Bipolar Electric-Field Switching of Perpendicular Magnetic Tunnel Junctions through Voltage-Controlled Exchange Coupling.

Perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields have extensive applications in energy-efficient memory and logic devices. Voltage-controlled magnetic anisotropy linearly lowers the energy barrier of the ferromagnetic layer via the electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here, we demonstrate a bipolar electric field effect switching of 100 nm p-MTJs with a synthetic antiferromagnetic free layer through voltage-controlled exchange coupling (VCEC). The switching current density, ∼1.1 × 105 A/cm2, is 1 order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that the electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition of the synthetic antiferromagnetic free layer and generates a fieldlike interlayer exchange coupling torque, which causes the bidirectional magnetization switching of p-MTJs. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications.

Spin canting across core/shell Fe3O4/MnxFe3-xO4 nanoparticles.

Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe3O4/MnxFe3-xO4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.

Tracking the Verwey Transition in Single Magnetite Nanocrystals by Variable-Temperature Scanning Tunneling Microscopy.

Variable-temperature scanning tunneling spectroscopy revealed a sharp Verwey transition in individual ∼10 nm magnetite nanocrystals prepared by the coprecipitation technique and embedded in the surface of a gold film. The transition was observed as a significant change in the electronic structure around the Fermi level, with an apparent band gap of ∼140-250 meV appearing below the transition temperature and a pseudogap of ∼75 ± 10 meV appearing above it. The transition temperature was invariably observed around 101 ± 2 K for different nanocrystals, as opposed to 123 K typically reported for stoichiometric bulk crystals. This suggests that the lowering of the transition temperature is an intrinsic finite size effect, probably due to the presence of the surface.

Patterning of sub-50 nm perpendicular CoFeB/MgO-based magnetic tunnel junctions.

Perpendicular magnetic tunnel junctions (p-MTJs) were patterned into nanopillars using electron-beam lithography to study their scaling and switching behaviour. Magnetoresistance measurements of annealed and unannealed p-MTJ films using scanning probe microscopy showed good agreement with Monte Carlo modeling. p-MTJ pillars demonstrated clear parallel magnetic states, both 'up' or both 'down' following AC-demagnetization. Significant variability in the resistance of p-MTJ pillars was observed and attributed to edge features generated during patterning or local inhomogeneity in the MgO layer.

Pattern transfer with stabilized nanoparticle etch masks.

Self-assembled nanoparticle monolayer arrays are used as an etch mask for pattern transfer into Si and SiO(x) substrates. Crack formation within the array is prevented by electron beam curing to fix the nanoparticles to the substrate, followed by a brief oxygen plasma to remove excess carbon. This leaves a dot array of nanoparticle cores with a minimum gap of 2 nm. Deposition and liftoff can transform the dot array mask into an antidot mask, where the gap is determined by the nanoparticle core diameter. Reactive ion etching is used to transfer the dot and antidot patterns into the substrate. The effect of the gap size on the etching rate is modeled and compared with the experimental results.

Ten-nanometer dense hole arrays generated by nanoparticle lithography.

Large area dense hole arrays with a feature size of ~10 nm were generated using self-assembled monolayers of nanoparticles as etch masks. To fabricate the hole arrays, monolayers of nanoparticles were irradiated by electron beam to turn surfactants into amorphous carbon, treated by acid to remove the nanoparticle cores, and then etched by CF(4) to deepen the holes. Evaporated gold films preferentially diffuse into the holes to generate gold nanoparticle arrays. However no obvious diffusion into holes was observed for a sputtered iron platinum film.

Ultra-large-area self-assembled monolayers of nanoparticles.

Large-area self-assembled monolayers of nanoparticles are fabricated on the surface of deionized water by controlled evaporation of nanoparticles dispersed in a binary solvent mixture. The difference in solvent volatility and partial coverage of the trough leads to a flux of nanoparticles toward the evaporation front. The monolayers are comprised of monodisperse magnetite and gold nanoparticles or slightly more polydisperse manganese oxide nanoparticles. The floating monolayers are transferred onto different substrates by the Langmuir-Schaefer method. Surfactants in the colloidal solution and substrate materials have significant impact on the monolayer formation. Bilayers of nanoparticles with different twist angles between layers are also obtained by double deposition.

Magnetophoresis of nanoparticles.

