We present experimental evidence on the physical origin of a magnetic dead layer (MDL) in manganite nanoparticles. The studied nanoparticles constitute the wall of La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3 manganite nanotubes. Magnetic properties analysis and high resolution transmission electron microscopy show a shell of approximately 2 nm thickness with different properties from the core. In this shell the atoms are in a noncrystalline array that perfectly explains the 50% reduction of the magnetization compared to the bulk. Moreover, we present experimental evidence that the internal magnetic structure of the MDL is constituted by small ferromagnetic clusters in a frustrated configuration.

We prepared synthetic hydroxyapatite [HAP; Ca5(PO4)(3-x)(CO3)x(OH)(1+x) (x = 0.3)] and then investigated this material's ability to remove trivalent antimony [Sb(III)] from water. The HAP was characterized by X-ray diffraction analysis, scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy. The sorption of Sb(III) to HAP was measured over an Sb(III) concentration range of 0.05-50 mg L(-1), at constant ionic strength (I = 0.01 mol dm(-3)) in equilibrated aqueous suspensions (34 g dm(-3)) for 5 < pH < 8 in vessels that were closed to the atmosphere. Under these conditions, we found that HAP particles were enriched in Ca after incongruent dissolution of the solid. More than 95% of the Sb(III) in solution adsorbed to the solid-phase HAP in <30 min. The equilibrium distribution of Sb(III) (solid vs liquid phase) was characterized by a Langmuir model with gamma(max) = 6.7 +/- 0.1 x 10(-8) mol m(-2) (1.4 +/- 0.2 x 10(-4) mol dm(-3) g(-1)) and K(ads) = 1.5 +/- 0.2 x 10(3) dm3 mol(-1). As Sb adsorption occurred, the pH of the isoelectric point (pH(iep)) of the HAP suspensions declined from 7.7 to 6.9. This finding supports the idea that the negative surface potential of the HAP increased due to the adsorption of Sb as a charged species. The decline in pH(iep) during Sb adsorption plus the thermodynamic description of the Sb(III)-HAP-H2O system suggest likely surface reactions for the interaction of Sb with HAP. We discuss the efficiency of Sb removal from water by HAP in the context of phosphate enrichment.

We present a study on the magnetic properties of manganite ultrafine grains assembled in nanotubes of 800 nm of external diameter and 6 -8 m length. The study includes two homogeneous ferromagnetic compounds, La 0.67 Ca 0.33 MnO 3 and La 0.67 Sr 0.33 MnO 3 , and one exhibiting ferromagnetic and charge-ordered phase coexistence, La 0.325 Pr 0.300 Ca 0.375 MnO 3 . From magnetic measurements, we conclude that the grains behave as single magnetic domains. Observations of dipolar interactions between magnetic grains are evidenced by isothermal remanent magnetization and direct current demagnetization experiments. These experiments suggest that the grain magnetic moments should be arranged in a fanning configuration at H = 0. Also, a uniaxial shape anisotropy was observed on previously aligned ferromagnetic nanotubes by ferromagnetic-resonance experiences. In La 0.325 Pr 0.300 Ca 0.375 MnO 3 , we observe distinctive features related to its inhomogeneous character as compared with the ferromagnetic homogeneous nanotubes.

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We report the synthesis of rare-earth manganese-oxide-based nanotubes. The pore wetting technique was used to obtain structures of nominal composition La0.325Pr0.300Ca0.375MnO3 with 800 nm external diameter, 4000 nm length, and wall thickness below 100 nm exhibiting magnetic and magnetoresistive behavior below 200 K, including nonvolatile memory. Walls are found to be formed by small crystals of approximately 20 nm. Structures obtained using different diameter of pores, as small as 100 nm, have a similar aspect ratio. Results show the realization of nanotubes of manganites exhibiting intrinsic phase separation.

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In this work, ZrO2−CeO2 mixed oxide nanotubes with 50, 70, and 90 mol % CeO2 were synthesized following a very simple, high yield procedure, and their properties were characterized by synchrotron radiation XRD and by high resolution electron microscopy. The 50, 70, and 90 mol % CeO2 nanotubes exhibited the tetragonal phase (t′-form and t′′-form, P42/nmc space group) or the cubic phase (Fm3m space group). The nanotube walls were composed of nanoparticles with an average crystallite size ranging from 4.7 to 7.6 nm. Electron microscopy observations confirmed the size of these nanoparticles by direct observation. The SEM and TEM results showed that individual nanotubes were composed of a curved sheet of these nanoparticles. By SEM analysis, the nanotubes were found to have lengths of around 1−8 μm, diameters of around 500 nm, and wall thicknesses of 20 nm. Elemental analysis showed that Ce:Zr ratios appeared to be constant across space, suggesting compositional homogeneity in the samples. The 90 mol % CeO2 nanotubes exhibited the highest value of specific surface area, 101 m2·g−1, which compared with about 28 m2·g−1 for the other two compositions.

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.