Ab initio investigations in magnetic oxides

It is argued that band theory can be used as a valuable tool to investigate and model properties of several classes of oxide systems. This theoretical framework is presented in the first part of the article with details on the density functional theory (DFT) and its approximations leading to the local spin density functional LSDF as well as on the common methods built around it (mainly the augmented spherial wave ASW method used throughout this work). The major part of the article is devoted to case studies meant to illustrate the wealth and limits of the LSDF in addressing the electronic and magnetic structures of a selected variety of transition metal oxides of technological importance. Such systems are iron monoxide “FeO” (mainly the high pressure form), the archetype of half metallic ferro magnets CrO₂, used in magnetic recording media, the simple and double perovskite derived systems: SrFeO₃ and La₂TIrO₆ (T=Mn, Fe, Co) as well as manganese oxides (CaMnO₃, Ca₄Mn₄O₁₀ and Tl₂Mn₂O₇) for which a giant magnetoresistive GMR behaviour was identified in recent years. The computed ground state magnetic configurations and electronic structure results are discussed and confrontations with experimental data are carried out when available. The chemical bonding properties are analysed, and a new conceptual approach is provided.     

Self-assembly of colloidal pyramids in magnetic fields

We study routes toward the construction of 2D colloidal pyramids. We find that magnetic beads may self-assemble into pyramids near a nonmagnetic 1D boundary as long as the number of beads in the pyramid does not exceed 10. We have also found that a strong magnetic field gradient could act as a boundary, thus assisting the self-assembly of magnetic colloids in water, and have observed the formation of stable microscopic pyramids within a certain magnetic field range. Our results indicate that colloidal pyramids can be formed in a number of ways by utilizing external fields.     

Millimeter scale alignment of magnetic nanoparticle functionalized microtubules in magnetic fields

The conjugation of 14 nm diameter CoFe2O4 nanoparticles to the surface of biotinylated microtubules enables their manipulation with externally applied magnetic fields of small, permanent NdFeB magnets. Microtubules are selectively patterned on kinesin motor-modified glass surfaces in coparallel arrays that mimic the orientation of the magnetic field lines over millimeter distances. The magnetic field is simultaneously used to increase surface loading of microtubules. We demonstrate that motility across the kinesin motor surface is retained following magnetic functionalization of the microtubules, while gliding speed is dependent on loading level of the neutravidin linker as well as magnetic nanoparticles.     

Resonant states of the hydrogen atom in strong magnetic fields

Resonant states of atomic hydrogen in strong magnetic fields have been computed by semiclassical methods. Eigenvalues are obtained by using an adiabatic separation of variables and standard WKB methods; these are confirmed by semiclassical quantization of numerically computed quasiperiodic trajectories. Large numbers of resonant states are found at B = 10 kT for L_z values above 20.     

Nematic reorientation in electric and magnetic fields

Homogeneous reorientation processes of two nematic liquid crystals in electric and magnetic fields have been observed using proton nuclear magnetic resonance spectroscopy (NMR). Using a recently developed experimental set-up, it is possible to study reorientation processes in liquid crystals by means of NMR experiments in a very flexible way. The time constant τ describing these processes has been determined as a function of the applied electric field. It emerges that the electric field cannot only be used to increase the reorientation time but also to slow the director reorientation by approximately one order of magnitude. Experimental data for 5CB and a fluorinated liquid crystal (BCH-5 FFF) are presented. The reorientation time measured as a function of the electric field can be used to calculate the rotational viscosity γ ₁. By repeating these experiments at different temperatures it was possible to investigate the temperature behaviour of γ ₁.     

The Breit interaction in external magnetic fields

The Breit energy of a system, which is shown to depend on the external magnetic vector potential, is exactly transformed, by applying Gordon decompositions, to a form explicitly exhibiting some of the field dependence. The result is used to derive the leading Breit contributions to NMR chemical shifts and atomic g factors.     

Electrical properties of NbO in high magnetic fields

The resistivity and Hall coefficients of polycrystalline samples, single crystals, and doped specimens of NbO_x (0.98 ? x ? 1.02) were measured in magnetic fields up to 160 kG at 4.2, 78, and 295 K. The material is found to be an excellent conductor. The results are interpreted in terms of a nearly free electron model in which holes and electrons contribute jointly to conduction processes. Orbital switching, magnetic breakdown effects, and perturbations arising from impurities are invoked to explain the observed magneto-resistance anomalies.     

Some aspects of TLC in homogenous magnetic fields

This article consists of two parts. First part is a short review about the role of magnetic phenomena in natural environment, human surroundings, and his activities such as science, engineering, and medicine. The second part of the article presents a set of experiments, their results, and data obtained in a static homogenous magnetic field, generated by a pair of permanent magnets and outside it. Adsorption chromatographic systems were investigated: as chromatographed substances – polyaromatic hydrocarbon (PAH), as stationary phase – silica gel 60, as monocomponent mobile phases – n‐hexane, n‐heptane, n‐octane, and benzene were used and binary mobile phases n‐hydrocarbons – benzene. Magnetic field influences retention and efficiency of investigated chromatographic systems. Experimental data analysis (R_F, N) allows us to propose some explanations of the differences between experiment results performed in induced magnetic field and outside it, and in consequence on the changes in the interfacial phenomena induced by field presence.     

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.     

