A. de Andrés

Low-temperature magnetoresistance in polycrystalline manganites: connectivity versus grain size

We have performed magnetic and transport measurements on La0.7Ca0.3MnO3 polycrystalline pellets and thin films. We propose a model that reproduces very well the low-field and high-field magnetoresistance at low temperature. The decrease of the resistance at high fields is modeled by the opening of new conduction channels in parallel due to the ordering of Mn spins blocked at the grain surface. We found that high-field magnetoresistance is independent of grain size, at least down to 12 nm, but depends strongly on the connectivity between grains.

Origin of the colossal dielectric response of Pr 0.6 Ca 0.4 Mn O 3

We report the detailed study of dielectric response of Pr(0.6)Ca(0.4)MnO(3) (PCMO), member of manganite family showing colossal magnetoresistance. Measurements have been performed on four polycrystalline samples and four single crystals, allowing us to compare and extract the essence of dielectric response in the material. High frequency dielectric function is found to be 30, as expected for the perovskite material. Dielectric relaxation is found in frequency window of 20Hz-1MHz at temperatures of 50-200K that yields to colossal low-frequency dielectric function, i.e. static dielectric constant. Static dielectric constant is always colossal, but varies considerably in different samples from 1000 until 100000. The measured data can be simulated very well by blocking (surface barrier) capacitance in series with sample resistance. This indicates that the large dielectric constant in PCMO arises from the Schottky barriers at electrical contacts. Measurements in magnetic field and with d.c. bias support this interpretation. Weak anomaly at the charge ordering temperature can also be attributed to interplay of sample and contact resistance. We comment our results in the framework of related studies by other groups.

Enhanced magnetoresistance in the complex perovskite LaCu3Mn4O12

Moderate-pressure techniques (P=2 GPa) have been used to prepare the complex LaCu3Mn4O12 perovskite. It has been characterized by neutron powder diffraction, magnetic, and magnetotransport measurements. This material is ferrimagnetic below TC=361 K. The magnetoresistance (MR) is enhanced with respect to that of CaCu3Mn4O12 due to the effective electronic injection that dramatically reduces the bulk resistivity, thus promoting the grain-boundary contribution to the electrical resistance. Values of low-field MR close to 3% at room temperature are achieved for magnetic fields of 1 T.

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.

Raman phonons and light scattering in RMnO3 (R=La, Pr, Nd, Ho, Er Tb and Y) orthorhombic and hexagonal manganites

Abstract RMnO3 compounds present the Pbnm orthorhombic structure for R=La, Pr, Nd while compounds with smaller ionic radii (R=Ho, Er Tb and Y) can be obtained either in the Pbnm or the P63cm hexagonal structure. Their magnetic and transport properties are strongly dependent on the rare earth and, therefore, on their detailed structure. We have studied these compounds through the Raman phonons of both structures as a function of the rare earth ion and temperature. The phonon frequencies of the main peaks of Pbnm structure have been correlated to structural characteristics, such as bond-lengths or bond angles, obtained from the analysis of neutron diffraction data. According to the structural and Raman behavior, these compounds can be classified into two groups: one includes La, Pr, and Nd, and the other includes the rest. This classification also correlates to the different behavior of their magneto transport properties when doped. A structural transition to a cubic or quasi-cubic perovskite, above 800 K, can be deduced from the phonon spectra for YMnO3 orthorhombic compound.

Effect of stoichiometry on the dynamic mechanical properties of LiNbO3

Acoustic and optic phonons have been measured by Brillouin and Raman spectroscopies in LiNbO3 samples with different stoichiometries. Consequently, the elastic constants have been determined by Brillouin scattering as a function of the stoichiometry. The C44 elastic constant presents, as a function of the stoichiometry, a clear minimum for the [Li]/[Nb]=1 in the melt sample. This behavior must be related to the existence and competition of two kind of defects, i.e., extended defects induced when growing samples not at the congruent composition, and point defects present in nonstoichiometric samples. The optic phonons seem to be dominated by the point defects. The linear increase of the phonon lifetimes, with respect to an ideal stoichiometric sample, can be due to the scattering by the static imperfections and to an increase of the anharmonic scattering. Several Raman peaks, which appear when the [Li]/[Nb] ratio decreases, are related to the presence of Nb in antisites. The width of some particular phonon...

