Synthesis, Structures, and Magnetic Behavior of a Series of Copper(II) Azide Polymers of Cu-4 Building Clusters and Isolation of a New Hemiaminal Ether as the Metal Complex

Four new neutral copper azido polymers, Cu-4(N-3)(8)(L-1)(2)](n) (1), Cu-4(N-3)(8)(L-2)(2)](n) (2), Cu-4(N-3)(8)(L-3)(2)](n) (3), and Cu-9(N-3)(18)(L-4)(4)](n) (4) L1-4 are formed in situ by reacting pyridine-2-carboxaldehyde with 22-(methylamino)ethyl]pyridine (mapy, L-1), N,N-dimethylethylenediamine (N,N-dmen, L-2), N,N-diethylethylenediamine (N,N-deen, L-3), and N,N,2,2-tetramethylpropanediamine (N,N,2,2-tmpn, L-4)], have been synthesized by using 0.5 mol equiv of the chelating tridentate ligands with Cu-(NO3)(2)center dot 3H(2)O and an excess of NaN3. Single-crystal X-ray structures show that the basic unit of these complexes, especially 1-3, contains very similar Cu-4(II) building blocks. The overall structure of 3 is two-dimensional, while the other three complexes are one-dimensional in nature. Complex 1 represents a unique example containing hemiaminal ether arrested by copper(R). Complexes 1 and 2 have a rare bridging azido pathway: both end-on and end-to-end bridging azides between a pair of Cu-II centers. Cryomagnetic susceptibility measurements over a wide range of temperature exhibit dominant ferromagnetic behavior in all four complexes. Density functional theory calculations (B3LYP functional) have been performed on complexes 1-3 to provide a qualitative theoretical interpretation of their overall ferromagnetic behavior.

Enhancement of uniaxial magnetic anisotropy in Fe thin films grown on GaAs(001) with an MgO underlayer

The understanding and control of anisotropy in Fe films grown on cubic systems such as GaAs and MgO has been of interest from the point of view of applications in devices. We report magnetic anisotropy studies on Fe/GaAs(001) and Fe/MgO/GaAs(001) prepared by pulsed laser deposition. In Fe/GaAs(001), magneto optical Kerr effect (MOKE) measurements revealed a dominant uniaxial anisotropy for Fe thickness less than 20 monolayers (ML) and this was confirmed by ferromagnetic resonance (FMR) studies. Multiple steps in the hysteresis loops were observed for Fe films of thickness 20 and 25 ML. Whereas, in Fe/MgO/GaAs(001), even at 25 ML of Fe, the uniaxial anisotropy remained dominant. The anisotropy constants obtained from FMR spectra have shown that the relative strength of uniaxial anisotropy is higher as compared to the cubic anisotropy constant in the case of Fe/MgO/GaAs(001). (C) 2011 American Institute of Physics. doi:10.1063/1.3556941]

Influence of the ferromagnetic layer on the pair breaking and low alternating current field magnetic response in superconductor/ferromagnet bilayer

We investigate the influence of the ferromagnetic layer on the magnetic and transport properties of YBa2Cu3O7-delta in YBa2Cu3O7-delta (YBCO)/La0.7Sr0.3MnO3 (LSMO) bilayers. The temperature dependent dc magnetization study reveals the presence of magnetic anisotropy in YBCO/LSMO bilayer as compared to the pure YBCO layer. The ac susceptibility study on YBCO/LSMO bilayers reveals stronger pinning and the temperature dependent critical current is found to be less prone to temperature. Besides, the current (I) dependent electrical transport studies on YBCO/LSMO exhibit a significant reduction in the superconducting T-c with increase in I and it follows I-2/3 dependence in accord with the pair breaking effect. The higher reduction of superconducting T-c in YBCO/LSMO is believed to be due to the enhanced pair-breaking induced by the spin polarized carriers being injected into the superconductor. (C) 2011 American Institute of Physics. doi: 10.1063/1.3560029]

Screw dynamo and the generation of nonaxisymmetric magnetic fields

A mechanism is presented here for the amplification of large-scale nonaxisymmetric magnetic fields as a manifestation of the dynamo effect. We generalize a result on restrictions of dynamo actions due to laminar flow originally derived by Zeldovich, Ruzmaikin, and Sokolov [Magnetic Fields in Astrophysics (Gordon and Breach, New York, 1983)]. We show how a screwlike motion having phi and z components of velocity can help to grow a magnetic field. This model postulates a large-scale flow having phi and z components with radial dependences (helical flow). Shear in the radial field, because of a near-flux-freezing condition, causes amplification of the phi component of the magnetic field. The radial and axial components grow due to the presence of turbulent diffusion. The shear in the large scale flow induces an indefinite growth of magnetic field without the a effect; nevertheless, turbulent diffusion forms an important part in the overall mechanism.

