W.A.C. Passos

Granularity in superconductors: intrinsic properties and processing-dependent effects

Abstract This contribution presents a selected set of results, obtained as part of a systematic investigation, evidencing that many effects exhibited by superconductors are distinct manifestations of granularity which, in turn, is envisaged as a break of symmetry. The Wohlleben effect, the “fishtail anomaly”, the magnetic remanence exhibited by Josephson junction arrays, and the jumps on the magnetic moment of superconducting samples of mesoscopic dimensions, are examples which we briefly review and discuss taking granularity as the basic ingredient. The emphasis of the present approach is to recognize the importance of granularity in every scenario intended to explain the magnetic properties of superconducting systems.

Remanent magnetization of high-temperature Josephson junction arrays

Abstract In this work we study the remanent magnetization exhibited by tridimensional disordered high-Tc Josephson junction arrays excited by an AC magnetic field. The effect, as predicte by numerical simulations and previously verified for a low-Tc array of Nb, occurs in a limited range of temperatures. We also show that the magnetized state can be excited and detected by two alternative experimental routines.

Field-induced networks of weak-links: an experimental demonstration that the paramagnetic Meissner effect is inherent to granularity

Abstract In this article we report a direct observation that the paramagnetic Meissner effect (PME, also called Wohlleben effect), presented by some superconducting samples, is an inherent consequence of granularity in superconductors. The experiments reported here were performed using high-quality thin films of Nb and YBa 2 Cu 3 O 7− δ . A network of randomly distributed SS ′ S weak-links was induced on the film by application of a small perpendicular DC magnetic field. The high demagnetization factor arising from this geometry, forces magnetic flux to penetrate into the sample, establishing a pattern of magnetic dendrites. By changing the external field we can adjust the critical current strength of the weak-links, thus controlling the magnetic response of the induced network. In this way we have tuned the temperature dependence of the field-cooled magnetization. An important conclusion supported by the experiments is that PME results from a competition between positive and negative magnetic responses generated by different levels of granularity in a multigranular system. This is in accordance with previous experiments correlating PME and the dynamic reentrance exhibited by a Josephson junction array, a particularly ordered granular system.

Critical current of tridimensional La1.85Sr0.15CuO4−δ disordered Josephson junction arrays – dependence with the magnetic field

Abstract This paper deals with the magnetic field ( H ) dependence of the critical current of a tridimensional disordered Josephson junction array. The average critical current of an array of La 1.85 Sr 0.15 CuO 4− δ is determined through the AC-susceptibility measured with an excitation field of amplitude h . The critical field H c , necessary to switch off supercurrents through the weak-links at the working temperature is also obtained. This configuration relies on χ ′′ ( h ) – instead of χ ′ ( H ) – to determine the critical currents, what proved to be particularly useful for experimental setups with excitation and applied fields parallel to each other, a case in which the static flux imposed to the array affects χ ′ much more severely than χ ′′ .

Magnetic field dependence of the critical current of tridimensional YBa2Cu3O7-δ Josephson junction arrays

Abstract In this paper, we determine the magnetic field dependence of the critical current of a tridimensional disordered Josephson junction array. A contactless configuration, employing measurements of the AC-susceptibility, is used to evaluate the average critical current of an array of YBa 2 Cu 3 O 7− δ . The critical field necessary to switch off supercurrents through the weak links at the working temperature is also obtained.

Universal temperature behavior of remanent magnetization observed in low-Tc and high-Tc Josephson junction arrays

Abstract A comparative study of the magnetic remanence exhibited by tridimensional Josephson junction arrays in response to an excitation with an AC magnetic field is presented. The observed temperature behavior of the remanence curves for disordered arrays fabricated from three different materials (Nb, YBa 2 Cu 3 O 7– δ and La 1.85 Sr 0.15 CuO 4– δ ) is found to follow the same universal law (based on the explicit temperature expressions for the activation energy and the inductance-dominated contribution to the magnetization of the array within the framework of the phase-slip model) regardless of the origin of the superconducting electrodes of the junctions which form the array.

Absolute measurements of the AC susceptibility of cylindrical samples of arbitrary aspect ratio: scaling of the external susceptibility

Summary form only given. Absolute values of the AC-susceptibility of a given material are usually obtained from measurements of the so-called external susceptibility of a sample which, to favor proper correction of demagnetizing effects, is frequently shaped as a cylinder or an ellipsoid of revolution. For long cylindrical samples, one may, alternatively, benefit from the fact that the one-dimensional problem of infinite cylinders has been addressed in the literature and used to relate the complex components of external and absolute susceptibilities. More recently, de Faria et al. have successfully extended the infinite cylinder case, deriving expressions for cylindrical samples of non-magnetic metals of any length in the low-frequency regime. In this contribution we report new results obtained for two sets of cylindrical samples: YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and electrolytic copper. In both cases, the low-frequency absolute susceptibility is related to the external susceptibilities of finite length cylinders through a single scaling function. This behavior, which seems to be universal, proved to be very well fitted by the same governing function previously proposed by de Faria and co-workers. This study extends also to the very small length side of the range, indicating limiting conditions for the ratio between sample and pickup coil dimensions.

Unusual magnetic relaxation behavior in La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3

We have carried out a systematic magnetic relaxation study, measured after applying and switching off a 5 T magnetic field to polycrystalline samples of La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3. The long time logarithmic relaxation rate ~LTLRR!, decreased from 10 to 150 K and increased from 150 to 195 K in La0.5Ca0.5MnO3. This change in behavior was found to be related to the complete suppression of the antiferromagnetic phase above 150 K and in the presence of a 5 T magnetic field. At 195 K, the magnetization first decreased, and after a few minutes increased slowly as a function of time. Moreover, between 200 and 245 K, the magnetization increased throughout the measured time span. The change in the slope of the curves, from negative to positive at about 200 K was found to be related to the suppression of antiferromagnetic fluctuations in small magnetic fields. A similar temperature dependence of the LTLRR was found for the Nd0.5Sr0.5MnO3 sample. However, the temperature where the LTLRR reached the minimum in Nd0.5Sr0.5MnO3 was lower than that of La0.5Ca0.5MnO3. This result agrees with the stronger ferromagnetic interactions that exist in Nd0.5Sr0.5MnO3 in comparison to La0.5Ca0.5MnO3. The above measurements suggested that the general temperature dependence of the LTLRR and the underlying physics were mainly independent of the particular charge ordering system considered. All relaxation curves could be fitted using a logarithmic law at long times. This slow relaxation was attributed to the coexistence of ferromagnetic and antiferromagnetic interactions between Mn ions, which produced a distribution of energy barriers.

Evaluating the critical current magnitude and distribution width of tridimensional Josephson junction arrays

Abstract In this contribution, we present a simple and effective procedure to determine the average critical current of a tridimensional disordered Josephson junction array (3D-DJJA). Using a contactless configuration we evaluate the average critical current and the typical width of the distribution through the analysis of the isothermal susceptibility response to the excitation field amplitude, χ AC ( h ). A 3D-DJJA fabricated from granular Nb is used to illustrate the method.