Magnetic iron oxide nanopowders produced by CO2 laser evaporation—‘In situ’ coating and particle embedding in a ceramic matrix

The CO₂ laser evaporation technique is not only well suited for the production of magnetic iron oxide nanopowders, but also allows for their conditioning. Two optional methods, the ‘in situ’ coating and the co-laser evaporation, are introduced. Laser-generated magnetic Fe_xO_y nanoparticles very frequently form chain-like structures, which were stabilized by ‘in situ’ coating with stearic acid. A first attempt was made to align these chains in a magnet field before the coating process. Homogeneous hematite/silica mixtures were co-laser evaporated in order to embed Fe_xO_y nanoparticles in a silica matrix. The produced nanopowders were analyzed with TEM, X-ray diffraction (XRD), and magnetic measurements.     

Electron spectroscopy and density-functional study of ‘ferric wheel’ molecules

The Li-centered ‘ferric wheel’ molecules with six oxo-bridged iron atoms form molecular crystals. We probed their electronic structure by X-ray photoelectron and soft X-ray emission spectroscopy, having calculated in parallel the electronic structure of a single ‘ferric wheel’ molecule from first-principles by tools of the density-functional theory, using, specifically, the Siesta method. The Fe local moments were found to be 4μ_B, irrespective of their mutual orientation. Neighbouring atoms, primarily oxygen, exhibit a noticeable magnetic polarization, yielding effective spin S=5/2 per iron atom, that can get inverted as a ‘rigid’ one in magnetic transitions. Corresponding energy preferences can be mapped onto the Heisenberg model with effective exchange parameter J of about −80 K.     

Room temperature photoluminescence from amorphous silicon nanoparticles in SiOx thin films

Amorphous SiO_x thin films with four different oxygen contents (x=1.15, 1.4, 1.5, and 1.7) have been prepared by thermal evaporation of SiO in vacuum and then annealed at 770 or 970 K in argon for various times ?40 min. The influence of annealing conditions and the initial film composition on photoluminescence (PL) from the annealed films has been explored. Intense room temperature PL has been observed from films with x?1.5, visible with a naked eye. It has been shown that PL spectra of most samples consists of two main bands: (i) a ‘green’ band centered at about 2.3 eV, whose position does not change with annealing conditions and (ii) an ‘orange-red’ band whose maximum moves from 2.1 to 1.7 eV with increasing annealing time and temperature and decreasing initial oxygen content. These observations have been explained assuming recombination via defect states in the SiO_x matrix for the first band and emission from amorphous Si nanoparticles for the second one.     

Magnetic phase diagram of double-layered La2−2xCa1+2xMn2O7 manganite

Double-layered manganite La_2−2xCa_1+2xMn₂O₇ have been synthesized for compositions ‘x’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘x’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘x’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.     

Magnetic hysteresis in type II superconductors

The theories governing the magnetic behavior of Type II superconductors assume reversible magnetization curves. However, due to imperfections and inhomogeneities, many superconductors possess hysteretic curves. A theory is proposed which allows a ‘reversible’ curve to be found from the two irreversible curves. The ‘reversible’ curve is an approximation to the exact Abrikosov reversible curve for H_c1 ≤ H ≤ H_c2. The theory agrees closely with experimental results.     

Controlling the crystal structure of Ni nanoparticles by the use of alkylamines

Ni nanoparticles were prepared via thermal decomposition of nickel acetate tetrahydrate in the presence of long-chain amines, which acted as both solvents and reducing agents. By tuning the reaction temperature, Ni nanostructures with either hcp or fcc crystal structure were obtained. In principle, higher temperatures favored the formation of hcp nanoparticles. The employment of additional surfactants such as 1-adamantanecarboxylic acid and trioctylphosphine-oxide facilitated the tuning of the particles’ growth limit. The size of the particles varied between 5 and 120 nm. The magnetic features of fcc-Ni nanoparticles were quite similar to the corresponding ‘bulk’ ones. On the other hand, the hcp-Ni particles showed weak magnetic features, reflected by low magnetization values, the absence of saturation magnetization and by blocking temperatures far below room temperature.     

The a.c. susceptibility third harmonic component of NdO1−0.14F0.14FeAs: A flux dynamic magnetic analysis

In this contribution we present the analysis of the third harmonic susceptibility data of the new superconductor NdFeAs_1−0.14F_0.14. ‘Cole-Cole’ polar plots respect to the magnetic frequency of the exciting field are presented and discussed. Data show that NdFeAs_1−0.14F_0.14 exhibit a ‘bulk pinning’ with a 3D flux dynamic character. A comparison of the responses of high T_c materials and an evaluation of theoretical critical states is also presented.     

