Magnetic coupling in the weac ferromagnet CuF2

CuF2 is known to be an antiferromagnetic compound with a weak ferromagnetism due to the anisotropy of its monoclinic unit cell (Dzialoshinsky-Moriya mechanism). We investigate the magnetic ordering of this compound by means of ab initio periodic unrestricted Hartree-Fock calculations and by cluster calculations which employ state-of-the-art configuration interaction expansions and modern density functional theory techniques. The combined use of periodic and cluster models permits us to firmly establish that the antiferromagnetic order arises from the coupling of one-dimensional subunits which themselves exhibit a very small ferromagnetic coupling between Cu neighbor cations. This magnetic order could be anticipated from the close correspondence between CuF2 and rutile crystal structures.

Ferromagnetic instabilities in neutron matter at finite temperature with the Skyrme interaction

The properties of spin-polarized neutron matter are studied at both zero and finite temperature using Skyrme-type interactions. It is shown that the critical density at which ferromagnetism takes place decreases with temperature. This unexpected behavior is associated to an anomalous behavior of the entropy that becomes larger for the polarized phase than for the unpolarized one above a certain critical density. This fact is a consequence of the dependence of the entropy on the effective mass of the neutrons with different third spin component. A new constraint on the parameters of the effective Skyrme force is derived if this behavior is to be avoided.

”Investigations on The Finite Size Effects of Some Inverse Spinels and Studies on Their Composites Based on Nitrile Rubber”

In the present study, nano particles of NiFe3O4, I_.l()5Feg5O4 and CoFegO4 are prepared by sol gel method. By appropriate heat treatments, particles of different grain sizes are obtained. The structural, magnetic and electrical measurements are evaluated as a function of grain size and temperature. NiFe3O4 prepared in the ultrafine regime are then incorporated in nitrile rubber matrix. The incorporation was carried out according to a specific recipe and for various loadings of magnetic fillers. The cure characteristics, magnetic properties, electrical properties and mechanical properties of these elastomer blends are carried out. The electrical permittivity of all the rubber samples in the X — band are also conducted

Hybrid Multiferroic Nanostructure with Magnetic−Dielectric Coupling

The development of methods to economically synthesize single wire structured multiferroic systems with room temperature spin−charge coupling is expected to be important for building next-generation multifunctional devices with ultralow power consumption. We demonstrate the fabrication of a single nanowire multiferroic system, a new geometry, exhibiting room temperature magnetodielectric coupling. A coaxial nanotube/nanowire heterostructure of barium titanate (BaTiO3, BTO) and cobalt (Co) has been synthesized using a template-assisted method. Room temperature ferromagnetism and ferroelectricity were exhibited by this coaxial system, indicating the coexistence of more than one ferroic interaction in this composite system

Multiferroic Behavior of Gd Based Manganite

Here we report the multiferroic nature of charge ordered manganite Gd0.5Sr0.5MnO3 for the first time. The temperature variation of dielectric constant shows broad relaxor type ferroelectric transition at around 210K and magnetization measurements shows weak ferromagnetism at 50K. The dielectric peak is very close to charge ordering temperature which is an evidence of the link between electronic state and increase of dielectric response. Butterfly variation of capacitance with voltage confirms ferroelectric nature of the sample at room temperature

Synthesis and characterization of 3d-transition metals doped ZnO thin films and nanostructures for possible spintronic applications

The objective of the present study is the formation of single phase Zn1−xTMxO thin films by PLD and increase the solubility limit of TM dopants. The TM doped ZnO nanostructures were also grown by hydrothermal method. The structural and morphological variation of ZnO:TM thin films and nanostructures with TM doping concentration is also investigated. The origin and enhancement of ferromagnetism in single phase Zn1−xTMxO thin films and nanostructures using spectroscopic techniques were also studied. The dependence of ablation parameters on the structural and optical properties of ZnO thin films were studied

Numerical simulation of magnetic interactions in polycrystalline YFeO3

The magnetic behavior of polycrystalline yttrium orthoferrite was studied from the experimental and theoretical points of view. Magnetization measurements up to 170 kOe were carried out on a single-phase YFeO3 sample synthesized from heterobimetallic alkoxides. The complex interplay between weak-ferromagnetic and antiferromagnetic interactions, observed in the experimental M(H) curves, was successfully simulated by locally minimizing the magnetic energy of two interacting Fe sublattices. The resulting values of exchange field (H-E = 5590 kOe), anisotropy field (H-A = 0.5 kOe) and Dzyaloshinsky-Moriya antisymmetric field (H-D = 149 kOe) are in good agreement with previous reports on this system. (C) 2007 Elsevier B.V. All rights reserved.

