Structural studies of NaPO3-MoO3 glasses by solid-state nuclear magnetic resonance and raman spectroscopy

Vitreous samples were prepared in the (100 - x)% NaPO3-x% MoO3 (0 <= x <= 70) glass-forming system by a modified melt method that allowed good optical quality samples to be obtained. The structural evolution of the vitreous network was monitored as a function of composition by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), Raman scattering, and solid-state nuclear magnetic resonance (NMR) for P-31, Na-23, and Mo-95 nuclei. Addition of MoO3 to the NaPO3 glass melt leads to a pronounced increase in the glass transition temperatures up to x = 45, suggesting a significant increase in network connectivity. For this same composition range, vibrational spectra suggest that the Mo6+ ions are bonded to some nonbridging oxygen atoms (Mo-O- or Mo=O bonded species). Mo-O-Mo bond formation occurs only at MoO3 contents exceeding x = 45. P-31 magic-angle spinning (MAS) NMR spectra, supported by two-dimensional J-resolved spectroscopy, allow a clear distinction between species having two, one, and zero P-O-P linkages. These sites are denoted as Q(2Mo)((2)), Q(1Mo)((2)), and Q(0Mo)((2)), respectively. For x < 0.45, the populations of these sites can be described along the lines of a binary model, according to which each unit of MoO3 converts two Q(nMo)((2)) sites into two Q((n+1)Mo)((2)) sites (n = 0, 1). This structural model is consistent with the presence of tetrahedral Mo(=O)(2)(O-1/2)(2) environments. Indeed, Mo-95 NMR data suggest that the majority of the molybdenum species are four-coordinated. However, the presence of additional six-coordinate molybdenum in the MAS NMR spectra indicates that the structure of these glasses may be more complicated and may additionally involve sharing of network modifier oxide between the network formers phosphorus and molybdenum. This latter hypothesis is further supported by Na-23{P-31} rotational echo double resonance (REDOR) data, which clearly reveal that the magnetic dipole-dipole interactions between P-31 and Na-23 are increasingly diminished with increasing molybdenum content. The partial transfer of modifier from the phosphate to the molybdate network former implies a partial repolymerization of the phosphate species, resulting in the formation of Q(nMo)((3)) species and accounting for the observed increase in the glass transition temperature with increasing MoO3 content that is observed in the composition range 0 <= x <= 45. Glasses with MoO3 contents beyond x = 45 show decreased thermal and crystallization stability. Their structure is characterized by isolated phosphate species [most likely of the P(OMo)(4) type] and molybdenum oxide clusters with a large extent of Mo-O-Mo connectivity.

Exchange narrowing effects in the EPR linewidth of Gd diluted in Ce compounds

Anomalous thermal behavior on the EPR linewidths of Gd impurities diluted in Cc compounds has been observed. In metals, the local magnetic moment EPR linewidth, Delta H, is expected to increase linearly with the temperature. In contrast, in CexLa1-xOs2 the Gd EPR spectra show a nonlinear increase. In this work, the mechanisms that are responsible for the thermal behavior of the EPR lines in CexLa1-xOs2 are examined. We show that the exchange interaction between the local magnetic moments and the conduction electrons are responsible for the narrowing of the spectra at low temperatures. At high temperatures, the contribution to the linewidth of the exchange interaction between the local magnetic moments and the Ce ions has an exponential dependence on the excitation energy of the intermediate valent ions. A complete fitting of the EPR spectra for powdered samples is obtained, (C) 1998 American Institute of Physics. [S0021-8979(98)39911-9].

Effect of spin-polarized subbands in the inhomogeneous hole gas providing the indirect exchange in GaMnAs bilayers

The magnetic order resulting from the indirect exchange in the metallic phase of a (Ga,Mn)As/GaAs double layer structure is studied via Monte Carlo simulation. The polarization of the hole gas is taken into account, establishing a self-consistency between the magnetic order and the electronic structure. The Curie-Weiss temperatures calculated for these low-dimensional systems are in the range of 50-80 K, and the dependence of the transition temperature with the GaAs separation layer is established. (C) 2003 Published by Elsevier B.V.