Iron oxide cores of 35 nm are coated with gold nanoparticles so that individual particle motion can be tracked in real time through the plasmonic response using dark field optical microscopy. Although Brownian and viscous drag forces are pronounced for nanoparticles, we show that magnetic manipulation is possible using large magnetic field gradients. The trajectories are analyzed to separate contributions from the different types of forces. With field gradients up to 3000 T/m, forces as small as 1.5 fN are detected.

Characterization of single-core magnetite nanoparticles for magnetic imaging by SQUID relaxometry.

Optimizing the sensitivity of SQUID (superconducting quantum interference device) relaxometry for detecting cell-targeted magnetic nanoparticles for in vivo diagnostics requires nanoparticles with a narrow particle size distribution to ensure that the Néel relaxation times fall within the measurement timescale (50 ms-2 s, in this work). To determine the optimum particle size, single-core magnetite nanoparticles (with nominal average diameters 20, 25, 30 and 35 nm) were characterized by SQUID relaxometry, transmission electron microscopy, SQUID susceptometry, dynamic light scattering and zeta potential analysis. The SQUID relaxometry signal (detected magnetic moment/kg) from both the 25 nm and 30 nm particles was an improvement over previously studied multi-core particles. However, the detected moments were an order of magnitude lower than predicted based on a simple model that takes into account the measured size distributions (but neglects dipolar interactions and polydispersity of the anisotropy energy density), indicating that improved control of several different nanoparticle properties (size, shape and coating thickness) will be required to achieve the highest detection sensitivity. Antibody conjugation and cell incubation experiments show that single-core particles enable a higher detected moment per cell, but also demonstrate the need for improved surface treatments to mitigate aggregation and improve specificity.

Plasmonic magnetic nanoparticles for biomedicine.

Core-shell nanoparticles containing both iron oxide and gold are proposed as a new tool for in vitro biosensing applications. The surface plasmon resonance of gold was used to track the positions of individual particles smaller than the optical diffraction limit. With a large magnetic field gradient from a tip made of a soft magnetic material, particles can be collected and later released.

Stabilization of superparamagnetic iron oxide core-gold shell nanoparticles in high ionic strength media.

Nanoparticles with monodisperse, spherical magnetic iron oxide cores and contiguous gold shells (Fe/Au NPs) have been synthesized in order to combine magnetophoretic responsiveness and localized surface plasmon resonance in a single nanoparticle. Such particles are sufficiently charged to be stable against flocculation in low ionic strength media, but they require surface modification to be stably dispersed in elevated ionic strength media that are appropriate for biotechnological applications. Dynamic light scattering and ultraviolet-visible spectrophotometry are used to monitor the colloidal stability of Fe/Au NPs in pH 7.4 phosphate buffered saline containing 154 mM NaCl (PBS). While uncoated particles flocculate immediately upon introduction to PBS, Fe/Au NPs with adsorbed layers of bovine serum albumin or the amphiphilic triblock copolymers Pluronic F127 and Pluronic F68 resist flocculation after more than 5 days in PBS. Adsorbed dextran allowed flocculation that was limited to the formation of small clusters, while poly(ethylene glycol) homopolymers ranging in molecular weight from 6000 to 100 000 were ineffective steric stabilizers. The effectiveness of adsorbed Pluronic copolymers as steric stabilizers was interpreted in terms of the measured adsorbed layer thickness and extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory predictions of interparticle interactions.

Optical and electron microscopy studies of Schiller layer formation and structure.

Iridescent Schiller layers were prepared by centrifugation of beta-FeOOH sols with an initial particle concentration of 10(14) particles/mL, reducing the Schiller layer formation time from over 2 months to 3 weeks. The formation and structure of the Schiller layers were investigated using optical and transmission electron microscopy. Microscopy studies revealed the self-assembly to proceed by the formation of two-dimensional particle arrays followed by the stacking of these arrays to form the final iridescent state. Varying the pH showed that Schiller layer formation occurs only in the pH range 1.4-2.0, indicating that electrostatic interactions play a pivotal role in the self-assembly. Decreasing the particle concentration of the sols was found to inhibit the assembly. DLVO theory and order-disorder phase transition models were found to be insufficient to accurately model the experimental behavior. Several approaches were investigated in an attempt to make ferrimagnetic arrays from the Schiller layers. The most promising was via electron beam irradiation, which transforms the beta-FeOOH into gamma-Fe(2)O(3) without altering the shape of the nanorods.

