1,721,175 research outputs found
Spin Dynamics in Mesoscopic Size Magnetic Systems: A H-1 NMR Study in Rings of Iron(III) Ions
No full textTwo magnetic mol. clusters contg. almost coplanar rings of iron (III) ions with spin S = 5/2 were studied by 1H NMR and relaxation measurements. The 1st system, which will be referred to as Fe6, is a mol. [NaFe6(OCH3)12(C17O4H15)6]+ClO4- or [NaFe6(OCH3)12(C15H11O2)6]+ClO4- or [LiFe6(OCH3)12(C15H11O2)6]+ClO4- while the 2nd type of ring, denoted Fe10, corresponds to the mol. [Fe10(OCH3)20(C2H2O2Cl)10]. The 1H NMR linewidth is broadened by the nuclear dipolar interaction and by the dipolar coupling of the protons with the iron (III) paramagnetic moment. The nuclear spin-lattice relaxation rate, T1-1, of the proton is a sensitive probe of the Fe spin dynamics. In both clusters, T1-1 decreases with decreasing temps. from room temp., goes through a peak just .ltorsim.30 K in Fe6 and 10 K in Fe10, and it drops exponentially to very small values at helium temp. The temp. dependence of the relaxation rate is discussed in terms of the fluctuations of the local spins within the allowed total spin configurations in the framework of the weak collision theory to describe the nuclear relaxation. The authors use the calcd. energy levels for the Fe6 ring based on a Heisenberg Hamiltonian and the value of J obtained from the fit of the magnetic susceptibility to describe semiquant. the behavior of T1-1 vs. T. The exponential drop of T1-1 at low temp. is consistent with a nonmagnetic singlet ground state sepd. by an energy gap from the 1st excited triplet state. The values obtained for the gap energies are ET/k = 12 K for Fe10 and ET/k = 38 K for Fe6 which are almost twice as big as the values deduced from susceptibility measurements. At all temps. the relaxation rate decreases with increasing magnetic field, i.e., NMR resonance frequency. This effect could be related to the long time persistence of the spin correlation functions typical of diffusive modes in low dimensional magnetic systems. It is argued that the data presented are a direct exptl. study of spin dynamics in mesoscopic spin rings and should afford a test for exact anal. and/or numerical solns
Review of NMR and μSR studies in the molecular nanomagnet Mn12-ac
No full textWe present a comprehensive review of the NMR and μSR studies performed in the molecular nanomagnet Mn12 a system characterized and widely studied by Prof. Gatteschi's group in Florence. The proton (1H, 2D) NMR, the 55Mn NMR and the μSR investigations have yielded important information regarding both static and dynamic magnetic properties of the molecule. The magnetic and quadrupole hyperfine interactions have been extracted from NMR data. The spin dynamics at high and intermediate temperature associated with the zero dimensionality and with the spin-phonon coupling has been studied together with the spin dynamics in the quantum tunneling low temperature regime. The local spin configuration in the giant S = 10 total spin ground state has been determined via 55Mn NMR in zero magnetic field and with fields parallel and perpendicular to the anisotropy axis. Finally a novel method is described to monitor the relaxation of the magnetization from the time evolution of the NMR spectrum
Tunneling dynamics of the magnetization in transverse field: 1H-NMR study in single crystal of Fe8
No full textThe proton spin-lattice relaxation rates (1/T1) have been measured as a function of transverse field in single crystal Fe8 cluster at T=1.5K. When the transverse field is applied along the medium axis of the cluster, a large enhancement of 1/T1 is observed around H=2.6T. The peculiar behavior of 1/T1 is well explained by taking into considerations the effect of enhancement of quantum tunneling by an application of the transverse field
NMR in magnetic molecular rings and clusters
No full textMolecular nanomagnets (MNM) are magnetic molecular clusters containing a limited number of transition ions in a highly symmetric configuration and coupled by strong exchange interaction (either ferromagnetic (FM) or more often antiferromagnetic (AFM)). The magnetic intermolecular interaction is very weak and thus the clusters behave as single nanosize units. NMR has proved to be an excellent probe to investigate the static magnetic properties and the spin dynamics of this new fascinating class of magnetic materials. The chapter contains a comprehensive review of the work performed in the last few years by the present authors with only a brief reference to work performed by other researchers. Most of the NMR measurements were performed on protons but important results were obtained also using other nuclei like 55Mn, 57Fe, 7Li, 23Na, 63Cu, 19F. In some cases the NMR was observed at low temperature in zero external field. Some novel NMR phenomena specific of the systems investigated were discovered and explained. For example in the anisotropic ferrimagnetic clusters Mn12 and Fe8, the ground state is a high total spin S = 10 state whereby the crystal field anisotropy generates an energy barrier typical of superparamagnets. It is shown how NMR and relaxation measurements can detect the microscopic local spin configuration in the ground state and the dynamics of quantum tunnelling of the magnetization (QMT). Another example is the case of the AFM rings, Fe10, Fe6 and Cr8, in which the ground state is a singlet, S = 0, separated from the first triplet excited state by an energy gap of about 5-10 K. By applying a magnetic field one can observe level crossing effects. These e.ects were studied by proton NMR and relaxation measurements vs field at low temperature (1.5-3 K). Finally, the nuclear relaxation rate as a function of temperature in the above mentioned AFM rings displays a field dependent peak at a temperature of the order of the exchange constant J, which can be fitted with a general scaling law. From these data, the lifetime broadening of the energy levels can be determined
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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