Observation of the Nonreciprocal Magnon Hanle Effect.

The precession of magnon pseudospin about the equilibrium pseudofield, the latter capturing the nature of magnonic eigenexcitations in an antiferromagnet, gives rise to the magnon Hanle effect. Its realization via electrically injected and detected spin transport in an antiferromagnetic insulator demonstrates its high potential for devices and as a convenient probe for magnon eigenmodes and the underlying spin interactions in the antiferromagnet. Here, we observe a nonreciprocity in the Hanle signal measured in hematite using two spatially separated platinum electrodes as spin injector or detector. Interchanging their roles was found to alter the detected magnon spin signal. The recorded difference depends on the applied magnetic field and reverses sign when the signal passes its nominal maximum at the so-called compensation field. We explain these observations in terms of a spin transport direction-dependent pseudofield. The latter leads to a nonreciprocity, which is found to be controllable via the applied magnetic field. The observed nonreciprocal response in the readily available hematite films opens interesting opportunities for realizing exotic physics predicted so far only for antiferromagnets with special crystal structures.

Carboxymethyl-Dextran-Coated Superparamagnetic Iron Oxide Nanoparticles for Drug Delivery: Influence of the Coating Thickness on the Particle Properties.

Carboxymethyl-dextran (CMD)-coated iron oxide nanoparticles (IONs) are of great interest in nanomedicine, especially for applications in drug delivery. To develop a magnetically controlled drug delivery system, many factors must be considered, including the composition, surface properties, size and agglomeration, magnetization, cytocompatibility, and drug activity. This study reveals how the CMD coating thickness can influence these particle properties. ION@CMD are synthesized by co-precipitation. A higher quantity of CMD leads to a thicker coating and a reduced superparamagnetic core size with decreasing magnetization. Above 12.5−25.0 g L−1 of CMD, the particles are colloidally stable. All the particles show hydrodynamic diameters < 100 nm and a good cell viability in contact with smooth muscle cells, fulfilling two of the most critical characteristics of drug delivery systems. New insights into the significant impact of agglomeration on the magnetophoretic behavior are shown. Remarkable drug loadings (62%) with the antimicrobial peptide lasioglossin and an excellent efficiency (82.3%) were obtained by covalent coupling with the EDC/NHS (N-ethyl-N'-(3-(dimethylamino)propyl)carbodiimide/N-hydroxysuccinimide) method in comparison with the adsorption method (24% drug loading, 28% efficiency). The systems showed high antimicrobial activity with a minimal inhibitory concentration of 1.13 µM (adsorption) and 1.70 µM (covalent). This system successfully combines an antimicrobial peptide with a magnetically controllable drug carrier.

Spray-Deposited Anisotropic Ferromagnetic Hybrid Polymer Films of PS-b-PMMA and Strontium Hexaferrite Magnetic Nanoplatelets.

Spray deposition is a scalable and cost-effective technique for the fabrication of magnetic hybrid films containing diblock copolymers (DBCs) and magnetic nanoparticles. However, it is challenging to obtain spray-deposited anisotropic magnetic hybrid films without using external magnetic fields. In the present work, spray deposition is applied to prepare perpendicular anisotropic magnetic hybrid films by controlling the orientation of strontium hexaferrite nanoplatelets inside ultra-high-molecular-weight DBC polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films. During spray deposition, the evolution of DBC morphology and the orientation of magnetic nanoplatelets are monitored with in situ grazing-incidence small-angle X-ray scattering (GISAXS). For reference, a pure DBC film without nanoplatelets is deposited with the same conditions. Solvent-controlled magnetic properties of the hybrid film are proven with solvent vapor annealing (SVA) applied to the final deposited magnetic films. Obvious changes in the DBC morphology and nanoplatelet localization are observed during SVA. The superconducting quantum interference device data show that ferromagnetic hybrid polymer films with high coercivity can be achieved via spray deposition. The hybrid films show a perpendicular magnetic anisotropy before SVA, which is strongly weakened after SVA. The spray-deposited hybrid films appear highly promising for potential applications in magnetic data storage and sensors.

