S. Taboada

Structural and magnetic properties of ultrathin epitaxial La0.7Ca0.3MnO3 manganite films: Strain versus finite size effects

We present a structural and magnetic study of La0.7Ca0.3MnO3 epitaxial films (the thickness ranges from 2.4 to 80 nm) on SrTiO3. Their structure, as obtained by x-ray diffraction, is orthorhombic with a 45° rotated 2×2 square lattice, referred to the SrTiO3 substrate, in the film plane. The 2.4 nm film adopts a different structure as evidenced by the extinction of particular diffraction peaks and supported by the behavior of the size of the in-plane structural domains. While no important structural changes are detected for the other films as the layer thickness is reduced, Tc decreases drastically while the remanent magnetization is constant. Tc follows the functional dependence of a mean field approximation for the limitation of the divergence of the spin–spin correlations by the film thickness.

Low field magnetoresistance at the metal–insulator transition in epitaxial manganite thin films

We present a study of the magnetotransport behavior around Tc of epitaxial La0.7Ca0.3MnO3 thin films with controlled chemical defects introduced in order to change the electron-lattice coupling (λ). We found unexpected enhancement of the very low field (below 250 Oe) magnetoresistance around Tc and relate it to a reduction of the magnetic domain size. This process is strongly favored by the presence of chemical defects that also promote weak localization of the carriers into polaron clusters that can be overcome by relatively low magnetic fields. This behavior is characteristic of temperatures near Tc where the bandwidth is reduced and localization, due to intrinsic disorder and chemical defects, is more probable. In addition, the increase of λ raises the polaron binding energy and reduces the temperature at which percolation occurs.

Excitation and desexcitation processes of Eu3+ luminescence in Eu2BaZnO5

Abstract Luminescence and Raman crystal field transitions of Eu 3+ ions in Eu 2 BaZnO 5 are investigated in off-resonance conditions for several excitation energies. Luminescence peaks are identified and classified into two groups (transitions from 5 D 0 and those from 5 D 1 or 5 D 2 ) as a function of their relative intensity versus temperature. Crystal field parameters are found to be very similar for both europium sites. Raman crystal field transitions are studied as a function of temperature. The increase of their intensity when temperature is lowered is explained by the change of the ground state population. The behaviour of the luminescence at several excitation energies has been explained by means of second-order processes. The lattice phonons and thermal population of the lowest excited multiplets have a determining role in the excitation processes of Eu luminescence.

Reflectance, luminescence and Raman experiments on crystal field transitions in

Three spectroscopic techniques have been combined to resolve the complex problem of crystal field levels in luminescence has been investigated in the off-resonance condition for several excitation energies as a function of temperature. Raman phonons and several Raman crystal field transitions are obtained. From reflectance experiments (between 1.25 and 3.7 eV), multiplet levels are determined. The schemes of the lowest electronic levels of both sites have been established combining data from these three spectroscopies which show a good agreement with the calculated values. Crystal field parameters are obtained in an approximation to a local symmetry. The high value of fourth-rank parameters indicates a partial covalent character of Eu-O bonds. The obtained spin-orbit coupling value, , is similar to those in other europium oxides. The variation of the Raman crystal field transition intensity with temperature presents an unexpected deviation with respect to the ground state population, specially pronounced in the low temperature range. The peaks corresponding to the three highest levels of the multiplet are anomalous probably due to some hybridization with the nearest oxygen levels. The relative intensity of luminescence peaks associated with transitions from and levels vary versus temperature as a function of the probability of the non-radiative decay. We propose a mechanism that can explain the observation of anti-Stokes luminescence even at low temperatures.

Transport properties and magnetoresistance of La-Ca manganites near the optimal doping concentrations

Abstract We review our work on the characterization and modeling of the extrinsic and intrinsic contributions to the magnetoresitance of La–Ca manganites near the optimal doping. Firstly, we propose a macroscopic model for the electronic transport that parametrically quantifies the intrinsic and extrinsic contributions to the magnetoresistance, when grain boundaries are present. Then, we complete this model by considering the effects of Coulomb blockade in the electronic transport at low temperatures in polycrystalline samples. Secondly, we explore at a microscopic scale the existence of intrinsic inhomogeneities and the possibility of controlling the electron–lattice coupling by introducing controlled chemical defects. Finally, we describe the effects that these artificially built-in barriers have on the magnetoresistive properties of the system at very low magnetic fields.

Excitation and relaxation processes of Eu3+ crystal field transitions in Eu2BaZnO5

Copyright (c) 1997 Elsevier Science B.V. All rights reserved.Eu 3+ luminescence for several excitation energies in off resonance conditions has been studied. Raman phonons and Raman crystal field transitions are also obtained. The reflectance between 1.25 and 3.7 eV shows the 5 D and upper multiplets up to 27 000 cm −1 . There are two Eu sites with a maximum level energy difference of 19 cm −1 at RT. We present most of the CF levels for 7 F J (J=0-4) and 5 D J . The analysis of the relative intensities of the phonons and luminescence peaks as a function of the excitation energy and the temperature indicates that complex processes are happening in the excitation and that the lattice phonons have a determining role in Eu luminescence.

Luminescence properties of Nd2BaZnO5

The complete fine structure of the first multiplets 4I92, 4I112, 4I132 and 4F32 of the crystal-field levels of Nd3+ ion in Nd2BaZnO5 was identified using temperature-dependent photoluminescence measurements.

Characterization and magnetic properties of R2−xCaxBaNiO5 oxides (R Y and Sm)

Abstract Mixed valence compounds of general formula R2−xCaxBaNiO5 were synthesized and X-ray diffraction data revealed that they crystallize with orthorhombic symmetry, space group Immm, with the Ca2+ and R3+ ions randomly distributed in the 4j crystallographic positions of the lattice. Systematic variation of the lattice parameters with an increase in x is observed. The magnetic properties of these doped oxides are quite different from those reported for pure R2BaNiO5 oxides. Doping produces a breakdown in the one-dimensional antiferromagnetic correlations of the nickel sublattice, and the data obtained appear to indicate the appearance of three-dimensional antiferromagnetic ordering for these doped samples at low temperatures.

Effect of the rare earth substitution on the optical phonons of LaRBaCuO5 (R=Nd and Eu) oxides

Abstract In this work we present, on the one hand, the effect in the Raman spectra is produced by the partial substitution of lanthanum by Nd or Eu in LaRBaCuO 5 . Because the atomic masses and ionic radii of these ions are quite similar the frequencies of the phonons principally related to them are not expected to vary significantly. In fact only small changes are observed in this direction. Nevertheless, substantial changes are observed in the intensities of the low frequency modes. The frequencies of the oxygen vibrations are also influenced by the distortion produced by the rare earth substitution. On the other hand, we report the IR reflectivity spectra of polycrystalline samples of pure and mixed compounds. The lowest frequency modes present small damping which permits us to observe variations in their frequencies corresponding to the different compounds. The effect of the quality of the pressed powder on the IR reflectivity spectra is discussed.