1,721,098 research outputs found
Analisi della vulnerabilità dei sistemi di distribuzione idrica rispetto a potenziali fenomeni di contaminazione
No full textDrinking Water Distribution Systems (DWDSs), as well as other complex systems are characterized by an intrinsic vulnerability which can be both quantitative and qualitative: the first one related to the possible lack of water and the second one related to presence of substances concentrations higher than the legal threshold. Assessing the DWDSs’ vulnerability and resilience in an objective way is necessary to understand the potential impacts of random disturbances, deliberate attacks, and natural disasters and to be aware of the system critical points that need to be monitored. The work that is presented, in collaboration with Cafc S.p.a. (the Udine water utililty), aims to be a preliminary study on the vulnerability assessment and reduction of a real DWDS, i.e. the Udine DWDS. A general assessment of the vulnerability of DWDS in Udine was made using indices and metrics derived from the Complex Networks Theory (CNT). By comparing these topological parameters with those of other real and literature networks, it was possible to understand the level of redundancy and robustness, as indicators of resilience of the network under consideration. This analysis, already in use for different types of network and based solely on its topology, is a fundamental step to have an overview of the network and understand the critical areas of improvement. From the interpretation of the results of this first analysis, it was clear that, in terms of operating conditions, a major improvement in the network would be achieved by reducing the failures research time. In order to reach this goal, it was decided to partitioning the network through a procedure based on the CNT. The Water Distribution Network Partitioning (WDNP), which aims to subdivide the network into hydraulically independent subsystems, has provided for the use of optimization procedures using Genetic Algorithms (GAs) operating on a suitably calibrated hydraulic network simulation model. The size and level of detail of this model has certainly been an added value throughout the study, which among the many examples of literature can be considered a rare case of applied engineering. Always considering the operating conditions, it was decided to study the qualitative vulnerability deriving from the Disinfection by-Products (DBPs). These substances, resulting from the reaction of the disinfectant with the natural organic matter (NOM) in the network, are harmful to human health. Therefore, knowing both the concentration of these substances in the DWDS and their variation with the initial concentration of disinfectant, it becomes possible to simultaneously assess and manage any critical issue. To this end, it was decided to build a chemical-hydraulic model, using the EPANET-MSX software, capable of predicting the decay of disinfectant and the formation of DBPs. Being the Udine network disinfected by chlorine dioxide, the expected DBPs are chlorite and chlorate whose concentrations have a legal limit of 0.7 mg/l. There is few literature on the subject, and the attempts to model these substances have significant approximations that reduce the quality of model predictions. Moreover, there are no cases of chemical modelling of these substances for real networks of the size and level of detail of the WDN of Udine. The results of the simulation, compared with the concentration values of the three chemical species measured in the real network have confirmed an excellent predictive ability of the model that was finally used to simulate a contamination scenario in order to assess the possibility of increased concentration of DBPs above the legal limit
Tailoring the exchange bias properties of Ni/NiO nanogranular samples by the structure control
No full textThe exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by an original method that combines mechanical milling and hydrogen partial reduction of NiO. In this procedure, precursor NiO powder is ball-milled to reduce the grain size to the nanometric scale; then, the milled powder is subjected to high-temperature treatments in H2, inducing the reduction to metallic Ni. Typically, the samples consist of Ni nanocrystallites (size of the order of 10 nm) dispersed in a nanocrystalline NiO matrix, as observed by electron microscopy (HRTEM) [1].
In particular, Ni/NiO samples have been prepared by annealing in H2, at selected temperatures (200 < Tann < 300 °C), NiO powder previously milled for 5, 10, 20 and 30 hours. The structural features of the samples have been investigated by X-ray diffraction and the low-temperature magneto-thermal behavior and EB properties have been analyzed by SQUID magnetometry.
