Search

The effect of Ni2+ substitution on the magneto-transport properties and the concomitant relationship with the magnetocaloric function of nano-sized La0.7Ca0.3Mn1−xNixO3 (x = 0, 0.02, 0.07, and 0.10) perovskite manganites are reported. All the samples were synthesized using the auto-combustion method. X-ray diffraction analysis studies confirmed the phase purity of the synthesized samples. The substitution of Mn3+ ions with Ni2+ ions in the La0.7Ca0.3MnO3 lattice was also verified using this technique. Rietveld analysis indicated that the volume of the unit cell increased with increasing Ni2+ content. Zero-field-cooled and field-cooled magnetization showed that all samples underwent a paramagnetic-ferromagnetic phase transition, which was concomitant with a metal-insulator transition. A small deviation between the zero-field-cooled and field-cooled magnetization curves was observed when the measurements were carried out in a field of 1000 Oe. The Curie temperature decreased systematically from 264 K for x = 0 to 174 K for x = 0.10. Probably the doping at the Mn3+ sites with Ni2+ ions in the La0.7Ca0.3MnO3 lattice weakened the Mn3+-O-Mn4+ double exchange interaction, which led to a decrease in the transition temperature. The metal-insulator transition also shifted to lower temperatures upon Ni2+ substitution, and the value of the resistivity increased. Different conduction mechanisms were found in different temperature regions. Important physical parameters such as the polaron activation energy were obtained from the fit of the models to the experimental data. Arrot’s plots revealed a second-order nature of the magnetic transition for all the samples, which was also confirmed by Landau’s theory and universal curves. The second-order character of the magnetic phase transition observed in the pristine La0.7Ca0.3MnO3 sample may be attributed to effects of the downsizing of the particle. Interestingly, a notable increase in the value of the magnetic entropy change was observed at Ni2+ doping levels as low as 2%. The magnetoresistance underwent a great change near the magnetic transition temperature, suggesting a close relationship between the magnetocaloric effect and magnetotransport properties in La0.7Ca0.3MnO3 manganites. Such behavior can be attributed to the spin order/disorder feature, which plays a crucial role in both effects. On the other hand, the value of the magnetoresistance of the pristine La0.7Ca0.3MnO3 sample increased upon Ni2+ doping, which is probably related to the downsizing of the particles.

In this work we analyze the role of the SiO2 layer in the functionality of Fe3O4/SiO2/Si heterostructures, which have been proved to present a strong potential for spin-based applications. Nevertheless, a complete control of the interfaces properties is fundamental for application. In this work, high quality heterostructures are fabricated avoiding chemical exchange and achieving good quality interfaces. The chemical interaction between the Fe3O4 and SiO2 layers during the heterostructures manufacture is deeply analyzed and its role on the transport, magnetic and magneto transport response is revealed. It is proven that during Fe3O4 deposition a competitive interplay happens between the catalytic action of Fe atoms, the transport of dissociated oxygen through SiO2 and the stabilization of Fe3O4. A defective silicon oxide layer is found to grow on top of the native SiO2 enabling the formation of single phase Fe3O4 layer. Such a defective layer and the granular character of the Fe3O4 determine the magnetic and transport response of the heterostructures. The present results prove that the defects in the SiO2 layer induce the switching of the MR sign, so that anomalous positive MR at RT exceeding 17% at 80 kOe is obtained in heterostructures with 19 nm thick magnetite layer, while conventional negative MR response is obtained for thicker films.