Nuclear magnetic resonance structural investigations of ammonia-doped fullerides

The dynamic and structural properties of the ammonia-doped superconducting fulleride (NH3)xNaK2C60 (0.5< or =x< or =1), well known for its anomalous decrease of transition temperature with doping, have been investigated using sodium and deuterium solid-state NMR techniques. The independence of 23Na quadrupole splitting from the ammonia content x, which, at the same time, substantially affects Tc, suggests a marginal role of the cation position in the superconducting mechanism. On the other hand, a strong reduction of the deuterium quadrupole coupling with respect to the free ammonia value denotes the presence of weak hydrogen bonds between the deuterium atoms and fullerene pi orbitals. Despite the bond weakness, as evinced by the lively ammonia rotational dynamics even at very low temperatures, the resulting electron localization could explain the observed Tc anomaly. The motion of the ND3-Na group (located in the compound's octahedral voids), as well as the evolution of the ammonia dynamics as a function of temperature, were determined from deuterium NMR line shape analysis and from detailed numerical simulations. While at the lowest measured temperatures only the ammonia rotation around its own C3 axis takes place, above approximately 25 and 70 K, respectively, also the wobbling of the C3 axis and the ND3 relocation become active, successfully modeled by a strongly correlated motion involving two different time scales.     

Investigations of the sodium and lithium D-lines in strong magnetic fields

Experimental and theoretical investigations of the splitting of the hyperfine structure of the sodium and lithium-D-lines in magnetic fields between 0 and 1 T were performed. In this magnetic field region the fine structure levelsJ=1/2 andJ=3/2 of the excited term² P begin to influence each other. In case of lithium crossings and anticrossings of hyperfine states stemming from different fine structure energy levelsJ=1/2 andJ=3/2 can be observed. The measurements were performed by laseratomic-beam spectroscopy in dependence on the applied external magnetic field strength. The experimental spectra were compared with computed spectra. Spectra were simulated by calculations using for the hyperfine hamiltonian two hyperfine constantsA andB in case of sodium and four hyperfine constantsa _c ,a _d ,a ₀ andb in case of lithium. Values for this constants could be derived by fitting the theoretical splittings to the experimental ones. For the first time theg _I — factor of sodium could be determined in a purely optical way.     

Magnetic properties of the fullerene organic compounds in strong magnetic fields

Magnetisation study of the C₆₀·TMTSF·2CS₂ molecular complex in magnetic field up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)₂C₆₀ at T=1.8 K for the frequency range 60–90 GHz in magnetic field up to 32 T provide the experimental evidence that a paramagnetic centers with the reduced g-factor values g<1 control magnetic properties of these solids. Anomalous g-factor values may be caused by dynamic Jan-Teller effect on the negative C₆₀ ions, which appear as defects in crystalline structure with a weak charge transfer.     

Investigations on the kinetics of electron transfer reactions in magnetic fields

There is a controversial debate if a magnetic field can influence the rate of electron transfer (ET) reactions. In this paper, we report kinetic measurements of the ET rate constants for the redox couples [IrCl6]2-/[IrCl6]3-, [Fe(CN)6]3-/[Fe(CN)6]4-, and [Fe(H2O)6]3+/[Fe(H2O)6]2+ in magnetic fields up to 1 T. To reduce effects arising from magnetically induced mass transport (magnetohydrodynamic effect), disk microelectrodes with a diameter of 50 microm were used in potentiodynamic (cyclic and linear sweep voltammetry) and in electrochemical impedance spectroscopy experiments. None of the investigated redox couples showed a magnetic field effect on the ET rate constant.     

Stability of the Rydberg atom in the crossed magnetic and electric fields

The stability of equilibrium positions of the Rydberg atom exposed to the uniform crossed electric and magnetic fields is analyzed. The dynamics of the system is described by an autonomous Hamiltonian depending on parameters a and f. By the normalization of the quadratic part of the Hamiltonian expansion in the neighborhood of the equilibrium position it is proved that for any f < 0 and ${1 \over 2} < a < {1 \over 2} + {{( - f)^{3/2} } \over {3\sqrt 3 }}$ , the equilibrium solution of the equations of motion is stable in Liapunov sense, while for f > 0 and a < 1/2, there is a domain of instability in the plain of parameters Ofa bounded by the curve d₃ = 0. In the domain of linear stability, it is proved that there are two curves in the plane Ofa, where the resonance conditions of third (ω₁ = 2ω₂) and fourth (ω₁ = 3ω₂) order are fulfilled. Moreover, by the normalization of the third‐ and fourth‐order terms in the Hamiltonian expansion it is proved that in the case of the third‐order resonance, the equilibrium position is unstable for all f > 0 different from f = 0.111572 and f = 0.281144, for which the stability takes place. In the case of the fourth‐order resonance, there are two intervals of parameters for which the equilibrium position is unstable. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Falling sphere viscometry in gravitational and magnetic fields

The classical falling sphere method based onStokes' law is well known. Using gravitational forces, however, restricts the range of measurements considerably. This advantage can be overcome by applying iron spheres in a magnetic field, which allows to measure polymer melts with viscosities up to ₀=10⁹ Pa s (with very strong magnets available even more). A special measuring cell designed for application in the magnetic field is tested with several silicon oils in the gravitational field too. In both cases wall and end corrections toStokes' law have to be considered and are of main concern. Another important point is a reliable temperature measurement. Results especially for the high viscosity region of polymer melts are given.Herrn Prof. Dr.K. Komarek zum 60. Geburtstag gewidmet.     

Micropatterning of cells using modulated magnetic fields

A new technique of cell micropatterning was presented. Mouse osteoblast cells (MC3T3-E1) were seeded on a substrate whose surface was exposed to a periodically modulated magnetic field (a line pattern with a 200- or 600-microm pitch) produced by a field modulator inserted into a homogeneous magnetic field of 1 T generated by an electromagnet. The cells were trapped consistent with the line profile of the modulated field. The trapping efficiency was enhanced by adding Mn(II)EDTA (paramagnetic) to the cultivation medium. The cells were subsequently incubated in the magnetic field. The same technique was applied to whole blood to pattern red blood cells.