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).     

Ferromagnetic Mn-doped Si0.3Ge0.7 nanodots self-assembled on Si(100)

Densely packed epitaxial Mn-doped Si(0.3)Ge(0.7) nanodots self-assembled on Si(100) have been obtained. Their structural properties were studied using reflection high-energy electron diffraction, energy dispersive x-ray diffraction, atomic force microscopy, extended x-ray absorption fine structure measurements and high-resolution transmission electron microscopy. Mn(5)Ge(1)Si(2) crystallites embedded in Si(0.3)Ge(0.7) were found. They exhibit a ferromagnetic behaviour with a Curie temperature of about 225?K.     

Magnetic anisotropy and magnetization dynamics of Fe nanoparticles embedded in Cr and Ag matrices

Static and dynamical magnetic properties of Fe nanoparticles (NPs) embedded in non-magnetic (Ag) and antiferromagnetic (Cr) matrices with a volume filling fraction (VFF) of 10% have been investigated. In both Fe@Ag and Fe@Cr nanocomposites, the Fe NPs have a narrow size distribution, with a mean particle diameter around 2 nm. In both samples, the saturation magnetization reaches that of Fe bulk bcc, suggesting the absence of alloying with the matrices. The coercivity at 5 K is much larger in Fe@Cr than in Fe@Ag as a result of the strong interaction between the Fe NPs and the Cr matrix. Temperature-dependent magnetization and ac-susceptibility measurements point out further evidence of the enhanced interparticle interaction in the Fe@Cr system. While the behaviour of Fe@Ag indicates the presence of weakly interacting magnetic monodomain particles with a wide distribution of blocking temperatures, Fe@Cr behaves like a superspin glass produced by the magnetic interactions between NPs.     

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.     

Superparamagnetic blocking and superspin-glass freezing in ultra small δ-(Fe(0.67)Mn(0.33))OOH particles

The magnetic properties of ultra-small (~2 nm) δ-(Fe(0.67)Mn(0.33))OOH nanoparticles prepared by a microemulsion technique have been investigated by magnetization and ac susceptibility measurements at variable frequency. The results provide evidence of two different magnetic regimes whose onset is identified by two maxima in the zero-field-cooled susceptibility: a large one, centered at ~150 K (T(mh)), and a narrow one at ~30 K (T(ml)). The two temperatures exhibit a different frequency dependence: T(mh) follows a Vogel-Fulcher law τ = τ(0)exp[(E(a)/k(B))/(T-T(0))], indicating a blocking of weakly interacting nanoparticle moments, whereas T(ml) follows a power law τ = τ(0)(T(g)/T(mν)-T(g))(α), suggesting a collective freezing of nanoparticle moments (superspin-glass state). This picture is coherent with the field dependence of T(ml) and T(mh) and with the temperature dependence of the coercivity, strongly increasing below 30 K.     

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.     

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.     

Mn–ferrite nanoparticles via reverse microemulsions: synthesis and characterization

Mn–ferrite nanoparticles were synthesized by thermal treatment at 800 °C of manganese and iron oxo-hydroxides obtained via water-in-oil microemulsions consisting of n-hexanol as continuous phase, cetyl trimethyl ammonium bromide (CTAB) as the cationic surfactant and aqueous solutions of metal salts and precipitant agent (tetramethyl ammonium hydroxide) as reagents. Nanoparticles were synthesized using a multi-microemulsion approach. Two different co-precipitation routes are described depending on the Fe(II) or Fe(III) precursor salts. The influence of salt concentration and digestion process on the final products was examined. The nanoparticles were characterized by X-ray diffraction accompanied by Rietveld analysis, transmission electron microscopy, thermal analysis, infrared spectroscopy, and SQUID magnetometry. In all the synthesis reported in this study MnFe₂O₄ was observed only after thermal treatment at 800 °C of the as-prepared precursors. Almost spherical nanocrystalline MnFe₂O₄ ranging from 12 to 39 nm was obtained starting from chlorides or mixed chloride–sulfate salts as precursors. Low values of reduced remanent magnetization (M _r/M _s) and coercive field (H _c) induce to believe that a fraction of superparamagnetic particle is present at room temperature.