Massimiliano Polichetti

Magnetic relaxation of type-II superconductors in a mixed state of entrapped and shielded flux

The magnetic relaxation has been investigated in type-II superconductors when the initial magnetic state is realized with entrapped and shielded flux contemporarily. This flux state is produced by an inversion in the magnetic field ramp rate due to, for example, a magnetic field overshoot or undershoot. The investigation has been faced both numerically and by measuring the magnetic relaxation in BSCCO tapes. Numerical computations have been performed in the case of an infinite thick strip and of an infinite slab, showing a quickly relaxing magnetization in the first seconds. As verified experimentally, the effects of the overshoot (or the undershoot) cannot be neglected simply by cutting the first

Detection of the flux creep regime in the AC susceptibility curves by using higher harmonic response

10\char21{}100\phantom{\rule{0.3em}{0ex}}\mathrm{s}

Epitaxial growth of La0.7Ba0.3MnO3 thin films on MgO substrates: Structural, magnetic, and transport properties

in the magnetic relaxation. On the other hand, at very long times, the magnetic states relax toward those corresponding to field profiles with only shielded flux or only entrapped flux, depending on the amplitude of the field change with respect to the full penetration field of the considered superconducting samples. In addition, we have performed numerical simulations in order to reproduce the relaxation curves measured on the BSCCO(2223) tapes; this allowed us to interpret correctly also the first seconds of the

Vortex pinning properties in Fe-chalcogenides

M(t)

Vortex dynamics and pinning properties analysis of MgB2 bulk samples by ac susceptibility measurements

curves.

Harmonics of the AC susceptibility for the study of I–V curves in melt grown YBCO

In order to investigate the dynamic properties of the flux line lattice, the temperature dependencies of the basic and .

Transition between the bragg glass and the disordered phase in Nb3Sn detected by third harmonics of the ac magnetic susceptibility

We report on structural, magnetic, and transport properties of La0.7Ba0.3MnO3 thin films, epitaxially grown on MgO substrates. Despite the quite similar structural features if compared to crystalline manganite films, our samples show a metal-insulator transition temperature of about 200K, sizeably lower than the bulk-value (TMI≃345K). Moreover, the magnetotransport properties show the absence of saturation in the magnetoresistance and localization phenomena at low temperatures (T<30K). The temperature behavior of the magnetization shows a Curie temperature Tc value above room temperature, ruling out effects due to oxygen deficiency. All these findings are analyzed in terms of possible physical mechanisms related to the growth in the presence of large mismatch between film and substrate lattice parameters.

Harmonics of the AC susceptibility as probes to differentiate the various creep models

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.

Response of glass and liquid phases in the vortex lattice to an external AC magnetic field at different frequencies

Flux line dynamics have been investigated on MgB2 bulk superconductors obtained by reactive liquid infiltration by measuring the ac magnetic susceptibility. The fundamental and third harmonics have been studied as a function of temperature, dc magnetic field, ac field amplitude and frequency. In order to determine the dynamical regimes governing the vortex motion, the experimental results have been compared with susceptibility curves obtained by numerical calculations of the non-linear diffusion equation for the magnetic field. The frequency behaviour of the third harmonic response, that cannot be explained by frequency dependent critical state models, has been related to the current dependence of the flux creep activation energy U(J) in the diffusion coefficient. In this way we have shown that the measured curves are correctly interpreted within the framework of a vortex glass description.

A study of coupling loss on bi-columnar BSCCO/Ag tapes through ac susceptibility measurements

Abstract The measurements of the first and the third harmonics of the AC magnetic susceptibility, performed on melt grown YBCO samples as a function of the temperature and the external parameters (frequency and amplitude of AC magnetic field, intensity of a DC field) were studied by using the Cole–Cole plot analysis. By considering the AC field dependence of the third harmonics Cole–Cole plots, we have extracted information about the peculiar behaviour of the I–V characteristics and, therefore, about the dissipative phenomena in the sample. The experimental results have been analysed with the help of both analytical and numerical curves. In particular, the use of a phenomenological expression for the resistivity allowed us to reproduce the main experimental features, including the observed invariance of the area inside the Cole–Cole plots and their behaviour as a function of the AC field.