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Influence of the interaction between the inter- and intragranular magnetic responses in the analysis of the ac susceptibility of a granular FeSe0.5Te0.5 superconductor
No full textThe temperature-dependent fundamental ac susceptibility of a granular superconductor in the absence of dc fields has been analyzed by developing a phenomenological model for effective magnetic fields, taking into account the influence of the magnetic interaction between the intergranular and the intragranular magnetizations due to demagnetizing effects. For this purpose a policrystal Fe-based superconductor FeSe0.5Te0.5 sample has been studied. By the frequency dependence of the peaks of the temperature-dependent imaginary part of the fundamental complex susceptibility, the dependence on temperature of the characteristic times both for intergranular and intragranular relaxations of magnetic flux are derived, and the corresponding relaxation processes due to combinations of the flux creep, the flux flow and the thermally activated flux flow regimes are identified on the basis of the effective magnetic fields both at the sample surface and at the grains' surfaces. Such characteristic times, through the Havriliak–Negami function, determine the temperature and the frequency dependences of the complex susceptibility. The comparison of the numerically obtained curves with the experimental ones confirms the relevance, for identifying the intergranular and intragranular contributions to the ac magnetic response and the corresponding flux dynamical regimes, of the interaction between the intergranular and intragranular magnetizations due to demagnetizing effects
Effect of disorder on the passage from bulk superconductivity to spin glass behaviour in RuSr2GdCu2O8
No full textIn this paper we analyse the structural and physical properties of rutheno-cuprate samples of the phase Ru–Gd(1212) subjected to different thermal treatments. We have observed the variations induced by these treatments by structural measurements (x-ray powder diffraction and high-resolution transmission electron microscopy) and physical measurements (dc magnetic measurements and
magnetoresistive measurements). Such variations involve both the intrinsic properties of the Ru–Gd 1212 phase
(for example the temperatures of the magnetic and superconducting ordering), and the extrinsic properties, i.e. those related to the microstructure of the samples,
such as the domain dimension, and to their granular nature. In particular, while a prolonged annealing in flowing oxygen induces an extended microstructural
homogeneity, annealing in vacuum, besides producing loss of oxygen, leads to the formation of intragranular disordered microdomains and extended planar defects.
We verify how the physical properties are dependent on the level of microstructural order that determines the passage from metallic to semiconducting behaviour and, possibly, from bulk to spin glass superconducting behaviour.
By saying ‘bulk’ superconducting behaviour we mean the behaviour commonly observed in high-Tc superconductors where, as is well known, granularity is present and intergranular and intragranular properties of the sample
may be observed: in appropriate experimental conditions it is possible to separate the two contributions to study the intrinsic properties of the material or to observe
the character of the Josephson coupling between grains (or clusters). In contrast, the ‘spin glass’ is modelled as superconducting grains weakly linked into closed
loops, where different supercurrent carrying states of nearly equal energy may install, producing, among other effects, metastable configurations. We suggest
that the microstructure is an important cause of the different behaviour of samples otherwise very similar: strong microstructural disorder decreases the domain size
inside the grains until they are comparable with the coherence length ξ (and much smaller than the penetration depth λ). In such conditions the superconducting
volume of the sample becomes smaller and smaller; granularity or weak-link behaviour becomes ubiquitously present and practically determines the physical properties in the whole phase diagram
Synthesis and characterisation of superconducting MgCNi3
No full textThe intermetallic perovskite MgCNi3, superconducting with a Tc of 8 K, is particularly interesting
from several points of view: in spite of the large amount of nickel present the compound is not
ferromagnetic; nevertheless band structure calculations put in evidence a Van Hove singularity just
above the Fermi level suggesting that MgCNi3 is on the border of magnetic instability and that may
