Sara A. Majetich

Superparamagnetic Photonic Crystals

Photonic crystals consisting of monodisperse superparamagnetic colloidal particles have been synthesized. The particles (see Figure) are obtained by emulsion polymerization of styrene in the presence of freshly precipitated surface-modified iron oxide nanoparticles. A magnetic field self-assembles the particles and controls the diffraction wavelength and crystal parameters of the array.

Gold-coated iron nanoparticles for biomedical applications

We describe the magnetic properties of gold-coated iron nanoparticles, and the effects of low pH and heat treatment. Acicular iron and spherical iron based nanoparticles were coated with a thin layer of gold. The morphology and magnetic properties of magnetic particles were examined using transmission electron microscopy and alternative gradient magnetometry. While the small spherical particles had relatively uniform layers coatings, the larger acicular particles had many gold clusters decorating the surface. The original acicular iron nanoparticles had a specific magnetic moment of 145 emu/g and a coercivity of 1664 Oe. Corrosion tests showed good corrosion resistance for gold-coated commercial iron particles even in a 1.0×10−3 M HCl solution at 80 °C for 12 h, compared with uncoated particles.

The 2014 Magnetism Roadmap

Magnetism is a very fascinating and dynamic field. Especially in the last 30 years it has experienced many major advances in the full range from novel fundamental phenomena to new products. Applications such as hard disk drives and magnetic sensors are part of our daily life, and new applications, such as in non-volatile computer random access memory, are expected to surface shortly. Thus it is timely for describing the current status, and current and future challenges in the form of a Roadmap article. This 2014 Magnetism Roadmap provides a view on several selected, currently very active innovative developments. It consists of 12 sections, each written by an expert in the field and addressing a specific subject, with strong emphasize on future potential. This Roadmap cannot cover the entire field. We have selected several highly relevant areas without attempting to provide a full review - a future update will have room for more topics. The scope covers mostly nano-magnetic phenomena and applications, where surfaces and interfaces provide additional functionality. New developments in fundamental topics such as interacting nano-elements, novel magnon-based spintronics concepts, spin-orbit torques and spin-caloric phenomena are addressed. New materials, such as organic magnetic materials and permanent magnets are covered. New applications are presented such as nano-magnetic logic, non-local and domain-wall based devices, heat-assisted magnetic recording, magnetic random access memory, and applications in biotechnology. May the Roadmap serve as a guideline for future emerging research directions in modern magnetism.

Photomagnetism and structure in cobalt ferrite nanoparticles

Light-induced changes in the coercivity ΔHc are reported for cobalt ferrite nanoparticles. These changes arise from charge transfer initiated by optical absorption near 2 eV. An insulator-to-metal transition at 170 K is an upper limit for significant ΔHc. The larger ΔHc for smaller (17 nm) particles was correlated with a distortion about the Co2+ ions at B sites, which increases the absorption probability.

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.

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.

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.

MAGNETIC PROPERTIES AND ORDERING IN C-COATED FEXCO1-X ALLOY NANOCRYSTALS

A surface-consistent refraction analysis automatically derives near surface corrections during seismic data processing. Residual time lags are evaluated in multiple CMP-offset-azimuth bins by similarity analysis with a pilot trace where a correlation window is centered at the refracted arrival. The similarity analysis may take the form of computerized cross-correlation, or other criteria such as semblance. The residuals are then used to build a system of linear equations that is simultaneously inverted for surface-consistent shot and receiver time shift corrections plus a possible subsurface residual term. The refraction analysis steps are completely automated and require a fraction of the time needed for conventional near surface analysis.

Functional magnetic nanoparticle assemblies: formation, collective behavior, and future directions.

This Perspective describes recent progress in the development of functional magnetic nanoparticle assemblies. After describing the formation of two- and three-dimensional particle arrays in terms of the size-dependent driving forces, we focus on magnetic nanoparticle arrays. We discuss how the self-organized structure can modify the magnetic behavior, relative to that of isolated particles. We highlight an important development, described in this issue of ACS Nano by Kostiainen and co-workers, who have demonstrated not only the novel aqueous self-assembly of magnetic particles but also controlled and reversible disassembly. Finally, we explore two inter-related future directions for self-assembly of magnetic nanoparticles: the formation of more complex, hierarchical structures and the integration of self-assembly with fabrication techniques for electronic devices.

Size and Concentration Effects on High Frequency Hysteresis of Iron Oxide Nanoparticles

High-frequency hysteresis measurements were performed on two different types of hydrosols of iron oxide particles: fully dispersed and highly clustered. The fully dispersed sol showed superparamagnetic behavior across the frequency range investigated, while the clustered sample exhibited significant hysteresis. The hysteretic losses were then directly related to magnetic hyperthermia measurements in a sinusoidal field of amplitude 10 mT at 140 kHz, where the clustered sample heated by up to 25degC in 1000 s while the dispersed sample at similar concentration showed no measurable heating.