Andrew Doran

Mechanism of calcite co-orientation in the sea urchin tooth.

Sea urchin teeth are remarkable and complex calcite structures, continuously growing at the forming end and self-sharpening at the mature grinding tip. The calcite (CaCO(3)) crystals of tooth components, plates, fibers, and a high-Mg polycrystalline matrix, have highly co-oriented crystallographic axes. This ability to co-orient calcite in a mineralized structure is shared by all echinoderms. However, the physico-chemical mechanism by which calcite crystals become co-oriented in echinoderms remains enigmatic. Here, we show differences in calcite c-axis orientations in the tooth of the purple sea urchin ( Strongylocentrotus purpuratus ), using high-resolution X-ray photoelectron emission spectromicroscopy (X-PEEM) and microbeam X-ray diffraction (muXRD). All plates share one crystal orientation, propagated through pillar bridges, while fibers and polycrystalline matrix share another orientation. Furthermore, in the forming end of the tooth, we observe that CaCO(3) is present as amorphous calcium carbonate (ACC). We demonstrate that co-orientation of the nanoparticles in the polycrystalline matrix occurs via solid-state secondary nucleation, propagating out from the previously formed fibers and plates, into the amorphous precursor nanoparticles. Because amorphous precursors were observed in diverse biominerals, solid-state secondary nucleation is likely to be a general mechanism for the co-orientation of biomineral components in organisms from different phyla.

Gradual Ordering in Red Abalone Nacre

Red abalone (Haliotis rufescens) nacre is a layered composite biomineral that contains crystalline aragonite tablets confined by organic layers. Nacre is intensely studied because its biologically controlled microarchitecture gives rise to remarkable strength and toughness, but the mechanisms leading to its formation are not well understood. Here we present synchrotron spectromicroscopy experiments revealing that stacks of aragonite tablet crystals in nacre are misoriented with respect to each other. Quantitative measurements of crystal orientation, tablet size, and tablet stacking direction show that orientational ordering occurs not abruptly but gradually over a distance of 50 microm. Several lines of evidence indicate that different crystal orientations imply different tablet growth rates during nacre formation. A theoretical model based on kinetic and gradual selection of the fastest growth rates produces results in qualitative and quantitative agreement with the experimental data and therefore demonstrates that ordering in nacre is a result of crystal growth kinetics and competition either in addition or to the exclusion of templation by acidic proteins as previously assumed. As in other natural evolving kinetic systems, selection of the fastest-growing stacks of tablets occurs gradually in space and time. These results suggest that the self-ordering of the mineral phase, which may occur completely independently of biological or organic-molecule control, is fundamental in nacre formation.

Tailoring the topology of an artificial magnetic skyrmion

Despite theoretical predictions, it remains an experimental challenge to realize an artificial magnetic skyrmion whose topology can be well controlled and tailored so that its topological effect can be revealed explicitly in a deformation of the spin textures. Here we report epitaxial magnetic thin films in which an artificial skyrmion is created by embedding a magnetic vortex into an out-of-plane aligned spin environment. By changing the relative orientation between the central vortex core polarity and the surrounding out-of-plane spins, we are able to control and tailor the system between two skyrmion topological states. An in-plane magnetic field is used to annihilate the skyrmion core by converting the central vortex state into a single domain state. Our result shows distinct annihilation behaviour of the skyrmion core for the two different skyrmion states, suggesting a topological effect of the magnetic skyrmions in the core annihilation process.

Observation of injection and pinning of domain walls in magnetic nanowires using photoemission electron microscopy

Photoemission electron microscopy is used to explore the injection and pinning of magnetic domain walls in 250-nm-wide, 20-nm-thick Permalloy nanowires. Domain walls are injected from a micron-sized elliptical nucleation pad at one end of the nanowire. A vortex-like structure is readily nucleated in this pad at low magnetic fields (<15Oe), whereas injection of a domain wall into the nanowire requires significantly larger fields (∼60Oe). Domain walls are pinned in the nanowire at notches patterned along the wire’s edges. The domain walls are observed to have vortex-like structures with chiralities that vary in successive experiments.

Electrically Driven Magnetic Domain Wall Rotation in Multiferroic Heterostructures to Manipulate Suspended On-Chip Magnetic Particles

In this work, we experimentally demonstrate deterministic electrically driven, strain-mediated domain wall (DW) rotation in ferromagnetic Ni rings fabricated on piezoelectric [Pb(Mg1/3Nb2/3)O3]0.66-[PbTiO3]0.34 (PMN-PT) substrates. While simultaneously imaging the Ni rings with X-ray magnetic circular dichroism photoemission electron microscopy, an electric field is applied across the PMN-PT substrate that induces strain in the ring structures, driving DW rotation around the ring toward the dominant PMN-PT strain axis by the inverse magnetostriction effect. The DW rotation we observe is analytically predicted using a fully coupled micromagnetic/elastodynamic multiphysics simulation, which verifies that the experimental behavior is caused by the electrically generated strain in this multiferroic system. Finally, this DW rotation is used to capture and manipulate micrometer-scale magnetic beads in a fluidic environment to demonstrate a proof-of-concept energy-efficient pathway for multiferroic-based lab-on-a-chip applications.

