Magneto-optical probe for investigation of multiphase fe oxide nanosystems

In iron oxide nanoparticles the transformation of the metastable magnetite phase to maghemite, through the oxidation of iron, often represents a major drawback for correct interpretation of their physical properties and their effective use in several applicative areas. To solve this issue we propose an innovative method for identifying the chemical composition of complex ferrite nanostructures based on magneto-optical (MO) spectroscopy. Spherical iron oxide nanoparticles with increasing size are investigated by magnetic circular dichroism (MCD) in the nUV-vis-nIR range (350-1000 nm). Particular attention is dedicated to the time evolution of the MO response, which is ascribable to the oxidation process of the nanomaterial. The measured MCD spectra show a complex line shape due to the overlap of several MO transitions. Deconvolution analysis of MCD hysteresis loops allows the interpretation of this complex response as the combination of two distinct contributions, originating from magnetite and maghemite domains in the nanoparticles. We consider this method a viable complement to conventional techniques for the discrimination of the two magnetic phases in nanostructured materials

X-Ray Detected Magnetic Hysteresis of Thermally Evaporated Terbium Double-Decker Oriented Films

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Local spin dynamics at low temperature in the slowly relaxing molecular chain [Dy(hfac)3NIT(C6H4OPh)]: A μ+ spin relaxation study

The spin dynamics of the molecular magnetic chain [Dy(hfac)3NIT(C6H4OPh)] were investigated by means of the Muon Spin Relaxation (\u3bc+SR) technique. This system consists of a magnetic lattice of alternating Dy(III) ions and radical spins, and exhibits single-chain-magnet behavior. The magnetic properties of [Dy(hfac)3NIT(C6H4OPh)] have been studied by measuring the magnetization vs. temperature at different applied magnetic fields (H = 5, 3500, and 16500 Oe) and by performing \u3bc+SR experiments vs. temperature in zero field and in a longitudinal applied magnetic field H = 3500 Oe. The muon asymmetry P(t) was fitted by the sum of three components, two stretched-exponential decays with fast and intermediate relaxation times, and a third slow exponential decay. The temperature dependence of the spin dynamics has been determined by analyzing the muon longitudinal relaxation rate \u3bbinterm(T), associated with the intermediate relaxing component. The experimental \u3bbinterm(T) data were fitted with a corrected phenomenological Bloembergen-Purcell-Pound law by using a distribution of thermally activated correlation times, which average to \u3c4 = \u3c40 exp(\u394/kBT), corresponding to a distribution of energy barriers \u394. The correlation times can be associated with the spin freezing that occurs when the system condenses in the ground state

Da Commons a Urban Commons. Complessità e contraddizioni nella traduzione di un concetto

La nozione di commons è stata tradotta, nelle ultime due decadi, dal campo di studio delle risorse naturali alla dimensione urbana. Come per ogni traduzione si perde qualcosa e si trova qualcos’altro. La teoria degli urban commons, ancora lontana dal presentarsi come completa o esaustiva, viene frequentemente accostata all’inflazionato slogan del dirittto alla città. Questo articolo vuole illustrare come e perché i concetti di commons, diritto alla città e urban commons sono collegati fra loro. Questo studio permetterà di evidenziarne le differenze e dunque di percepirne le complessità e le contraddizioni. La teoria degli urban commons verrà inquadrata attarverso la cornice teorica del concetto di commons, studiato da Hardin e Ostrom, attraverso le nozioni di abitudine e performance articolate da Hardt e Negri e infine attraverso la metafora della soglia sviluppata da Stavrides. Nelle conclusioni dell’articolo verra evidenziata la natura strettamente relazionale degli urban commons, e verranno proposte delle relazioni fra questa teoria e i concetti di processo, tempo e democrazia sfuggente.The notion of commons has been translated, in the last two decades, from the field of natural resources to the urban dimension. As every translation goes, something is lost, and something can be found. The theory of urban commons, far from being complete or exhaustive, has been also linked to the inflated slogan of the right to the city. This article will try to illustrate how and why the concepts of commons, right to the city and urban commons are linked together. This insight will allow us to understand the differences between these concepts and, therefore, will point out any complexities and contradictions. Urban commons will be addressed through the theoretical frame of the concept of commons, studied by Hardin and Ostrom, through the notions of habit and performance as articulated by Hardt and Negri, and through the metaphor of the threshold developed by Stavrides. The conclusion of the article will highlight the relational nature of the urban commons, and the findings will outline their relation with the concepts of process, time and fugitive democracy

