1,721,196 research outputs found
Hydrogen Detection In Ruthenium Oxide Layers By Means of the H-1(n-15,alpha-gamma)c-12 Nuclear-reaction
No full textInterplay between GMR intensity and efficiency in the FeAg nanogranular system
No full textThe dependence of electronic conductivity on the electron spin state was observed in nanogranular systems where magnetic (M) nanoparticles are dispersed into a non-magnetic (NM) matrix. The nanogranular systems display a spin-dependent electronic resistivity that remarkably decreases if an external magnetic field (H) is applied, i.e. they show the so called giant magnetoresistance (GMR) [1]. GMR is ascribable to the magnetic ordering effect induced by H, so the higher the degree of disorder at zero field the larger the GMR effect. When the magnetoresistive properties of an ensemble of independent nanogranular particles [2] are considered, GMR is proportional to (m/msat)^2, where m is the sample magnetization. The proportionality constant γ can be interpreted as the GMR efficiency, i.e. as the overall GMR variation in correspondence with a unit change of reduced square magnetization, so it can be seen as an indication of how effective is the granular structure in producing GMR. We investigate how γ and the GMR intensity change with the Fe concentration.
We focused on FexAg1-x nanogranular films, where x is the Fe atomic relative concentration and ranges from 0 up to 0.50, as measured by Rutherford Backscattering Spectrometry. The samples were deposited on Si substrates using dc-magnetron sputtering in cosputtering configuration and Ar atmosphere. The Fe-Ag phase diagram indicates that the two elements are not mutually soluble for any relative concentration but thanks to that out-of-equilibrium deposition technique it is possible to produce a system that at room temperature behaves like a magnetic nanogranular one [3].
X-ray diffraction measurements have been performed to investigate the structural properties of the samples. The granular films exhibit three different kind of structures: for x 0.32 there are bcc Fe cluster and Fe-Ag saturated solid solution. On the other hand, for all the concentrations, magnetization data show the presence of Fe precipitates whose size increases with x and the Mössbauer investigation confirms this picture. The GMR intensity is maximum for x = 0.32, while the maximum of γ is observed for x = 0.26. The maximum GMR effect is the best arrangement between a structure that displays the γ maximum, a not-saturated solid solution with very small Fe clusters and a structure with a high concentration of magnetic material. In particular, when the maximum GMR effect is observed, the distance between the magnetic clusters is of the order of the electron spin diffusion length.
[1] A. E. Berkowitz et al., Phys. Rev. Lett. 68 (1992) 3745.
[2] S. Zhang and P. M. Levy, J. Appl. Phys. 73 (1993) 5315
[3] J.Q. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982
GMR effect across the transition from diluted to granular-like effect scattering centers
No full textGiant magnetoresistive effect (GMR) in nanostructured materials made of magnetic (M) and non-magnetic (NM) species is ascribed to spin-dependent scattering (SDS) at the M/NM interface [1]. It has recently been shown that this effect appears even when the size of the magnetic regions goes to zero, i.e. the SDS is observed for M clusters made of a few atoms [2].
In this work, we investigate how the GMR efficiency, γ, i.e. the change in GMR for a unit change of magnetisation, varies as we reduce the size of SDS centres, i.e. as we start from a nanogranular magnetic system and we progres-sively reduce grain size. We study this topic in FexAg100-x co-sputtered thin films. Along with the reduction, the system first displays dipolar magnetic in-teractions and then it enters a frustrated regime. When the system undergoes this transition, shows a maximum, so we focus on the corresponding struc-tural modifications.
The volume relative Fe content, x, ranged from 8 up to 40 and was measured with Rutherford Backscattering Spectrometry. The evolution of Fe atoms con-figuration was studied with magnetisation data recorded in the zero-field and the field cooled (ZFC/FC) configurations as well as using X-Ray diffraction measurements. GMR data recorded at 4 and 300 K as a function of x will be presented and compared with the corresponding samples magnetic structure evolution deduced from ZFC/FC and X-Ray data.
[1] P. M. Levy, Sol. Stat. Phys. 47, 367 (1994)
[2] P. Allia et al., Phys. Rev. B 67, 174412 (2003
Concentration dependence of interclusters interaction role in sputtered Fe-Ag nanogranular samples
No full textAn ideal nanogranular sample is an ensemble of independent nanosized magnetic (M) particles dispersed in a non magnetic (NM) matrix. The magnetic particles can be seen as large magnetic moments; in case their dynamics is affected just by thermal energy contributions, the whole system is referred to as a superparamagnet (SP) [1]. Real systems, due to growth conditions, often differ from this picture and interparticle magnetic interactions, e.g. dipolar or exchange ones, are one of the main sources of deviation from the ideal SP behaviour.
If M and NM metallic species are used, the samples display a spin-dependent electronic resistivity that remarkably decreases if an external magnetic field, H, is applied, i.e. they show the so called giant magnetoresistance (GMR) [1]. GMR is ascribable to the magnetic ordering effect induced by H, so the higher the degree of disorder at zero field the larger the GMR effect. Interactions induce correlations among the magnetic moments, in particular when H is small, viz. an higher degree of order. As a consequence, the overall resistivity change is reduced. It’s therefore important to study interactions effects when H ≈ 0.
In this work, we have studied dc-cosputtered nanogranular FexAg100-x thin films with a volume Fe concentration, x, varying from 10 up to 30 as measured by Rutherford Backscattering Spectrometry. At room temperature, for x < 20 a SP behaviour is observed. A recently devel-oped model [2], based on the simultaneous investigation of magnetic and GMR data, has pointed out that magnetic interactions affect samples dynamics for all concentrations. The correlation length, λ, is always larger than particles average distance and increases with tem-perature and x [2]. These systems are therefore suitable to study interparticle interactions and their effect on low-field magnetic configuration. The investigation was performed with sus-ceptibility measurements in field-cooled (FC) and zero-field-cooled (ZFC) configuration, re-laxation and Mössbauer measurements; X-Ray diffraction data were collected, as well.
When x < 18, FC and ZFC data display the typical lambda shape but, for temperatures lower than the blocking temperature, FC signal displays an unexpected maximum at about 40 K. This effect is less and less pronounced as x increases and it vanishes starting from x = 18. The comparison between ZFC/FC curves and magnetic relaxation data confirms that interparticle interactions have a remarkable influence on low-field dynamics and this finding is supported by low-temperature Mössbauer measurements. However, the kind of interactions seems to change with x. Indeed, for low Fe concentration the samples possibly behave like a cluster-glass system, where frustrated interactions produce the FC maximum. Whilst approaching x ≈ 18, the interactions turn to dipolar and for higher concentrations the samples approach a re-entrant ferromagnetic behaviour. Eventually, X-Ray diffraction data suggest that the whole transition is related to the effects induced on samples structure/morphology by the change in iron concentration.
[1] A. E. Berkowitz et al, Phys. Rev. Lett. 68 (1992) 3745
[2] P. Allia, M. Coisson, F. Spizzo, P. Tiberto, and F. Vinai, Phys. Rev. B 73 (2006) 05440
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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