Surface distortion of Fe dot-decorated TiO2 nanotubular templates using time-of-flight grazing incidence small angle scattering.

Physical properties of nanoclusters, nanostructures and self-assembled nanodots, which in turn are concomitantly dependent upon the morphological properties, can be modulated for functional purposes. Here, in this article, magnetic nanodots of Fe on semiconductor TiO2 nanotubes (TNTs) are investigated with time-of-flight grazing incidence small-angle neutron scattering (TOF-GISANS) as a function of wavelength, chosen from a set of three TNT templates with different correlation lengths. The results are found corroborating with the localized scanning electron microscopy (SEM) images. As we probe the inside and the near-surface region of the Fe-dotted TNTs with respect to their homogeneity, surface distortion and long-range order using TOF-GISANS, gradual aberrations at the top of the near-surface region are identified. Magnetization measurements as a function of temperature and field do not show a typical ferromagnetic behavior but rather a supermagnetic one that is expected from a nonhomogeneous distribution of Fe-dots in the intertubular crevasses.

Self-Assembly of Large Magnetic Nanoparticles in Ultrahigh Molecular Weight Linear Diblock Copolymer Films.

The development of diblock copolymer (DBC) nanocomposite films containing magnetic nanoparticles (NPs) with diameters (D) over 20 nm is a challenging task. To host large iron oxide NPs (Fe3O4, D = 27 ± 0.6 nm), an ultrahigh molecular weight (UHMW) linear DBC polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) is used as a template in the present work. Due to hydrogen bonding between the carboxylic acid ligands of the NPs and the ester groups in PMMA, the NPs show an affinity to the PMMA block. The localization of the NPs inside the DBC is investigated as a function of the NP concentration. At low NP concentrations, NPs are located preferentially at the interface between PS and PMMA domains to minimize the interfacial tension caused by the strong segregation strength of the UHMW DBC. At high NP concentrations (≥10 wt %), chain-like NP aggregates (a head-to-tail orientation) are observed in the PMMA domains, resulting in a change of the morphology from sphere to ellipsoid for part of the PMMA domains. Magnetic properties of the hybrid films are probed via superconducting quantum interference device magnetometry. All hybrid films show ferrimagnetism and are promising for potential applications in magnetic data storage.

Printed Thin Diblock Copolymer Films with Dense Magnetic Nanostructure.

Thin hybrid films with dense magnetic structures for sensor applications are printed using diblock copolymer (DBC) templating magnetic nanoparticles (MNPs). To achieve a high-density magnetic structure, the printing ink is prepared by mixing polystyrene- block-poly(methyl methacrylate) (PS- b-PMMA) with a large PS volume fraction and PS selective MNPs. Solvent vapor annealing is applied to generate a parallel cylindrical film morphology (with respect to the substrate), in which the MNP-residing PS domains are well separated by the PMMA matrix, and thus, the formation of large MNP agglomerates is avoided. Moreover, the morphologies of the printed thin films are determined as a function of the MNP concentration with real and reciprocal space characterization techniques. The PS domains are found to be saturated with MNPs at 1 wt %, at which the structural order of the hybrid films reaches a maximum within the studied range of MNP concentration. As a beneficial aspect, the MNP loading improves the morphological order of the thin DBC films. The dense magnetic structure endows the thin films with a faster superparamagnetic responsive behavior, as compared to thick films where identical MNPs are used, but dispersed inside the minority domains of the DBC.

Magnetic nanoparticle-containing soft-hard diblock copolymer films with high order.