The structure and composition of the Ni/NiO samples can be satisfactorily controlled during the synthesis procedure by varying both Tann and the milling time of the precursor NiO powders [2]. By increasing this last parameter, the mean grain size of the NiO phase reduces down to the final value of 16 nm and the microstrain increases, which is consistent with an enhancement of the structural disorder. The structure of the milled NiO matrix strongly affects the process of nucleation and growth of the Ni nanocrystallites, so that, Tann being equal, the amount and the mean grain size DNi of the Ni phase vary substantially in samples having different milling times. Such features of the Ni phase determine the extent of the Ni/NiO interface and consequently the magnitude of the exchange field Hex: the highest value (~ 940 Oe) has been measured at T = 5 K in a sample containing ~7 wt % Ni and with DNi = 19 nm. However, in Ni/NiO samples with very different structural characteristics and different values of Hex at T = 5 K, the EB effect vanishes at the same temperature (~ 200 K) and the same thermal dependence of Hex is observed. We consider that the evolution of the EB effect with temperature is ultimately determined by the microstructure of the Ni/NiO interface, which cannot be substantially modified by changing the synthesis parameters, milling time and Tann.
[1] L. Del Bianco, F. Boscherini, A.L. Fiorini, M. Tamisari, F. Spizzo, M. Vittori Antisari, E. Piscopiello, Phys. Rev. B 77 (2008) 094408
[2] L. Del Bianco, F. Spizzo, M. Tamisari, J. Magn. Magn. Mater. (2009, in press
Provenance of obsidians from the neolithic archaeological site of San Martino Spadafora (Messina - Italy) by Mössbauer spectroscopy
No full textIn the San Martino of Spadafora area (Messina-Italy), during the proceedings of the third line of the pipeline Eni-Snam, a Neolithic site was found, related to two diverse phases of occupation: Ancient Neolithic Age (of stentinelliana culture) and Late Neolithic (of Diana culture). Almost all lithic artifacts discovered in that site are made of obsidian (Neolithic black gold), a volcanic rock of dark color very rich of SiO2, used during the neolithic age to realize tools and objects. Due to the vicinity to the Aeolian Islands, the obsidians may came directly from Lipari; however, the chemical analysis point out slight differences in composition with respect to the Lipari outcrops. Therefore, a compositional comparison was performed, to see if the discovered obsidians have an extra territorial origin or if the differences are ascribable to lavic flows covered by successive eruptions. This allows to check whether San Martino site is part of a complex exchange network that encouraged the arrival of raw material from afar. The structural and compositional characteristics that made interesting to exchange high value obsidians, not only for their use but also for their symbolic and aesthetic value, may be highlighted, as well.
We investigated the composition and provenance of different obsidian samples, S1 – S12, obtained from obsidian artifacts resulting from industry lithic chipped excavations. They were analyzed by transmission Mössbauer spectroscopy measurements performed at room temperature with a 57Co in Rh source; the spectrometer was calibrated using an α-Fe foil.
The typical shape of the spectrum consists of an asymmetrical doublet. The presence of sextet subspectra, possibly ascribable to the presence of magnetite and/or hematite contributions [1,2], was not observed. The experimental spectra were simulated using three different quadrupole doublets (QD) as in obsidians the presence of two Fe2+ sites and one Fe3+ site is found [1,2,3]. Symmetrical quadrupole doublets were used, as a preferential texture is not expected [4]. The possible presence of QD distributions was considered, as well, but that did not improve the fitting quality, so each of the Fe ions chemical/structural environment seems to be rather uniformly reproduced within the sample. In this contribution, the results of the analysis performed on the obsidian samples will be presented and compared with the results obtained from obsidians coming from Aeolian islands and from other obsidian sites of the Mediterranean area. In particular, the Mössbauer data collected on S1- S12 display a negligible spread, indicating a common source for what concerns the artifacts from San Martino site.
[1] A. Bustamante, M. Delgado, M. R. Lattini, A. V. Bellido, Hyperfine Interactions 175 (2007) 43.
[2] M. Duttine, R. B. Scorzelli, G. Popeau, A. Bustamante, A. V. Bellido, M. R. Lattini, N. Guillaume Gentil, Hyperfine Interactions 175 (2007) 85.
