Showing papers by "F. J. Bermejo published in 2006"
TL;DR: The effect of Fe doping on the magnetic properties of massive magnetoresistance perovskites is studied by muon spin rotation spectroscopy and macroscopic static magnetization measurements as discussed by the authors.
Abstract: The effect of Fe doping on the magnetic properties of ${\mathrm{La}}_{0.7}{\mathrm{Pb}}_{0.3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.2)$ colossal magnetoresistance perovskites is studied by muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ spectroscopy and macroscopic static magnetization measurements. The latter quantities show a cusp at temperatures below that signaling magnetic ordering (Curie temperature ${T}_{C}$) as well as a kink at lower temperatures which are taken as signatures of magnetic irreversibility. Random substitution of Mn by Fe ions gives rise to antiferromagnetic couplings mediated by conventional superexchange interaction. This manifests itself by the decrease of Curie temperatures and a concomitant decrease of the spontaneous magnetization with increased doping. The muon relaxation data under zero and applied longitudinal fields is interpreted in terms of progressive spin freezing concomitant with a reduction of the ferromagnetic component as Fe substitution is increased. The spin dynamics as ${T}_{C}$ is crossed from above shows clear departures from those expected for exchange-coupled Heisenberg or Ising systems but rather it displays features common to magnetically interacting-cluster systems. This effect leads to the appearance of a peak in the muon relaxation rate versus temperature curves that is located at temperatures below that signaling magnetic ordering $({T}_{C})$. Finally, direct evidence of the existence of antiferromagnetic and ferromagnetically short-range-ordered regions within the $x=0.2$ sample is provided by polarized neutron diffraction. The picture that emerges from our study identifies competing positive (double exchange) and negative (superexchange) interactions at random sites as the entities responsible for the low-temperature freezing monitored by using $\ensuremath{\mu}\mathrm{SR}$ spectroscopy.
26 citations
TL;DR: In this paper, a comparative study of the excitations in bulk and liquid confined within the pores of Mobile Crystalline Material-41 was presented. But the results were limited to the range of wave vectors where collective excitations can be followed up.
Abstract: We present a comparative study of the excitations in bulk and liquid ${\mathrm{D}}_{2}$ confined within the pores of MCM-41. The material (Mobile Crystalline Material-41) is a silicate obtained by means of a template that yields a partially crystalline structure composed by arrays of nonintersecting hexagonal channels of controlled width having walls made of amorphous ${\mathrm{SiO}}_{2}$. Its porosity was characterized by means of adsorption isotherms and found to be composed by a regular array of pores having a narrow distribution of sizes with a most probable value of $2.45\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The assessment of the precise location of the sample within the pores is carried out by means of pressure isotherms. The study was conducted at two pressures which correspond to pore fillings above the capillary condensation regime. Within the range of wave vectors where collective excitations can be followed up $(0.3\ensuremath{\leqslant}Q\ensuremath{\leqslant}3.0\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1})$, we found confinement brings forward a large shortening of the excitation lifetimes that shifts the characteristic frequencies to higher energies. In addition, the coherent quasielastic scattering shows signatures of reduced diffusivity.
17 citations
TL;DR: In this article, a set of momentum distributions of liquid para-hydrogen were determined by means of inelastic neutron scattering under applied pressures ranging from 1 to 80 bars, at T = 16.5 K. The results depict significant pressure dependences for all single-particle quantities such as the momentum distributions and average kinetic energies as well as for the final state effects.
Abstract: Momentum distributions of liquid para-hydrogen were determined by means of inelastic neutron scattering under applied pressures ranging from 1 to 80 bars, at T=16.5 K. The data processing procedure involves the parametrization of the dynamic structure factor and yields a set of momentum distributions as functions of the density. The results depict significant pressure dependences for all single-particle quantities such as the momentum distributions and average kinetic energies as well as for the final-state effects. The obtained results enable us to quantify the departure of the momentum distributions from classical Maxwell-Gauss shape. Such observations are then rationalized with the help of calculations that were carried out in terms of the correlated density matrix formalism.
