Erik Shipton

Cargo-towing fuel-free magnetic nanoswimmers for targeted drug delivery.

Fuel-free nanomotors are essential for future in-vivo biomedical transport and drug-delivery applications. Herein, the first example of directed delivery of drug-loaded magnetic polymeric particles using magnetically driven flexible nanoswimmers is described. It is demonstrated that flexible magnetic nickel-silver nanoswimmers (5-6 μm in length and 200 nm in diameter) are able to transport micrometer particles at high speeds of more than 10 μm s(-1) (more than 0.2 body lengths per revolution in dimensionless speed). The fundamental mechanism of the cargo-towing ability of these magnetic (fuel-free) nanowire motors is modelled, and the hydrodynamic features of these cargo-loaded motors discussed. The effect of the cargo size on swimming performance is evaluated experimentally and compared to a theoretical model, emphasizing the interplay between hydrodynamic drag forces and boundary actuation. The latter leads to an unusual increase of the propulsion speed at an intermediate particle size. Potential applications of these cargo-towing nanoswimmers are demonstrated by using the directed delivery of drug-loaded microparticles to HeLa cancer cells in biological media. Transport of the drug carriers through a microchannel from the pick-up zone to the release microwell is further illustrated. It is expected that magnetically driven nanoswimmers will provide a new approach for the rapid delivery of target-specific drug carriers to predetermined destinations.

Dichroic coherent diffractive imaging

Understanding electronic structure at the nanoscale is crucial to untangling fundamental physics puzzles such as phase separation and emergent behavior in complex magnetic oxides. Probes with the ability to see beyond surfaces on nanometer length and subpicosecond time scales can greatly enhance our understanding of these systems and will undoubtedly impact development of future information technologies. Polarized X-rays are an appealing choice of probe due to their penetrating power, elemental and magnetic specificity, and high spatial resolution. The resolution of traditional X-ray microscopes is limited by the nanometer precision required to fabricate X-ray optics. Here we present a novel approach to lensless imaging of an extended magnetic nanostructure, in which a scanned series of dichroic coherent diffraction patterns is recorded and numerically inverted to map its magnetic domain configuration. Unlike holographic methods, it does not require a reference wave or precision optics. In addition, it enables the imaging of samples with arbitrarily large spatial dimensions, at a spatial resolution limited solely by the coherent X-ray flux, wavelength, and stability of the sample with respect to the beam. It can readily be extended to nonmagnetic systems that exhibit circular or linear dichroism. We demonstrate this approach by imaging ferrimagnetic labyrinthine domains in a Gd/Fe multilayer with perpendicular anisotropy and follow the evolution of the domain structure through part of its magnetization hysteresis loop. This approach is scalable to imaging with diffraction-limited resolution, a prospect rapidly becoming a reality in view of the new generation of phenomenally brilliant X-ray sources.

Compact X-pinch based point x-ray source for phase contrast imaging of inertial confinement fusion capsules

Results from experiments performed to characterize plastic capsules containing foam layers are presented. A compact X-pinch pulser with a footprint <1m2 having a peak current of 80kA and a rise time of 50ns was used. Various wire materials including tungsten, molybdenum, and aluminum were employed. Results with plastic capsules (1mm diameter, 20μm thick wall with 80μm foam inside the capsule) show phase contrast effects at the edges of the wall due to the foam, which mimics the ice inside the shell. The sharpness of the image reveals a source less than 2μm in size and x-ray diodes show a pulse length of ∼10ns. The small source size allows high-resolution phase contrast imaging of capsules. The x-ray pulse from an X-pinch is sufficiently short to avoid the motional blurring due to cryogenic system vibrations, which is not possible with low flux sources.

Suppression of the perpendicular anisotropy at the CoO Néel temperature in exchange-biased CoO/[Co/Pt] multilayers

We performed high field torque magnetometry measurements on CoO/[Co/Pt] magnetic multilayers that exhibit perpendicular exchange bias. We find that the antiferromagnet CoO layers strongly modify the uniaxial anisotropy of the multilayer structures. The strongest effects due to the CoO layers occur in the vicinity of the Neel temperature, where we observe a suppression of the first-order anisotropy and a smaller enhancement of the second-order anisotropy. This results in a nonmonotonic variation of the anisotropy with temperature and for selected samples a transition from perpendicular to in-plane and back to perpendicular anisotropy with increasing temperature.

