Dispersion of magnetic susceptibility and the microstructure of magnetic fluid

The microstructure of magnetic fluid produced on the basis of kerosene with oleic acid as a stabilizer is studied experimentally. An analytical procedure based on the known dependence of the time of Brownian relaxation of the magnetic moment of the colloidal particle on its size and the expansion of a low-frequency spectrum of dynamic susceptibility into the series of Debye functions is used. Magnetic susceptibility is measured at frequencies from 10 Hz to 100 kHz and temperatures from 225 to 360 K for colloidal solutions with the volume fraction of magnetite from 0.08 to 0.17. The clusters with uncompensated magnetic moments and sizes varying from 50 to 70 nm that are three-or fourfold larger than the mean diameter of a single colloidal particle are found. It is revealed that characteristic sizes of clusters are virtually independent of temperature and concentration of colloidal particles. The contribution of clusters to the equilibrium susceptibility of magnetic fluid grows exponentially with decreasing temperature, being manyfold larger at low temperatures than that of single particles. The obtained temperature dependence of equilibrium susceptibility is compared with that predicted from current theoretical models.     

Detailed magnetic study on the formato-bridged MOFs with anion-tunable magnetic properties

Detailed studies of the structures, magnetic properties and photodimerization of a series of formato-bridged MOFs with the general formula M2(HCOO)3(4,4′-bpe)3(H2O)3(X) (4,4′-bpe = 4,4′-bipyridylethylene, M = Mn (1-X-), X- = ClO4-, NO3-, BF4-, I-, Br-; M = Co (2-X-), X- = ClO4-, NO-3; M = Zn (3-X-), X- = NO3-) were reported. Careful magnetic measurements on an ori- ented single crystal of 1-ClO4- determined the spin-flop magnetic phase diagram and some intrinsic parameters, such as the intralayer coupling J, the anisotropy field HA and the exchange field HE. Different anions can remarkably tune the magnetic properties of 1-X-, especially the critical fields of the spin-flop transition. Compound 2-ClO4- remained paramagnetic down to 2 K.     

On the possibility of structural and magnetic ordering in magnetic colloids

The aggregation stability of a magnetic colloid at an excess content of a surfactant is studied. The presence of aggregates with a nonzero magnetic moments is revealed; on this basis, magnetic ordering of magnetic particles in them is regarded as possible. The possible mechanisms of the formation of periodic structural lattices appearing under the action of a direct electric field on the magnetic colloid are studied. A fundamental difference between the structurization processes induced by a surfactant and by an electric field is noted: structurization processes occurring at the excess of surfactant may be associated with the flocculation, whereas such processes proceeding under the effect of an electric field may be due to phase separation of the colloid.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 161–166.Original Russian Text Copyright © 2005 by Dikanskii, Vegera, Zakinyan, Nechaeva, Gladkikh.     

Spectroscopic investigation of magnetic dipole allowed transitions through the magnetic transition moment

An experimental technique for detecting magnetic dipole allowed transitions by their dispersion-induced circular dichroism (DICD) is discussed. The DICD spectra of three nitrogen heterocycles (pyridine, pyrazine and tetraphenylporphyrin) are shown to be dominated by the n — π^* transitions, as distinct from the normal absorption spectra which are dominated by the electric dipole allowed π — π^* bands.     

Phenylboronic acid-functionalized cross-linked chitosan magnetic adsorbents for the magnetic solid-phase extraction of benzoylurea pesticides

In this study, a 4-formylphenylboronic acid-modified cross-linked chitosan magnetic nanoparticle (FPBA@CCHS@Fe₃O₄) was fabricated. The synthesized material was utilized as the magnetic solid-phase extraction adsorbent for the enrichment of six benzoylurea pesticides. In addition to B-N coordination, FPBA@CCHS@Fe₃O₄ interacts with benzoylureas through hydrogen bonds and π-π stacking interaction on account of rich active groups (amino and hydroxyl) and aromatic rings in structure. Compared to traditional extraction methods, less adsorbent (20 mg) and reduced extraction time (3 min) were achieved. The adsorbent also exhibited good reusability (no less than 10 times). Coupled with a high-performance liquid chromatography–diode array detector, satisfactory recoveries (89.1–103.9%) and an acceptable limit of detection (0.2–0.7 μg/L) were obtained. Under optimized conditions, the established method was successfully applied to the tea infusion samples from six major tea categories with acceptable recoveries ranging from 76.8 to 110%, indicating its application potential for the quantitative detection of pesticides in complex matrices.     