Ferromagnetic low-temperature phase (LTP) of manganese bismuth MnBi (at.%) alloy has been prepared by DC magnetron sputtering onto glass by alternative deposition of Bi and Mn with in-situ annealing. Modifying the temperature during deposition and/or during post-growth annealing leads to different microstructures ranging from quasi-isotropic isolated MnBi micro-particles to larger platelet-like individual ones up to highly coalesced textured film-like areas. These microstructural variations determine the magnetic behaviour, including change in the easy axis direction and possibility of varying coercivity from 4 to 16 kOe at room temperature. In all the cases, coercivity has a positive temperature coefficient, reaching 29 kOe at 500 K, being among the largest reported coercivity for MnBi. Isolated LTP-MnBi microstructures, being homogeneously distributed onto glass, have been grown while avoiding the presence of unreacted Bi regions between the MnBi structures. The possibility of tuning microstructural and magnetic properties, in combination with their time stability, makes them interesting candidates for high temperature permanent magnet applications., This research has been supported by MINECO (Ministerio de Economía y Competitividad) at IMDEA Nanociencia through ENMA-National project (MAT2014-56955-R) and at ICMM-CSIC by MAT2014-52405-C2-2-R; by MINECO through M-era.Net Programme: NEXMAG project (PCIN-2015-126); and by Regional Government (Comunidad de Madrid): NANOFRONTMAG (Ref. S2013/MIT-2850). EC acknowledges MINECO for the Formación Posdoctoral (JdC) program (FPDI-2013).

We present a new protocol to grow large-area, high-quality single-layer graphene on Cu foils at relatively low temperatures. We use C60 molecules evaporated in ultra high vacuum conditions as carbon source. This clean environment results in a strong reduction of oxygen-containing groups as depicted by X-ray photoelectron spectroscopy (XPS). Unzipping of C60 is thermally promoted by annealing the substrate at 800 °C during evaporation. The graphene layer extends over areas larger than the Cu crystallite size, although it is changing its orientation with respect to the surface in the wrinkles and grain boundaries, producing a modulated ring in the low energy electron diffraction (LEED) pattern. This protocol is a self-limiting process leading exclusively to one single graphene layer. Raman spectroscopy confirms the high quality of the grown graphene. This layer exhibits an unperturbed Dirac-cone with a clear n-doping of 0.77 eV, which is caused by the interaction between graphene and substrate. Density functional theory (DFT) calculations show that this interaction can be induced by a coupling between graphene and substrate at specific points of the structure leading to a local sp3 configuration, which also contribute to the D-band in the Raman spectra., Financial support from EU Horizon 2020 research and innovation program under grant agreement No. 696656 (GrapheneCore1-Graphene-based disruptive technologies) and Spanish MINECO (grants MAT2014-54231-C4-1-P,MAT2014-52405-C2-2-R, CSD2009-00013, MAT2015-64110) is acknowledged. JA and NR are supported by the FPI program from MINECO (BES-2012-058600 and BES-2015-072642, respectively). CM and JIM acknowledge the financial support by the “Ramón y Cajal” Program of MINECO (RYC-2014-16626 and RYC-2015-17730, respectively). JIM and GS acknowledge funding from the ERC-Synergy Program (grant ERC-2013-SYG-610256 Nanocosmos). JIM thanks CTI-CSIC for use of computing resources. MK and VV acknowledge funding from ERC-CZ (LL1301) project. GO-I acknowledges FCT for his grant (SFRH/BPD/90562/2012).

We present the structural and magnetic properties of a novel series of La0.67Sr0.22Ba0.11Mn1-xFexO3 (0 ≤ x ≤ 0.3) perovskites prepared by the sol–gel method. These oxides were characterized by x-ray (XRD), neutron powder diffraction (NDP) at room temperature and magnetization measurements versus temperature and various applied magnetic fields. The NPD data, very sensitive to the octahedral tilting, show a crystallographic phase transition from an orthorhombic structure (Pnma) for x = 0 to a rhombohedral structure (R-3c) for Fe-doped samples. Magnetic data show that x = 0 and x = 0.1 perovskites exhibit a paramagnetic–ferromagnetic transition at low temperature, while for 0.2 ≤ x ≤ 0.3 a strong divergence between ZFC and FC curves suggest the presence of antagonistic antiferromagnetic and ferromagnetic interactions. The magnetic entropy change (|ΔSmax|) takes values of 2.46 J kg−1 K−1, 2.43 J kg−1 K−1 and 0.91 J kg−1 for x = 0, x = 0.1 and 0.2, respectively at 5 T. The relative cooling power (RCP) amounts 169 J Kg−1, 241 J Kg−1 and 70 J Kg−1 at 5 T for x = 0, 0.1, 0.2 respectively. These values are compared favorably with those of some others reported manganites, making La0.67Sr0.22Ba0.11Mn0.9Fe0.1O3 a promising candidate for magnetic refrigeration., We acknowledge the financial support of the Spanish Ministry of Economy and Competitivity the project MAT2013-41099-R.

