R.D. Sánchez

Fe and Mo Valences in Sr2FeMoO6

Abstract The valences of Fe and Mo in Sr 2 FeMoO 6 are investigated by means of X-ray absorption and photoemission spectroscopies. We find that the d occupations are similar to those of Fe in Fe 2 O 3 and Mo in MoO 3 . Considering the covalency, however, we argue that the ground state is closer to a Fe +3 (3 d 5 )–Mo +5 (4 d 1 ) configuration than to a Fe +2 (3 d 6 )–Mo +6 (4 d 0 ) configuration.

Tunneling barrier in nanoparticle junctions of La2/3(Ca,Sr)1/3MnO3: Nonlinear current–voltage characteristics

We study the nonlinear current–voltage (I–V) characteristics and analyze the voltage-dependent tunneling conductance in nanoparticles of La2/3A1/3MnO3 (A=Ca, Sr). The powders were prepared by different wet-chemical routes and low calcination temperatures were used to obtain an average particle size D≈30 nm. The data are comprehensively explained in terms of the tunneling picture, which allows one to estimate the height of the grain boundary insulating barrier (φ) for each sample. For constant D, our results show that the sample preparation route is mainly responsible for the value of φ in nanoparticles, while the Coulomb gap in the Coulomb blockade regime is ∼3 times higher for Sr- than for Ca-doping. We also show that a small fraction of the barriers contribute to the nonlinear transport, and the current is mainly carried through low-resistive percolated paths. In addition, despite the different barrier strengths, the low-field magnetoresistance (LFMR) is similar for all samples, implying that φ is not t...

Specific heat, magnetic susceptibility and electrical resistivity measurements on LaNiO3

Abstract Measurements of specific heat, magnetic susceptibility and electrical resistivity on LaNiO 3 are reported. No evidence of magnetic ordering or metal-insulator transitions is found. From the measured electronic specific heat parameter γ and the Pauli susceptibility χ P , a Stoner enhancement parameter is derived. Comparison with band structure calculations performed by other workers is also made.

Magnetic behavior of single La0.67Ca0.33MnO3 nanotubes: Surface and shape effects

We report magnetization experiments in two magnetically isolated ferromagnetic nanotubes of perovskite La0.67Ca0.33MnO3. The results show that the magnetic anisotropy is determined by the sample shape, although the coercive field is reduced by incoherent magnetization reversal modes. The temperature dependence of the magnetization reveals that the magnetic behavior is dominated by grain surface properties. These measurements were acquired using a silicon micromechanical oscillator working in its resonant mode. The sensitivity was enough to measure the magnetic properties of these two samples with a mass lower than 14pg and to obtain for the first time the magnetization loop for one isolated nanotube.

R - M interactions in (R = Y or Gd; M=Cu or Zn)

(R = Gd or Y; M = Cu or Zn) oxides have been studied using the specific heat, DC magnetic susceptibility and electron paramagnetic resonsance (EPR). For one member of the series without a magnetic moment at M, namely , measurements reveal long-range antiferromagnetic order at K, much lower than the Curie - Weiss temperature of K. This indicates the existence of competing interactions that introduce a large degree of magnetic frustration in the system. For the Cu - Cu interactions are responsible for the broad maximum in the magnetic contribution to the specific heat centred at K that stretches beyond the instrumental limit of 25 K. The strong Cu - Cu interactions also present in , combined with the Gd - Cu interaction, polarize the Gd sublattice, giving measurable contributions to the specific heat in the same temperature range. In addition, they broaden the Gd EPR line and saturate its integrated intensity. The ordering temperature of Gd ions is raised to K.

Electronic structure of Sr2FeMoO6

Abstract We have used electron spectroscopies to investigate the electronic structure of the double perovskite Sr2FeMoO6. The valence-band photoemission spectra present a well-defined cut-off at the Fermi level, indicative of the metallic character of the material. The O 1s X-ray absorption spectrum presents three peaks, which are in good correspondence with the main structures in the unoccupied density-of-states of DF-LDA calculations. The electron energy-loss spectra present two structures which are also interpreted in terms of transitions between the bands obtained in the DF-LDA calculations.

Hole-doping dependence of percolative phase separation in Pr 0.5 − δ Ca 0 . 2 + δ Sr 0.3 MnO 3 around half doping

We address the problem of the percolative phase separation in polycrystalline samples of

Effects of Cation Vacancies in the Phonon Raman Spectra of LaMnO3

{\mathrm{Pr}}_{0.5\ensuremath{-}\ensuremath{\delta}}{\mathrm{Ca}}_{0.2+\ensuremath{\delta}}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}

Synthesis, Characterization, and Nanocatalysis Application of Core–Shell Superparamagnetic Nanoparticles of Fe3O4@Pd

for

Oxygen vacancies effect on phase separation in Pr0.5Ca0.2Sr0.3MnO3−δ

\ensuremath{-}0.04l~\ensuremath{\delta}l~0.04