Similarity solution of unsteady boundary layer equations with a magnetic field

The unsteady laminar incompressible boundary layer flow of an electrically conducting fluid in the stagnation region of two-dimensional and axisymmetric bodies with an applied magnetic field has been studied. The boundary layer equations which are parabolic partial differential equations with three independent variables have been reduced to a system of ordinary differential equations by using suitable transformations and then solved numerically using a shooting method. Here, we have obtained new solutions which are solutions of both the boundary layer and Navier-Stokes equations.

Unsteady compressible boundary layer flow in the stagnation region of a sphere with a magnetic field

Abstract: An analysis is performed to study the unsteady compressible laminar boundary layer flow in the forward stagnation-point region of a sphere with a magnetic field applied normal, to the surface. We have considered the case where there is an initial steady state that is perturbed by the step change in the total enthalpy at the wall. The nonlinear coupled parabolic partial differential equations governing the flow and heat transfer have been solved numerically using a finite-difference scheme. The numerical results are presented, which show the temporal development of the boundary layer. The magnetic field in the presence of variable electrical conductivity causes an overshoot in the velocity profile. Also, when the total enthalpy at the wall is suddenly increased, there is a change in the direction of transfer of heat in a small interval of time.

Magnetic properties of mechanically alloyed nanocrystalline Ni3Fe

Disordered nanocrystalline Ni3Fe alloy was prepared by mechanical alloying of elemental powders. X-ray diffractograms show the formation of Ni3Fe single phase. The chemical composition and morphology of the powder have been obtained by using EDAX and SEM analysis respectively. While the saturation magnetisation decreases with milling time, the coercivity increases. The width of the hyperfine field distributions obtained from Mossbauer studies shows that the alloy is highly disordered Atomic ordering is found to take place at a faster rate compared to that in the bulk alloy. (C) 1999 Acta Metallurgica Inc.

Magneto-structural correlation in (mu-alkoxo/hydroxo)(mu-carboxylato)dicopper(II) systems: Synthesis, X-ray structure and magnetic properties of aquo(mu-hydroxo) (mu-arylcarboxylato)bis(N,N,N ',N '-tetramethylethane-1,2-diamine)dicopper(II) diperchlorate

Asymmetrically dibridged dicopper(II) complexes, [Cu-2(OH)(O2CC6H4-p-Me)(tmen)(2)(H2O)](ClO4)(2) (1) and [Cu-2(OH)(O2CC6H4-p-OMe)(tmen)(2)(H2O)](ClO4)(2) (2) (tmen = N,N,N',N'-tetramethylethane-1,2-diamine), were prepared and structurally characterized. Complex 1 crystallizes in the monoclinic space group P2(1)/a with a = 17.718(2), b = 9.869(1), c = 19.677(2) Angstrom, beta = 115.16(1)degrees, V = 3114.3(6) Angstrom(3) and Z = 4. The structure was refined to R(wR(2)) = 0.067(0.178). Complex 2 crystallizes in the monoclinic space group P2(1)/a with a = 17.695(3), b = 9.574(4), c = 20.104(2) Angstrom, beta = 114.18(1)degrees, V = 3107(1) Angstrom(3) and Z = 4. The final residuals are R(wR(2)) = 0.067(0.182). The complexes have a [Cu-2(mu-OH)(mu-OH)(mu-O2CAr)](2+) core with tmen Ligands occupying the terminal sites of the core. In addition, one copper is axially bound to a water molecule. The Cu ... Cu distances and the Cu-OH Cu angles in the core are 3.394(1) Angstrom, 124.4(2)degrees for 1 and 3.374(1) Angstrom, 123.3(3)degrees for 2. The complexes show axial X-band EPR spectral features in methanol glass at 77 K giving g(perpendicular to) = 2.02, g(parallel to) = 2.3 (A(parallel to) = 165 x 10(-4) cm(-1)) and a visible band near similar to 630 nm in methanol. The complexes are weakly antiferromagnetic. A theoretical fit of the magnetic susceptibility data in the temperature range 40-295 K gives -J = 10 cm(-1), g = 2.05 for 1 and -J = 10 cm(-1), g = 2.0 for 2. Plots of -2J versus the Cu-OH-Cu angle (phi) in this class of asymmetrically dibridged dicopper(II) complexes having d(x2-y2)-d(x2-y2) magnetic orbitals show a linear magneto-structural correlation: -2J(cm(-1)) = 11.48 phi(deg) - 1373.

