Francesco Congiu

ZnFe2O4 nanoparticles dispersed in a highly porous silica aerogel matrix: a magnetic study

We report the detailed structural characterization and magnetic investigation of nanocrystalline zinc ferrite nanoparticles supported on a silica aerogel porous matrix which differ in size (in the range 4-11 nm) and the inversion degree (from 0.4 to 0.2) as compared to bulk zinc ferrite which has a normal spinel structure. The samples were investigated by zero-field-cooling-field-cooling, thermo-remnant DC magnetization measurements, AC magnetization investigation and Mössbauer spectroscopy. The nanocomposites are superparamagnetic at room temperature; the temperature of the superparamagnetic transition in the samples decreases with the particle size and therefore it is mainly determined by the inversion degree rather than by the particle size, which would give an opposite effect on the blocking temperature. The contribution of particle interaction to the magnetic behavior of the nanocomposites decreases significantly in the sample with the largest particle size. The values of the anisotropy constant give evidence that the anisotropy constant decreases upon increasing the particle size of the samples. All these results clearly indicate that, even when dispersed with low concentration in a non-magnetic and highly porous and insulating matrix, the zinc ferrite nanoparticles show a magnetic behavior similar to that displayed when they are unsupported or dispersed in a similar but denser matrix, and with higher loading. The effective anisotropy measured for our samples appears to be systematically higher than that measured for supported zinc ferrite nanoparticles of similar size, indicating that this effect probably occurs as a consequence of the high inversion degree.

Mössbauer spectroscopic investigation of some iron-containing sodium phosphate glasses

Abstract 57Fe Mossbauer spectroscopy has been used to investigate the effect of an addition of an increasing amount of Fe2O3 on the iron environment in a number of sodium phosphate glasses of composition (1 - x)NaPO3 · xFe2O3 (x = 0.05, 0.1, 0.15), prepared in air under the same conditions. High-spin ferric and ferrous iron in octahedral coordination is present in all the samples. Two sites, with different disorder and octahedral distortion, are occupied by ferric ions. An increasing iron content results in an increase in the distortion and disorder of the ferric sites and, in addition, in a redistribution of the iron atoms between the two ferric sites.

Local order in amorphous Fe2Zr prepared by mechanical alloying and mechanical grinding

Abstract Amorphous Fe2Zr samples were prepared by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying from elemental powders. The structural changes during milling were monitored by X-ray diffraction. Mossbauer absorption permitted us to follow the transition from the magnetically ordered phase of iron to the paramagnetic state of the final amorphous products. X-ray and neutron diffraction, coupled with Reverse Monte Carlo simulation, were used to extract the partial structure factors for the two samples. The results for the two alloys are compared and discussed.

Mössbauer spectroscopic investigation of iron in sodium phosphate glasses

Abstract We studied three sodium phosphate glasses (1 − x ) NaPO 3 + Fe 2 O 3 ( x = 0.05, 0.1, 0.15) prepared in air in the same conditions using 57 Fe Mossbauer spectroscopy in order to investigate the effect of a growing Fe 2 O 3 addition on the features of iron sites. High spin ferric and ferrous iron in octahedral coordination is present in all the samples. We found that the distortion, compared to a regular octahedron, of the ferric and ferrous iron sites increases from the low to the high iron content samples.

Magnetic properties of pseudomorphic epitaxial films of Pr0.7Ca0.3MnO3 under different biaxial tensile stresses

In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn atom along the easy axis. On the base of high resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviours are discussed in terms of the crystallographic constraints imposed by the epitaxy of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the tensile strain on the film c-axis, lying within the substrate plane, favours the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.

Investigation of cobalt–iron alloy nanoparticles in silica matrix by X-ray diffraction and Mössbauer spectroscopy

Abstract Nanocrystals of Co100−xFex with different nominal iron content (x=50, 70, 100) embedded in amorphous silica matrix were prepared by the sol–gel method and thermal treatment in hydrogen atmosphere. Different salts (acetates and nitrates) were used as metal precursors. The samples were characterized by X-ray diffraction and Mossbauer spectroscopy in order to verify the formation of the Fe–Co alloy and investigate its structure and composition. XRD patterns only for acetate samples exhibit peaks belonging to a body centered cubic phase, expected for the Fe–Co alloy at the investigated compositions, and also the lattice parameter values confirm the alloy formation. The average particle size obtained by the Warren–Averbach analysis turned out 9.6(5) nm. In agreement with these results, Mossbauer spectra show the formation of an ordered component with isomer shift and hyperfine fields typical of the Fe–Co alloy only for acetate samples.

Mössbauer Investigation of γ-Fe2O3 Nanocrystals in Silica Matrix Prepared by the Sol-gel Method

A series of Fe2O3-SiO2 nanocomposites (25 weight % of Fe2O3) has been prepared using a sol-gel method. Samples showing different features were obtained by varying the evaporation conditions acting on the surface / volume ratio of the starting sol. The samples were investigated using Mössbauer spectroscopy at low temperature. The Fe2O3 nanoparticles have been identified as maghemite ( γ-Fe2O3) particles. The samples show a superparamagnetic behavior with a blocking temperature that depends on the average particle size.

Mössbauer investigation of rare earth sites in europium containing glasses

Abstract Samples of crystalline and amorphous trivalent europium metaphosphate were examined by 151 Eu Mossbauer spectroscopy. The disorder of the lanthanide sites in the glass has been investigated by comparing the glass to the corresponding crystalline material. In the crystalline and amorphous samples, the rare earth is accommodated in a site on average distorted with respect to cubic symmetry; this site has no threefold or fourfold symmetry axis. In the glass, the structural disorder has been evaluated by broadening the Lorentzian component of the quadrupole multiplet with respect to the crystal. The spectrum of the amorphous sample was also analysed using a convolution method, which reproduces the distribution of the quadrupole interaction parameter due to the structural disorder.

Hyperfine Interactions at Lanthanide Sites in Europium Doped Oxide Glasses

Abstract A series of europium doped Na2O-B2O3-SiO2, PbO-GeO2 and ZnOTeO2 glasses was investi-gated by means of151 Mössbauer spectroscopy. The distortion of the Eu sites has been evaluated in the Na2O-B2O3-SiO2 glasses by means of the quadrupolar interaction parameter and the asym-metry parameter; the disorder has been estimated by the line width. The occupancy of the sites in the borosilicate glasses is discussed. The correlation of the isomer shift with the optical basicity of the glass is discussed in terms of the degree of covalence of the Eu-O bond.

Magnetic Properties and Electronic Structures of Compounds from the Hf-Co Phase System

Magnetic measurements of molar susceptibilities as a function of temperature in the temperature range between 5 and 400 K were performed for the intermetallic compounds Hf2Co and HfCo2, using a SQUID magnetometer. The density of states at the Fermi level for Hf2Co was evaluated from the measured spin paramagnetic susceptibility. In addition, band structure calculations using the augmented plane waves plus local orbitals (APW+lo) method as implemented in the WIEN2k programme package for the two compounds were done. The obtained results were compared with the measured data.