[3] R. B. Scorzelli et al., C. R. Acad. Sci. Paris 332 (2001) 769.
[4] U. Gonser, Mössbauer Spectroscopy, Springer-Verlag (1975
Spin-dependent conductivity of nanosized magnetic inhomogeneities
No full textSpin-dependent scattering (SDS) originates at the interface between magnetic (M) and non-magnetic (NM) regions, and gives rise to the giant magnetoresistive effect (GMR), that is observed when M and NM regions are interleaved at the nanoscale level. The GMR intensity, i.e. the change in resistivity observed due to the application of an external magnetic field, is affected by the SDS strength that, in its turn, is inversely proportional to the lateral size of the magnetic regions [1]. Such a lateral scale is the effective size of the magnetic regions [2], Deff, that is the result of the effect of magnetic interactions on the real size of the magnetic regions, D. Similarly, if the magnetic morphology of the system is not uniform, the SDS strength changes with the lateral scale of the magnetization inhomogeneities [3].
The dependence of GMR on the external field is the counterpart of Deff, so direct indications about D are not easily accessed. However, if we resort to the GMR efficiency, gamma, [4], i.e. the change in GMR for a unit change of squared magnetization, the comparison between the values of gamma measured at low and at high applied field as a function of Deff enables one to evidence D with respect to Deff. Indeed, if Deff is larger than D, as soon as the external applied field overcomes the effect of magnetic interactions, the efficiency of the magnetic structure is expected to change, as the large effective magnetic volumes break into smaller parts. As a consequence of that, the lateral scale of the system decreases, and gamma is expected to increase accordingly.
In this work, we study different FexAg1-x nanogranular systems, where x is the relative Fe atomic concentration, 0.1 < x < 0.5. Under equilibrium conditions, Fe and Ag are not miscible, so using an out-of-equilibrium technique, in our case dc-magnetron sputtering, we obtain a deep intermixing of the two species. In this way, as a function of x, different samples with a different average magnetic length scale, namely with a different Deff, can be produced [3]. The evolution of the magnetic morphology of the systems was followed with zero-field-cooled and field-cooled magnetization measurements. Magnetization and GMR loops were recorded at two different temperatures, at 300 K and at 4 K. Indeed they represent two conditions were the contribution of interparticle interactions to systems dynamics is expected to be different. In this way, the effect of Deff can be better appreciated. We present the gamma dependence on x, measured both at low and at high applied field, gamma_L and gamma_H, respectively. For low values of x, gamma_L and gamma_H display the same dependence as a function of x, whilst for higher values gamma_L shows a broad maximum whilst gamma_L has a monotonic dependence that eventually approaches saturation. These data are presented and discussed and compared to magnetization loops and diffraction data in order to give an estimation of the Deff of the different samples.
[1] S. Zhang and P. M. Levy, J. Appl. Phys. 73, 5315, 1993.
[2] P. Allia, M. Coisson, F. Spizzo, P. Tiberto, F. Vinai, Phys. Rev. B 73 (2006) 054409
[3] P. Vavassori, E. Angeli, D. Bisero, F. Spizzo, F. Ronconi, J. Magn. Magn. Mater. 262 (2003) 52
[4] M. Tamisari, F. Spizzo, F. Ronconi, M. Sacerdoti, G. Battaglin, submitted to Journal of Applied Physics
Magnetoresistance of granular Ni/NiO tuned by exchange anisotropy
No full textThe GMR behavior of nanogranular samples is ruled by the interplay of their structural and magnetic features, the two properties being strictly interlaced. The structural features of a given sample not being easily adjustable (at least, in a reversible way), the GMR response is hardly tunable unless a method is envisaged of modifying the magnetic configuration only. For this purpose, a magnetic tool would be needed that acts on the nanoscale.
