Temperature dependence of the magnetization of canted spin structures

Numerous studies of the low-temperature saturation magnetization of ferrimagnetic nanoparticles and diamagnetically substituted ferrites have shown an anomalous temperature dependence. It has been suggested that this is related to freezing of canted magnetic structures. We present models for the temperature dependence of the magnetization of a simple canted spin structure in which relaxation can take place at finite temperatures between spin configurations with different canting angles. We show that the saturation magnetization may either decrease or increase with decreasing temperature, depending on the ratio of the exchange coupling constants. This is in agreement with experimental observations.     

Magnetic Properties of Nanoparticles of Antiferromagnetic Materials

Hyperfine Interactions - The magnetic properties of antiferromagnetic nanoparticles have been studied by Mössbauer spectroscopy and neutron scattering. Temperature series of Mössbauer...     

Uncertainty budget for determinations of mean isomer shift from Mössbauer spectra

The magnetite/maghemite content within iron oxide nanoparticles can be determined using the mean isomer shift (\(\overline {\delta }\)). However, accurate characterisation of the composition is limited by the uncertainty associated with \(\overline {\delta }\). We have identified four independent sources of uncertainty and developed a quantitative expression for the uncertainty budget. Sources of uncertainty are categorised as follows: that from the fitting of the Mössbauer spectrum (σ _fit), that of the calibration of the α-Fe reference spectrum (σ _cal), thermal corrections to the spectrum due to second order Doppler shift (SODS) (σ _Δδ ) and other experimental errors (σ _err). Each contribution is discussed in detail using ⁵⁷Fe Mössbauer spectra obtained from an iron oxide nanoparticle system at temperatures between 16 K and 295 K on different spectrometers in two different laboratories.     

Interparticle interactions in agglomerates of alpha-Fe2O3 nanoparticles: influence of grinding

We have chemically prepared a sample of antiferromagnetic alpha-Fe2O3 nanoparticles by a gel-sol technique. M?ssbauer spectra of the as-prepared sample showed that superparamagnetic relaxation was suppressed due to strong magnetic interparticle interactions even at room temperature. However, subsequent grinding of the sample by hand in a mortar for some minutes resulted in fast superparamagnetic relaxation of some of the particles. The effect was even more dramatic if the alpha-Fe2O3 powder was ground for a longer time or together with nonmagnetic eta-Al2O3 nanoparticles. Similar effects were found after low-energy ball milling. Thus it is found that the agglomeration of the nanoparticles during preparation under wet conditions results in strong magnetic interparticle interaction, but a relatively gentle mechanical treatment is sufficient to break up the agglomerates, resulting in much weaker interactions. We show that these effects can also be seen when a soil sample containing magnetic nanoparticles is ground.     

LC–MS/MS Method for Simultaneous Determination of Monoethanol- and Dimethylnitramine in Aqueous Soil Extracts

Monoethanol (MEA)- and dimethyl (DMA)-nitramines are by-products of amine-based post-combustion CO₂ capture (PCCC) processing, and are potentially carcinogenic. The compounds are challenging to measure, also with LC–tandem mass spectrometry (MS/MS), attributed to their high polarity and extreme proneness to matrix effects. In contrast to related methods, the MEA- and DMA-nitramines were simultaneously determined in aqueous soil extracts in less than 10 min using a 1 mm × 150 mm Atlantis^® T3 (3 µm) C18 column. A mobile phase of water/methanol (90/10, v/v) and 2 mM acetic acid allowed for electrospray ionization (ESI) of both analytes [in contrast to the need for both ESI and atmospheric pressure chemical ionization (APCI) in related methods]. Polarity switching electrospray was required for the simultaneous detection of the analytes, and concentration limits of detection (LODs) in the aqueous soil extracts were ≤5.0 µg L^?1 using an injection volume of 20 μL and no prior enrichment step. Matrix effects were compensated for using isotope-labelled internal standards, and satisfactory precision and linearity were obtained (within- and between-day precisions ≤19%, r ² ≥ 0.995 for concentrations up to 500.0 µg L^?1). To avoid signal decrease over time when measuring DMA-nitramine alone, the use of polarity switching was beneficial, in addition to frequent cleaning of the ion transfer capillary. The validated method can be used to determine nitramines in aqueous soil extracts, which is of importance as soil sorption is a determinant of the compounds’ environmental fate.     

