52 research outputs found
Measuring Lateral Magnetic Structure in Thin Films Using Polarized Neutron Reflectometry
Polarized neutron reflectometry (PNR) has long been applied to measure the
magnetic depth profile of thin films. In recent years, interest has increased
in observing lateral magnetic structures in a film. While magnetic arrays
patterned by lithography and submicron-sized magnetic domains in thin films
often give rise to off-specular reflections, micron-sized ferromagnetic domains
on a thin film produce few off-specular reflections and the domain distribution
information is contained within the specular reflection. In this paper, we will
first present some preliminary results of off-specular reflectivity from arrays
of micron-sized permalloy rectangular bars. We will then use specular
reflections to study the domain dispersion of an exchange-biased Co/CoO bilayer
at different locations of the hysteresis loop.Comment: 10 pages, 3 figurres, PNCMI 2002, Juelich, German
Ferromagnetic Domain Distribution in Thin Films During Magnetization Reversal
We have shown that polarized neutron reflectometry can determine in a
model-free way not only the mean magnetization of a ferromagnetic thin film at
any point of a hysteresis cycle, but also the mean square dispersion of the
magnetization vectors of its lateral domains. This technique is applied to
elucidate the mechanism of the magnetization reversal of an exchange-biased
Co/CoO bilayer. The reversal process above the blocking temperature is governed
by uniaxial domain switching, while below the blocking temperature the reversal
of magnetization for the trained sample takes place with substantial domain
rotation
Annealing-Dependent Magnetic Depth Profile in Ga[1-x]Mn[x]As
We have studied the depth-dependent magnetic and structural properties of
as-grown and optimally annealed Ga[1-x]Mn[x]As films using polarized neutron
reflectometry. In addition to increasing total magnetization, the annealing
process was observed to produce a significantly more homogeneous distribution
of the magnetization. This difference in the films is attributed to the
redistribution of Mn at interstitial sites during the annealing process. Also,
we have seen evidence of significant magnetization depletion at the surface of
both as-grown and annealed films.Comment: 5 pages, 3 figure
Magnetic and chemical nonuniformity in Ga[1-x]Mn[x]As films as probed by polarized neutron and x-ray reflectometry
We have used complementary neutron and x-ray reflectivity techniques to
examine the depth profiles of a series of as-grown and annealed Ga[1-x]Mn[x]As
thin films. A magnetization gradient is observed for two as-grown films and
originates from a nonuniformity of Mn at interstitial sites, and not from local
variations in Mn at Ga sites. Furthermore, we see that the depth-dependent
magnetization can vary drastically among as-grown Ga[1-x]Mn[x]As films despite
being deposited under seemingly similar conditions. These results imply that
the depth profile of interstitial Mn is dependent not only on annealing, but is
also extremely sensitive to initial growth conditions. We observe that
annealing improves the magnetization by producing a surface layer that is rich
in Mn and O, indicating that the interstitial Mn migrates to the surface.
Finally, we expand upon our previous neutron reflectivity study of
Ga[1-x]Mn[x]As, by showing how the depth profile of the chemical composition at
the surface and through the film thickness is directly responsible for the
complex magnetization profiles observed in both as-grown and annealed films.Comment: Now Published in Physical Review
Enantioselective, intermolecular benzylic C–H amination catalysed by an engineered iron-haem enzyme
C–H bonds are ubiquitous structural units of organic molecules. Although these bonds are generally considered to be chemically inert, the recent emergence of methods for C–H functionalization promises to transform the way synthetic chemistry is performed. The intermolecular amination of C–H bonds represents a particularly desirable and challenging transformation for which no efficient, highly selective, and renewable catalysts exist. Here we report the directed evolution of an iron-containing enzymatic catalyst—based on a cytochrome P450 monooxygenase—for the highly enantioselective intermolecular amination of benzylic C–H bonds. The biocatalyst is capable of up to 1,300 turnovers, exhibits excellent enantioselectivities, and provides access to valuable benzylic amines. Iron complexes are generally poor catalysts for C–H amination: in this catalyst, the enzyme's protein framework confers activity on an otherwise unreactive iron-haem cofactor
Characterization of the Decaheme c-Type Cytochrome OmcA in Solution and on Hematite Surfaces by Small Angle X-Ray Scattering and Neutron Reflectometry
The outer membrane protein OmcA is an 85 kDa decaheme c-type cytochrome located on the surface of the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1. It is assumed to mediate shuttling of electrons to extracellular acceptors that include solid metal oxides such as hematite (α-Fe2O3). No information is yet available concerning OmcA structure in physiologically relevant conditions such as aqueous environments. We purified OmcA and characterized its solution structure by small angle x-ray scattering (SAXS), and its interaction at the hematite-water interface by neutron reflectometry. SAXS showed that OmcA is a monomer that adopts a flat ellipsoidal shape with an overall dimension of 34 × 90 × 65 Å3. To our knowledge, we obtained the first direct evidence that OmcA undergoes a redox state-dependent conformational change in solution whereby reduction decreases the overall length of OmcA by ∼7 Å (the maximum dimension was 96 Å for oxidized OmcA, and 89 Å for NADH and dithionite-reduced OmcA). OmcA was also found to physically interact with electron shuttle molecules such as flavin mononucleotide, resulting in the formation of high-molecular-weight assemblies. Neutron reflectometry showed that OmcA forms a well-defined monomolecular layer on hematite surfaces, where it assumes an orientation that maximizes its contact area with the mineral surface. These novel insights into the molecular structure of OmcA in solution, and its interaction with insoluble hematite and small organic ligands, demonstrate the fundamental structural bases underlying OmcA's role in mediating redox processes
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