Fluorochrome-functionalized magnetic nanoparticles for high-sensitivity monitoring of the polymerase chain reaction by magnetic resonance

Easy to find: magnetic nanoparticles bearing fluorochromes (red) that intercalate with DNA (green) form microaggregates with DNA generated by the polymerase chain reaction (PCR). These aggregates can be detected at low cycle numbers by magnetic resonance (MR).     

Investigation of the magnetic properties of ferritin by AFM imaging with magnetic sample modulation

Individual ferritin molecules can be sensitively detected using magnetic sample modulation (MSM) combined with contact mode atomic force microscopy (AFM). To generate an oscillating magnetic field, an alternating current (AC) was applied to a solenoid placed within the base of the AFM sample stage. When a modulated electromagnetic field is applied to samples, ferromagnetic and paramagnetic nanomaterials are induced to vibrate. The flux of the AC electromagnetic field causes the ferritin samples to vibrate with corresponding rhythm and periodicity of the applied field. This motion can be detected and mapped using contact mode AFM with a soft, nonmagnetic cantilever. Changes in the phase and amplitude of the periodic motion of the sample are sensed by the tip to selectively map vibrating magnetic nanomaterials. Particle lithography was used to create nanopatterned test platforms of ferritin for MSM measurements. Regularly spaced structures of proteins provide precise reproducible dimensions for multiple successive surface measurements at dimensions of tens of nanometers. Figure Ring patterns of ferritin were used as nanoscale test platforms to characterize magnetic properties at the level of individual proteins with AFM imaging

Effect of an oscillating magnetic field on the release properties of magnetic collagen gels

The paper describes the effect of an oscillating magnetic field (OMF) on the morphology and release properties of collagen gels containing magnetic nanoparticles and microparticles and fluorescent drug analogues. Collagen gels were prepared through fibrillogenesis of collagen in the presence of iron oxide magnetic particles averaging 10 nm or 3 mum in diameter and rhodamine-labeled dextran (Dex-R) of molecular weights between 3000-70 000 g/mol. Dextran molecules effectively simulate protein-based drugs, since they have similar molecular weights and dimensions. The paper discusses the effect of an OMF on the release properties of the gels and proposes an empirical model to predict the release rate. It also demonstrates the self-repair capability of collagen gels following the structural damage caused by an OMF.     

Role of the electron transfer and magnetic exchange interactions in the magnetic properties of mixed-valence polyoxovanadate complexes

Modeling the properties of high-nuclearity, high-electron-population, mixed-valence (MV) magnetic systems remains one of the open challenges in molecular magnetism. In this work, we analyze the magnetic properties of a series of polyoxovananadate clusters of formula [V 18O 42] (12-) and [V 18O 42] (4-). The first compound is a fully localized spin cluster that contains 18 unpaired electrons located at the metal sites, while the second one is a MV cluster with 10 unpaired electrons largely delocalized over the 18 metal sites. A theoretical model that takes into account the interplay between electron transfer and magnetic exchange interactions is developed to explain the unexpected enhancement of the antiferromagnetic coupling when the number of unpaired electrons is reduced from 18 to 10 in these clusters. In the MV area, these systems represent the most complex magnetic clusters studied theoretically so far. Because of the high complexity of the systems, the number of relevant parameters is too large for a conventional model Hamiltonian approach. We therefore perform a theoretical study that combines ab initio calculations with the model Hamiltonian. In this way, we use ab initio calculations performed on small fragments of the cluster to lower the degrees of freedom of the parameter set of the model Hamiltonian that operates in the whole MV cluster. This approach shows the usefulness of combining ab initio calculations with model Hamiltonians in order to explore the magnetic properties of large and complex molecular systems, emphasizing the key role played by the electron transfer in these model magnetic materials.     

Slow magnetic relaxation in a cobalt magnetic chain

A homospin ladder-like chain, [Co(Hdhq)(OAc)](n) (1; H(2)dhq = 2,3-dihydroxyquinoxaline), shows a single-chain-magnet-like (SCM-like) behavior with the characteristics of frequency dependence of the out-of-phase component in alternating current (ac) magnetic susceptibilities and hysteresis loops.     

Joule heating influence on the vitality of fungi in pulsed magnetic fields during magnetic permeabilization

Generation of the pulsed magnetic fields that are 5–6 orders of magnitude higher than the geomagnetic field requires switching of high pulsed currents. As a result, the occurrence of the Joule heating in the inductors limits the possible biological applications of the pulsed magnetic fields. This work is focused on the investigation of the generated Joule heating inside the inductors of different shapes. The analysis of the Joule heating influence on the vitality of biological objects during magnetic permeabilization is presented. The biological objects that are used in the study are the pathogenic fungi Candida albicans and Trichophyton rubrum, which are the common cases for human infections. The finite element method analysis of the pulsed inductors and the experimental results with the selected pathogenic fungi are overviewed. The limitations of the magnetic permeabilization technique due to the generated Joule heating are identified.     