La0.5Ca0.5MnO3 and La0.5Ca0.4Ag0.1MnO3 ceramics have been prepared by sol–gel methods and its magnetic properties, and electronic transport properties have been experimentally investigated. X-ray diffraction data reveal that all samples crystallize in a orthorhombic structure with Pnma space group. Magnetic measurements of La0.5Ca0.5MnO3 show multiple magnetic transitions, a paramagnetic to ferromagnetic and a ferromagnetic to charge-ordered antiferromagnetic. Two metal–semiconductor transitions (Tρ1 and Tρ2) are observed in the electrical resistivity. The electrical resistivity decreases when an external magnetic field is applied rendering a magnetoresistive effect of the order of 75% at the charge-ordered temperature. All the samples clearly reveal the unusual low temperature resistivity minimum, presumably due to the combined effect of weak localization, electron–electron and electron–phonon scattering. The present results are discussed and possible explanations were given based on the related theory and previous reported results., This paper within the frame work of collaboration is supported by the Tunisian Ministry of Higher Education and Scientific Research Grants MAT2011-27470-C02-02 and CSD2009-00013 from the Spanish MINECO are acknowledged.

8 pags, 9 figs, 4 tabs, The transparent semiconductor ZnSnO with cubic spinel structure and the isostructural ZnTiO have been magnetically doped with Co. ZnCoSnO and ZnCoTiO exhibit ferrimagnetism below T ≈ 13 K and T ≈ 17 K. Ferrimagnetic moments are evident in M vs H curves below T by small hysteresis. Fits to strictly linear Curie-Weiss plots above T give μ ≈ 4.86 μ and ≈4.91 μ for ZnCoSnO and ZnCoTiO, above theoretical predictions. Impedance spectroscopy data from sintered ceramic can be fitted with a standard equivalent circuit model based on two RC elements for bulk and GB areas. The relative dielectric permittivity of the bulk is ≈20 and ≈30 for ZnSnO and ZnTiO. The semiconducting ZnCoSnO and ZnCoTiOceramics exhibit bulk resistivity of ≈1 10 Ω cm and ≈1 10 Ω cm at 560 K (287 °C), and bulk activation energies of E ≈ 1.2 eV and 1.1 eV., R.S. acknowledges a travel grant (Convenio Internacional) from the Universidad Complutense de Madrid to visit the USACH in Chile. JPG thanks the Spanish MINECO for granting a Juan de la Cierva fellowship. Spanish MINECO grant MAT2014-52405-C2-2-R is also acknowledged.

The effect of composition and particle size on the electrical and magnetic behavior of ZnxCo1-xFe2O4 spinels (x = 0.2 and 0.3) has been studied. Powdered samples of these ferrites have been synthetized by the liquid mix technique and hydrothermal method (in sub and supercritical conditions), leading to average particle sizes of ca. 50 and 10 nm, respectively. They have been characterized by means of X-ray diffraction, Thermogravimetric analysis, Energy-Dispersive X-ray Spectroscopy and impedance and magnetic measurements. Permittivity values up to ca. 500 were registered at 375 K, which remained almost constant at moderate frequencies, between 10 3 and 106 Hz. Stabilization of polarization phenomena is very sensitive to grain size and composition. Dielectric behavior evolves to a relaxor ferroelectric response when grain size becomes nanometric and, particularly, when the sample shows high monodispersion. The conduction mechanism and type of majority charge carriers have been established from Seebeck measurements. The x = 0.3 sample, prepared in supercritical water for the first time, exhibits homogeneous particle size distribution, superparamagnetic behavior and Curie temperature lower than those corresponding to similar microsized samples. The electronic response of the ferrites obtained in supercritical conditions is interpreted considering the possible short scale polarization of nanodomains. © 2014 Elsevier B.V. All rights reserved., Authors are grateful to the CAI centers of the UCM (XRD and electron microscopy) and financial support from Spanish Ministerio de Ciencia e Innovación, Grants MAT2010-20117, MAT2011-27470-C02-02 and CSD2009-00013.