Arquitetura D.I.Y. e urbanismo tático: uma abordagem leve na transformação da cidade

Como é salientado no início do livro "Tactical Urbanism"1 , "tático" significa:"relativo a ações de pequena escala que servem um propósito maior".Um tipo de urbanismo que procura as soluções dos problemas compequenas intervenções. No mesmo sentido, a Arquitetura DIY procuraincentivar a mudança das atitudes das pessoas num local, com projetosque são muitas vezes pequenos e temporários.Estas duas abordagens desenvolvem-se nos espaços públicos da cidadecom a ajuda dos moradores do local onde se desenvolve o projeto.Por vezes os projetos são construídos diretamente pelos arquitetos e peloshabitantes.A palavra "tático" e a expressão "do it yourself" realçam que a distância entreos cidadãos, o arquiteto e o projeto final é muito mais curta nestas práticasdo que na arquitetura convecional. O projeto, nestas visões da arquitetura,é muitas vezes o produto de um grupo de arquitetos, no qual a equipa énecessária no desenho, na estabelecimento das relações humanas e, porfim, na construção.A arquitetura DIY e o Urbanismo tático são duas práticas em rápida evolução.São adotadas cada vez mais para resolver alguns problemas da cidadecontemporânea na América do Norte e na Europa. Os dois continentes,com as suas contradições, são os limites geográficos desta investigação.De facto a velocidade de transformação da cidade é muito mais rápida doque é a capacidade de intervenção da Arquitetura e do Urbanismo no sentidoclássico. É precisamente pelo aumento desta velocidade da transformaçãoque tais abordagens são mais difundidas.A entidade profissional do arquiteto altera-se perante estas práticas,encontrando-se mais próximo das pessoas e do projeto. Estas duasproblemáticas, a transformação da cidade e esta nova dimensão do"arquiteto", são o objeto de estudo desta dissertação.As it is explained in the beginning of the book "Tactical Urbanism"1 tacticalmeans:"of relating to small-scale actions serving a larger purpose"A kind of urbanism that look for solutions of problems with small interventions.In the same direction the practice of DIY Architecture tries to improve thechange of behavior of the people in a place, with projects that are oftensmall and temporaries.These two approaches's fieldwork is the public space of cities, and aredeveloped with the help of the inhabitants of the place where the projectis located. Sometimes these projects are directly realized by the designersand inhabitants their selves.The word "tactical" and the expression "do it yourself" highlight that thedistance between citizens, architects and the final project is shorter in thesepractices then what usually happens in the orthodox architectural practice.In fact these projects are always and clearly a product of a group of people,where a team is needed in the design, in establishing humans relations, andin the end in the building process.DIY Architecture and Tactical Urbanism are two branches of the field inrapid evolution. They are used more and ore in solving problems of NorthAmerican and Europeans contemporary cities. These two continents, withtheir contradictions, are the geographical borders of this study.The speed of the transformation of cities is faster than the capability ofinterventions of orthodox Architecture and Urbanism. It's because of thisgrowing speed of transformation that these two approaches are becomingmore usual.The operative dimension of the architect changes in these practices, he'scloser to people and to the project. These two practices, the transformationof cities and this new dimension of the "architect" are the main object of thisthesis

Chemical state of arsenic and copper in enargite: evidences from EPR and X-ray absorption spectroscopies, and SQUID magnetometry

