Comparative study on structure and magnetic properties of polycrystalline PrxY1-xBa2Cu3O7-delta prepared in oxygen and argon atmosphere

We report a systematic study on the influence of the synthesis routes on the structural and magnetic properties of polycrystalline PrxY1-xBa2Cu3O7-delta. We have prepared high-quality samples of this material by following a sol-gel method based on heat treatment in both inert argon and oxygen atmospheres in order to compare their effect on the formation of the superconducting phase using X-ray powder diffraction. Magnetic measurements (DC and AC susceptibility) clearly demonstrate that, for the same concentration of Pr, the superconducting transition temperature markedly increases in all samples prepared in argon atmosphere, including pure Pr-123. (C) 2012 Elsevier B.V. All rights reserved.

Study of the oxygen vacancy influence on magnetic properties of Fe- and Co-doped SnO2 diluted alloys

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(V (O))(0.02), where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 mu(B)/cell, for Fe and Co. Two metastable states, with 0 and 4 mu(B)/cell were found for Fe, and a single value, 3 mu(B)/cell, for Co. The spin-crossover energies (E (S)) were calculated. The values are E (S) (0/2) = 107 meV and E (S) (4/2) = 25 meV for Fe. For Co, E (S) (3/1) = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and E (S) change. For iron, a new state appears, and the state with zero magnetic moment disappears. The ground state is 4 mu(B)/cell instead of 2 mu(B)/cell, and the energy E (S) (2/4) is 30 meV. For cobalt, the ground state is then found with 3 mu(B)/cell and the metastable state with 1 mu(B)/cell. The spin-crossover energy E (S) (1/3) is 21 meV. Our results suggest that these materials may be used in devices for spintronic applications that require different magnetization states.

Thermal stability and the magnetic properties of hybrid vanadium oxide-tetradecylamine nanotubes

Vanadium oxide nanotubes (NTs) were synthesized by the sol-gel method followed by a long-term hydrothermal treatment. The obtained nanotubes have a multiwall structure, and 70% of vanadium ions are in the V4+ state. This percentage was derived by evaluating three components of the magnetic susceptibility; namely, (i) the paramagnetic Curie-Weiss behavior, (ii) antiferromagnetic dimers, and (iii) magnetic trimers. The as-made NTs were annealed in situ in the cavity of the electron paramagnetic resonance (EPR) spectrometer. The line shape changes irreversibly at 390 K, and the EPR susceptibility presents an anomaly at 425 K. These changes are interpreted as a partial oxidation of the V4+ ions and consequently a decrease in the concentration of the magnetic species. The quantification of the V4+ ions of the annealed NTs reveals a diminution to 39% of V4+, a weakening of the Curie-Weiss and antiferromagnetic dimers contributions, and the suppression of magnetic trimers. Vibrational studies confirm the decrease of V4+ amount. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4749417]

Synthesis, crystal structure and magnetic properties of the helical oxalate-bridged copper(II) chain {[(CH3)(4)N](2)[Cu(C2O4)(2)] center dot H2O}(n)

The preparation, crystal structure and magnetic properties of a new oxalate-containing copper(II) chain of formula {[(CH3)(4)N](2)]Cu(C2O4)(2)] center dot H2O}(n) (1) [(CH3)(4)N+ = tetramethylammonium cation] are reported. The structure of 1 consists of anionic oxalate-bridged copper(II) chains, tetramethylammoniun cations and crystallization water molecules. Each copper(II) ion in 1 is surrounded by three oxalate ligands, one being bidentate and the other two exhibiting bis-bidenate coordination modes. Although all the tris-chelated copper(H) units from a given chain exhibit the same helicity, adjacent chains have opposite helicities and then an achiral structure results. Variable-temperature magnetic susceptibility measurements of 1 show the occurrence of a weak ferromagnetic interaction through the oxalate bridge [J = +1.14(1)cm(-1), the Hamiltonian being defined as H = -J Sigma nm S-i . S-j]. This value is analyzed and discussed in the light of available magnetostructural data for oxalate-bridged copper(H) complexes with the same out-of-plane exchange pathway. (C) 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Effects of the Compaction Pressure on the Magnetic Properties of a Sintered Fe-Based Alloy

The structural and magnetic properties of a Fe-based alloy before and after sintering have been analyzed. X ray diffraction measurements confirm the deformation of the magnetic particles in the compacted samples. After sintering, hysteresis energy dissipation, remanence and intrinsic coercivity differ by less than 10% as porosity changes from 15 to 7%.