We show that it is possible to tune the GMR of nanogranular systems through the exchange anisotropy, opening new perspectives towards the control of the magnetoresistive performance of properly engineered nanoparticle assemblies
Spin-dependent transport in granular films with mixed length-scale magnetic coherence
No full textWe investigated the magnetoresistance effect in presence of mixed length-scale magnetic coherence in systems displaying coexistence of ferromagnetic- and superparamagnetic-like behaviors. In spite of their different microstructure, all the samples investigated show magnetoresistance curves vs. sample magnetization with a characteristic feature: the magnetoresistance remains unchanged (viz., equal to its value for M = 0) over a wide range of magnetization values. The results are interpreted considering the interplay between the different magnetic ordering scales probed by the spin-dependent scattering and the magnetization reorientation processes and the presence in the films of magnetic coherence at different length-scales. From the comparison of the results with a phenomenological model, developed for dealing with local variations of magnetic ordering, we determined the field evolution of the ratio between the characteristic length-scale for magnetic coherence and spin diffusion length in the different samples
Coupling between exchange bias effect and giant magnetoresistance in a Ni/NiO nanogranular sample
No full textWe have studied the magnetotransport properties of a Ni/NiO nanogranular sample, showing exchange bias (EB) effect. The sample was obtained by milling precursor NiO powder for 30 hours to reduce the mean grain size to ~ 20 nm; then, the milled powder was annealed in H2 for 30 minutes at the temperature of 250 °C, to induce the partial reduction to metallic Ni. Typically, samples prepared by this procedure consist of Ni nanocrystallites dispersed in a nanocrystalline NiO matrix [1]. As for this sample, X-ray diffraction analysis reveals that the Ni phase is about 30 volume % and the mean grain size DNi = 14 nm.
The magnetoresistance (MR) was measured as a function of T and H on a cold-compacted Ni/NiO specimen, using the standard four-probe technique, in a SQUID magnetometer operating in the 5-300 K temperature range at a maximum applied field H = 50 kOe. Magnetization loops, M(H), and MR(H) loops were measured both after zero-field-cooling (ZFC) and after field-cooling (FC) in Hcool = + 20 kOe, from T = 300 K down to selected measuring temperatures T.
At T = 5 K, the sample exhibits EB effect, as revealed by the horizontal shift of the FC M(H) loop (the exchange field Hex ~ 460 Oe); it decreases with increasing T and is almost vanished at T = 250 K.
The low value of the electric resistance R at T = 5 K (~ 0.5 Ohm) and its 20% increase with increasing T up to 300 K indicate the existence of a metallic conduction channel across the sample. We define MR = [R(H)-R(0)]/R(0), where R(0) is the highest resistance value. At T = 5 K, a negative MRmax ~ – 0.40 % is measured at H = 50 kOe, both in the mode with the current flowing perpendicular to H and in the mode with the current parallel to H. This implies that the effect is of the type usually referred to as Giant-MR and is caused by the electronic scattering from the nonaligned magnetic moments of the Ni nanocrystallites. Thus, despite the creation of a percolation metallic network allowing electronic transport, the Ni nanocrystallites, or at least a part of them, act as separated ferromagnetic entities and are not magnetically coupled. In fact, in this last condition, they would form a ferromagnetic network, unsuitable for the observation of Giant-MR.
At T = 5 K, the FC MR(H) loop appears shifted along the H axis, in good agreement with the value of Hex, and the effect exhibits the same thermal dependence as derived from the M(H) loops. Hence, at all T, a strict coupling exists between EB and MR effects in the investigated sample.
Finally, the MR effect is seen to increase with increasing T and, at T = 300 K, MRmax ~ – 0.57%. This anomalous behaviour is explained considering that a structural NiO disordered phase, with glassy magnetic properties, surrounds the Ni nanocrystallites [1]. At very low T, below ~ 50 K, the magnetic moments of this phase are completely frozen and even under the maximum H a poor alignment is attained. However, with increasing T, the magnetic moments become progressively unfrozen and, for applied field H > 20 kOe, they provide a substantial contribution to MR.
