Danilo Zola

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

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

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

Evidence for low-field crossover in the vortex critical velocity of type-II superconducting thin films

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

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

M(t)

Study of morphological and structural properties of multi-seeded NdBaCuO samples

curves.

Magnetic Instability in

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.

{\hbox{MgB}}_{2}

We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux flow state related to the vortex critical velocity v* is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence v* ~ B^-1/2, in the low field range a new cross-over field, Bcr1, is observed below which v* decreases by further lowering the external magnetic field B. We ascribe this unexpected cross-over to vortex channeling due to a fan-like penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of Bcr1 becomes a direct evidence of a general feature in type-II superconducting films at low fields, that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.

Monofilamentary Tapes

Coupling losses were studied in composite tapes containing superconducting material in the form of two separate stacks of densely packed filaments embedded in a metallic matrix of Ag or Ag alloy. This kind of sample geometry is quite favourable for studying the coupling currents and, in particular, the role of superconducting bridges between filaments. By using the ac susceptibility technique, the electromagnetic losses as a function of ac magnetic field amplitude and frequency were measured at the temperature T = 77 K for two tapes with different matrix composition. The length of the tapes was varied by subsequent cutting in order to investigate its influence on the dynamics of magnetic flux penetration. The geometrical factor χ0 which takes into account the demagnetizing effects was established from ac susceptibility data at low amplitudes. Losses versus frequency dependencies have been found to agree nicely with the theoretical model developed for round multifilamentary wires. Applying this model, the effective resistivity of the matrix was determined for each tape by using only measured quantities. For the tape with a pure silver matrix its value was found to be larger than that predicted by the theory for given metal resistivity and filamentary architecture. In contrast, in the tape with a Ag/Mg alloy matrix, an effective resistivity much lower than expected was determined. We explain these discrepancies by taking into account the quality of the interface between the superconducting filaments and the normal matrix. In the case of a soft matrix of pure Ag, this is of poor quality, while the properties of the alloy matrix seem to provoke the extensive creation of intergrowths which can be actually observed in these kind of tapes.

Crystal structures, magnetic and superconducting properties of the RuSr2NdCu2Ox and RuSr2GdCu2Oy compounds

In this work, we report on the morphological and structural properties of multi-seeded melt textured NdBaCuO bars, using MgO seeds. We have characterised the samples by x-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. In particular by these techniques we analysed the characteristics of boundaries joining adjacent domains. For 2 centimetres spacings between adjacent seeds, good texturing and absence of Nd422-rich quenched liquid deposits or other spurious phases near the boundaries have been observed. On the other side a serious mechanical fragility, sensibly enhanced along the boundary regions, is found. The superconducting properties, such as critical temperature and critical current density have been investigated by means of magnetic measurements.

Analysis of coupling losses in multifilamentary untwisted BSCCO/Ag tapes through a.c. susceptibility measurements

The magnetic instability in MgB2 monofllamentary tapes with nickel alloy matrix has been investigated by measuring hysteresis loops and magnetic relaxation. The measurements have been performed in the temperature range from 2 K up to 39 K, applying an external magnetic field up to 20 kOe both parallel and perpendicular to the filament direction. We have analysed the magnetic instability by using an adiabatic and dynamical approach and taking also into account the effects of the samples geometry on the magnetic properties. In this work we discuss the observed dependence of the flux jumps on the temperature and on the applied magnetic field taking into account the recent results on the magnetic instability in a superconductor placed in a perpendicular magnetic field.