Heusler-based synthetic antiferrimagnets.

Antiferromagnet spintronic devices eliminate or mitigate long-range dipolar fields, thereby promising ultrafast operation. For spin transport electronics, one of the most successful strategies is the creation of metallic synthetic antiferromagnets, which, to date, have largely been formed from transition metals and their alloys. Here, we show that synthetic antiferrimagnetic sandwiches can be formed using exchange coupling spacer layers composed of atomically ordered RuAl layers and ultrathin, perpendicularly magnetized, tetragonal ferrimagnetic Heusler layers. Chemically ordered RuAl layers can both be grown on top of a Heusler layer and allow for the growth of ordered Heusler layers deposited on top of it that are as thin as one unit cell. The RuAl spacer layer gives rise to a thickness-dependent oscillatory interlayer coupling with an oscillation period of ~1.1 nm. The observation of ultrathin ordered synthetic antiferrimagnets substantially expands the family of synthetic antiferromagnets and magnetic compounds for spintronic technologies.

Surface Initiated Polymer Thin Films for the Area Selective Deposition and Etching of Metal Oxides.

The area selective growth of polymers and their use as inhibiting layers for inorganic film depositions may provide a valuable self-aligned process for fabrication. Polynorbornene (PNB) thin films were grown from surface-bound initiators and show inhibitory properties against the atomic layer deposition (ALD) of ZnO and TiO2. Area selective control of the polymerization was achieved through the synthesis of initiators that incorporate surface-binding ligands, enabling their selective attachment to metal oxide features versus silicon dielectrics, which were then used to initiate surface polymerizations. The subsequent use of these films in an ALD process enabled the area selective deposition (ASD) of up to 39 nm of ZnO. In addition, polymer thickness was found to play a key role, where films that underwent longer polymerization times were more effective at inhibiting higher numbers of ALD cycles. Finally, while the ASD of a TiO2 film was not achieved despite blanket studies showing inhibition, the ALD deposition on polymer regions of a patterned film produced a different quality metal oxide and therefore altered its etch resistance. This property was exploited in the area selective etch of a metal feature. This demonstration of an area selective surface-grown polymer to enable ASD and selective etch has implications for the fabrication of both micro- and nanoscale features and surfaces.

Chiral domain wall motion in unit-cell thick perpendicularly magnetized Heusler films prepared by chemical templating.

Heusler alloys are a large family of compounds with complex and tunable magnetic properties, intimately connected to the atomic scale ordering of their constituent elements. We show that using a chemical templating technique of atomically ordered X'Z' (X' = Co; Z' = Al, Ga, Ge, Sn) underlayers, we can achieve near bulk-like magnetic properties in tetragonally distorted Heusler films, even at room temperature. Excellent perpendicular magnetic anisotropy is found in ferrimagnetic X3Z (X = Mn; Z = Ge, Sn, Sb) films, just 1 or 2 unit-cells thick. Racetracks formed from these films sustain current-induced domain wall motion with velocities of more than 120 m s-1, at current densities up to six times lower than conventional ferromagnetic materials. We find evidence for a significant bulk chiral Dzyaloshinskii-Moriya exchange interaction, whose field strength can be systematically tuned by an order of magnitude. Our work is an important step towards practical applications of Heusler compounds for spintronic technologies.

Evidence for Ionic Liquid Gate-Induced Metallization of Vanadium Dioxide Bars over Micron Length Scales.

It has recently been shown that the metal-insulator transition in vanadium dioxide epitaxial films can be suppressed and the material made metallic to low temperatures by ionic liquid gating due to migration of oxygen. The gating is only possible on certain crystal facets where volume channels along the VO2's rutile c-axis intersect the surface. Here, we fabricate bars with the c-axis in plane and oriented parallel to or perpendicular to the length of the bars. We show that only bars with the c-axis perpendicular to the bars, for which the volume channels are accessible from the sides of the bar, can be metallized by ionic liquid gating. Moreover, we find that bars up to at least 0.5 µm wide can be fully gated, demonstrating the possibility of the electric field induced migration of oxygen over very long distances, ∼5 times longer than previously observed.

Pattern placement accuracy in block copolymer directed self-assembly based on chemical epitaxy.

The realization of viable designs for circuit patterns using the dense features formed by block copolymer directed self-assembly (DSA) will require a precise and quantitative understanding of self-assembled feature registration to guiding templates or chemical prepatterns. Here we report measurements of DSA placement error for lamellar block copolymer domains indexed to specific lines in the surface chemical prepattern for spatial frequency tripling and quadrupling. These measurements are made possible by the use of an inorganic domain-selective prepattern material that may be imaged upon polymer removal after DSA and a prepattern design incorporating a single feature serving as an in situ registration mark that is identifiable by pattern symmetry in both the prepattern and resulting self-assembled pattern. The results indicate that DSA placement error is correlated with average prepattern line width as well as prepattern pitch uniformity. Finally, the magnitude of DSA placement error anticipated for a uniform, optimized prepattern is estimated.

Demonstration of electrooptic modulation at 2165nm using a silicon Mach-Zehnder interferometer.

