A. Galluzzi

Vortex pinning properties in Fe-chalcogenides

Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the first step in producing wires and/or tapes for practical high power conductors. Here we present the results of a comparative study of pinning properties in iron-chalcogenides, investigating the flux pinning mechanisms in optimized Fe(SeTe x ) and FeSe samples by current–voltage characterization, magneto-resistance and magnetization measurements. In particular, from Arrhenius plots in magnetic fields up to 9 T, the activation energy is derived as a function of the magnetic field, whereas the activation energy as a function of temperature, is derived from relaxation magnetization curves. The high pinning energies, high upper critical field versus temperature slopes near critical temperatures, and highly isotropic pinning properties make iron-chalcogenide superconductors a technological material which could be a real competitor to cuprate high temperature superconductors for high field applications.

Critical current and flux dynamics in Ag-doped FeSe superconductor

The measurements of DC magnetization as a function of the temperature M(T), magnetic field M(H), and time M(t) have been performed in order to compare the superconducting and pinning properties of an undoped FeSe0.94 sample and a silver doped FeSe0.94 + 6 wt% Ag sample. The M(T) curves indicate an improvement of the superconducting critical temperature and a reduction of the non-superconducting phase Fe7Se8 due to the silver doping. This is confirmed by the field and temperature dependent critical current density Jc(H,T) extracted from the superconducting hysteresis loops at different temperatures within the Bean critical state model. Moreover, the combined analysis of the Jc(T) and of the pinning force Fp(H/Hirr) indicate that the pinning mechanisms in both samples can be described in the framework of the collective pinning theory. The U*(T, J) curves show a pinning crossover from an elastic creep regime of intermediate size flux bundles, for low temperatures, to a plastic creep regime at higher temperatures for both the samples. Finally, the vortex hopping attempt time has been evaluated for both samples and the results are comparable with the values reported in the literature for high Tc materials.

Synthesis and Characterization of Magnetic Nanocomposites for Environmental Remediation

In the present study, the effect of nano-magnetite (Fe3O4) content on mechanical and magnetic properties of polypropylene matrix is investigated. Magnetite nanoparticles were successfully synthesized by co- precipitation while the composites were prepared by an ex situ processing method involving solvent casting followed by compression molding. The nanoparticles and resulting nanocomposites were characterized by X- ray diffraction, thermogravimetric analysis, scanning electron microscopy, tensile testing and vibrating sample magnetometry. It was found that composites have tailorable mechanical and magnetic properties dependent on the content of magnetic filler. Increase of concentration of magnetite particles provides a significant increase of Young’s modulus without affecting the yield strength and the ductility. As regards the magnetic properties, nanocomposites having 10 wt% of nanoparticles exhibited a superparamagnetic behaviour that can be exploited in environmental applications.

Fabrication and Characterization of Sintered Iron-Chalcogenide Superconductors

Iron-chalcogenide superconductors are appealing for high-magnetic-field applications due to their promising superconducting properties, i.e., extremely high upper critical and irreversibility field.In this paper, the first achievements obtained from sintering polycrystalline samples are presented. The sintering of FeSe polycrystalline powders and pellets was obtained in several steps starting from stoichiometric quantities of freshly polished powders shots; after grinding, powders were loaded into cleaned and dried silica tubes sealed under vacuum for subsequent heat treatments in order to obtain the superconducting phase. Samples were then characterized from the structural, transport, and magnetic points of view. From the structural characterization, it has been discovered that, in addition to the presence of the desired tetragonal FeSe phase, hexagonal phase and several impurities are present in the samples; the lack of homogeneity inside the samples is confirmed by the superconductive characterization: despite the small undeniable enhancement of the results with the application of subsequent HTs and, nearby, a resistive superconductive onset of 12 K, the transition is still broad after the third HT.

Mixed state properties of iron based Fe(Se,Te) superconductor fabricated by Bridgman and by self-flux methods

The superconducting and transport properties of iron based Fe(Se,Te) superconductors fabricated by means of Bridgman (B) and Self-flux (S) methods have been compared using dc Magnetization (M) measurements as a function of temperature (T) and magnetic field (H). The M(T) measurements performed in Zero Field Cooling-Field Cooling conditions show higher critical temperature Tc and a lower spurious magnetic background signal for the sample (B) rather than the (S) one. By considering the superconducting M(H) hysteresis loops, the sample (B) shows a stronger superconducting signal together with the presence of a peak effect. The field and temperature dependence of the critical current densities Jc are extracted from the superconducting hysteresis loops M(H) within the Bean critical state model, and the high ratio between the J c B and the J c S, relative to the two typologies of samples, together with the comparison between their upper critical field Hc2, points out that the Bridgman method is most attractive for exploiting superconducting and transport properties in view of applications.The superconducting and transport properties of iron based Fe(Se,Te) superconductors fabricated by means of Bridgman (B) and Self-flux (S) methods have been compared using dc Magnetization (M) measurements as a function of temperature (T) and magnetic field (H). The M(T) measurements performed in Zero Field Cooling-Field Cooling conditions show higher critical temperature Tc and a lower spurious magnetic background signal for the sample (B) rather than the (S) one. By considering the superconducting M(H) hysteresis loops, the sample (B) shows a stronger superconducting signal together with the presence of a peak effect. The field and temperature dependence of the critical current densities Jc are extracted from the superconducting hysteresis loops M(H) within the Bean critical state model, and the high ratio between the J c B and the J c S, relative to the two typologies of samples, together with the comparison between their upper critical field Hc2, points out that the Bridgman method is most...