The effect of additional Si and SiGe layers on the confinement potential of GeMn diluted ferromagnetic semiconductor

In this work we analyze the spin-polarized charge density distribution in the GeMn diluted ferromagnetic semiconductors (DFS). The calculations are performed within a self-consistent k.p method, in which the exchange correlation effects in the local density approximation, as well as the strain effects due to the lattice mismatch, are taken into account. Our findings show that the extra confinement potential provided by the barriers and the variation of the Mn content in the DFS are responsible for a separation between the different spin charge densities, giving rise to higher mobility spin-polarized currents or high ferromagnetism transition temperatures systems. (c) 2008 Elsevier B.V. All rights reserved.

Collapse of rho (xx) Ringlike Structures in 2DEGs Under Tilted Magnetic Fields

In the quantum Hall regime, the longitudinal resistivity rho (xx) plotted as a density-magnetic-field (n (2D) -B) diagram displays ringlike structures due to the crossings of two sets of spin split Landau levels from different subbands [see, e.g., Zhang et al., in Phys. Rev. Lett. 95:216801, 2005. For tilted magnetic fields, some of these ringlike structures ""shrink"" as the tilt angle is increased and fully collapse at theta (c) a parts per thousand 6A degrees. Here we theoretically investigate the topology of these structures via a non-interacting model for the 2DEG. We account for the inter Landau-level coupling induced by the tilted magnetic field via perturbation theory. This coupling results in anticrossings of Landau levels with parallel spins. With the new energy spectrum, we calculate the corresponding n (2D) -B diagram of the density of states (DOS) near the Fermi level. We argue that the DOS displays the same topology as rho (xx) in the n (2D) -B diagram. For the ring with filling factor nu=4, we find that the anticrossings make it shrink for increasing tilt angles and collapse at a large enough angle. Using effective parameters to fit the theta=0A degrees data, we find a collapsing angle theta (c) a parts per thousand 3.6A degrees. Despite this factor-of-two discrepancy with the experimental data, our model captures the essential mechanism underlying the ring collapse.

Spin-polarization effects in homogeneous and non-homogeneous diluted magnetic semiconductor heterostructures

Spin polarization is a key characteristic in developing spintronic devices. Diluted magnetic heterostructures (DMH), where subsequent layers of conventional and diluted magnetic semiconductors (DMS) are alternate, are one of the possible ways to obtain it. Si being the basis of modern electronics, Si or other group-IV DMH can be used to build spintronic devices directly integrated with conventional ones. In this work we study the physical properties and the spin-polarization effects of p-type DMH based in group-IV semiconductors (Si, Ge, SiGe, and SiC), by performing self-consistent (k) over right arrow . (p) over right arrow calculations in the local spin density approximation. We show that high spin polarization can be maintained in these structures below certain values of the carrier concentrations. Full spin polarization is attained in the low carrier concentration regime for carrier concentrations in the DMS layer up to similar to 2.0 x 10(19) cm(-3) for Si and up to similar to 6.0 x 10(19) cm(-3) for SiC. Partial, but still important spin polarization can be achieved for all studied group-IV DMH, with the exception of Ge for carrier concentrations up to 6.0 x 10(19) cm(-3). The role played by the effective masses and the energy splitting of the spin-orbit split-off hole bands is also discussed throughout the paper.

Preparação de óxidos mistos de níquel e zinco nanoparticulados a partir de combustíveis alternativos