Correlation of properties of Brazilian Haplustalfs with magnetic susceptibility measurements

Magnetic susceptibility (chi, mass specific) is useful for easy indirect estimation of other soil properties at a low cost. The aim of this study was to assess the use of chi as measured with an analytical balance for predicting properties with a substantial influence on the management of Typic Haplustalfs in southern Brazil. To achieve this 48 topsoil samples were taken at the intersection points in a rectangular grid of 20 m x 20 m cells, with 38 of these used for calibration and 10 for validation in regression analyses. The obtained chi values were slightly higher than, and highly correlated (r = 0.970; P < 0.001) with those measured with a susceptibility meter. Highly significant (P < 0.001) correlations were also found between chi and other soil properties relevant to soil classification and management such as clay content (r = 0.68), cation exchange capacity (r = 0.62), P sorption capacity (r = 0.76) and haematite content (r = 0.82). Results from a principal component analysis of eight properties important for soil classification explained 11% of the variance in the data set. The good predictive ability of chi was consistent with current knowledge on the formation pathways for pedogenic ferrimagnets. In summary, chi, which can be readily measured with an analytical balance, has the potential for quantifying soil attributes and may therefore be used in pedotransfer functions.

Central control of cardiorespiratory interactions in fish

Fish control the relative flow rates of water and blood over the gills in order to optimise respiratory gas exchange. As both flows are markedly pulsatile, close beat-to-beat relationships can be predicted. Cardiorespiratory interactions in fish are controlled primarily by activity in the parasympathetic nervous system that has its origin in cardiac vagal. preganglionic neurons. Recordings of efferent activity in the cardiac vagus include units firing in respiration-related bursts. Bursts of electrical stimuli delivered peripherally to the cardiac vagus or centrally to respiratory branches of cranial, nerves can recruit the heart over a range of frequencies. So, phasic, efferent activity in cardiac vagi, that in the intact fish are respiration-related, can cause heart rate to be modulated by the respiratory rhythm. In elasmobranch fishes this phasic activity seems to arise primarily from central feed-forward interactions with respiratory motor neurones that have overlapping distributions with cardiac neurons in the brainstem. In teleost fish, they arise from increased levels of efferent vagal activity arising from reflex stimulation of chemoreceptors and mechanoreceptors in the orobranchial, cavity. However, these differences are largely a matter of emphasis as both groups show elements of feed-forward and feed-back control of cardiorespiratory interactions. (C) 2008 Elsevier GmbH. All rights reserved.

On the role of synchrony for neuron-astrocyte interactions and perceptual conscious processing

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

EPR linewidth of local magnetic moments diluted in Ce intermediate-valence compounds

Anomalous thermal behavior on the EPR linewidths has been observed for Gd impurities diluted in CexA1-xBn (A=La,Y, B=Ir,Os,Rh,Pd) intermediate-valence compounds. In this work we show that the exchange interaction between the local magnetic moments and the intermediate-valence host ions has an important contribution to the relaxation rates of the local moments. We calculated the relaxation, using the Redfield formalism and the ideas contained in the interconfigurational fluctuation model of Hirst. We show that the exchange interaction contribution has an exponential dependence on the excitation energy of the intermediate-valence ions. © 1992 The American Physical Society.

Renormalization of the one-pion-exchange interaction

A renormalization scheme for the nucleon-nucleon (NN) interaction based on a subtracted T-matrix equation is proposed and applied to the one-pion-exchange potential supplemented by contact interactions. The singlet and triplet scattering lengths are given to fix the renormalized strengths of the contact interactions. With only one scaling parameter (μ), the results show an overall very good agreement with neutron-proton data, particularly for the observables related to the triplet channel. The agreement is qualitative in the 1 S0 channel. Between the low-energy NN observables we have examined, the mixing parameter of the 3S1-3D1 states is the most sensitive to the scale. The scheme is renormalization group invariant for μ → ∞. © 1999 Elsevier Science B.V. All rights reserved.