Synthesis and characterization of paramagnetic microparticles through emulsion-templated free radical polymerization.

A novel method is described for the preparation of high-magnetization paramagnetic microparticles functionalized with a controlled density of poly(ethylene glycol) (PEG) and carboxyl groups. These microparticles were synthesized using four steps: (1) creation of an oil-in-water emulsion in which hydrophobic iron oxide nanoparticles and a UV-activated initiator were distributed in hexane; (2) formation of uniform microparticles through emulsion homogenization and evaporation of hexane; (3) functionalization of the microparticle with a PEG-functionalized surfactant and acrylic acid; and (4) polymerization of the microparticles. Characterization of the microparticles with electron microscopy and light scattering revealed that they were composed of densely packed iron oxide nanoparticles and that the size of the microparticles may be controlled through the pore size of the membrane used to homogenize the emulsion. The concentration of surfactant and acrylic acid used in the third processing step was found to determine the surface chemistry, iron content, and magnetization of the microparticles. Increasing the PEG surfactant to acrylic acid ratio resulted in higher PEG surface densities, lower iron content, and lower magnetization. The resulting microparticles were readily functionalized with antibodies and showed a low propensity for nonspecific protein adsorption. We believe that these microparticles will be useful for magnetic tweezers measurements and bioanalytical devices that require microparticles with a high magnetization.

TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties.

Nanoscale Fe0 particles are a promising technology for in situ remediation of trichloroethene (TCE) plumes and TCE-DNAPL source areas, butthe physical and chemical properties controlling their reactivity are not yet understood. Here, the TCE reaction rates, pathways, and efficiency of two nanoscale Fe0 particles are measured in batch reactors: particles synthesized from sodium borohydride reduction of ferrous iron (Fe/B) and commercially available particles (RNIP). Reactivity was determined under iron-limited (high [TCE]) and excess iron (low [TCE]) conditions and with and without added H2. Particle efficiency, defined as the fraction of the Fe0 in the particles that is used to dechlorinate TCE, was determined under iron-limited conditions. Both particles had a core/shell structure and similar specific surface areas (approximately 30 m2/g). Using excess iron, Fe/B transformed TCE into ethane (80%) and C3-C6 coupling products (20%). The measured surface area normalized pseudo-first-order rate constant for Fe/B (1.4 x 10(-)2 L.h(-1).m(-2) is approximately 4-fold higher than for RNIP (3.1 x 10-(3) L.h(-1).m(-2). All the Fe0 in Fe/B was accessible for TCE dechlorination, and 92 +/- 0.7% of the Fe0 was used to reduce TCE. For Fe/B, H2 evolved from reduction of water (H+) was subsequently used for TCE dechlorination, and adding H2 to the reactor increased both the dechlorination rate and the mass of TCE reduced, indicating that a catalytic pathway exists. RNIP yielded unsaturated products (acetylene and ethene). Nearly half (46%) of the Fe0 in RNIP was unavailable for TCE dechlorination over the course of the experiment and remained in the particles. Adding H2 did not change the reaction rate or efficiency of RNIP. Despite this, the mass of TCE dechlorinated per mass of Fe0 added was similar for both particles due to the less saturated products formed from RNIP. The oxide shell composition and the boron content are the most likely causes for the differences between the particle types.

Magnetic interactions of iron nanoparticles in arrays and dilute dispersions.

The magnetic properties of monodisperse Fe nanoparticles with over 4 orders of magnitude difference in concentration are studied by a combination of ordinary and remanent hysteresis loops, zero field cooled magnetization as a function of temperature, and magnetic relaxation rates. We compare the behavior of dilute dispersions with different concentrations, dispersions, and arrays made from the same particles, and nanoparticle arrays with different particle sizes and separations. The results are related to theoretical predictions and are used to create a unified picture of magnetostatic interactions within the assemblies.

Mesoscopic monodisperse ferromagnetic colloids enable magnetically controlled photonic crystals.

We report here the first synthesis of mesoscopic, monodisperse particles which contain nanoscopic inclusions of ferromagnetic cobalt ferrites. These monodisperse ferromagnetic composite particles readily self-assemble into magnetically responsive photonic crystals that efficiently Bragg diffract incident light. Magnetic fields can be used to control the photonic crystal orientation and, thus, the diffracted wavelength. We demonstrate the use of these ferromagnetic particles to fabricate magneto-optical diffracting fluids and magnetically switchable diffracting mirrors.