For sensor applications, superparamagnetic anisotropy is an indispensable property, which is typically achieved by employing an external field to guide the arrangement of magnetic nanoparticles (NPs). In the present investigation, the diblock copolymer polystyrene-block-poly(N-isopropylacrylamide) (PS-b-PNIPAM) is printed as a template to localize magnetic iron oxide NPs without any external field. Via microphase separation, cylindrical nanostructures of PS in a PNIPAM matrix are obtained, aligned perpendicular to the substrate. Since the magnetite NPs (Fe3O4) are functionalized with hydrophobic organic chains showing affinity to the PS blocks, they can selectively aggregate inside the PS cylinders. Moreover, solvent vapor annealing allows the achievement of nanostructures inside the hybrid system with a very high order, even at a high NP loading. Therefore, NPs can accumulate within PS domains to form perpendicularly aligned aggregates with high periodicity. The magnetic properties of the hybrid films are determined at various temperatures in two orthogonal directions (with PS cylinders vertical and parallel to the applied magnetic field). All hybrid films show superparamagnetism and a remarkable magnetic anisotropy is achieved at certain NP concentrations. This investigation shows a facile route to prepare superparamagnetic films with magnetic anisotropy and offers a novel possibility to future magnetic sensor fabrication.

Structural and magnetic properties of cobalt iron disulfide (CoxFe1-xS2) nanocrystals.

We report on synthesis and investigation of nanocrystalline cobalt-iron-pyrites with an emphasis on nanocrystal structure, morphology and magnetic behavior. The nanocrystals (NCs) were 5-25 nm in diameter as characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). With an increase in Fe fraction, X-ray diffraction and small-angle-X-ray scattering (SAXS) showed a systematic decrease in lattice constant, primary grain/NC size (15 to 7 nm), and nanoparticle (NP) size (70 to 20 nm), respectively. The temperature dependence of the DC magnetization and AC susceptibility versus frequency revealed a number of magnetic phases in Co x Fe1-xS2. Samples with x = 1 and x = 0.875-0.625 showed evidence of superspin glass (SSG) behavior with embedded ferromagnetic (FM) clusters of NPs. For x = 0.5, samples retained their mixed phases, but showed superparamagnetic (SPM) behavior with antiferromagnetic clusters suppressing magnetic dipolar interactions. Below x = 0.5, the pyrites show increasing paramagnetic character. We construct a phase diagram, which can be understood in terms of competition between the various dipolar, exchange, inter- and intracluster interactions. Our results suggest that NC size and shape can be tuned to engineer spin-polarized ferromagnetism of n-doped iron pyrite.

Printed Thin Magnetic Films Based on Diblock Copolymer and Magnetic Nanoparticles.

Printing techniques have been well established for large-scale production and have developed to be effective in controlling the morphology and thickness of the film. In this work, printing is employed to fabricate magnetic thin films composed of polystyrene coated maghemite nanoparticles (γ-Fe2O3 NPs) and polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer. By applying an external magnetic field during the print coating step, oriented structures with a high content of nanoscale magnetic particles are created. The morphology of the magnetic films and the arrangement of NPs within the polymer matrix are characterized with real and reciprocal space techniques. Due to the applied magnetic field, the magnetic NPs self-assemble into microscale sized wires with controlled widths and separation distances, endowing hybrid films with a characteristic magnetic anisotropy. At the nanoscale level, due to the PS coating, the NPs disperse as single particles at low NP concentrations. The NPs self-assemble into nanosized clusters inside the PS domains when the NP concentration increases. Due to a high loading of uniformly dispersed magnetic NPs across the whole printed film, a strong sensitivity to an external magnetic field is achieved. The enhanced superparamagnetic property of the printed films renders them promising candidate materials for future magnetic sensor applications.

Arrangement of Maghemite Nanoparticles via Wet Chemical Self-Assembly in PS-b-PNIPAM Diblock Copolymer Films.