Influence of nearest-neighbor Coulomb interactions on the phase diagram of the ferromagnetic Kondo model

The influence of a nearest-neighbor Coulomb repulsion of strength V on the properties of the ferromagnetic Kondo model is analyzed using computational techniques. The Hamiltonian studied here is defined on a chain using localized S = 1/2 spins, and one orbital per site. Special emphasis is given to the influence of the Coulomb repulsion on the regions of phase separation recently discovered in this family of models, as well as on the double-exchange-induced ferromagnetic ground state. When phase separation dominates at V= 0, the Coulomb interaction breaks the large domains of the two competing phases into small islands of one phase embedded into the other. This is in agreement with several experimental results, as discussed in the text. Vestiges of the original phase separation regime are found in the spin structure factor as incommensurate peaks, even at large values of V. In the ferromagnetic regime close to density n = 0.5, the Coulomb interaction induces tendencies to charge ordering without altering the fully polarized character of the state. This regime of charge-ordered ferromagnetism may be related with experimental observations of a similar phase by Chen and Cheong [Phys. Rev. Lett. 76, 4042 (1996)]. Our results reinforce the recently introduced notion [see, e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)] that in realistic models for manganites analyzed with unbiased many-body techniques, the ground state properties arise from a competition between ferromagnetism and phase-separation - charge-ordering tendencies. ©1999 The American Physical Society.

Multiperiodic magnetic structures in Hubbard superlattices

We consider fermions in one-dimensional superlattices (SL's), modeled by site-dependent Hubbard-U couplings arranged in a repeated pattern of repulsive (i.e., U>0) and free (U=0) sites. Density matrix renormalization group diagonalization of finite systems is used to calculate the local moment and the magnetic structure factor in the ground state. We have found four regimes for magnetic behavior: uniform local moments forming a spin-density wave (SDW), floppy local moments with short-ranged correlations, local moments on repulsive sites forming long-period SDW's superimposed with short-ranged correlations, and local moments on repulsive sites solely with long-period SDW's; the boundaries between these regimes depend on the range of electronic densities ρ and on the SL aspect ratio. Above a critical electronic density, ρ↑↓, the SDW period oscillates both with ρ and with the spacer thickness. The former oscillation allows one to reproduce all SDW wave vectors within a small range of electronic densities, unlike the homogeneous system. The latter oscillation is related to the exchange oscillation observed in magnetic multilayers. A crossover between regimes of thin to thick layers has also been observed.

Magnetic resonance study of the crystallization behavior of InF3-based glasses doped with Cu2+, Mn2+ and Gd3+

The crystallization of fluoroindate glasses doped with Gd3+, Mn2+ and Cu2+ heat treated at different temperatures, ranging from the glass transition temperature (Tg) to the crystallization temperature (Tc), are investigated by electron paramagnetic resonance (EPR) and 19F nuclear magnetic resonance (NMR). The EPR spectra indicate that the Cu2+ ions in the glass are located in axially distorted octahedral sites. In the crystallized glass, the g-values agreed with those reported for Ba2ZnF6, which correspond to Cu2+ in a tetragonal compressed F- octahedron and to Cu2+ on interstitial sites with a square-planar F- co-ordination. The EPR spectra of the Mn2+ doped glasses exhibit a sextet structure due to the Mn2+ hyperfine interaction. These spectra suggest a highly ordered environment for the Mn2+ ions (close to octahedral symmetry) in the glass. The EPR spectra of the recrystallized sample exhibit resonances at the same position, suggesting that the Mn2+ ions are located in sites of highly symmetric crystalline field. The increase of the line intensity of the sextet and the decrease of the background line in the thermal treated samples suggest that the Mn2+ ions move to the highly ordered sites which contribute to the sextet structure. The EPR spectra of the Gd3+ doped glasses exhibit the typical U-spectrum of a s-state ion in a low symmetry site in disordered systems. The EPR of the crystallized glasses, in contrast, have shown a strong resonance in g ≈ 2.0, suggesting Gd3+ ions in environment close to cubic symmetry. The 19F NMR spin-lattice relaxation rates were also strongly influenced by the crystallization process that takes over in samples annealed above Tc. For the glass samples (doped or undoped) the 19F magnetization recoveries were found to be adjusted by an exponential function and the spin-lattice relaxation was characterized by a single relaxation time. In contrast, for the samples treated above Tc, the 19F magnetization-recovery becomes non-exponential. A remarkable feature of our results is that the changes in the Cu2+, Mn2+, Gd3+ EPR spectra and NMR relaxation, are always observed for the samples annealed above Tc. © 2006 Elsevier B.V. All rights reserved.

