Giorgio Concas

Superparamagnetic behaviour of γ-Fe2O3 nanoparticles dispersed in a silica matrix

The structural and magnetic properties of two Fe2O3–SiO2 nanocomposites, containing respectively 16.9 and 28.5 wt.% Fe2O3, were investigated. The samples were synthetized by a sol–gel method, using ethylene glycol as a solvent, and heating the gels gradually to 900°C. The procedure allowed us to obtain γ-Fe2O3 nanoparticles homogeneously dispersed in the amorphous silica matrix. The particles have a narrow size distribution and mean sizes from 3 to 6 nm depending on the iron oxide content. The magnetic properties of the samples were investigated by static and dynamic susceptibility measurements. All the samples showed superparamagnetic behaviour. The superparamagnetic relaxation was investigated also by Mossbauer spectroscopy. Hysteresis loops were measured at 2.5 K and both samples showed high values of coercive field. The role of magnetic interparticle interactions on the magnetic properties is discussed.

Nanoscopic coexistence of magnetism and superconductivity in YBa2Cu3O6+x detected by muon spin rotation.

We performed zero and transverse field muon spin rotation experiments on a large number of YBa2Cu3O6+x samples. We detect the coexistence of antiferromagnetic (AF) short range magnetism with superconductivity below T(f) < or = 10 K in compositions 0.37 < or = x < or = 0.39. Most muons experience local AF fields, even when a SQUID detects a full superconducting volume fraction, which points to a local minimal interference organization of short AF stripes embedded in the superconductor. A detailed phase diagram is produced and the consequences of the minimal interference are discussed.

How to tailor maghemite particle size in γ-Fe2O3–SiO2 nanocomposites

A sol–gel procedure for the tailoring of maghemite particle size in silica-based nanocomposites is proposed. The preparation method allows control of the gelation time, which is varied from 2 to 16 days in samples having the same concentration of iron oxide (25%). Superparamagnetic particles of γ-Fe2O3 with sizes in the 2.5–5.6 nm range were found in all the samples, as confirmed by TEM, Mossbauer spectroscopy and magnetic measurements. The particle size was independent of the porosity of the silica host matrix, but strongly dependent on the amount of solvent trapped inside the gels. The solvent plays an important role, favouring the formation of Fe3O4 nanoparticles as an intermediate step before the final oxidation to maghemite.

Investigation of the precursors of γ-Fe2O3 in Fe2O3/SiO2 nanocomposites obtained through sol-gel

Abstract The structure and the magnetic properties of a series of Fe 2 O 3 –SiO 2 nanocomposites (16.9 and 28.5 wt% of Fe 2 O 3 /(Fe 2 O 3 +SiO 2 )), prepared by a sol–gel method and submitted to thermal treatments in a temperature range 300–700°C, were investigated through X-ray diffraction (XRD), transmission electron microscopy (TEM), Mossbauer spectroscopy and magnetometry. Poorly crystallized particles, belonging to the 2-line ferrihydrite phase, were found in all the samples and recognized as precursors of the γ-Fe 2 O 3 that form in Fe 2 O 3 –SiO 2 nanocomposites at T >700°C. The result was achieved through a detailed comparison of structural and magnetic properties of nanocomposites with a bulk iron (III) oxide hydrated reference sample.

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.

Structural features of a Eu3+ doped nuclear glass and gels obtained from glass leaching

Abstract In order to describe the rare earth environment in an aluminoborosilicate nuclear glass and its evolution with water leaching, Eu 3+ was used as a luminescent structural probe. Two types of Eu 3+ environments were observed in the glass as well as in the gels (originating from leached glass). A study of the B 2 crystal field parameters allows one to identify one site as a silicate type. We showed moreover that the second site is modified with leaching from a borate to an aluminate site. Asymmetry ratio from photoluminescence and Mossbauer analyses demonstrated a decrease of the average distortion of the Eu 3+ sites in the gels. Indeed, gel can be described as an amorphous matrix but less disordered than glass. Another important result is that the lanthanide environment is not influenced by the leaching mode.

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.

Halogen-bonding in a new family of tris(haloanilato)metallate(III) magnetic molecular building blocks

Here we report on new tris(haloanilato)metallate(III) complexes with general formula [A]3[M(X2An)3] (A = (n-Bu)4N(+), (Ph)4P(+); M = Cr(III), Fe(III); X2An = 3,6-dihalo derivatives of 2,5-dihydroxybenzoquinone (H4C6O4), chloranilate (Cl2An(2-)), bromanilate (Br2An(2-)) and iodanilate (I2An(2-))), obtained by a general synthetic strategy, and their full characterization. The crystal structures of these Fe(III) and Cr(III) haloanilate complexes consist of anions formed by homoleptic complexes formulated as [M(X2An)3](3-) and (Et)3NH(+), (n-Bu)4N(+), or (Ph4)P(+) cations. All complexes exhibit octahedral coordination geometry with metal ions surrounded by six oxygen atoms from three chelate ligands. These complexes are chiral according to the metal coordination of three bidentate ligands, and both Λ and Δ enantiomers are present in their crystal lattice. The packing of [(n-Bu)4N]3[Cr(I2An)3] (5a) shows that the complexes form supramolecular dimers that are held together by two symmetry related I···O interactions (3.092(8) Å), considerably shorter than the sum of iodine and oxygen van der Waals radii (3.50 Å). The I···O interaction can be regarded as a halogen bond (XB), where the iodine behaves as the XB donor and the oxygen atom as the XB acceptor. This is in agreement with the properties of the electrostatic potential for [Cr(I2An)3](3-) that predicts a negative charge accumulation on the peripheral oxygen atoms and a positive charge accumulation on the iodine. The magnetic behaviour of all complexes, except 5a, may be explained by considering a set of paramagnetic non-interacting Fe(III) or Cr(III) ions, taking into account the zero-field splitting effect. The presence of strong XB interactions in 5a are able, instead, to promote antiferromagnetic interactions among paramagnetic centers at low temperature, as shown by the fit with the Curie-Weiss law, in agreement with the formation of halogen-bonded supramolecular dimers.

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.

Anisotropic exchange interaction between nonmagnetic europium cations in Eu2O3

The electronic structure of the cubic and (high pressure) hexagonal phases of Eu2O3 have been investigated by mean of full potential linearized augmented plane wave calculations, within the LDA+U method. For the cubic phase, the comparison between ferromagnetic and antiferromagnetic calculations shows that the exchange interaction is very weak and is therefore expected to have a negligible effect on the magnetic susceptibility. This is consistent with the experimental behavior of the susceptibility of solid solutions of Eu2O3 into A2O3 (A=Y, Lu, Sc). The calculations performed for the high pressure hexagonal phase, on the other hand, show that there is an antiferromagnetic exchange interaction between nearest neighbor Eu ions, which should have a sizeable effect on the susceptibility. Our results allow us to discuss the existing theories.