[1] L. Del Bianco, F. Boscherini, A.L. Fiorini, M. Tamisari, F. Spizzo, M. Vittori Antisari, E. Piscopiello, Phys. Rev. B 77 (2008), 09440
Evolution from multilayer to granular behavior via Cobalt layers fragmentation in Co/Cu multilayers
No full textWe have analyzed the structural evolution of Co(t(Co))/Cu(4t(Co)) multilayers when t(Co) ranges from 12 to 2 Angstrom. The investigation has been performed by studying their magnetization (both at room and low temperatures) and giant magnetoresistance. In the intermediate range of thickness. we observe mixed multilayer granular features yielding an interesting behavior of magnetoresistance curves related to the length scale of the spin-dependent scattering processes. At sufficiently low Co thickness. the samples exhibit non-interacting superparamagnetic behavior (at room temperature). The analysis of the low-temperature data (coercive field and of the ratio between the remnant and saturation magnetization) of the superparamagnetic samples allowed us to address the effects related to particles size distribution on the temperature dependence of coercive field and of the ratio between the remnant and saturation magnetization
GMR effect across the transition from multilayer to granular structure
No full textWe have studied the evolution of giant magnetoresistance in magnetic multilayers obtained by alternately growing thin cobalt and copper layers having proportional thickness.Co nominal layers thickness was progressively reduced from 12 (A down to 2 ( A.This decrease produces a fragmentation of the Co layers and the evolution of samples magnetic properties indicates a transition from multilayer to granular behavior.Magnetic and magnetoresistive data of the granular phase have been analyzed as a function of Co thickness in order to determine particle size distribution
Production and magnetic characterization of exchange-coupled NiFe/IrMn and IrMn/NiFe films
No full textNowadays, several advanced magnetic and magnetoelectronic devices rely on interface exchange coupling (EC) between different magnetic phases (for example, magnetoresistive recording read heads, magnetoresistive random access memories, spring magnets, magnetic tunnel junctions, etc). The great challenge for optimizing the performances of such devices is to control the EC strength in order to tune the anisotropy, which represents the key technological parameter for any applications of magnetic materials. In fact, the exchange interaction across the ferromagnetic(FM)/antiferromagnetic(AF) interface gives rise to an additional source of anisotropy (exchange anisotropy), determining the onset of the exchange bias effect and producing significant changes in the coercivity (HC).
In this context, we have studied EC in continuous films with AF/FM and FM/AF configurations, grown by dc-magnetron sputtering in Ar atmosphere. As AF phase we have used Ir25Mn75 (IrMn), whilst we have used Ni80Fe20 (Py) as FM. All the samples have been deposited on a 5 nm thick Cu underlayer to favor a fcc (111) orientation and have been capped with a Cu overlayer (5 nm).
We have studied and compared the effects of EC on the macroscopic magnetic properties (exchange fied Hex, HC and squareness) of a number of samples grown in different values of a magnetic field applied along the film plane (Hdep= 0, 200, 800 Oe) and having different thickness of the AF layer tAF (3, 6 and 10 nm; the thickness of FM was 5 nm in all the samples).
Hysteresis loops have been measured by magneto-optic Kerr effect (MOKE) and SQUID in the 5-300 K temperature range, after cooling in zero-field or in a field Hcool = 100 Oe from T = 400 K.
At T = 300 K, the highest Hex (~150 Oe) has been measured in the Si/Cu/NiFe/IrMn/Cu sample with tAF = 10 nm (HC ~ 25 Oe); Hex goes to zero at TB ~ 390 K. In the same configuration, for tAF = 6 nm, TB ~ 370 K, (at T = 300 K, Hex ~ 100 Oe, HC ~ 60 Oe), whereas for tAF = 3 nm TB ~ 230 K (at T = 300 K, HC ~ 20 Oe).
In all the samples, both Hex and HC increase with decreasing T especially for T < 100 K and, at T = 5 K, the highest Hex (~ 1000 Oe) is measured for tAF = 3 nm (in this sample, HC ~ 640 Oe).
For T 100 K, Hex and Hc assume similar values, irrespective of the configuration.
This research work has been carried out in the framework of the project FIRB2010 “Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices” – NANOREST
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