D. Rinaldi

Further results on cerium fluoride crystals

Abstract A systematic investigation of the properties of cerium fluoride monocrystals has been performed by the “Crystal Clear” collaboration in view of a p

Alternating current magnetic susceptibility measurements in La1−xSrxCoO3 (x⩽0.30) below 300 K

A dynamical magnetic susceptibility study on La1−xSrxCoO3 samples obtained by a coprecipitation method is reported for x⩽0.30. For x<0.20 differences between zero field cooled and field cooled dc magnetic susceptibility appear at a temperature Ta below the Curie point, TC≈240 K, as well as a magnetic freezing point at a temperature Tb<Ta. Alternating current magnetic susceptibility shows two maxima at these temperatures, which is interpreted as the evidence of different interaction processes between ferromagnetic clusters mediated by a matrix of changing spin-state Co3+ ions. For x⩾0.20 an unique blocking process seems to take place due to the percolation of the ferromagnetic clusters, making unimportant the contribution of the matrix.

Superparamagnetic blocking and superspin-glass freezing in ultra small δ-(Fe0.67Mn0.33)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.

Phase separation as origin of the magnetic anomalies in La0.85Sr0.15CoO3

The dependence of the ac-magnetic susceptibility of La0.85Sr0.15CoO3 on the annealing temperature used during synthesis is addressed. Such dependence has been previously attributed to compositional inhomogeneities. Nevertheless, the presence of distinct phases with different chemical compositions is excluded after explorations by several techniques. Instead it is proposed that an electronic phase separation takes place in the material, whose state is changed after charge-carrier redistributions due to the thermal history of the samples, analogously to parent manganese-based perovskites.

Study of the sintering behaviour of MgB2 superconductor during hot-pressing

Abstract The densification behaviour of MgB 2 was studied when commercial powder was hot-pressed in the temperature range 1070–1190 °C. The mechanisms active during sintering were investigated by continuously recording the shrinkage vs. time and elaborating the data on the basis of Kingery’s model for liquid phase sintering. XRD and SEM analysis on final dense bodies were used to evaluate secondary phases formation during sintering and the effect of magnesium sublimation. AC magnetic susceptibility measurements were also performed to correlate the microstructural and morphological modifications induced by hot-pressing and the superconducting properties.

Phase separation, thermal history and magnetic behaviour of Sr doped LaCoO3

High-resolution transmission electron microscopy observations and magnetic ac susceptibility measurements have been carried out to study the changes in the magnetic properties of La1-xSrxCoO3 samples (x≤0.3) processed at different annealing temperatures, ranging from 1273 to 1573 K. Although neutron diffraction excludes the coexistence of distinct phases with different chemical composition, room temperature micrographs taken along the [0,-1,1] rhombohedral zone-axis show bright fringes of different widths, extending over regions with sizes ranging from 8 to 40 nm. These regions are detected in all the samples, whatever the Sr content or the annealing temperature Ta. However, their number depends strongly on the processing conditions and becomes smaller and smaller as Ta increases. The periodicity of the bright fringes cannot be accounted for, either by variations in the sample thickness or by crystallographic defects, but it can be simulated if the presence of aperiodically alternate Sr-rich and La-rich (0,1,1) planes is assumed. On the other hand, the susceptibility measurements show that the amount of ferromagnetic phase present in each specimen depends on its thermal history. This suggests that the segregation of a second electronic phase takes place, the nucleation of which is influenced by the spatial homogeneity of the dopant distribution.

THERMAL TREATMENT DEPENDENCE OF THE DYNAMIC MAGNETIC BEHAVIOR OF GD2CUO4

dc and ac magnetization measurements in Gd2CuO4 ceramic samples synthesized at different annealing temperatures show that their magnetic properties are influenced by the thermal treatment. The changes consist of variations of the onset point for differences between zero field cooled and field cooled dc magnetizations, in the coercive fields, and in the position and frequency dependence of a maximum in the real part of ac susceptibility. An explanation suggesting a temperature‐dependent homogeneity of the oxygen distortions causing weak ferromagnetism in the CuO2 planes is given.

PHOTOELASTICITY FOR THE INVESTIGATION OF INTERNAL STRESS IN BGO SCINTILLATING CRYSTALS

Abstract Quality control of scintillating crystals production requires the on-line monitoring of the state of internal stress levels. The determination of the photoelastic constant in crystals is fundamental in this measurement, rendered necessary by the crystal fragility, a limiting factor in the construction of calorimeters (radiation detectors consisting in large arrays of crystals). In this work high-density BGO (Bismuth Germanate Bi3Ge4O12) crystals for calorimeters have been studied by means of photoelasticity. A test bench, based on a circular polariscope, an image acquisition system, a data processing unit and a suitable mechanical loading unit have been developed and applied to crystal sample testing. The results provide new data on photoelasticity of BGO and outline the basic features for the construction of an apparatus for on-line inspection of crystal quality.

Quality inspection of anisotropic scintillating lead tungstate (PbWO4) crystals through measurement of interferometric fringe pattern parameters

Abstract Scintillating crystals are widely used as detectors in radiographic systems, computerized axial tomography devices and in calorimeters employed in high-energy physics. This paper results from a project motivated by the development of the CMS calorimeter at CERN, which will make use of a large number of scintillating crystals. In order to prevent crystals from breaking because of internal residual stress, a quality control system based on optic inspection of interference fringe patterns was developed. The principle of measurement procedures was theoretically modelled, and then a dedicated polariscope was designed and built, in order to observe the crystals under induced stresses or to evaluate the residual internal stresses. The results are innovative and open a new perspective for scintillating crystals quality control: the photoelastic constant normal to the optic axis of the lead tungstate crystals (PbWO4) was measured, and the inspection procedure developed is applicable to mass production, not only to optimize the crystal processing, but also to establish a quality inspection procedure.

Static and Dynamic Magnetic Properties of Melt-Spun Granular Cu100-xCox Alloys

The magnetic properties of granular Cu-Co alloys produced by melt-spinning have been investigated by DC and AC susceptibility measurements. The results indicate the occurrence of blocking processes of interacting superparamagnetic particles. The effects of annealing by Joule heating at different current densities are discussed in relation to the giant magnetoresistance properties of the system.