We demonstrate electrooptic modulation at a wavelength of 2165nm, using a free-carrier injection-based silicon Mach-Zehnder modulator. The modulator has a V(π)∙L figure of merit of 0.12V∙mm, and an extinction ratio of -23dB. Optical modulation experiments are performed at bitrates up to 3Gbps. Our results illustrate that optical modulator design methodologies previously developed for telecom-band devices can be successfully applied to produce high-performance devices for a silicon nanophotonic mid-infrared integrated circuit platform.

Ordered nanopillar structured electrodes for depleted bulk heterojunction colloidal quantum dot solar cells.

A bulk heterojunction of ordered titania nanopillars and PbS colloidal quantum dots is developed. By using a pre-patterned template, an ordered titania nanopillar matrix with nearest neighbours 275 nm apart and height of 300 nm is fabricated and subsequently filled in with PbS colloidal quantum dots to form an ordered depleted bulk heterojunction exhibiting power conversion efficiency of 5.6%.

Plasmonic nanohybrid with ultrasmall Ag nanoparticles and fluorescent dyes.

We investigate a hybrid nanocomposite combining fluorescent dyes and ultrasmall (<3 nm) silver nanocrystals in a block copolymer micelle. Although the metal nanoparticles are significantly smaller than the electromagnetic skin depth, we observe a modification of the exciton lifetime and the nonradiative energy transfer among the dyes. This behavior is absent in a control experiment with dyes whose energetic levels are far from the plasmonic resonance, establishing the plasmonic nature of the interaction.

Monodisperse cobalt ferrite nanomagnets with uniform silica coatings.

Ferro- and ferrimagnetic nanoparticles are difficult to manipulate in solution as a consequence of the formation of magnetically induced nanoparticle aggregates, which hamper the utility of these particles for applications ranging from data storage to bionanotechnology. Nonmagnetic shells that encapsulate these magnetic particles can reduce the interparticle magnetic interactions and improve the dispersibility of the nanoparticles in solution. A route to create uniform silica shells around individual cobalt ferrite nanoparticles--which uses poly(acrylic acid) to bind to the nanoparticle surface and inhibit nanoparticle aggregation prior to the addition of a silica precursor--was developed. In the absence of the poly(acrylic acid) the cobalt ferrite nanoparticles irreversibly aggregated during the silica shell formation. The thickness of the silica shell around the core-shell nanoparticles could be controlled in order to tune the interparticle magnetic coupling as well as inhibit magnetically induced nanoparticle aggregation. These ferrimagnetic core-silica shell structures form stable dispersion in polar solvents such as EtOH and water, which is critical for enabling technologies that require the assembly or derivatization of ferrimagnetic particles in solution.

Self-assembled ferrimagnet--polymer composites for magnetic recording media.

A self-assembled magnetic recording medium was created using colloidal ferrimagnetic building blocks. Monodisperse cobalt ferrite nanoparticles (CoFe(2)O(4)) were synthesized using solution-based methods and then stabilized in solution using the amphiphilic diblock copolymer, poly(acrylic acid)-b-poly(styrene) (PAA-PS). The acid groups of the acrylate block bound the polymer to the nanoparticle surface via multivalent interactions, while the styrene block afforded the magnetic nanoparticle--polymer complex solubility in organic solvents. Moreover, the diblock copolymer improved the colloidal stability of the ferrimagnetic CoFe(2)O(4) nanoparticles by reducing the strong interparticle magnetic interactions, which typically caused the ferrimagnetic nanoparticles to irreversibly aggregate. The nanoparticle--polymer complex was spin-coated onto a silicon substrate to afford self-organized thin film arrays, with the interparticle spacing determined by the molecular weight of the diblock copolymer. The thin film composite was also exposed to an external magnetic field while simultaneously heated above the glass transition temperature of poly(styrene) to allow the nanoparticles to physically rotate to align their easy axes with the direction of the magnetic field. In order to demonstrate that this self-assembled ferrimagnet--polymer composite was suitable as a magnetic recording media, read/write cycles were demonstrated using a contact magnetic tester. This work provides a simple route to synthesizing stabilized ferrimagnetic nanocrystals that are suitable for developing magnetic recording media.

Microwave-assisted synthesis of monodispersed CdTe nanocrystals.

High quality and monodispersed CdTe nanocrystals with tunable emission spectra ranging from 516 nm to 650 nm were synthesized by a highly reproducible microwave method.

CMOS-integrated high-speed MSM germanium waveguide photodetector.

A compact waveguide-integrated Germanium-on-insulator (GOI) photodetector with 10 +/- 2fF capacitance and operating at 40Gbps is demonstrated. Monolithic integration of thin single-crystalline Ge into front-end CMOS stack was achieved by rapid melt growth during source-drain implant activation anneal.

Increased tunneling magnetoresistance using normally bcc CoFe alloy electrodes made amorphous without glass forming additives.

Using cross-section transmission electron microscopy we show that films of CoFe alloys, sandwiched between two conventional amorphous materials, are amorphous when less than approximately 25-30 A thick. When these amorphous layers are integrated into magnetic tunnel junctions with amorphous alumina tunnel barriers, significantly higher tunneling magnetoresistance is found compared to when these layers are made crystalline (e.g., by heating or by thickening them). We postulate that this is likely due to changes in interfacial bonding at the alumina-CoFe interface. Indeed, x-ray emission spectroscopy shows a significant increase in the Fe, but not the Co, 3d density of states at the Fermi energy for thin amorphous CoFe layers.