Asymptotic approach for the nonlinear equatorial long wave interactions

In the present work we use an asymptotic approach to obtain the long wave equations. The shallow water equation is put as a function of an external parameter that is a measure of both the spatial scales anisotropy and the fast to slow time ratio. The values given to the external parameters are consistent with those computed using typical values of the perturbations in tropical dynamics. Asymptotically, the model converge toward the long wave model. Thus, it is possible to go toward the long wave approximation through intermediate realizable states. With this approach, the resonant nonlinear wave interactions are studied. To simplify, the reduced dynamics of a single resonant triad is used for some selected equatorial trios. It was verified by both theoretical and numerical results that the nonlinear energy exchange period increases smoothly as we move toward the long wave approach. The magnitude of the energy exchanges is also modified, but in this case depends on the particular triad used and also on the initial energy partition among the triad components. Some implications of the results for the tropical dynamics are disccussed. In particular, we discuss the implications of the results for El Nĩo and the Madden-Julian in connection with other scales of time and spatial variability. © Published under licence by IOP Publishing Ltd.

Bright solitons in quasi-one-dimensional dipolar condensates with spatially modulated interactions

We introduce a model for the condensate of dipolar atoms or molecules, in which the dipole-dipole interaction (DDI) is periodically modulated in space due to a periodic change of the local orientation of the permanent dipoles, imposed by the corresponding structure of an external field (the necessary field can be created, in particular, by means of magnetic lattices, which are available to the experiment). The system represents a realization of a nonlocal nonlinear lattice, which has a potential to support various spatial modes. By means of numerical methods and variational approximation (VA), we construct bright one-dimensional solitons in this system and study their stability. In most cases, the VA provides good accuracy and correctly predicts the stability by means of the Vakhitov-Kolokolov criterion. It is found that the periodic modulation may destroy some solitons, which exist in the usual setting with unmodulated DDI and can create stable solitons in other cases, not verified in the absence of modulations. Unstable solitons typically transform into persistent localized breathers. The solitons are often mobile, with inelastic collisions between them leading to oscillating localized modes. © 2013 American Physical Society.

Supramolecular assemblies and magnetic behaviors of the M(II)/p-aminopyridine/malonate (M = Ni, Mn, Cu, Co) systems

The coordination compounds [Ni(μ-mal)(apy)2(H 2O)]·2.8H2O (1), [Mn(μ-mal)(H2O) 2] (2), (apyH)2[Cu(μ-mal)2] (3) and (apyH)2[Co(mal)2(H2O)2] (4) (mal = malonate, apy = p-aminopyridine) have been synthesized and characterized by elemental analysis, vibrational spectroscopy, single crystal X-ray diffraction and magnetometry. With exception of 4, the malonate group acts as bridging ligand leading to the formation of one-dimensional polymeric chains. In compound 1 it was observed the coordination of the p-aminopyridine in the axial positions of the distorted octahedral coordination sphere. The solid-state structure exhibits a high complex 3D network formed by several supramolecular interactions. Magnetic properties were determined for all members of the series and indicate that the materials behave are normal paramagnets, except the Mn polymer 2 which exhibits an antiferromagnetic ground state. © 2013 Elsevier Ltd. All rights reserved.

Intrinsic magnetic properties of BixCo2-xMnO 4 spinels obtained by short-time etching

We report the structural and magnetic properties of Co2MnO 4, partially substituted by Bi at the octahedral site. Bismuth enhances ferromagnetism due to a decrease of the Co2+-Co2+ antiferromagnetic interactions and an increase of the Mn3+-Mn 4+ exchanges. Spurious phases (magnetic and/or nonmagnetic oxides) can easily form because of the large differences between the ionic radii of Bi3+ and Co3+, hiding or altering the intrinsic physical properties of the main BixCo2-xMnO4 phase. An easy way to eliminate the secondary phases is using acid reagents. Short-time etching of Bi0.1Co1.9MnO4 using nitric acid was successfully used, keeping most of the properties of the initial compound, with no alteration of the crystallographic structure. Final stoichiometry was respected (∼Bi0.08Co1.82MnO4), meaning that the material after etching definitely contains bismuth elements in its structure and the observed properties are intrinsic to the oxide spinel. Additional experiments were performed as a function of the synthesis conditions, showing that an optimal pH value of 7 allowed the best magnetic response of the non-doped material. © 2013 Elsevier B.V. All rights reserved.