A new combustion route to γ-Fe2O3 synthesis

A new combustion route for the synthesis of γ-Fe ₂ O ₃ is reported by employing purified a-Fe ₂ O ₃ as a precursor in the present investigation. This synthesis which is similar to a self propagation combustion reaction, involves fewer steps, a shorter overall processing time, is a low energy reaction without the need of any explosives, and also the reaction is completed in a single step yielding magnetic iron oxide i.e. γ-Fe ₂ O ₃ .The as synthesized γ-Fe ₂ O ₃ is characterized employing thermal, XRD, SEM, magnetic hysteresis, and density measurements. The effect of ball-milling on magnetic properties is also presented.     

Oscillatory interlayer magnetic coupling and induced magnetism in Fe/Nb multilayers

We present an ab initio calculation of interlayer magnetic coupling for Fe/Nb multilayers using the self-consistent full-potential linearized augmented-plane-wave (FLAPW) method. For this calculation, we have constructed supercells consisting of bcc Fe and Nb multilayers in Fe/Nb/Fe sandwich geometry stacked along (001) direction. In the supercells two Fe layers are separated by Nb layers ranging from 1 to 11 layers. We have calculated the total energy of the system as a function of Nb spacer layer thickness. For each spacer layer thickness, we have done three calculations corresponding to para, ferro and antiferromagnetic ordering of Fe atoms. The interlayer magnetic coupling is obtained from the energy difference of the systems in which Fe layers are antiferromagnetically and ferromagnetically ordered. We find that the interlayer magnetic coupling oscillates with increasing Nb spacer thickness in agreement with the experimental results. The induced magnetic moment is also found to be oscillating with increasing Nb spacer layer thickness.     

Fe–Co magnetic nanoclusters by density functional theory calculations

ABSTRACT

We present density functional theory results referring to the structural, electronic and magnetic properties of 13, 55, 147 and 309 Fe–Co (magnetic–magnetic) icosahedral nanoclusters (ICO) comparing with our previous results on Fe–Cu (magnetic–nonmagnetic). It came out that the Fe atoms always favour the edge surface sites exhibiting higher average magnetic moment (MM) for Fe and FeCo ICOs than FeCu while the local Fe MM is greater for FeCu₁₂ and Fe₆Cu₄₉ ones. This is due to the strong hybridisation of Fe 3d–Co 3d states, while in Fe–Cu the Fe spin down states are restricted close to fermi without been affected by the corresponding Cu states. These results could be used for the design of environmental sustainable smart magnetic alloys.

This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.     

Simple explanation for the reentrant magnetic phase transition in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.41</Subscript>Ca<Subscript>0.09</Subscript>MnO<Subscript>3</Subscript> perovskite

The reentrant magnetic phase transition in Pr _0.5Sr_0.41Ca_0.09MnO₃ perovskite is explained using the Ising spin model on the square lattice with mixed ferromagnetic and antiferromagnetic exchange interactions. It is shown using numerical calculations that this effect is strongly affected by the external magnetic field and lattice disorder.     

The Cyclotron Waves in Layered Conductors

We have studied theoretically the propagation of electromagnetic waves in layered conductors placed in a strong magnetic field B ₀. It is shown that the wave process is very sensitive to the orientation of B ₀ with respect to the layers. The weakly damping modes with the frequencies near the cyclotron resonance frequency can propagate at orientations of the magnetic field vector, for which the drift velocity of charge carriers along the B ₀-direction is close to zero. The cyclotron waves spectrum is analyzed with regard to the Fermi-liquid interaction of electrons.     

3D physical modeling of anisotropic grain growth at high temperature in local strong magnetic force field

Abstract

A new mechanism based on the effect of local magnetic forces on diffusing ions around a growing ferromagnetic precipitate is proposed. A 3D simulation based only on physical parameters is undertaken in which main assumption is of interface limited growth controlled by the effect of both curvature and local magnetic field distortion. Although usually neglected in magnetic field effect mechanisms, it is shown that these local magnetic forces acting on a single paramagnetic ion can change markedly affect the growth process and induce strong shape anisotropy.     

Thermopower of Carbon Nanotubes in a Magnetic Field

Abstract

The thermopower of a quantum nanotube in magnetic field was investigated. We obtained the convenient analytic formula for the thermopower. The temperature dependence of the thermopower is studied and the influence of the magnetic field on the thermopower is examined. Oscillations in the thermopower were investigated.     

