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Keavney, David

Publications and source records attributed to Keavney, David.

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

X‐ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin‐Coupled Molybdenum(III) Center

Abstract Nitrogenase enzymes catalyze the reduction of atmospheric dinitrogen to ammonia utilizing a Mo‐7Fe‐9S‐C active site, the so‐called FeMoco cluster. FeMoco and an analogous small‐molecule (Et 4 N)[(Tp)MoFe 3 S 4 Cl 3 ] cubane have both been proposed to contain unusual spin‐coupled Mo III sites with an S (Mo)=1/2 non‐Hund configuration at the Mo atom. Herein, we present Fe and Mo L 3 ‐edge X‐ray magnetic circular dichroism (XMCD) spectroscopy of the (Et 4 N)[(Tp)MoFe 3 S 4 Cl 3 ] cubane and Fe L 2,3 ‐edge XMCD spectroscopy of the MoFe protein (containing both FeMoco and the 8Fe‐7S P‐cluster active sites). As the P‐clusters of MoFe protein have an S =0 total spin, these are effectively XMCD‐silent at low temperature and high magnetic field, allowing for FeMoco to be selectively probed by Fe L 2,3 ‐edge XMCD within the intact MoFe protein. Further, Mo L 3 ‐edge XMCD spectroscopy of the cubane model has provided experimental support for a local S (Mo)=1/2 configuration, demonstrating the power and selectivity of XMCD.

Kowalska, Joanna K.↗
DOE OSTI2019

X‐ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin‐Coupled Molybdenum(III) Center

Abstract Nitrogenase enzymes catalyze the reduction of atmospheric dinitrogen to ammonia utilizing a Mo‐7Fe‐9S‐C active site, the so‐called FeMoco cluster. FeMoco and an analogous small‐molecule (Et 4 N)[(Tp)MoFe 3 S 4 Cl 3 ] cubane have both been proposed to contain unusual spin‐coupled Mo III sites with an S (Mo)=1/2 non‐Hund configuration at the Mo atom. Herein, we present Fe and Mo L 3 ‐edge X‐ray magnetic circular dichroism (XMCD) spectroscopy of the (Et 4 N)[(Tp)MoFe 3 S 4 Cl 3 ] cubane and Fe L 2,3 ‐edge XMCD spectroscopy of the MoFe protein (containing both FeMoco and the 8Fe‐7S P‐cluster active sites). As the P‐clusters of MoFe protein have an S =0 total spin, these are effectively XMCD‐silent at low temperature and high magnetic field, allowing for FeMoco to be selectively probed by Fe L 2,3 ‐edge XMCD within the intact MoFe protein. Further, Mo L 3 ‐edge XMCD spectroscopy of the cubane model has provided experimental support for a local S (Mo)=1/2 configuration, demonstrating the power and selectivity of XMCD.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure

In the bulk, LaCoO 3 (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. Here, these results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO 6 octahedra in combination with the ferroelastic order parameter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Switchable orbital polarization and magnetization in strained LaCo O 3 films

Strain engineering of epitaxial heterostructures offers opportunities to control the orbital degree of freedom by lifting the degeneracy of e g states. In this work, we show that the orbital occupation in LaCoO 3 (LCO) films can be switched between two degenerate e g bands with epitaxial strain. The orbital polarization of nearly -100% (or 100%) is controlled by depleting occupation of the d x2-y2 (or d( 3z2-r2 ) orbital entirely in LCO for large compressive (or moderate tensile) strain. The change of electronic configuration associated with the spin-state transition modulates the magnetization of strained LCO films. Under compressive strain, LCO films exhibit a small magnetization without long-range ferromagnetic ordering. With tensile-strain increases, the magnetization of LCO films increases and reaches the maximum value when the bonding angle (Co-O-Co) is close to 180° and the in-plane bond length (Co-O) is unstretched. Our results highlight the role of octahedral distortion and spin-state crossover in tailoring the magnetic properties of cobaltite thin films, indicating an attractive route to deliberately control the orbital polarization that can be tuned to maximize the functionality of oxide heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Intercalated rare-earth metals under graphene on SiC

Intercalation of rare earth metals (RE = Eu, Dy, and Gd) is achieved by depositing the RE metal on graphene that is grown on silicon-carbide (SiC) and by subsequent annealing at high temperatures to promote intercalation. STM images of the films reveal that the graphene layer is defect free and that each of the intercalated metals has a distinct nucleation pattern. Intercalated Eu forms nano-clusters that are situated on the vertices of a Moirè pattern, while Dy and Gd form randomly distributed nano-clusters. X-ray magnetic circular dichroism (XMCD) measurements of intercalated films reveal the magnetic properties of these REs nano-clusters. Furthermore, field dependence and temperature dependence of the magnetic moments extracted from the XMCD show paramagnetic-like behaviors with moments that are generally smaller than those predicted by the Brillouin function. XMCD measurements of RE-oxides compared with those of the intercalated REs under graphene after exposure to air for months indicate that the graphene membranes protect these intercalants against oxidation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Intercalated europium metal in epitaxial graphene on SiC

X-ray magnetic circular dichroism (XMCD) reveals the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nanoclusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M 4,5 edges at T = 15 K, our samples show paramagnetic-like behavior with distinct anomaly at T ≈ 90 K, which may be related to the Nèel transition, T N = 91 K, of bulk metal Eu. Here, we find no evidence of ferromagnetism due to EuO or antiferromagnetism due to Eu 2 O 3 , indicating that the graphene layer protects the intercalated metallic Eu against oxidation over months of exposure to atmospheric environment.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural perturbations of epitaxial α-(Fe{sub 1−x}V{sub x}){sub 2}O{sub 3} thin films driven by excess oxygen near the surface

We examine the structure and composition of phase-pure epitaxial α-(Fe{sub 1−x}V{sub x}){sub 2}O{sub 3} thin films deposited on α-Al{sub 2}O{sub 3}(0001) substrates by oxygen-plasma-assisted molecular beam epitaxy for 0 ≤ x ≤ ∼0.5. The films crystallize in the corundum lattice, with vanadium substituting for iron throughout. Vanadium cations exhibit the expected 3+ charge state in the bulk, but exhibit higher valences nearer to the surface, most likely because of excess oxygen in interstitial sites near the surface. The extent of vanadium oxidation beyond the 3+ state is inversely proportional to x. The gradation of vanadium valence with depth has an impact on local bonding geometries, and could be highly significant in this material's efficiency as a photocatalyst.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Room-Temperature Multiferroic Hexagonal LuFeO3

We observed the coexistence of ferroelectricity and weak ferromagnetism at room temperature in the hexagonal phase of LuFeO3 stabilized by epitaxial thin film growth. While the ferroelectricity in hexagonal LuFeO3 can be understood as arising from its polar structure, the observation of weak ferromagnetism at room temperature is remarkable considering the frustrated triangular spin structure. An explanation of the room temperature weak ferromagnetism is proposed in terms of a subtle lattice distortion revealed by the structural characterization. The combination of ferroelectricity and weak ferromagnetism in epitaxial films at room temperature offers great potential for the application of this novel multiferroic material in next generation devices.

Cheng, Xuemei↗