The structure and magnetic behavior of hybrid films composed of maghemite (γ-Fe2O3) nanoparticles (NPs) and an asymmetric diblock copolymer (DBC) polystyrene61-block-polyN-isopropylacrylamide115 are investigated. The NPs are coated with PS chains, which allow for a selective incorporation inside the PS domains at different NP concentrations. Upon incorporation of low amounts of NPs into the DBC thin films, the initial parallel (to film surface) cylinder morphology changes to a well ordered, perpendicularly oriented one. The characteristic domain distance of the DBC is increased due to the swelling of the PS domains with NPs. At higher NP concentrations, the excess NPs which can no longer be embedded in the PS domains, are accumulated at the film surface, and NP aggregates form. Irrespective of NP concentration, a superparamagnetic behavior of the metal oxide-DBC hybrid films is found. Such superparamagnetic properties make the established hybrid films interesting for high density magnetic storage media and thermoresponsive magnetic sensors.

Self-assembly of diblock copolymer-maghemite nanoparticle hybrid thin films.

The arrangement of maghemite (γ-Fe2O3) nanoparticles (NPs) in poly(styrene-d8-block-n-butyl methacrylate) P(Sd-b-BMA) diblock copolymer (DBC) films via a self-assembly process was investigated toward the fabrication of highly ordered maghemite-polymer hybrid thin films. The resulting thin films exhibited a perforated lamella with an enrichment layer containing NPs as investigated with X-ray reflectometry, scanning electron microscopy, atomic force microscopy, and time-of-flight grazing incidence small angle neutron scattering as a function of the NP concentrations. The NPs were selectively deposited in the PSd domains of the DBC during the microphase separation process. At low NP concentrations, the incorporation of the NPs within the DBC thin films resulted in an enhanced microphase separation process and formation of highly oriented and ordered nanostructured hybrid films. At higher NP concentrations, the aggregation of the NPs was dominating and large sized metal oxide clusters were observed. The superparamagnetic properties of the metal oxide-polymer hybrid films at various NP concentrations were probed by a superconducting quantum interference device magnetometer, which shows that the hybrid films are highly attractive for optical devices, magnetic sensors, and magnetic recording devices.

Nano- and microstructures of magnetic field-guided maghemite nanoparticles in diblock copolymer films.

The control over the alignment of nanoparticles within a block copolymer matrix was investigated for different external magnetic fields with respect to producing well-aligned, highly oriented metal-oxide-polymer nanopatterns. Hybrid films were prepared by solution casting under a range of external magnetic fields. The nano- and microstructure of maghemite nanoparticles within poly(styrene-b-methyl methacrylate) diblock copolymer films as a function of the nanoparticle concentration was studied using optical microscopy, atomic force microscopy, scanning electron microscopy, and grazing incidence small-angle X-ray scattering. Because of a polystyrene (PS) coating, the nanoparticles are incorporated in the PS domains of the diblock copolymer morphology. At higher nanoparticle concentrations, nanoparticle aggregates perturb the block copolymer structure and accumulate at the films surface into wire-shaped stripes. These wire-shaped nanoparticle aggregates form mainly because of the competition between nanoparticle-polymer friction and magnetic dipolar interaction. The magnetic behavior of the hybrid films was probed at different temperatures for two orthogonal directions (with the line-shaped particle aggregates parallel and perpendicular to the magnetic field). The hybrid film systems show superparamagnetic behavior and remarkable shape anisotropy that render them interesting for magnetic applications.

Local charge and spin currents in magnetothermal landscapes.

A scannable laser beam is used to generate local thermal gradients in metallic (Co2FeAl) or insulating (Y3Fe5O12) ferromagnetic thin films. We study the resulting local charge and spin currents that arise due to the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE), respectively. In the local ANE experiments, we detect the voltage in the Co2FeAl thin film plane as a function of the laser-spot position and external magnetic field magnitude and orientation. The local SSE effect is detected in a similar fashion by exploiting the inverse spin Hall effect in a Pt layer deposited on top of the Y3Fe5O12. Our findings establish local thermal spin and charge current generation as well as spin caloritronic domain imaging.

Contamination of U.S. butter with polybrominated diphenyl ethers from wrapping paper.