On the influence of magnetic field processing on the texture,phase assemblage and properties of low aspect ratio Bi2Sr2CaCu2Ox/AgMg wire

Abstract

Bi₂ Sr₂ CaCu₂ O_x/AgMg conductors are potentially important for many applications up to 20 K, including magnets for cryogen-free magnetic resonance imaging and high field nuclear magnetic resonance research. One promising approach to increased critical current density is partial-melt processing in the presence of a magnetic field which has been shown to enhance c-axis texturing of wide, thin tape conductors. Here, we report on low aspect ratio rectangular conductors processed in an 8 T magnetic field. The magnetic field is applied during different stages of the heat treatment process. The conductors are electrically characterized using four-point critical current measurements as a function of magnetic field and magnetic field orientation relative to the conductor. The superconductive transition and magnetization hysteresis are measured using a SQUID magnetometer. The microstructures are characterized using scanning electron microscopy and energy dispersive spectroscopy and analyzed using digital image processing. It is found that the presence of a magnetic field during split melt processing enhances the electrical transport and magnetic behavior, but that the anisotropy is not consistently affected. The magnetic field also affects development of interfilamentary Bi2212 bridges, and that this depends on the initial shape of the Bi2212 filament. At least two behaviors are identified; one impacts the oxide phase assemblage and the other impacts textured growth.     

Magnetic Field Gradients and Particle Density Effects in Second-harmonic Generation in Sodium Vapour

Abstract

A single-frequency c.w. dye laser has been used to study the properties of second-harmonic generation in sodium vapour induced by a magnetic field. Theoretical modelling shows that the phase as well as the magnitude of the generated second harmonic depends on the magnetic field strength. By using an appropriate magnetic field gradient it is thus possible to reduce the effects of phase-velocity mismatching, and this has been demonstrated experimentally. The effects of buffer gas pressure on second-harmonic generation have also been studied.     

Numerical simulation of chainlike cluster movement of feeble magnetic particles by induced magnetic dipole moment under high magnetic fields

Abstract

In this paper, the motion of a chainlike cluster of feeble magnetic particles induced by high magnetic field is discussed on the basis of the results of numerical simulations. The simulations were performed on glass particles with a diameter of 0.8 mm; and the viscosity, applied magnetic field and magnetic properties of the surrounding medium were changed. In addition to the magnetic field and the difference in magnetic susceptibility between the particles and the surrounding medium, the obtained results indicate that the viscosity is an essential factor for the formation of the chainlike alignment of feeble magnetic particles. We also carried out simulations using glass particles with a smaller diameter of 0.1 mm. Chainlike clusters were produced similar to those of ferromagnetic particles formed in a ferromagnetic fluid.     

Anisotropic elasticity of magnetically ordered agarose gel

Abstract

The physical properties of agarose gel prepared under strong magnetic fields were investigated. The storage modulus was measured by the reflection method with an ultrasonic pulse. The measurement results of the gel's elasticity indicate that agarose gel has anisotropic properties. The elasticity and its anisotropy depend on the concentration of the gel and the magnetic field to which it is exposed. The experimental results indicate that the anisotropic network structure of the gel is induced by the exposure to the magnetic field during gelation. The gelation mechanism under a magnetic field is discussed.     

Formation of a honeycomb domain structure in magnetic films

An analysis is made of the possible formation of a honeycomb domain structure directly from a labyrinth structure in uniaxial magnetic films exposed to a series of uniform magnetic field pulses. It is shown that the scenario for the formation of a honeycomb structure depends on the pulse parameters and on the magnitude of the static magnetizing magnetic field. Pis’ma Zh. Tekh. Fiz. 23, 46–51 (May 12, 1997)     

A Multi-oscillator Ring Laser with Zeeman Bias

Abstract

A laboratory multi-oscillator ring laser of monoblock construction employing a non-planar cavity and the Zeeman effect on the helium-neon discharge is used to bias oppositely travelling modes from lock-in. A theoretical model of the laser is used to explain the behaviour with respect to gain, tuning and magnetic field. Methods for stabilizing the gyro operating point and correcting for ambient magnetic fields are presented.     