A Compact X-pinch X-ray Source for Characterization of Inertial Confinement Fusion Capsules

We present initial results from experiments performed to characterize plastic capsules using a compact x-pinch pulser with a floor space < 1m 2 . The pulser produces 80 kA current with a rise time of 40 ns. Various wire materials including tungsten, molybdenum and aluminum were used. X-pinch length was varied to obtain maximum x-ray yield and photon energies. X- rays in 5-9 keV energy range were used for the phase contrast radiography of plastic shells. Results with plastic capsules (1 mm diameter, 20 µm thick wall) show a phase contrast effect at the edges of the wall. The sharpness of the image reveals source size of less than 3 µm.

Core-Shell Structured Nanowire Spin Valves

Nanowire based magnetic spin valves utilizing a core-shell device architecture about free-standing Ni nanowires have been fabricated and characterized. Devices containing sequential shell layers of CoO(10 nm)-Co(5 nm)-Cu(5 nm)-Co(5 nm) deposited through sputter deposition around the chemical-vapor-deposited Ni core nanowires exhibit a giant magnetoresistance effect of ~9%, which matches that of the corresponding planar thin film multilayer. The Ni nanowires which serve as the device platforms are obtained in diameters ranging from 100 nm through 300 nm and typical heights of 20 micrometers or greater. Since the nanowires are oriented vertically upon Si/SiO2 growth substrates they allow the creation of core-shell spin valve devices with an orientation, aspect ratio, and distribution difficult to achieve with conventional thin film methodologies. The demonstrated, fully-functional, nanowire-based spin valve establishes the viability of magnetic multi-layer device structures in the core-shell implementation. Devices of this nature have potential in various applications including high-acuity magnetic field sensing.

Development of an in situ peak intensity measurement method for ultraintense single shot laser-plasma experiments at the Sandia Z petawatt facility

Using the physical process of ultraintense field ionization of high charge states of inert gas ions, we have developed a method of peak intensity measurement at the focus of high energy short pulse lasers operating in single shot mode. The technique involves detecting ionization products created from a low pressure gas target at the laser focus via time of flight detector. The observation of high ion charge states collected by the detector yields peak intensity at the focus when compared with the results obtained from well established tunnel ionization models. An initial peak intensity measurement of 5×1016Wcm−2 was obtained for a 1.053μm center wavelength, 0.4J pulse with 1ps pulse duration focused with an f∕5.5 off-axis parabola. Experiments with multijoule level, 500fs laser pulses are on the way.

Experimental observations of transport of picosecond laser generated electrons in a nail-like target

The transport of relativistic electrons, generated by the interaction of a high intensity (2×1020W∕cm2) laser, has been studied in a nail-like target comprised of a 20μm diameter solid copper wire, coated with ∼2μm of titanium, with an 80μm diameter hemispherical termination. A ∼500fs, ∼200J pulse of 1.053μm laser light produced by the Titan Laser at Lawrence Livermore National Laboratory was focused to a ∼20μm diameter spot centered on the flat face of the hemisphere. Kα fluorescence from the Cu and Ti regions was imaged together with extreme ultraviolet (XUV) emission at 68 and 256eV. Results showed a quasiexponential decline in Kα emission along the wire over a distance of a few hundred microns from the laser focus, consistent with bulk Ohmic inhibition of the relativistic electron transport. Weaker Kα and XUV emission on a longer scale length showed limb brightening suggesting a transition to enhanced transport at the surface of the wire.

Tunable resonant properties of perpendicular anisotropy [Co/Pd]/Fe/[Co/Pd] multilayers

We describe the static and dynamic magnetic behaviors of Fe films (thicknesses 2, 4, and 6 nm) sandwiched between Co/Pd multilayers with strong perpendicular magnetic anisotropy. Out-of-plane measurements of both magnetization and ferromagnetic resonance confirm well-defined Fe layer response modified by large perpendicular exchange field arising from the coupling with the Co/Pd. The field/frequency dispersion is linear for all samples with field intercepts increasing with Fe layer thickness. Analysis in terms of shape anisotropy and interfacial exchange model yields a large out-of-plane interfacial coupling of ∼3.0–3.7 erg/cm2 that is mediated by the coupling across thin Pd layers. The value of this interface exchange is also shown to be tunable with interfacial Pd thickness.

Torque magnetometry of perpendicular anisotropy exchange-spring heterostructures

The field-induced magnetic configurations in a [Co/Pd]15 /TbFeCo exchange-spring system with perpendicular magnetic anisotropy are studied using torque magnetometry. The experimental results are compared to a 1D micromagnetic simulation. The good agreement between experiments and simulations allows us to deduce the evolution of the in-depth magnetic configuration as a function of the applied field orientation and amplitude. The chirality transition of the interfacial domain wall developing in the structure can also be determined with this technique.