Studies on magnetic properties of MnTi1-xNbxO3 system

Synthesis and characterization of electrical and magnetic properties of ilmenite phases of the type MnTi1-xNbxO3 have been carried out. Single phase materials could be obtained for 0.0 less than or equal to x less than or equal to 0.25. The electrical conductivity increases with increasing Nb content. Magnetic susceptibility studies show that the phases exhibit 2D antiferromagnetic behavior. The magnetic susceptibility data has been analyzed using Fisher's specific heat to determine the long range ordering temperature, (C) 1998 Academic Press.

On the large-scale diffuse magnetic field of the sun - II. The contribution of active regions

Dikpati and Choudhuri (1993, 1995) developed a model for the poleward migration of the weak diffuse magnetic field on the Sun's surface. This field was identified with the poloidal component produced by the solar dynamo operating at the base of the convection zone, and its evolution was studied by considering the effects of meridional circulation and turbulent diffusion. The earlier model is extended in this paper by incorporating the flux from, the decay of tilted active regions near the solar surface as an additional source of the poloidal field. This extended model can now explain various low-latitude features in the time-latitude diagram of the weak diffuse fields. These low-latitude features could not be accounted for in the earlier model, which was very successful in modeling the behavior at high latitudes. The time-latitude diagrams show that regions of a particular polarity often have 'tongues' of opposite polarity. Such tongues can be produced in the theoretical model by incorporating fluctuations in the source term arising out of the decaying active regions.

Structural instability of the charge ordered compound Nd0.5Sr0.5MnO3 under a magnetic field

We report an anomalous magnetostriction behavior of the charge ordered compound Nd0.5Sr0.5MnO3. We have found that the applied magnetic field not only gives rise to a large negative magnetoresistance but also produces a huge positive magnetovolume effect. This unusual effect is explained considering that the applied magnetic field induces a structural transition at which the volume drastically increases. This effect is also seen in the anisotropic magnetostriction which shows clear anomalies at the field induced transition.

Synthesis, crystal structure and magnetic properties of a polymeric copper(II) Schiff-base complex having binuclear units covalently linked by isonicotinate ligands

The polynuclear copper(II) complex [{Cu2L(O2CC5H4N)}. C2H5OH](x) (1), where H3L is a 1∶2 Schiff base derived from 1,3-diaminopropan-2-ol and salicylaldehyde, has been prepared and structurally characterized. The structure consists of a one-dimensional zigzag chain in which the binuclear [Cu2L](+) units are covalently linked by isonicotinate ligands to give a syndiotactic arrangement of the copper ions protruding outside the chain. In the basic unit, the copper(II) centres are bridged by an alkoxo and a carboxylato ligand, giving a Cu ... Cu distance of 3.492(3) Angstrom and a Cu-O-Cu angle of 130.9(2)degrees. While one copper centre has a square-planar geometry, the other copper is square-pyramidal with the pyridine nitrogen being the axial ligand. The visible electronic spectrum of 1 shows a broad d-d band at 615 nm. The complex shows a rhombic X-band EPR spectral pattern in the polycrystalline phase at 77 K. Magnetic susceptibility measurements in the temperature range 22 to 295 K demonstrate the antiferromagnetic behaviour of 1. A theoretical fit to the magnetic data is based on a model assuming 1 as an equimolar mixture of copper atoms belonging to an antiferromagnetically coupled one-dimensional Heisenberg chain with the other copper atoms outside the chain behaving like paramagnetic centres.

The onset of oscillatory Marangoni convection in a two-layer system of conducting fluid in the presence of a uniform magnetic field

A combination of numerical and analytical techniques is used to analyse the effect of magnetic field and encapsulated layer on the onset of oscillatory Marangoni instability in a two layer system. Oscillatory Marangoni instability is possible for a deformed free surface only when the system is heated from above. It is observed that the existence of a second layer has a positive effect on Marangoni overstability with magnetic field whereas it has an opposite effect without magnetic field.

Unsteady flow and heat transfer on a semi-infinite flat plate with an aligned magnetic field

The unsteady laminar boundary layer flow of an electrically conducting fluid past a semi-infinite flat plate with an aligned magnetic field has been studied when at time t > 0 the plate is impulsively moved with a constant velocity which is in the same or opposite direction to that of free stream velocity. The effect of the induced magnetic field has been included in the analysis. The non-linear partial differential equations have been solved numerically using an implicit finite-difference method. The effect of the impulsive motion of the surface is found to be more pronounced on the skin friction but its effect on the x-component of the induced magnetic field and heat transfer is small. Velocity defect occurs near the surface when the plate is impulsively moved in the same direction as that of the free stream velocity. The surface shear stress, x-component of the induced magnetic field on the surface and the surface heat transfer decrease with an increasing magnetic field, but they increase with the reciprocal of the magnetic Prandtl number. However, the effect of the reciprocal of the magnetic Prandtl number is more pronounced on the x-component of the induced magnetic field. (C) 1999 Elsevier Science Ltd. All rights reserved.