Structural evolution in mechanical alloying of Co and B powders

X-ray diffraction and scanning electron microscopy were used to monitor the mechanical alloying of four different mixtures of cobalt and boron with atomic compositions Co₅₀B₅₀, Co₆₇B₃₃, Co₇₅B₂₅ and Co₈₀B₂₀, respectively. The process induces amorphization reactions depending on the boron content; an almost complete amorphization was reached for the Co₆₇B₃₃ and Co₈₀B₂₀ samples, where only minor traces of unreacted cobalt are present. Extended X-ray absorption fine structure data collected on the most amorphous sample confirmed the diffraction results. In all the samples, the formation of t-Co₂B was detected.     

CoFe2O4 nanocrystalline powders prepared by citrate-gel methods: Synthesis, structure and magnetic properties

Nanocrystalline CoFe₂O₄ powders were prepared by decomposition of metal ion citrate precursors. Four samples were synthesized from precursor solutions having different pH values in the range <1–7.0. The powders were characterized by X-ray Diffraction, Thermogravimetry, Differential Thermal Analysis, N₂ physisorption and Transmission Electron Microscopy. Magnetic properties were explored by a SQUID magnetometer. Three out of the four samples, coming from solutions of pH 2, 4 and 7, were produced by an autocombustion reaction and are very similar as regards average size of the nanoparticles (about 20 nm), their morphology and the magnetic properties, while the fourth sample was produced by a slower thermal decomposition and is composed of smaller nanoparticles (about 10 nm).     

An X-ray diffraction study of sodium borate glasses

X-ray diffraction measurements have been performed on two pairs of sodium borate glasses of differing compositions. Glasses of given composition were obtained with two different thermal histories. The radial distribution functions of samples of the same composition appear similar. The short range order is not affected by the range of quenching rates explored.     

New Synthesis of Ferrite–Silica Nanocomposites by a Sol–Gel Auto-Combustion

A sol–gel autocombustion method was used to synthesize nanometric metal-oxide powders, and was extended for the first time to prepare ferrite–silica nanocomposites. The gels obtained by mixing suitable amounts of citric acid, metal nitrates, ammonia (pure phases) and tetraethylortosilicate (nanocomposites) were converted directly to ferrite (either γ-Fe₂O₃ or CoFe₂O₄) or ferrite–silica composites through a rapid autocombustion reaction. The combustion involves a thermally induced autocatalytic oxidation–reduction reaction between the nitrate and the citrate ions. The sample characterization by X-ray diffraction, transmission electron microscopy and N₂ physisorption measurements revealed nanosized pure phase powders and nanocomposites in which small spherical nanoparticles (mean size 3.5 and 5.0nm, respectively for the γ-Fe₂O₃and CoFe₂O₄) are homogeneously dispersed over a mesoporous silica matrix.     

Spin-canting and magnetic anisotropy in ultrasmall CoFe2O4 nanoparticles

The magnetic properties of cobalt ferrite nanoparticles dispersed in a silica matrix in samples with different concentrations (5 and 10 wt% CoFe2O 4) and same particle size (3 nm) were studied by magnetization, DC and AC susceptibility, and Mossbauer spectroscopy measurements. The results indicate that the particles are very weakly interacting. The magnetic properties (saturation magnetization, anisotropy constant, and spin-canting) are discussed in relation to the cation distribution.     

Magnetic properties of cobalt ferrite-silica nanocomposites prepared by a sol-gel autocombustion technique

The magnetic properties of cobalt ferrite-silica nanocomposites with different concentrations (15, 30, and 50 wt %) and sizes (7, 16, and 28 nm) of ferrite particles have been studied by static magnetization measurements and Mossbauer spectroscopy. The results indicate a superparamagnetic behavior of the nanoparticles, with weak interactions slightly increasing with the cobalt ferrite content and with the particle size. From high-field Mossbauer spectra at low temperatures, the cationic distribution and the degree of spin canting have been estimated and both parameters are only slightly dependent on the particle size. The magnetic anisotropy constant increases with decreasing particle size, but in contrast to many other systems, the cobalt ferrite nanoparticles are found to have an anisotropy constant that is smaller than the bulk value. This can be explained by the distribution of the cations. The weak dependence of spin canting degree on particle size indicates that the spin canting is not simply a surface phenomenon but also occurs in the interiors of the particles.     

Simple and fast preparation of pure maghemite nanopowders through sol–gel self-combustion

Pure maghemite nanopowders made up of nanocrystals with average size of 19 nm was prepared by a simple sol–gel self combustion process. The gel pH and the primer temperature turned out the key parameters for the obtaining of the maghemite phase, that often is accompanied by the most thermodynamically stable hematite. Pure maghemite was achieved only with a gel pH value of 7 and with a primer temperature between 290 and 325 °C. XRD and IR pointed out the formation of maghemite with tetragonal structure and HRTEM indicated the high degree of crystallinity of the powder. Mossbauer measurements allowed to confirm the presence of maghemite phase with Fe(octa):Fe(tetra) ratio of 1.62 which is very close to the theoretical value and the presence spin canting strongly dependent on applied magnetic field. This picture is confirmed by dc magnetic measurements.     

Cationic distribution and spin canting in CoFe2O4 nanoparticles

CoFe(2)O(4) nanoparticles (D(NPD) ~6 nm), prepared by a thermal decomposition technique, have been investigated through the combined use of dc magnetization measurements, neutron diffraction, and (57)Fe M?ssbauer spectrometry under high applied magnetic field. Despite the small particle size, the value of saturation magnetization at 300 K (M(s) ?= 70 A m(2) kg(-1)) and at 5 K (M(s) ?= 100 A m(2) kg(-1)) are rather close to the bulk values, making the samples prepared with this method attractive for biomedical applications. Neutron diffraction measurements indicate the typical ferrimagnetic structure of the ferrites, showing an inversion degree (γ(NPD) = 0.74) that is in very good agreement with cationic distribution established from low temperature (10 K) M?ssbauer measurements in high magnetic field (γ(moss) = 0.76). In addition, the in-field M?ssbauer spectrum shows the presence of a non-collinear spin structure in both A and B sublattices. The results allow us to explain the high value of saturation magnetization and provide a better insight into the complex interplay between cationic distribution and magnetic disorder in ferrimagnetic nanoparticles.