17 citations
TL;DR: In this article, the primary spectrometer of ILL's thermal three-axis spectrometers has undergone a major rebuild during the last few years, and the performance of the new instrument was highlighted in terms of flexibility and luminosity.
Abstract: The primary spectrometer of ILL's thermal three-axis spectrometer IN8 has undergone a major rebuild during the last few years. We here summarise the characteristics of the new instrument and highlight its enhanced performance in terms of flexibility and luminosity. Flux measurements indicate an increase of monochromatic flux at the sample position of × 3 – × 5 depending on monochromator and wavelength as compared to the previous instrument IN8B. We also highlight the scientific impact of the instrument as well as further instrumental developments envisaged.
15 citations
TL;DR: In this paper, the authors studied the collective excitations of liquid and glassy tellurium by means of inelastic neutron scattering, and reported on the dynamics of liquid Te as measured at two different temperatures, just above melting (T{sub m}=723 K) and at roughly 1000 K as well as the glass that is studied at room temperature.
Abstract: The spectra of collective excitations of liquid and glassy tellurium have been studied by means of inelastic neutron scattering. Here we report on the dynamics of liquid Te as measured at two different temperatures, just above melting (T{sub m}=723 K) and at {approx}1000 K as well as the glass that is studied at room temperature. Estimates for the velocity of propagating excitations for both temperatures have been obtained from the experimental data, and a contrasting behavior is found with respect to anomalies shown by the adiabatic sound velocity measured by ultrasound methods. The origin of such differences is finally discussed.
14 citations
TL;DR: In this paper, the primary spectrometer of the ILL's thermal-beam three-axis spectrometers was rebuilt to increase the instrument flexibility as well as the monochromatic flux at the sample position.
Abstract: The primary spectrometer of the ILL's thermal-beam three-axis spectrometer IN8 has recently been rebuilt to increase the instrument flexibility as well as the monochromatic flux at the sample position. Further improvement of the instrument is now envisaged and consists in the installation of a multianalyser detector system to multiplex the secondary spectrometer. This will enable one to obtain images of selected regions in (Q,ω) space. The design of such a multiplexed secondary spectrometer is based on analytical results reflecting the geometrical considerations and computer simulations performed using the neutron ray-tracing package McStas.
13 citations
TL;DR: In this article, the effect of Fe doping on the magnetic properties of La 0.7 Pb 0.3 Mn 1 - x Fe x O x, x = 0 and x = 1.2 perovskites is studied using muon spectroscopy and macroscopic static magnetization measurements.
Abstract: A study on the effect of Fe doping on the magnetic properties of La 0.7 Pb 0.3 Mn 1 - x Fe x O x , x = 0 and x = 0.2 perovskites is conducted using muon spectroscopy and macroscopic static magnetization measurements. For x = 0 , magnetization curves show a clear ferromagnetic component while a 20% Fe doping leads to the appearance of a kink in zero-field curves at low temperatures, attributed to progressive spin freezing together with a reduction of the ferromagnetic component. On dynamic grounds, we show that this effect translates into the appearance of non-exponential relaxation as T c is crossed from above.
3 citations
TL;DR: The capability of muon spin rotation and relaxation to explore dynamics in the vicinity of the glass transition is illustrated by results pertaining to three materials exhibiting two different glass-forming abilities as discussed by the authors.
Abstract: The capability of muon spin rotation and muon spin relaxation to explore dynamics in the vicinity of the glass transition is illustrated by results pertaining to three materials exhibiting two different glass-forming abilities. Measurements under transverse magnetic fields enable us to monitor the dynamics of muonium-labelled closed-shell molecules within the microsecond range. The results display the onset of stochastic molecular motions taking place upon crossing from below the glass-transition temperature. In turn, the molecular dynamics of radicals formed by addition of atomic muonium to unsaturated organic molecules can also be explored up to far shorter times by means of relaxation measurements under longitudinal fields. The technique is then shown to be capable of singling out stochastic reorientational motions from others, which usually are strongly coupled to them and usually dominate the material response when measured using higher-frequency probes such as neutron and light scattering.
2 citations