Synthesis of pillared clays assisted by microwaves

Abstract Microwave (MW) irradiation was used to synthesize aluminum-pillared clays (MW-Al-PILCs) by replacing the conventional calcination of Al-PILCs precursors (pre-Al-PILCs). X-ray diffraction (XRD) patterns and nitrogen adsorption/desorption isotherms showed that the MW-irradiated pre-Al-PILCs gave a series of MW-Al-PILC materials exhibiting some characteristics comparable to Al-PILCs obtained by conventional heating. However, differential thermal analysis (DTA), thermogravimetric analysis (TGA), impedance spectroscopy, 27 Al MAS NMR, and intercalative adsorption of methylene blue (MB), indicated that the MW-Al-PILC samples were intermediate between Al-pre-PILCs and conventional Al-PILCs. The 27 Al MAS NMR spectra showed that in MW-Al-PILCs obtained after 18 min of MW irradiation, the Al IV /Al VI ratio was 1/9, which is comparable to that in Al-pre-PILCs (8% Al IV ; 92% Al VI ) and significantly different from the typical values of conventional Al-PILCs (23% Al IV ; 77% Al VI ). It is assumed that the MW-induced calcination gives PILCs containing a fraction of residual, hydroxylated Al-polycations. The good thermal stability of the MW-Al-PILCs supports their possible use as acid catalysts.

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

Abstract:We have studied the behavior of the Raman phonons of a stoichiometric LaMnO3 single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow Tc. The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around Tc can be explained by local melting of the orbital order that begins quite below Tc and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions.

Magnetism and structure in hexagonal Fe/Ru superlattices with short periodicity

Abstract We present recent results on the structure and on the magnetic properties of Fe/Ru superlattices. Detailed knowledge of the local structure is obtained from X-ray diffraction, including in the anomalous mode, and by EXAFS, thus bringing evidence that Fe layers correspond to an expansion exceeding 10%. The magnetic state is mostly characterized by Mossbauer spectroscopy and also by magnetization measurements, thus demonstrating that the internal Fe layer carry a moment of about 2μ B , whereas the two interfacial layers bear no moment. Except for the extension of the non-magnetic interface, these results are in agreement with theoretical predictions of Kubler, which indicate an high-spin metastable state for such an expanded hcp Fe phase.

Long-Lasting Phosphorescent Pigments of the Type SrAl2O4: Eu2+,R3+(R = Dy, Nd) Synthesized by the Sol-Gel Method

Phosphorescent pigments have been investigated for many years, being the SrAl2O4:Eu2+,R3+ with a structure of the stuffed trydimite type the one which exhibits better luminescent properties. The ceramic synthesis of this pigment requires, however, high temperatures during the firing process, reason why it is necessary to use mineralizers that give rise to problems not only in the firing process but also from an environmental point of view. Besides, a mixture of N2-H2 is needed to reduce the Eu (III) to Eu (II) during the heat treatment.This paper reports the synthesis of the compound by means of an alternative method to the ceramic route. In this case the used method is a variant of the Pechinis one because ethilenglycol is not utilized and a resin, difficult to manipulate, is not formed. The use of gaseous currents is not required, since the organic matter of the gel itself generates the reducing atmosphere. Once mixed the different solutions, the gel formation takes place by the addition of citric acid and further elimination of the solvent. The gel is dried, fired and analyzed by X-ray diffraction. Luminescence related to Eu3+ 4f crystal field transitions shows a drastic reduction of trivalent Eu concentration when codoping with Dy or Nd. The characteristic broad phosphorescence band centered at 520 nm is observed under ultraviolet illumination and is enhanced in Dy codoped pigments.