Magnetic Properties of Liquid-Phase Sintered CoFe2O4 for Application in Magnetoelastic and Magnetoelectric Transducers

Cobalt ferrite is a ferrimagnetic magnetostrictive ceramic that has potential application in magnetoelastic and magnetoelectric transducers. In this work, CoFe2O4 was obtained using a conventional ceramic method and Bi2O3 was used as additive in order to obtain liquid-phase sintered samples. Bi2O3 was added to the ferrite in amounts ranging from 0.25 mol% to 0.45 mol% and samples were sintered at 900 degrees C and 950 degrees C. It was observed the presence of Bi-containing particles in the microstructure of the sintered samples and the magnetostriction results indicated microstructural anisotropy. It was verified that it is possible to get dense cobalt ferrites, liquid-phase sintered, with relative densities higher than 90% and with magnetostriction values very close to samples sintered without additives.

Electronic and magnetic properties of SnO2/CrO2 thin superlattices

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2) n (SnO2) n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2) n (SnO2) n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2) n (SnO2) n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Magnetic Properties of γ - Fe2O3 Nanoparticles at the Verge of Nucleation Process.

A low-energy new method based in a one-step synthesis at room temperature produces very small maghemite nanopar ticles. The fast neutralization reaction (co-precipitation) of a ferric solution (FeCl3 aqueous) in a basic medium (NH4OH concentrated) produces an intermediate phase, presumably two-line ferrihydrite, that in oxidizing conditions is transformed to maghemite nanopar ticles. That “primordial soup” is characterized by small atom arrangements that are the base for maghemite tiny crystals. The final product of the reaction was characterized by X-ray diffraction, high-resolution transmission electron microscopy, X-ray absorption fine structure, Mössbauer spectroscopy, and magnetometry.

Field dependent transition to the non-linear regime in magnetic hyperthermia experiments: Comparison between maghemite, copper, zinc, nickel and cobalt ferrite nanoparticles of similar sizes

Further advances in magnetic hyperthermia might be limited by biological constraints, such as using sufficiently low frequencies and low field amplitudes to inhibit harmful eddy currents inside the patient's body. These incite the need to optimize the heating efficiency of the nanoparticles, referred to as the specific absorption rate (SAR). Among the several properties currently under research, one of particular importance is the transition from the linear to the non-linear regime that takes place as the field amplitude is increased, an aspect where the magnetic anisotropy is expected to play a fundamental role. In this paper we investigate the heating properties of cobalt ferrite and maghemite nanoparticles under the influence of a 500 kHz sinusoidal magnetic field with varying amplitude, up to 134 Oe. The particles were characterized by TEM, XRD, FMR and VSM, from which most relevant morphological, structural and magnetic properties were inferred. Both materials have similar size distributions and saturation magnetization, but strikingly different magnetic anisotropies. From magnetic hyperthermia experiments we found that, while at low fields maghemite is the best nanomaterial for hyperthermia applications, above a critical field, close to the transition from the linear to the non-linear regime, cobalt ferrite becomes more efficient. The results were also analyzed with respect to the energy conversion efficiency and compared with dynamic hysteresis simulations. Additional analysis with nickel, zinc and copper-ferrite nanoparticles of similar sizes confirmed the importance of the magnetic anisotropy and the damping factor. Further, the analysis of the characterization parameters suggested core-shell nanostructures, probably due to a surface passivation process during the nanoparticle synthesis. Finally, we discussed the effect of particle-particle interactions and its consequences, in particular regarding discrepancies between estimated parameters and expected theoretical predictions. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi. org/10.1063/1.4739533]

Direct Access to Oxidation-Resistant Nickel Catalysts through an Organometallic Precursor

The synthesis of nickel catalysts for industrial applications is relatively simple; however, nickel oxidation is usually difficult to avoid, which makes it challenging to optimize catalytic activities, metal loadings, and high-temperature activation steps. A robust, oxidation-resistant and very active nickel catalyst was prepared by controlled decomposition of the organometallic precursor [bis(1,5-cyclooctadiene)nickel(0)], Ni(COD)(2), over silica-coated magnetite (Fe3O4@SiO2). The sample is mostly Ni(0), and surface oxidized species formed after exposure to air are easily reduced in situ during hydrogenation of cyclohexene under mild conditions recovering the initial activity. This unique behavior may benefit several other reactions that are likely to proceed via Ni heterogeneous catalysis.

