ABSTRACT. Polycrystalline samples of LaFeO3 and GdFeO3 were synthesized by the molten salt method. Some properties and the quality of the resulting compounds were investigated. The crystal structure and purity of the samples was determined through X-ray diffraction and Rietveld analysis. The vibrational properties were characterized by Raman and IR spectroscopy. Mössbauer spectroscopy was used to determine the ionic state of the Fe ions and the internal hyperfine magnetic fields Considerable reduction of the heat treatment (temperature and time) for the reaction to take place was achieved without detriment of the quality of the compounds.

Polycrystalline samples of FeSe 1-x Te x (x = 0.00, 0.25, 0.50, 0.75 and 1.00) were synthesized by solid-state reaction to study the effects of substituting Se with Te in the system. The magnetization properties of the resulting compounds were investigated and the crystallographic structures of the samples analyzed through X-ray diffraction.Mössbauer spectroscopy was used to determine the ionic state of the Fe ions and the hyperfine fields. The magnetic susceptibility curves of the samples with x = 0.25, 0.50 and 0.75 show superconducting behavior. The lattice parameters and the cell volume increase monotonically with increasing Te concentration and the Mössbauer spectra reveal the absence of internal magnetic hyperfine fields.

No abstract

No abstract

No abstract

MFe 2 O 4 (M = Ni and Zn) nanoparticles were prepared by the hydrothermal method. The obtained samples were characterized by X-ray and electron diffraction, Scanning and Transmission Electron Microscopy and Mössbauer spectroscopy. The transmission images show homogeneous shape and particle size ranging from 10 to 40 nm, depending on the nature of the M cation. Mössbauer spectroscopy yields to a ratio of occupancy between the A and B sites of 0.7 in the case of NiFe 2 O 4 oxide. DC magnetization (2-300 K) measurements reveal a superparamagnetic behaviour for the ZnFe 2 O 4 sample with a blocking temperature of 20 K. By contrast, in the case of the NiFe 2 O 4 ferrite the blocking temperature appears to be above 300 K and at lower temperature, it shows a ferrimagnetic behaviour arising from the superexchange interactions that take place in this inverse spinel. Mössbauer spectroscopy results confirm the bulk magnetic measurements.Keywords: ferrites, nanoparticles, superparamagnetism, ferrimagnetism, inverse spinel. Nanopartículas de ferrita MFe 2 O 4 (M = Ni y Zn): síntesis hidrotermal y propiedades magnéticas.Se han preparado mediante el método hidrotermal nanopartículas de ferritas MFe 2 O 4 (M = Ni, Zn). Las muestras obtenidas fueron caracterizadas mediante difracción de rayos X y electrones, microscopía electrónica de transmisión y barrido y espectroscopia Mössbauer. Las imágenes de transmisión muestran partículas de forma y tamaño homogéneo de 10 a 40 nm según la naturaleza del catión M. La espectroscopia Mössbauer revela una relación de ocupación entre los sitios A y B por los átomos de hierro de 0.7 en el caso del óxido NiFe 2 O 4 . Las medidas de magnetización DC (2 -300 K) muestran un comportamiento superparamagnético para la muestra ZnFe 2 O 4 con una temperatura de bloqueo de 20 K. En el caso de las nanopartículas de NiFe 2 O 4 la temperatura de bloqueo parece estar por encima de los 300 K mostrando por debajo de la misma, comportamiento ferrimagnético provocado por las interacciones de superintercambio que tienen lugar en esta espinela inversa. Los resultados de espectroscopia Mössbauer confirman los datos de las medidas magnéticas.Palabras clave: ferritas, nanopartículas, superparamgnetismo, ferrimagnetismo, espinela inversa.

No abstract

Individual single-walled carbon nanotubes (SWCNTs) were synthesized on the patterned water-soluble catalyst by thermal chemical vapor deposition. The individual SWCNTs were obtained by introducing polyvinylpyrrolidone (PVP) as a dispersant. The number of SWCNTs between two electrodes were approximately 1-2 with an average diameter of about 1.7 nm and a yield of forming electrodes of nearly 70%. The PVP played an important role in dispersing catalysts and suppressing the active sites to limit the number of SWCNTs during synthesis, which is a critical condition for fabrication of field effect transistors (FETs). The measured I-V characteristics of the over layer-deposited electrodes revealed a clear gating effect in large portion, in good agreement with Raman observations in several excitation energies. The patterning procedure, catalyst preparation, and growth condition for fabrication of the SWCNT-FET were further discussed.