Dual‐Function Cobalt–Nickel Nanoparticles Tailored for High‐Temperature Induction‐Heated Steam Methane Reforming

The tailored chemical synthesis of binary and ternary alloy nanoparticles with a uniform elemental composition is presented. Their dual use as magnetic susceptors for induction heating and catalytic agent for steam reforming of methane to produce hydrogen at temperatures near and above 800 °C is demonstrated. The heating and catalytic performance of two chemically synthesized samples of CoNi and Cu ? CoNi are compared and held against a traditional Ni‐based reforming catalyst. The structural, magnetic, and catalytic properties of the samples were characterized by X‐ray diffraction, elemental analysis, magnetometry, and reactivity measurements. For induction‐heated catalysts, the conversion rate of methane is limited by chemical reactivity, as opposed to the case of traditional externally heated reformers where heat transport limitations are the limiting factor. Catalyst production by the synthetic route allows controlled doping with miniscule concentrations of auxiliary metals.     

Optimization of capillary array electrophoresis single-strand conformation polymorphism analysis for routine molecular diagnostics

Mutation screening is widely used for molecular diagnostics of inherited disorders. Furthermore, it is anticipated that the present and future identification of genetic risk factors for complex disorders will increase the need for high-throughput mutation screening technologies. Capillary array electrophoresis (CAE) SSCP analysis is a low-cost, automated method with a high throughput and high reproducibility. Thus, the method fulfills many of the demands to be met for application in routine molecular diagnostics. However, the need for performing the electrophoresis at three temperatures between 18 degrees C and 35 degrees C for achievement of high sensitivity is a disadvantage of the method. Using a panel of 185 mutant samples, we have analyzed the effect of sample purification, sample medium and separation matrix on the sensitivity of CAE-SSCP analysis to optimize the method for molecular diagnostic use. We observed different effects from sample purification and sample medium at different electrophoresis temperatures, probably reflecting the complex interplay between sequence composition, electrophoresis conditions and sensitivity in SSCP analysis. The effect on assay sensitivity from three different polymers was tested using a single electrophoresis temperature of 27 degrees C. The data suggest that a sensitivity of 98-99% can be obtained using a 10% long chain poly-N,N-dimethylacrylamide polymer.     

Colloidal Flower‐Shaped Iron Oxide Nanoparticles: Synthesis Strategies and Coatings

The assembly of magnetic cores into regular structures may notably influence the properties displayed by a magnetic colloid. Here, key synthesis parameters driving the self‐assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi‐core nanoparticles are determined. In addition, a self‐consistent picture that explains the collective magnetic properties exhibited by these complex assemblies is achieved through structural, colloidal, and magnetic means. For this purpose, different strategies to obtain flower‐shaped iron oxide assemblies in the size range 25–100 nm are examined. The routes are based on the partial oxidation of Fe(OH)₂, polyol‐mediated synthesis or the reduction of iron acetylacetonate. The nanoparticles are functionalized either with dextran, citric acid, or alternatively embedded in polystyrene and their long‐term stability is assessed. The core size is measured, calculated, and modeled using both structural and magnetic means, while the Debye model and multi‐core extended model are used to study interparticle interactions. This is the first step toward standardized protocols of synthesis and characterization of flower‐shaped nanoparticles.     

Models for Ground Water Flow: A Numerical Comparison Between Richards' Model and the Fractional Flow Model

In this paper we compare two models for flow in porous media. The first is the well known Richards' equation, which is based on the assumption that the air in the unsaturated zone has infinite mobility. This means that it models a single phase. In the second and more general full two-phase approach, the air is considered as a separate phase. Here, we use the fractional flow equation.We study the difference between the two models numerically by varying the relative contribution of the different physical terms (the gravity and the total velocity) in the fractional flow equation. Richards' equation is considered as the limit of the fractional flow approach when the mobility of the air-phase tends to infinity. In particular, we are interested in determining the parameter intervals where the two models differ significantly, and we will quantify the asymptotic behavior.The equations are studied in the two-dimensional (2D) case. The study is based on a relative permeability depending quadratically on the saturation, and a capillary pressure expressed by a cubic function of the saturation.