The chemical state of elements in enargite, Cu(3)AsS(4), has been traditionally interpreted assigning the monovalent (+1) state to copper, pentavalent (+5) to arsenic, and divalent (-2) to sulphur. However, there are some evidences that make this interpretation not fully satisfying. These uncertainties reflect on the understanding of the oxidation process of enargite, that has important implications for both environmental issues and mineral processing. To clarify the issue, our study involves the combined use of X-ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, and SQUID magnetometry. XANES spectra at the Cu K-edge for enargite and other copper sulphides show the expected affinities between the spectra of enargite and chalcopyrite (although the pre-edge feature typical of chalcopyrite is absent in enargite), reflecting the structural similarities of these two minerals; the spectra are, on the other hand, fairly similar to tetrahedrite, whose structure is different. The spectral shapes of bornite, chalcocite, and covellite are distinctly different from enargite. The edge position of enargite is consistent with the presence of monovalent copper, in spite of some small differences with respect to other sulphides where Cu is considered monovalent. At the As K-edge, the most striking feature is the clear difference in the edge position of enargite with respect to As(V) compounds such as calcium arsenate, and the similarity with As(III) compounds such as sodium arsenite. On the other hand, the edge position for enargite occurs at higher energies than those of tennantite, where As is regarded as trivalent, and of realgar (As(4)S(4)), thus supporting the view that in enargite arsenic is in a higher valence state. Enargite does not show a recognizable EPR spectrum in the X band region; however, the mineral shows a rather strong bulk magnetisation, equivalent (in the hypothesis of divalent Cu) to 0.05 unpaired electron per formula unit. The most likely explanation of this behaviour is the occurrence of small amounts of clustered paramagnetic impurities (e. g., divalent Fe). In summary, to the extent that nominal valence states can describe complex chemical bonding such as in sulphides, the results of this study rule out the occurrence of a significant fraction of Cu(II) in enargite, and support the "traditional" assignment of a monovalent state to Cu, and pentavalent to As

Magnetic properties and cation ordering in two synthetic bornite samples, Cu5(Fe, Mn)S4

none5siThe magnetic properties of a synthetic bornite, Cu5FeS4, and of a Mn-bearing synthetic bornite were investigated using Superconducting Quantum Interference Device (SQUID) magnetometry and room temperature X-band Electron Paramagnetic Resonance (EPR) spectroscopy. Samples were synthesised from the elements using conventional dry methods. Chemical and phase compositions of the samples were confirmed by means of Electron Probe Micro-Analysis (EPMA) and by powder X-Ray Diffraction (XRD). Experimental results, interpreted using spectral simulations, highlight the superexchange nature of ionic interactions between paramagnetic centres and the role played by partial intervalence charge transfer between the Fe(III) and Cu(I) species in natural and synthetic bornite. The cation distribution in synthetic samples was shown to be different from the natural sample, in spite of the close similarity in chemical composition and structure.mixedBORGHERESI, MIRIA; DI BENEDETTO, FRANCESCO; CANESCHI, ANDREA; ROMANELLI, MAURIZIO; David J. VaughanBorgheresi, Miria; DI BENEDETTO, Francesco; Caneschi, Andrea; Romanelli, Maurizio; David J., Vaugha

Addressing the Influence of Localized Plasmon Resonance on the Magneto-Optical Properties of Cobalt Ferrite Nanoparticles

The optical and magneto-optical (MO) properties of magneto-plasmonic nanocomposite films made up of a transparent polymer with a dispersion of cobalt ferrite (CFO) nanoparticles (NPs) and different concentrations of Au NPs are investigated. The volumetric concentrations of CFO and Au NPs, around 3%, and below 7‰ respectively, are below the percolation limit, and hence the nanocom-posite films constitute models for investigating the influence of the electromagnetic field generated at the surface plasmon resonance of Au NPs on the magneto-optical properties of CFO NPs. The plasmon resonance is present in these magneto-plasmonic composites, red-shifted with respect to the bare Au NPs and covering the spectral region where charge-transfer and crystal field MO transitions can be excited. Moreover, the magneto-optical hysteresis loops were measured in the whole spectral region. We observe that the hysteresis loops shape is a fingerprint of the different MO transitions of the CFO NPs. The strength of the MO peak around 750 nm, corresponding to the Crystal Field transition is damped respect to the corresponding peak of the CFO NPs. The strength of this peak evolves non-monotonically with the Au NPs concentration. On the other hand, the MO band around 550 nm, excited by Charge Transfer transitions, changes sign when Au NPs are present. In addition, a second MO contribution is observed. Our results demonstrate that the interactions between plasmon resonance and MO effects are not only determined by the stronger local electromagnetic fields at the resonance but they depend on the type MO transition that is involved in these oxides. This study helps to understand and design the magneto plasmonic nano-structures and applications, for example in biomedicine and sensing, in which random and weak dipolar interparticle interactions between plasmonic and magnetic nanostructures are present