This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after 24 months of embargo period, The antiferromagnet Mn5Si3 has recently attracted attention because a noncollinear spin arrangement has been shown to produce a topological anomalous Hall effect and an inverse magnetocaloric effect. Here we synthesize single crystals of Mn5Si3 using flux growth. We determine the phase diagram through magnetization and measure the magnetoresistance and the Hall effect. We find the collinear and noncollinear antiferromagnetic phases at low temperatures and, in addition, a third magnetic phase, in between the two antiferromagnetic phases. The latter magnetic phase might be caused by strain produced by Cu inclusions. This suggests that fluctuations of the mixed character magnetic ordering in this compound can be easily quenched by stress, This work was supported by the Spanish MINECO (Consolider Ingenio Molecular Nanoscience CSD2007-00010 program, FIS2017-84330-R, MDM-2014-0377, MAT2014-52405-C2-2-R, FJCI-2015-25427 and CSD2009-00013), by the Comunidad de Madrid through program NANOMAGCOST-CM (S2018 NMT-4321) and MAD2D-CM (S2013/MIT-3007) and by EU (Graphene Core1 contract No. 696656, Nanopyme FP7-NMP-2012 SMALL-6 NMP3-SL-2012 310516 and COST CA16218)

Neutron strain scanning measurements on a eutectoid steel rod that has been subjected to standard industrial coiling and straightening operations are presented. Strains were determined non-destructively using two different diffractometers, one at a steady-state neutron source and the other at a pulsed spallation neutron source, with measurements made in both the ferrite and cementite components of the pearlitic microstructure. The residual stress state is explained in terms of a simplified analytical model for a two-phase material, which takes into account the successive loading operations contributing to residual stress. The results show that residual stresses generated by bending–straightening operations are significant and are likely to play an important role in the mechanical properties of the final wires.

Residual stress profiles in the three principal directions in a cold-drawn pearlitic rod were calculated by a three-dimensional finite element simulation and measured by synchrotron X-ray diffraction. It has been shown that anisotropy of cold-drawn pearlite has to be considered for accurate simulations of residual stresses of steel wires. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Residual stress profiles have been measured in the ferrite and cementite phases of a cold-drawn eutectoid steel rod by neutron and synchrotron X-ray diffraction in three orientations (axial, radial and hoop). Neutron diffraction was employed to measure the ferrite stresses, whereas synchrotron radiation was used for ferrite and cementite stresses. Experimental results in the ferrite phase showed excellent agreement between both experimental techniques when gauge volume effects were accounted for. Axial cementite stresses were always tensile, with a maximum value close to 1700 MPa at the rod surface. Radial and hoop cementite stresses were compressive along the diameter of the rod, with a minimum of -1900 MPa at the rod center. A 3D-finite element simulation of the macro residual stresses resulting from cold-drawing showed remarkable agreement with those determined from the experimental measurements in the ferrite and cementite phases.

Local deviations from the average magnetic and atomic structure of La 0.67 Ca 0.33 MnO 3 are explored by combining neutron diffraction and reverse Monte Carlo techniques. Short range FM Mn–Mn spin pair correlations are found to be a majority well above T c , while AF correlations between first neighbour pairs of Mn ions develop preferentially around T c and within those pairs with a distance larger than the average first neighbour Mn–Mn distance. These results agree with the predictions of current theories on phase transitions in double exchange systems.