be driven to ferromagnetism by partial substitution of Mg with a monovalent metal as Li or Na. The
material is also interesting because of the presence of two kinds of pinning centres, grain boundaries
and nanometer-scale precipitates inside the grains, combination which is not found in other metallic
superconductors. We have synthesized polycrystalline samples with composition MgCNi3 by a solid
state route. Both the resistive and the magnetic measurements show a double superconductive
transition, the first one occurring at a temperature of 10 K, higher than that reported in literature (8
K) and a second one at 6 K. High temperature resistivity shows a metallic behaviour
Orthorhombic lattice deformation of GdSr2RuCu2O8 from high-resolution transmission electron microscopy and x-ray powder diffraction analysis
No full textGdSr2RuCu2O8 has been synthesized using a solid state reaction and has been characterized from the structural and physical points of view. The samples exhibit the well-known superconducting and magnetic behavior; in addition, an orthorhombic modification of the GdSr2RuCu2O8 lattice has been detected by both high resolution transmission electron microscopy (HRTEM) and x-ray powder diffraction (XRPD) analysis. Rietveld refinement of the XRPD data has been successfully performed in the orthorhombic Pmmm space group yielding an a0xb0xc0(=3 a0) type cell, with octahedral RuO6 polyhedra slightly distorted and rotated around c, b, and a
Variations in structural and physical properties of RuSr2GdCu2O8 samples submitted to annealing and deoxygenation procedures
No full textIn this work we report on the variations observed in the properties of sintered RuSr2GdCu2O8 (Ru-1212) subjected to annealing and deoxygenation routes. These treatments change in an important way both the structural features as
observed by X-ray diffraction data, Rietveld refinement and TEM, and the physical properties as observed by transport
and magnetic measurements. All these data are presented and discussed
Dependence of the structural and physical properties of Ru-1212 compound on the thermal treatment and oxygen content
No full textWe present structural and physical data obtained on Ru (Gd)-1212 compounds submitted, after solid state preparation, to different annealing procedures and thermal treatment. The samples were characterized by XRPD and TEM analysis. In addition structural refinement was performed according to the Rietveld method. Variations of both the superconducting and magnetic properties are presented and discussed
Synthesis effects on the magnetic and superconducting properties of RuSr2GdCu2O8
No full textA systematic study on the synthesis of the Ru-1212 compound by preparing a series of samples that were annealed at increasing temperatures and then quenched has been performed. It results that the optimal temperature for the annealing lies around 1060-1065 degreesC; a further temperature increase worsens the phase formation. Structural order is very important and the subsequent grinding and annealing improves it. Even if from the structural point of view the samples appear substantially similar, the physical characterizations highlight great differences both in electrical and magnetic properties related to intrinsic properties of the phase as well as to the connection between the grains as inferred from the resistive and the Curie-Weiss behaviour at high temperature as well as in the visibility of field cooled and zero field cooled magnetic signals
Superparamagnetic behavior of ferromagnetic nanoclusters in RuSr2GdCu2O8 and RuSr2Gd1.6Ce0.4Cu2O10 samples observed by AC and DC magnetic measurements
No full textAC and DC magnetic measurements have been performed on polycrystalline RuSr2GdCu2O8 and RuSr2Gd1.6Ce0.4Cu2O10 samples with the aim to study the complex magnetic structure of these compounds. Our results indicate that RuSr2GdCu2O8, in addition to the well-known antiferromagnetic ordering, exhibits another magnetic feature at temperature slightly higher than the antiferromagnetic ordering temperature, that we attribute to the presence of ferromagnetic nanoclusters that exhibit superparamagnetic behavior. This hypothesis is supported by magnetization decay measurements at various temperatures that show slow decays typical of superparamagnetic systems. The phase RuSr2Gd1.6Ce0.4Cu2O10 shows an even more complicated magnetic behavior, but also in it we observe slow magnetization decays. Therefore, also for this phase we hypothesize the presence of ferromagnetic nanoclusters with blocking temperature higher with respect to that of all the other magnetic structure
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
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