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Magnetization experiments are performed on two magnetically isolated ferromagnetic nanotubes of perovskite [La.sub.0.67][Ca.sub.0.33]Mn[O.sub.3]. The temperature dependence of the magnetization has shown that the magnetic behavior is dominated by grain surface properties.

Abstract: We studied electric and magnetic properties of polycrystalline La0.325Pr0.300Ca0.375MnO3, a manganese oxide-based compound exhibiting intrinsic multiphase coexistence of submicrometric ferromagnetic, charge-ordered and charge-disordered regions. Time relaxation in both resistivity and magnetization were observed in a broad temperature range, signaling that coexisting phases are out of equilibrium. Different regimes of instability are presented and discussed. [Copyright &y& Elsevier]

(U,Gd)O2 sintered pellets are fabricated by different methods. The homogeneity characterisation of the Gd content seems to be necessary for a production control to qualify the process and the final product obtained. In this paper, we propose an analysis of the X-ray diffraction powder patterns through the Rietveld method, in which the differences between the experimental and the calculated data proposed from a crystalline structure model are evaluated. This result allows us to determine the cell parameters, that can be correlated with the Gd concentration, and the existence of other phases with different Gd contents. [Copyright &y& Elsevier]

We report the synthesis of rare-earth manganese-oxide-based nanotubes. The pore wetting technique was used to obtain structures of nominal composition La[sub 0.325]Pr[sub 0.300]Ca[sub 0.375]MnO[sub 3] 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. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: When a uniaxial magnetic field is applied to a non-magnetic dispersive medium filled with magnetic nanoparticles, they auto-assemble into thin needles parallel to the field direction, due to the strong dipolar interaction among them. We have prepared in this way magnetically oriented nanocomposites of nanometer-size CoFe2O4 particles in a polydimethylsiloxane polymer matrix, with 10%w/w of magnetic particles. We present the characteristic magnetic relaxation curves measured after the application of a magnetic field forming an angle α with respect to the needle direction. We show that the magnetic viscosity (calculated from the logarithmic relaxation curves) as a function of α presents a minimum at α=0, indicating slower relaxation processes associated with this configuration of fields. The results seems to point out that the local magnetic anisotropy of the nanoparticles is oriented along the needles, resulting in the macroscopic magnetic anisotropy observed in our measurements. [Copyright &y& Elsevier]

Abstract: Perovskite-type mixed oxides, such as La1− x Sr x Co1− y Fe y O3 compounds, are at the present time one of the most widely used materials for cathode of intermediate temperature solid-oxide fuel cells (SOFCs). A significant increase of the specific area of the cathode can be achieved by preparing structures on the nanometric scale. In this work, nanocrystalline tubular structures of La0.6Sr0.4CoO3 and La0.6Sr0.4Co0.2Fe0.8O3 were synthesized by denitration, using polycarbonate porous templates, microwave irradiation and a further calcination at 800°C. The shape and size of the tubes are determined by the characteristics of the template pores. The final thermal treatment is needed to obtain the desired perosvkite-type crystal structure. The resulting products were studied by means of X-ray diffraction and electron microscopy techniques. All rights reserved [Copyright &y& Elsevier]

Abstract: Magnetic properties of a single nanotube whose walls are constituted by nanograins of La2/3Sr1/3MnO3 are studied by means of Monte Carlo (MC) simulation. The system is considered as composed by ferromagnetic grains which couple via dipolar interaction. The grain size distribution is obtained from experimental measurements and the inter-grain distance distribution is obtained from a study of distance distribution among magnetic particles distributed in the tube walls. We show the magnetization behavior for a tube of 700nm of diameter and 3.5μm length. We discuss the simulation for different temperatures and external fields. As a main result, we show that the distribution of inter-granular distances has a unique behavior for tubular structures of similar diameter/length aspect ratio, independent of their sizes. This scaling relation allows us to perform the simulations using a tube of smaller dimensions. We succeed in explaining the magnetization curves, finding that dipolar interaction is necessary to explain the experimental behavior and that the grains behave as having magnetic dead layers. [Copyright &y& Elsevier]