XANES and EXAFS measurements has been performed in the 10–300 K temperature range in order to investigate the local order around the Fe and Cu ions in two samples with different oxygen stoichiometry: PrBaCuFeO 5.00 and PrBaCuFeO 5.25 . XANES allows determining that extra-oxygen is coordinated to the Fe ions and is not coordinated to the Cu ions. EXAFS data have been analysed in order to obtain the temperature behaviour of the Debye–Waller factor related to the oxygen surrounding the Fe and Cu ions. Additionally, low temperature resistivity data are reported for both samples, they show a variable range hoping mechanism for the conductivity.

Neutron diffraction measurements have been carried out on the REST diffractometer (Studsvik) on curved components fabricated by cold working bimetallic tubes. The tubes consist of two layers (an outer one of austenitic steel and an inner one of ferritic steel) with a total wall thickness of 6 mm. Both austenitic and ferritic components have been scanned at different positions, throughout the material thickness and for three orientations of the component axes with respect to the neutron momentum transfer vector. Due to the complicated geometry of the problem, Bragg peak data have been corrected for non-uniform illumination of gauge volume, neutron absorption and instrumental effects by means of a Monte Carlo simulation code which incorporates details of the diffractometer and a realistic model of the specimen. We report on estimates of the stress tensor components.

Deep inelastic neutron scattering (DINS) measurements on a hydrogenous glass, previously studied by conventional neutron spectroscopy and computer simulation techniques, reveal, by comparison with the spectra of the same sample after crystallization, some distinctive features in the neutron Compton profiles (NCS), the origin of which can be traced down to the different effective temperatures (kinetic energies) experienced by the particle in crystalline and glassy environments.

An overview of recent results on the high-frequency collective atomic motions in disordered molecular systems (liquids and glasses) is presented. The main emphasis is on the comparison of neutron scattering and computer simulation results which enables the unambiguous identification of the relevant dynamic processes. A brief account regarding the theoretical modeling of spectral lineshapes is also given, and applications to several specific cases are described in order to illustrate the capabilities of coherent inelastic neutron scattering for obtaining reliable information about the collective dynamics of these systems at microscopic time and length scales.

The coherent inelastic response from liquid deuterium in three different thermodynamic states has been measured on a thermal three axis neutron spectrometer. The observed dispersive behaviour has been analyzed using a damped harmonic oscillator model. The momentum transfer dependencies of the renormalized frequencies and damping coefficients are compared and related to the results from correlated density matrix formalism.

The quasielastic spectra of liquid and plastic-crystalline oxygen are shown to be dominated by the magnetic response. An approximate separation of the nuclear and magnetic responses from the total, unpolarized neutron double differential cross-sections is achieved by means of a subtraction of the structural contributions calculated from a Molecular Dynamics simulation. The magnetic scattering spectra are then shown to reproduce the wavevector dependence of the inelastic intensities associated with the magnetic excitations measured in a previous study, and are analyzed in terms of a model for an exchange-coupled paramagnet. Finally, the close resemblance between the magnetic response of the liquid and of the plastic crystal γ-phase are discussed.

An overview of the present status of the theory of neutron scattering from disordered molecular systems (liquids and glasses), with the main emphasis on the theoretical modelling of spectral lineshapes, is presented. Applications to several specific cases are described in order to illustrate the capabilities of Quasielastic Neutron Scattering (QENS) for obtaining reliable information about the stochastic dynamics of these systems.

Neutron inelastic triple-axis spectra of CD 3 OD/6%D 2 O in the glassy and liquid phases are reported. Both the dispersion and damping of the sound modes show characteristic features as the temperature is lowered to the glassy regime. Furthermore, analysis of the observed scattering intensities reveals the presence in the glass of a second component centred at zero frequency which is not observed in the liquid.

