010; 97(25):252502-252502-3. · 3.79 Impact Factor
  • Sumit Saxena, Trevor A. Tyson, Ezana Negusse
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    ABSTRACT: A study of the local structure of graphene oxide is presented. Graphene oxide is understood to be partially oxidized graphene. Absorption peaks corresponding to interlayer states suggest the presence of pristine graphitic nanoislands in graphene oxide. Site-projected partial density of states of carbon atoms bonded to oxygen atoms suggests that the broadening of the peak due to interlayer states in the carbon K-edge spectrum of graphene oxide is predominantly due to formation of epoxide linkages. Density functional theory suggests that multilayers of graphene oxide are linked by peroxide-like linkages.Keywords (keywords): graphene oxide; nanotechnology; XANES; density functional theory; electronic structure
    The Journal of Physical Chemistry Letters. 11/2010;
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    Sumit Saxena, Trevor A Tyson, Ezana Negusse
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    ABSTRACT: A study of the local structure of graphene oxide is presented. Graphene oxide is understood to be partially oxidized graphene. Absorption peaks corresponding to interlayer states suggest the presence of pristine graphitic nanoislands in graphene oxide. Site-projected partial density of states of carbon atoms bonded to oxygen atoms suggests that the broadening of the peak due to interlayer states in the carbon K-edge spectrum of graphene oxide is predomi-nantly due to formation of epoxide linkages. Density functional theory suggests that multilayers of graphene oxide are linked by peroxide-like linkages. SECTION Nanoparticles and Nanostructures G raphene oxide 1 (GO) is currently an intense area of research due to its electronic properties. Preliminary investigations have demonstrated its potential appli-cations in technologically important multidisciplinary re-search such as the development of transparent and flexible electronic 2 devices and nanomechanical devices, 3 and func-tionalized GO has been used in drug delivery of water-insoluble cancer drugs. 4 Recent studies have shown nonlinear effects in GO in the nanosecond and picoseconds domains, 5
    Journal of Physical Chemistry Letters 11/2010; 1:3433-3437. · 6.59 Impact Factor
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    ABSTRACT: The degradation mechanisms in ion conducting materials, including solid oxide fuel cells (SOFC), are still of high interest in current energy research, especially with regards to material and interface stability, fuel impurities and impurities originating from sealing or interconnect materials. A common practice is the use of symmetric half-cells (e.g. cathode/electrolyte/cathode) to determine interface stability via cross section Energy Dispersive x-ray Spectroscopy line scans and overpotential magnitude and degradation via AC-impedance spectroscopy. Using these electrically driven half-cells, we have developed a new method to directly measure degradation due to oxygen ion flow through ion conducting materials and their associated interface structures. By using X-ray absorption spectroscopy of cells before and after oxygen ion flow (800 C for 100 hours), we determined that the valence state of Cr that migrated from a metallic interconnect into the porous cathode changes in valence from +3 (Cr2O3) to +6 (CrO3), depending on the direction of the oxygen ion flow. This observation is strong evidence of the influence of the oxygen ion flux on the degradation mechanisms of ion conducting materials.
    03/2010;
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    ABSTRACT: We have investigated the atomic structure of graphene oxide by DFT calculations. Our spin restricted ab-initio density functional calculations have shown that the oxygen link to the graphene basal plane using epoxide bonds. The flat graphene sheets are found to deform by buckling due to formation of C -- O -- C linkages normal to the plane of the graphene sheets. The calculations were compared with x-ray diffraction and x-ray spectroscopic measurements. Tentative models of the structure are proposed based on the combined DFT and experimental data.
    03/2010;
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    ABSTRACT: We have studied the effect of annealing on the interface magnetization in a CoFeB/MgO structure which models the lower electrode in a magnetic tunnel junction device. We find that MgO deposition causes Fe to diffuse toward the CoFeB/MgO interface, where it preferentially bonds with oxygen to form a Fe-O-rich interfacial region with reduced magnetization. After annealing at 375 °C the compositional inhomogeneity remains; Fe is reduced back to a ferromagnetic metallic state and the full interfacial magnetization is regained.