Experimental study of the temperature stability of magnetic field sensors based on Bi12SiO20 crystals

The temperature characteristics of the conversion coefficient of the sensitive element on a fiber-optic magnetic field probe based on a Bi₁₂SiO₂₀ crystal are studied experimentally. A laboratory model of the sensitive element of a magnetic field sensor is built, and the temperature drift of the conversion coefficient is found to be ∼0.15% over the temperature interval from +15 to +70 °C. Pis’ma Zh. Tekh. Fiz. 24, 26–33 (June 12, 1998)     

Paraconductivity in type-II superconductors

The additional conductivity due to the fluctuation of the order parameter in dirty type-II superconductors is reconsidered. It is shown that in a magnetic fieldH slightly above the upper critical field the electric conductivity is strongly anisotropic depending on the relative direction of the magnetic field and the electric current. In the parallel case (i.e.,H J) the conductivity diverges like(H–H _c2)^–3/2, while in the perpendicular case (H J) it diverges like(H–H _c2)^–1/2.     

Magnetic field effects on electric behavior of [Fe(CN)6]3− at bare and membrane-coated electrodes

Abstract

The cyclic voltammetric behavior of [Fe(CN)₆]^3? was investigated under homogeneous magnetic fields perpendicular to the electrode surface in order to determine the effects of magnetic fields on the distribution of an Fe²⁺/Fe³⁺ redox couple. The cathodic current was enhanced much more than the anodic current by a homogeneous magnetic field, suggesting that the concentration gradient of paramagnetic [Fe(CN)₆]^3? and diamagnetic [Fe(CN)₆]^4? formed at an electrode surface may also contribute to the asymmetric current. The apparent diffusion coefficient of the redox couple increased by over 30% in both cathodic and anodic processes upon applying a magnetic field. For a gold electrode coated with dioctadecyldimethylammonium, the application of a magnetic field perpendicular to the surface increased the peak-to-peak separation, and enhanced the asymmetric current. It is inferred that the application of a magnetic field promotes the electron-tunneling process by tilting chain molecules in the barrier membrane.     

Structural control of Fe-based alloys through diffusional solid/solid phase transformations in a high magnetic field

Abstract

A magnetic field has a remarkable influence on solid/solid phase transformations and it can be used to control the structure and function of materials during phase transformations. The effects of magnetic fields on diffusional solid/solid phase transformations, mainly from austenite to ferrite, in Fe-based alloys are reviewed. The effects of magnetic fields on the transformation temperature and phase diagram are explained thermodynamically, and the transformation behavior and transformed structures in magnetic fields are discussed.     

Polarization Effect in ESR of Co Doped CuGeO<Subscript>3</Subscript>

Electron spin resonance of a single crystal of CuGeO₃ doped with 2% of Co has been studied at f = 99 GHz in temperature range 1.8–50 K. Contributions to ESR absorption from Cu²⁺ chains and from Co²⁺ ions were derived. It is found that functions obtained for ESR integrated intensities: Curie-Weiss for Cu²⁺ (χCu ∼ C _Cu/(T + Θ), with Θ = 92 K) and Curie for Co²⁺ (χCo ∼ C _Co/T) are well consistent with temperature dependence of static magnetic susceptibility. Strong dependence of ESR absorption on polarization of oscillating magnetic field was discovered for Co²⁺ contribution. Polarization effect was studied for magnetic field applied along a, b and c directions. Values of g-factors of resonance lines are presented.     

Using magnetic birefringence to determine the molecular arrangement of supramolecular nanostructures

Abstract

Supramolecular aggregates can be aligned in solution using a magnetic field. Because of the optical anisotropy of the molecular building blocks, the alignment results in an anisotropic refractive index of the solution parallel and perpendicular to the magnetic field. We present a model for calculating the magnetic birefringence, using solely the magnetic susceptibilities and optical polarizabilities of the molecules, for any molecular arrangement. We demonstrate that magnetic birefringence is a very sensitive tool for determining the molecular organization within supramolecular aggregates.     

The damping of helicon waves in a tipped magnetic field

The damping of helicon waves in indium was measured under nonlocal conditions with a variable angle between the wave vectorq and the magnetic field. In contrast to the predictions of the free-electron theory, the damping was not a monotonic function of the angle and exhibits considerable structure, which is attributed to minima in the Landau damping for certain orientations of the magnetic field. Minima in the damping are to be expected for such field directions as give rise to cyclotron orbits having a substantial number of electron states with orbital velocities perpendicular toq. In simple situations the critical orientations of the magnetic field may be deduced from a given Fermi surface by a geometrical construction. The construction has been extended to indium, and the tipping angles found are in reasonable agreement with the experiment.Based on a thesis to be submitted to the Senate of the Technion, Israel Institute of Technology, in partial fulfillment of the requirements for D.Sc. degree.