OBJECTIVES: Our aim was to report the first known incidence of U.S. butter contamination with extremely high levels of polybrominated diphenyl ethers (PBDEs). METHODS: Ten butter samples were individually analyzed for PBDEs. One of the samples and its paper wrapper contained very high levels of higher-brominated PBDEs. Dietary estimates were calculated using the 2007 U.S. Department of Agriculture Loss-Adjusted Food Availability data, excluding the elevated sample. RESULTS: The highly contaminated butter sample had a total upper bound PBDE level of 42,252 pg/g wet weight (ww). Levels of brominated diphenyl ether (BDE)-206, -207, and -209 were 2,000, 2,290, and 37,600 pg/g ww, respectively. Its wrapping paper contained a total upper-bound PBDE concentration of 804,751 pg/g ww, with levels of BDE-206, -207, and -209 of 51,000, 11,700, and 614,000 pg/g, respectively. Total PBDE levels in the remaining nine butter samples ranged from 180 to 1,212 pg/g, with geometric mean of 483 and median of 284 pg/g. Excluding the outlier, total PBDE daily intake from all food was 22,764 pg/day, lower than some previous U.S. dietary intake estimates. CONCLUSION: Higher-brominated PBDE congeners were likely transferred from contaminated wrapping paper to butter. A larger representative survey may help determine how frequently PBDE contamination occurs. Sampling at various stages in food production may identify contamination sources and reduce risk.

Perfluorinated compounds, polychlorinated biphenyls, and organochlorine pesticide contamination in composite food samples from Dallas, Texas, USA.

OBJECTIVES: The objective of this article is to extend our previous studies of persistent organic pollutant (POP) contamination of U.S. food by measuring perfluorinated compounds (PFCs), organochlorine pesticides, and polychlorinated biphenyls (PCBs) in composite food samples. This study is part of a larger study reported in two articles, the other of which reports levels of polybrominated diphenyl ethers and hexabromocyclododecane brominated flame retardants in these composite foods [Schecter et al. 2010. Polybrominated diphenyl ethers (PBDEs) and hexabromocyclodecane (HBCD) in composite U.S. food samples, Environ Health Perspect 118:357-362]. METHODS: In this study we measured concentrations of 32 organochlorine pesticides, 7 PCBs, and 11 PFCs in composite samples of 31 different types of food (310 individual food samples) purchased from supermarkets in Dallas, Texas (USA), in 2009. Dietary intake of these chemicals was calculated for an average American. RESULTS: Contamination varied greatly among chemical and food types. The highest level of pesticide contamination was from the dichlorodiphenyltrichloroethane (DDT) metabolite p,p -dichlorodiphenyldichloroethylene, which ranged from 0.028 ng/g wet weight (ww) in whole milk yogurt to 2.3 ng/g ww in catfish fillets. We found PCB congeners (28, 52, 101, 118, 138, 153, and 180) primarily in fish, with highest levels in salmon (PCB-153, 1.2 ng/g ww; PCB-138, 0.93 ng/g ww). For PFCs, we detected perfluorooctanoic acid (PFOA) in 17 of 31 samples, ranging from 0.07 ng/g in potatoes to 1.80 ng/g in olive oil. In terms of dietary intake, DDT and DDT metabolites, endosulfans, aldrin, PCBs, and PFOA were consumed at the highest levels. CONCLUSION: Despite product bans, we found POPs in U.S. food, and mixtures of these chemicals are consumed by the American public at varying levels. This suggests the need to expand testing of food for chemical contaminants.

Polybrominated diphenyl ethers (PBDEs) and hexabromocyclodecane (HBCD) in composite U.S. food samples.