X-ray Spectromicroscopy Study of Protein Adsorption to a Polystyrene-Polylactide Blend

Synchrotron-based X-ray photoemission electron microscopy (X-PEEM) was used to study the adsorption of human serum albumin (HSA) to polystyrene-polylactide (40:60 PS-PLA, 0.7 wt %) thin films, annealed under various conditions. The rugosity of the substrate varied from 35 to 90 nm, depending on the annealing conditions. However, the characteristics of the protein adsorption (amounts and phase preference) were not affected by the changes in topography. The adsorption was also not changed by the phase inversion which occurred when the PS-PLA substrate was annealed above T(g) of the PLA. The amount of protein adsorbed depended on whether adsorption took place from distilled water or phosphate buffered saline solution. These differences are interpreted as a result of ionic strength induced changes in the protein conformation in solution.

X-ray microscopy studies of protein adsorption on a phase segregated polystyrene/polymethylmethacrylate surface. 2. Effect of pH on site preference.

X-ray photoemission electron microscopy (XPEEM) using synchrotron radiation illumination has been used to study the adsorption of human serum albumin (HSA) onto a phase segregated polystyrene/polymethylmethacrylate (PS/PMMA) blend surface from solutions of five different pH values. The absolute coverage of albumin on each of three chemically distinct components of the surface, PS domains, PMMA domains, and the interface between the domains, was determined from a quantitative analysis of C 1s image sequences. At all pH values, the preferred adsorption site is the interface. At neutral pH (7.0), albumin showed a slight preference for PS regions relative to PMMA. At strongly acidic pH (2.0) and strongly basic pH (10.0), similar amounts of albumin adsorb on the PS and PMMA regions. However, at pH 4.0, the amount of albumin adsorbed on PMMA domains is approximately 1.6 times greater than that on PS domains, while at pH 8.6 the amount of albumin adsorbed on PMMA is one-half that adsorbed on PS domains. The pH dependence of the site preference is rationalized in terms of the known changes of albumin conformation with pH [Peters, T., Jr. All About Albumin: Biochemistry, Genetics, and Medical Applications; Academic Press: New York, 1995]. We infer from our results that the site preference of albumin adsorption on PS/PMMA blends is related mainly to changes in hydrophobic interactions, which are driven by pH-dependent electrostatic effects, that is, changes to the protein surface structure as the charge on the protein changes. The results provide insight into changes in the secondary structure of albumin in acid and basic media.

Oxygen Spectroscopy and Polarization-Dependent Imaging Contrast (PIC)-Mapping of Calcium Carbonate Minerals and Biominerals

X-ray absorption near-edge structure (XANES) spectroscopy and spectromicroscopy have been extensively used to characterize biominerals. Using either Ca or C spectra, unique information has been obtained regarding amorphous biominerals and nanocrystal orientations. Building on these results, we demonstrate that recording XANES spectra of calcium carbonate at the oxygen K-edge enables polarization-dependent imaging contrast (PIC) mapping with unprecedented contrast, signal-to-noise ratio, and magnification. O and Ca spectra are presented for six calcium carbonate minerals: aragonite, calcite, vaterite, monohydrocalcite, and both hydrated and anhydrous amorphous calcium carbonate. The crystalline minerals reveal excellent agreement of the extent and direction of polarization dependences in simulated and experimental XANES spectra due to X-ray linear dichroism. This effect is particularly strong for aragonite, calcite, and vaterite. In natural biominerals, oxygen PIC-mapping generated high-magnification maps of unprecedented clarity from nacre and prismatic structures and their interface in Mytilus californianus shells. These maps revealed blocky aragonite crystals at the nacre-prismatic boundary and the narrowest calcite needle-prisms. In the tunic spicules of Herdmania momus, O PIC-mapping revealed the size and arrangement of some of the largest vaterite single crystals known. O spectroscopy therefore enables the simultaneous measurement of chemical and orientational information in CaCO3 biominerals and is thus a powerful means for analyzing these and other complex materials. As described here, PIC-mapping and spectroscopy at the O K-edge are methods for gathering valuable data that can be carried out using spectromicroscopy beamlines at most synchrotrons without the expense of additional equipment.

Cascade-like signal propagation in chains of concave nanomagnets

We lithographically control the anisotropy properties of single-domain nananomagnets for use in emerging nanomagnetic logic applications. By defining concave-shaped nanomagnets to enhance the effect of configurational anisotropy, we induce the property of dual-axis remanence needed for high-speed and reliable operation of nanomagnetic logic circuits. Magneto-optical measurements verify the anisotropy properties of isolated concave nanomagnets, and photoelectron emission microscopy measurements verify signal propagation in chains of concave nanomagnets.

Nanoscale x-ray magnetic circular dichroism probing of electric-field-induced magnetic switching in multiferroic nanostructures

The magnetic structure as well as its response to an external electric field were studied in ferrimagnetic CoFe2O4 nanopillars embedded in an epitaxial ferroelectric BiFeO3 film using photoemission electron microscopy and x-ray magnetic circular dichroism. Magnetic switching was observed in both Co and Fe magnetic sublattices after application of an electric field. About 50% of the CoFe2O4 nanopillars were measured to switch their magnetization with the electric field, implying an elastic-mediated electric-field-induced magnetic anisotropy change.