    Applied Physics Letters 02/2010; 96(9):092501-092501-3. · 3.79 Impact Factor
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    ABSTRACT: Investigations of the impact of Cr2O3 overlayers on the oxygen self diffusion in two SOFC materials were conducted to gain insight into the Cr poisoning mechanism at the cathode side of solid oxide fuel cells (SOFCs) with stainless steel interconnects. High density Y0.15Zr0.85O2 (YSZ) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) sintered pellets were covered with 3 to 30nm Cr overlayers that were subsequently oxidized, forming Cr2O3. Standard 18O tracer diffusion experiments at 800°C were performed and ToF-SIMS profiling revealed that the oxygen ion diffusion coefficients were unaffected by the thin Cr2O3 overlayers, which is predictable since they are a bulk property, but the extracted effective surface exchange coefficients varied with Cr2O3 overlayer thickness. Solid-state reaction measurements and electronic structure considerations concerning the surface exchange, led to the conclusion that the observed oxygen uptake hindrance for Cr2O3 capped LSCF and the slight increase of the surface exchange coefficient for Cr2O3 capped YSZ can be attributed to the electronic properties of Cr2O3. A critical thickness for Cr2O3 was determined to be 12nm where the transition from decreasing cathode-performance to a Cr2O3-property-governed regime occurs.
    Solid State Ionics 01/2010; 181(13):640-645. · 2.05 Impact Factor
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    ABSTRACT: We present work done on EuO films with thicknesses varying from 10 to 60 Å grown as a stepped wedge on Si / SiO <sub>2</sub>/ Cr (20 Å )/ Cu (90 Å ) and capped with Y(20 Å)/Al(80 Å). The films were characterized by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the europium M<sub>5</sub> and copper L<sub>3</sub> edges. The films' high quality and consistent magnetic properties were confirmed by superconducting quantum interference device magnetometry, which revealed a constant saturation moment independent of film thickness. XAS at the Cu  L<sub>3</sub> edge showed that the bottom Cu electrode is metallic (oxidation free). We report an XMCD intensity of 52% (±4.3), in excellent agreement with theoretical calculations.
    Journal of Applied Physics 05/2009; · 2.21 Impact Factor
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    ABSTRACT: A large exchange splitting of the conduction band in ultrathin films of the ferromagnetic semiconductor EuO was determined quantitatively, by using EuO as a tunnel barrier and fitting the current-voltage characteristics and temperature dependence to tunneling theory. This exchange splitting leads to different tunnel barrier heights for spin-up and spin-down electrons and is large enough to produce a near-fully spin-polarized current. Moreover, the magnetic properties of these ultrathin films (<6 nm) show a reduction in Curie temperature with decreasing thickness, in agreement with theoretical calculation [R. Schiller, Phys. Rev. Lett. 86, 3847 (2001)10.1103/Phys. Rev. Lett.86.3847].
    Physical Review Letters 10/2008; 101(14):147201. · 7.73 Impact Factor
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    ABSTRACT: We have measured the field dependent roughness of the magnetic interface of CoFe and CoFeB films covered by MgO by using diffuse X-ray resonant magnetic scattering (diffuse-XRMS) of circular polarized light. The samples studied were 3.0 nm of either a Co(70)Fe(30) or a Co(60)Fe(20)B(20) film covered by 1.8 nm MgO created by UHV sputtering system (ANELVA C-7100). By comparing the specular scattering map and the diffuse scattering map for a large range of incidence angles for photons resonantly tuned near the Co L3-edge as the applied magnetic field is swept through the coercive field, we have determined the chemical and magnetic roughness as a function of applied field. For the CoFe films, the magnetic scattering of the X-rays increases significantly as the film passes through the coercive field where the magnetic in-plane correlations are relatively short range, indicating the presence of small magnetic domains during moment reversal. For the CoFeB films, there is no significant increase in magnetic scattering at the coercive field, consistent with large domain switching.
    03/2008;