The field of "Materials Chemistry" has been developing in recent years and there has been a great increase of interest in the synthesis and chemical and physical properties of new inorganic solids. New routes of synthesis and synthesis modified has been developed with the aim not only to optimize the processes in laboratory scale, but also on an industrial scale, and make them acceptable by current environmental legislation. The phenomenology of current solid state chemistry properties coupled with the high temperature superconductivity, ferromagnetism, porosity molecular and colors are evidence affected by the synthesis method, which in turn can influence the technological application of these materials. From this understanding, mixed oxides of nickel and zinc nanoparticulate were synthesized by microwave-assisted combustion route using three specific types of organic fuels employing the weight ratios 1:1/2 and 1:1 of cation metallic/fuel, in order to investigate the influence of such proportions to obtain the solids. The new fuels were chosen to replace, for example, urea or glycine that are the fuels most commonly preferred in this kind of synthesis. The powders without heat treatment were studied by Thermogravimetric analysis (TGA), X-Ray Diffraction (XRD) and then calcined at 900°C. After heat treatment, the samples were characterized by analysis of X Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The modified synthesis route porposed was effective for obtaining powders. Both the alternative fuels chosen as the different weight ratios employed, influenced in the morphology and obtaining oxides

Study of interstitial oxygen mobility in RuSr2GdCu2O8 by internal friction

The discovery of the spatial uniform coexistence of superconductivity and ferromagnetism in ruthenocuprates, RuSr2GdCu2O8 (Ru-1212), has spurred an extraordinary development in the study of the competition between magnetism and superconductivity. However, several points of their preparation process and characterization that determine their superconductive behaviour are still obscure. The improvement of sample preparation conditions involves some thermal treatments in inert atmosphere. Anelastic spectroscopy measurements were made using an inverted torsion pendulum, operating with an oscillation frequency of 38 Hz, temperature in the 90 and 310 K range, heating rate of 1 K/min, and vacuum better than 10(-3) Pa. The results show anelastic relaxation peaks at 210 K related to the presence of interstitial oxygen atoms. The peaks decrease significantly with the oxygen loss caused by the heat treatments in vacuum, appearing again after the annealing of the sample in an oxygen atmosphere. These observed peaks are clearly related to the additional oxygen atoms, with activation energy 0.13 and 0.36 eV, and can be explained in terms by diffusional jumps of interstitial oxygen in the RuO2 planes. (C) 2009 Elsevier B.V. All rights reserved.

Anelastic characterization and superconducting properties of RuSr2GdCu2O8+delta

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Síntese, caracterização e estudo das propriedades magnéticas de ortoferritas de lantânio dopadas com manganês

Orthoferrites AFeO3 (A = rare earth) are an important class of perovskite oxides that exhibit weak ferromagnetism. These materials find numerous applications as chemical sensors, cathodes for fuel cells and catalysis, which make them interesting from the standpoint of science and technology. Their structural, electrical and magnetic properties are dependent on many factors such as the preparation method, heat treatment conditions, chemical composition and replacement of cations in sites A and/or B. In this paper, LaFe1-xMnxO3 (0 ≤ x ≤ 1) orthoferrites-type was prepared by Pechini method and Microwave-assisted combustion reaction in order to evaluate the influence of synthesis route on the formation of oxide, as well as the effect of parcial replacement of iron by manganese and heat treatment on the magnetic properties. The precursor powders were calcined at 700°C, 900°C, 1100°C and 1300°C for 4 hours and they were characterized by the techniques: Thermogravimetric analysis (TGA), X ray diffraction (XRD), Refinement by Rietveld method, Scanning electron microscopy (SEM), Reduction temperature programmed (RTP) and Magnetic hysteresis measurements performed at room temperature. According to the XRD patterns, the formation of perovskite phase with orthorhombic structure was observed for the systems where 0 ≤ x ≤ 0.5 and rhombohedral for x = 1. The results also showed a decrease of lattice parameters with the parcial replacement of iron by manganese and consequently a reduction in cell volume. The hysteresis curves exhibited weak ferromagnetism for the systems prepared by both synthesis methods. However, a dependence of magnetization as a function of dopant content was observed for samples produced by Pechini method. As for the systems prepared by combustion reaction, it was found that the secondary phases exert a strong influence on the magnetic behavior

Magnetic properties of poly(3-hexylthiophene)

A room temperature ferromagnetic phase is observed in samples of poly(3-hexylthiophene) partially doped with ClO (4) over bar. The magnetic behavior presents a strong dependence on the sample preparation conditions, in particular, a dependence with the final potential of the sample after reduction. The origin of the ferromagnetism is proposed to be associated with interactions between spin 1/2 polarons formed in the polymeric chain upon doping. The dependence of saturation and spontaneous magnetization as the function of the final potential after reduction shows a way to control the magnetic properties of this polymer. (C) 2008 Elsevier B.V. All rights reserved.