Preparation and Structural Characterization of Vulcanized Natural Rubber Nanocomposites Containing Nickel-Zinc Ferrite Nanopowders

Single-phase polycrystalline mixed nickel-zinc ferrites belonging to Ni0.5Zn0.5Fe2O4 were prepared on a nanometric scale (mean crystallite size equal to 14.7 nm) by chemical synthesis named the modified poliol method. Ferrite nanopowder was then incorporated into a natural rubber matrix producing nanocomposites. The samples were investigated by means of infrared spectroscopy, X-ray diffraction, scanning electron microscopy and magnetic measurements. The obtained results suggest that the base concentration of nickel-zinc ferrite nanoparticles inside the polymer matrix volume greatly influences the magnetic properties of nanoconnposites. A small quantity of nanoparticles, less than 10 phr, in the nanocomposite is sufficient to produce a small alteration in the semi-crystallinity of nanocomposites observed by X-ray diffraction analysis and it produces a flexible magnetic composite material with a saturation magnetization, a coercivity field and an initial magnetic permeability equal to 3.08 emu/g, 99.22 Oe and 9.42 X 10(-5) respectively.

On the energy conversion efficiency in magnetic hyperthermia applications: A new perspective to analyze the departure from the linear regime

The success of magnetic hyperthermia cancer treatments rely strongly on the magnetic properties of the nanoparticles and their intricate dependence on the externally applied field. This is particularly more so as the response departs from the low field linear regime. In this paper we introduce a new parameter, referred to as the efficiency in converting electromagnetic energy into thermal energy, which is shown to be remarkably useful in the analysis of the system response, especially when the power loss is investigated as a function of the applied field amplitude. Using numerical simulations of dynamic hysteresis, through the stochastic Landau-Lifshitz model, we map in detail the efficiency as a function of all relevant parameters of the system and compare the results with simple-yet powerful-predictions based on heuristic arguments about the relaxation time. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4705392]

Prismatic magnetite magnetosomes from cultivated Magnetovibrio blakemorei strain MV-1: a magnetic fingerprint in marine sediments?

The magnetic properties (first-order reversal curves, ferromagnetic resonance and decomposition of saturation remanent magnetization acquisition) of Magnetovibrio blakemorei, a cultivated marine magnetotactic bacterium, differ from those of other magnetotactic species from sediments deposited in lakes and marine habitats previously studied. This finding suggests that magnetite produced by some magnetotactic bacteria retains magnetic properties in relation to the crystallographic structure of the magnetic phase produced and thus might represent a magnetic fingerprint for a specific magnetotactic bacterium. The use of this fingerprint is a non-destructive, new technology that might allow for the identification and presence of specific species or types of magnetotactic bacteria in certain environments such as sediments.

Spin-glass-like behavior of uncompensated surface spins in NiO nanoparticulated powder

Nickel oxide nonoparticles successfully synthesized by a polymer percursor method are studied in this work. The analysis of X-ray powder diffraction data provides a mean crystallite size of 22 +/- 2 nm which is in a good agreement with the mean size estimated from transmission electron microscopy images. Whereas the magnetization (M) vs. magnetic field (H) curve obtained at 5 K is consistent with a ferromagnetic component which coexists with an antiferromagnetic component, the presence of two peaks in the zero-field-cooled trace suggests the occurrence of two blocking process. The broad maximum at high temperature was associated with the thermal relaxation of uncompensated spins at the particle core and the low temperature peak was assigned to the freeze of surface spins clusters. Static and dynamic magnetic results suggest that the correlations of surface spins clusters show a spin-glass-like below T-g = 7.3 +/- 0.1 K with critical exponents zv = 9.7 +/- 0.5 and beta = 0.7 +/- 0.1, which are consistent with typical reported for spin-glass systems. (C) 2012 Elsevier B.V. All rights reserved.

Electrodeposition of catalytic and magnetic gold nanoparticles on dendrimer-carbon nanotube layer-by-layer films

Magnetic and catalytic gold nanoparticles were electrodeposited through potential pulse on dendrimer-carbon nanotube layer-by-layer (LbL) films. A plasmon absorption band at about 550 nm revealed the presence of nanoscale gold in the film. The location of the Au nanoparticles in the film was clearly observed by selecting the magnetic force microscopy mode. To our knowledge, this is the first report on the electrochemical synthesis of magnetic Au nanoparticles. In addition to the magnetic properties, the Au nanoparticles also exhibited high catalytic activity towards ethanol and glycerol oxidation in alkaline medium.