Non-Abelian hydrodynamics and the flow of spin in spin-orbit coupled substances

Motivated by the heavy ion collision experiments there is much activity in studying the hydrodynamical properties of non-Abelian (quark-gluon) plasmas. A major question is how to deal with color currents. Although not widely appreciated, quite similar issues arise in condensed matter physics in the context of the transport of spins in the presence of spin-orbit coupling. The key insight is that the Pauli Hamiltonian governing the leading relativistic corrections in condensed matter systems can be rewritten in a language of SU(2) covariant derivatives where the role of the non-Abelian gauge fields is taken by the physical electromagnetic fields: the Pauli system can be viewed as Yang-Mills quantum-mechanics in a ‘fixed frame’, and it can be viewed as an ‘analogous system’ for non-Abelian transport in the same spirit as Volovik’s identification of the He superfluids as analogies for quantum fields in curved space time. We take a similar perspective as Jackiw and coworkers in their recent study of non-Abelian hydrodynamics, twisting the interpretation into the ‘fixed frame’ context, to find out what this means for spin transport in condensed matter systems. We present an extension of Jackiw’s scheme: non-Abelian hydrodynamical currents can be factored in a ‘non-coherent’ classical part, and a coherent part requiring macroscopic non-Abelian quantum entanglement. Hereby it becomes particularly manifest that non-Abelian fluid flow is a much richer affair than familiar hydrodynamics, and this permits us to classify the various spin transport phenomena in condensed matter physics in an unifying framework. The “particle based hydrodynamics” of Jackiw et al. is recognized as the high temperature spin transport associated with semiconductor spintronics. In this context the absence of faithful hydrodynamics is well known, but in our formulation it is directly associated with the fact that the covariant conservation of non-Abelian currents turns into a disastrous non-conservation of the incoherent spin currents of the high temperature limit. We analyze the quantum-mechanical single particle currents of relevance to mesoscopic transport with as highlight the Ahronov-Casher effect, where we demonstrate that the intricacies of the non-Abelian transport render this effect to be much more fragile than its abelian analog, the Ahronov-Bohm effect. We subsequently focus on spin flows protected by order parameters. At present there is much interest in multiferroics where non-collinear magnetic order triggers macroscopic electric polarization via the spin-orbit coupling. We identify this to be a peculiarity of coherent non-Abelian hydrodynamics: although there is no net particle transport, the spin entanglement is transported in these magnets and the coherent spin ‘super’ current in turn translates into electric fields with the bonus that due to the requirement of single valuedness of the magnetic order parameter a true hydrodynamics is restored. Finally, ‘fixed-frame’ coherent non-Abelian transport comes to its full glory in spin-orbit coupled ‘spin superfluids’, and we demonstrate a new effect: the trapping of electrical line charge being a fixed frame, non-Abelian analog of the familiar magnetic flux trapping by normal superconductors. The only known physical examples of such spin superfluids are the ³He A- and B-phase where unfortunately the spin-orbit coupling is so weak that it appears impossible to observe these effects.     

Numerical modeling of magnetic drug targeting

A special focused magnet, designed for the use in the magnetic targeted drug delivery system, was constructed. The theoretical calculation of the adhesion condition for a magnetic fluid drop in magnetic field with obtained design showed that the constructed focused magnet generates a sufficient magnetic force for the capture of a magnetic drop on the vessel wall and can be used 1.5–2 cm deeper in an organism compared with the prism permanent magnet, which can enable non-invasivity of the magnetic drug targeting procedure. The maximal values for magnetic field and gradient of magnetic field are 0.38 T and 101 T/m, respectively.     

Collimation testing using wedge plate lateral shearing interferometry and Fourier fringe analysis

For checking the collimation of an optical beam Fourier fringe analysis has been incorporated into the wedge plate interferometric setup. Typical interferograms corresponding to ‘in-focus’, ‘at-focus’ and ‘out-of-focus’ positions of an optical beam have been recorded. As per the testing procedure, FFT of the recorded interferometer is computed digitally, and necessary processing for direct determination of phase is undertaken. Finally, the phase data is unwrapped and plotted as a function of pixel position along the direction perpendicular to the shear. The slope of the phase provides the information regarding collimation position of the collimator. As the collimation position is detected by the direct measurement of the phase over the whole area of the interferogram, high accuracy, reliability and precision are achieved.