Well defined collective excitations up to a momentum transfer value of 3.8 A -1 have been found in liquid deuterium 0.5 K above the melting point. The derived dispersion curve shows a remarkable similarity with the one observed for liquid 4 He in the normal liquid phase, with a well defined minimum at a momentum transfer of 2.08 A -1 corresponding to a gap of 30 K. A qualitative comparison is made with recent calculations on liquid para-hydrogen at the triple point.

Neutron inelastic triple-axis spectra of CD 3 OD/6% D 2 O in the glassy and liquid phases are reported. Both the dispersion and damping of the sound modes show characteristic features as the temperature is lowered down into the glassy regime. Also the analysis of the observed scattering intensities evidences the presence in the glass of a second component centered at zero frequency which is not observed in the liquid.

The crystal field (CF) levels of the ground state multiplets of Ho and Er in R2BaNiO5 have been studied combining inelastic neutron scattering and photoluminescence experiments at temperatures above and below the 1D to 3D magnetic transitions (at 53 and 32 K, respectively) at which the Ni and R sublattices order antiferromagneticaly. In Ho compound 3D magnetic peaks have been detected well above TN. Energy shift and splitting of several Kramers doublets is observed in Er compound below the magnetic transition due to the local magnetic field at Er sites. No dispersion is detected for the CF transitions. The evolution with temperature of the energy and width of the inelastic and elastic peaks is investigated. The lowest R3+ crystal field levels have been obtained in Ho compound at 0, 1.3, 2.7, 11.6 and 18.1 meV and in Er compound at 0, 4.9, 8.2, 10.4 and 16.6 meV above TN.

The coherent dynamical response of liquid deuterium 0.5 K above the melting temperature has been studied by means of triple-axis inelastic scattering and time-of-flight techniques. A model scattering law to represent the single-particle and low energy collective dynamics has been tested by analysing the time-of-flight spectra. On the other hand, well defined collective excitations up to a momentum transfer of 3.8A-1 have been found in the triple-axis experiments. The derived dispersion curve shows a well defined minimum at 2.08A-1 with a gap of 30 K.

A brief account of recent studies on the coherent inelastic response from representative examples of molecular systems in liquid and glassy states is presented.

Neutron strain scanning measurements have been performed on straight and curved parts of a component manufactured from bimetallic tubes. Instrumental and sample-related effects contributing to non-strain induced Bragg peak shifts have been corrected for by means of Monte Carlo simulations which incorporate details of the various diffractometer elements.

The spin dynamics of the 1D antiferromagnetic Heisenberg system Y2BaNi1−xZnxO5 has been investigated by thermal (for x = 0) and cold (for x = 0.01 and 0.03) neutron scattering. The former set of measurements enables us to study the temperature dependence of the transitions from the ground state to states above the Haldane gap. The latter, low incident energy measurements, reveal a quasielastic component in the spectra which shows an activated temperature behaviour and a strong depedence on the chain length. We suggest a mechanism which could be the origin of the present observations.

The results of a neutron scattering investigation on the spin and structural dynamics of oxygen in the liquid and orientationally disordered phases are presented. The presence of magnetic excitations is inferred from the analysis of the widths of the unpolarized spectra as well as from the wave vector dependence of the inelastic intensities.

A procedure for the analysis of neutron single-differential scattering cross sections measured by diffraction experiments in amorphous solid or liquid molecular systems is presented. The method incorporates a convenient scheme for applying the inelasticity or Placzek corrections up to a first order. A substantial improvement over conventional fitting procedures is illustrated by the analysis of a reactor and pulsed neutrons datasets. Finally, the dangers of performing oversimple corrections as well as the need of incorporating additional information in the separation between “self” and “distinct” terms is discussed.

The ν 6 IR band of SiH 3 F has been recorded with a resolution of 0.06 cm −1 , and analysed with available microwave data. The analysis takes in consideration the Coriolis perturbation with ν 3 and the q + 6 l-doubling term. A set of vib-rotational parameters are determined by least squares fitting of the data. The IR contour is simulated and compared to the experimental contour with good agreement in general.