OBJECTIVES: This study was designed to update previous U.S. market basket surveys of levels and polybrominated diphenyl ether (PBDE) dietary intake calculations. This study also quantifies hexabromocyclododecane (HBCD) levels in U.S.-purchased foods for the first time and estimates U.S. dietary intake of HBCD. This is part of a larger market basket study reported in two companion articles, of current levels of certain persistent organic pollutants (POPs) PBDEs, HBCD, perfluorinated compounds, polychlorinated biphenyls, and pesticides in composite food samples collected in 2008-2009. METHODS: In this study, we measured concentrations of 24 PBDE congeners and total HBCD in composite samples of 31 food types (310 samples). U.S. dietary intake of PBDEs and HBCD was estimated referencing the most current U.S. Department of Agriculture loss-adjusted food availability report. RESULTS: Total PBDE concentrations in food varied by food type, ranging from 12 pg/g wet weight (ww) in whole milk to 1,545 pg/g ww in canned sardines and 6,211 pg/g ww in butter. Total HBCD concentrations also varied substantially within and among food groups, ranging from 23 pg/g in canned beef chili to 593 pg/g in canned sardines. HBCD was not detected in any dairy samples. Dietary intake of all PBDE congeners measured was estimated to be 50 ng/day, mostly from dairy consumption but also from meat and fish. HBCD intake was estimated at 16 ng/day, primarily from meat consumption. CONCLUSION: PBDEs and HBCDs currently contaminate some food purchased in the United States, although PBDE intake estimated in this study is lower than reported in our previous market basket surveys. HBCD is in food at higher levels than expected based on previously reported levels in milk and blood compared with PBDE levels and is comparable to European levels.

Discussion of "Polybrominated diphenyl ethers in aircraft cabins--a source of human exposure?" by Anna Christiansson et al. [Chemosphere 73(10) (2008) 1654-1660].

This paper presents new data on the levels of polybrominated diphenyl ethers (PBDEs) in American airline workers. This pilot study did not find elevated total PBDEs in the blood of nine flight attendants and one aircraft pilot who have worked in airplanes for at least the past 5 years. These findings are not consistent with the findings of elevated blood levels of PBDEs from the 2008 Christiansson et al. publication "Polybrominated diphenyl ethers in aircraft cabins - A source of human exposure?" We agree that more research needs to be done on larger, more representative samples of airline workers to better characterize exposure of airline workers and other frequent flyers to PBDEs.

Hydrogen control of ferromagnetism in a dilute magnetic semiconductor.

We show that upon exposure to a remote dc hydrogen plasma, the magnetic and electronic properties of the dilute magnetic semiconductor Ga1-xMnxAs change qualitatively. While the as-grown Ga1-xMnxAs thin films are ferromagnetic at temperatures T less, similar 70 K, the samples are found to be paramagnetic after the hydrogenation, with a Brillouin-type magnetization curve even at T=2 K. Comparing magnetization and electronic transport measurements, we conclude that the density of free holes p is significantly reduced by the plasma process, while the density of Mn magnetic moments does not change.

Reductive dehalogenation of chlorinated dioxins by an anaerobic bacterium.

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) are among the most notorious environmental pollutants. Some congeners, particularly those with lateral chlorine substitutions at positions 2, 3, 7 and 8, are extremely toxic and carcinogenic to humans. One particularly promising mechanism for the detoxification of PCDDs and PCDFs is microbial reductive dechlorination. So far only a limited number of phylogenetically diverse anaerobic bacteria have been found that couple the reductive dehalogenation of chlorinated compounds--the substitution of a chlorine for a hydrogen atom--to energy conservation and growth in a process called dehalorespiration. Microbial dechlorination of PCDDs occurs in sediments and anaerobic mixed cultures from sediments, but the responsible organisms have not yet been identified or isolated. Here we show the presence of a Dehalococcoides species in four dioxin-dechlorinating enrichment cultures from a freshwater sediment highly contaminated with PCDDs and PCDFs. We also show that the previously described chlorobenzene-dehalorespiring bacterium Dehalococcoides sp. strain CBDB1 (ref. 3) is able to reductively dechlorinate selected dioxin congeners. Reductive dechlorination of 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD) demonstrates that environmentally significant dioxins are attacked by this bacterium.