Spin dynamics of the radical pair in a low magnetic field studied by the transient absorption detected magnetic field effect on the reaction yield and switched external magnetic field

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.     

Dissipative structures formed in the layer of magnetic suspension upon the action of rotating elliptically polarized magnetic field

Structural transformations that take place in an ensemble of spherical particles suspended in a liquid layer with finite thickness are studied upon the action of rotating elliptically polarized magnetic field with the polarization plane perpendicular to the plane layer. Results of physical and numerical studies of the transformation of dissipative structure upon variations in the polarization ellipse of magnetic field are reported.     

Controlling the magnetic field distribution on the micrometer scale and generation of magnetic bead patterns for microfluidic applications

As is well known, controlling the local magnetic field distribution on the micrometer scale in a microfluidic chip is significant and has many applications in bioanalysis based on magnetic beads. However, it is a challenge to tailor the magnetic field introduced by external permanent magnets or electromagnets on the micrometer scale. Here, we demonstrated a simple approach to controlling the local magnetic field distribution on the micrometer scale in a microfluidic chip by nickel patterns encapsulated in a thin poly(dimethylsiloxane) (PDMS) film under the fluid channel. With the precisely controlled magnetic field, magnetic bead patterns were convenient to generate. Moreover, two kinds of fluorescent magnetic beads were patterned in the microfluidic channel, which demonstrated that it was possible to generate different functional magnetic bead patterns in situ, and could be used for the detection of multiple targets. In addition, this method was applied to generate cancer cell patterns.     

Calculation of the magnetic properties of C-H bonds and their contribution to the magnetic susceptibility of saturated hydrocarbons

The magnetic susceptibility tensors of the C-H bonds in the CH₄, C₂H₆, C₃H₈, n-C₅H₁₂, iso-C₅H₁₂, and neo-C₅H₁₂ molecules have been calculated by the method of varying the vector potential with the use of a multiparameteric gradient-transformation function, which was constructed in the form of a polynomial in spherical coordinates and takes into account the electron correlation. The influence of the electron correlation on the magnetic properties of C-H bonds is greater than that in homonuclear molecules (H₂) and bonds (C-C) and increases strongly with enhancement of the electron correlation in the wave function not perturbed by an external magnetic field. In contrast to the previously established identity of the magnetic properties of C-C bonds, the magnetic properties of C-H bonds depend both on the structure and geometry of the molecule as a whole and on the location of the bond itself in the molecule. The values of the mean susceptibility of the molecules considered calculated in the additive approximation are in good agreement with the experimental values.     

The magnetic properties of the manganese-gallium-aluminum-germanium system

The properties of solid solutions of composition MnGa_1?xAl_xGe formed from ferromagnetic MnGaGe (T_c = 185°C) and MnAlGe (T_c = 245°C) have been examined. In the range, 0 < x < 0.1, the Curie temperature and saturation magnetization are not a monotonic function of x as might be expected for a solid solution of two ferromagnets. Instead, for x = 0.025, the Curie temperature is actually ～77°C lower than that of MnGaGe. Despite the similarity in the structure and lattice constants of MnGaGe and MnAlGe, aluminum rich compositions, e.g., x = 0.95 show no solid solution. Attempts to prepare other ferromagnetic ternary Mn (IIIA) (IVA) compounds have not been successful.     

The mechanism of the avian magnetic compass

The avian magnetic compass was analyzed by testing migratory birds, using their orientation as an indicator. These tests revealed some remarkable properties of the avian magnetic compass: (1) It is an inclination compass’, (2) it is light-dependent, with (3) receptors located in the right eye. These characteristics are in agreement with the Radical Pair model proposed by Ritz et al. (2000). Using the same experimental set-up, we tested the model by behavioral spectroscopy’, exposing migratory birds to radiofrequency fields of different frequencies and intensities. Such fields affected the orientation only when applied at an angle to the field lines. Tests with different frequencies led to an estimate of the life time of the crucial radical pair between 2-10 μs. We also could identify an extremely sensitive resonance at the Larmor frequency, which implies specific properties of the radical pair. Cryptochromes, a blue-light absorbing photopigment, has been proposed to be the receptor-molecule; it has been found to be present in the retina of birds.