Dr. Ion IVAN

Design of products involves functional and sensory aspects, where surfaces play an important role. This study uses (i) sensory attributes to show that tactile sensation is highly dependent on surface roughness, and (ii) variation in pupil diameter to suggest that roughness close to fingerprint geometry causes less arousal. A panel of over 30 participants explored six plexiglass surfaces with different roughness generated by variations in milling speed and depth. The pattern obtained on the samples is periodic in one direction, with an average wavelength between 113 mu m to 600 mu m and an average height between 13 mu m and 123 mu m. During a blind touch, the sensory attributes of smoothness, grip and quality of each sample were evaluated by the panellists, as well as the emotional attributes of valence and arousal. The evolution of pupil diameter over time was also recorded, and its average value during the first two seconds of touch was considered as a new emotional attribute. These attributes made it possible to calculate six centred indicators, ranging between -1 and 1, for each panellist and each sample. Statistical analysis of these indicators showed that the declared valence is correlated with smoothness, grip, and quality, all gradually decreasing as roughness increases. These results will allow product designers to improve the hedonic experience of future users. To more precisely analyse arousal, valence, and the evolution of pupil diameter, three of the six samples, manufactured with the same cutting tool, were considered. Valence and arousal appeared relatively difficult to verbalised, but the pupil diameter allowed the samples to be discriminated. The sample with a roughness close to the geometry of the fingerprint appeared to be the least emotional.

In this study we present the effect of the irradiation of sintered MgB2 with protons of intermediate energies, from 8.6 to 15.07 MeV, at a constant fluence of 2.86 x 10(16) p/cm(2). We found clear evidence that the disorder generated by irradiation leads to a weak suppression of the critical temperature T-c and an increase of the critical current density J(c) with increasing proton energy, as expected. However, it was found that thermomagnetic instabilities and vortex creep phenomena strongly depend on the ratio between proton range and the sample thickness. The number of the macroscopic flux jumps (MFJ) is highest and manifest up to 28 K in the sample with the proton range shorter than sample thickness (P-1) and decrease as the proton energy increases. Similarly, the relaxation rate of the irreversible magnetization is the lowest in the sample P-1 and increases with proton energy. We tentatively attribute this effect to the protons that stop within MgB2 and interact with the local structure/atomic composition.

The paper presents the features of the magnetization decay of proton-irradiated MgB2 bulk samples. Irradiation was performed with protons of energies between 8.6 and 15.07 MeV at different fluences. The creep activation energy is sensitive to the ratio between proton range and sample thickness. Experimental data show that the creep exponents are different from the theoretical predictions. When the proton range is shorter than the sample thickness (protons of 8.6 MeV), the thermomagnetic instabilities are strongly emphasized and affect the creep behavior.

The study of granularity in superconducting films by using AC susceptibility has a crucial role in the development of and improvement in the ReBCO-coated conductors, which are a constantly evolving reality in the modern power applications of superconductivity. Specifically, the study of the granularity is essential because the ReBCO superconducting wires and tapes are far from the regularity of a single crystal while they often present an inter- and intragranular contribution to the critical current density. On the other hand, the AC susceptibility is a key part of the characterization of a granular sample because this technique is very sensitive to the presence of granularity in the superconductors and, moreover, the study of its first harmonic allows for determining pivotal properties such as the pinning energy as well as the dissipation processes acting in the sample. The pinning energy values and the granularity of an YBCO thin film have been studied by means of AC susceptibility measurements as a function of the AC amplitude, temperature, and DC field. In particular, the first harmonic imaginary component of the AC susceptibility chi 1 '' related to the dissipation processes of the sample has been studied. First, starting from the Brandt approach, the critical current density J(c) and the pinning energy U of the sample have been extracted at 77 K by using the chi 1 '' measurements as a function of the AC amplitude at different AC frequencies and DC fields. From these measurements, a first signal of granularity appears. In order to confirm it, the temperature dependence of the chi 1 '' at different DC fields has been studied and a contribution deriving from the inter- and intragranular part of the sample has emerged. By taking the temperature corresponding to the crossover between the two contributions at the different DC fields, the intergranular and intragranular response has been separated. Successively, the temperature has been fixed to 77 K, together with an AC frequency equal to 1597.9 Hz, and the chi 1 '' as a function of the DC field at different AC amplitudes has been analyzed showing a clear presence of granularity in all the curves. By drawing the contour plot of the chi 1 '' with the DC and AC values, it was possible to determine the best parameters to put at 77 K in order to exploit the material for applications.

Among many iron-based superconductors, isovalently substituted BaFe2(As1-xPx)2 displays, for x approximate to 0.3, apart from the quite usual Second Magnetization Peak (SMP) in the field dependence of the critical current density, an unusual peak effect in the temperature dependence of the critical current density in the constant field, which is related to the rhombic-to-square (RST) structural transition of the Bragg vortex glass (BVG). By using multi-harmonic AC susceptibility investigations in three different cooling regimes-field cooling, zero-field cooling, and field cooling with measurements during warming up-we have discovered the existence of a temperature region in which there is a pronounced magnetic memory effect, which we attributed to the direction of the structural transition. The observed huge differences in the third harmonic susceptibility at low and high AC frequencies indicates the difference in the time-scale of the structural transition in comparison with the timescale of the vortex excitations. Our findings show that the RST influence on the vortex dynamics goes beyond the previously observed influence on the onset of the SMP.

For practical applications of superconductors, understanding the vortex matter and dynamics is of paramount importance. An important issue in this context is the transition of the vortex glass, which is a true superconducting phase, into a vortex liquid phase having a linear dissipation. By using multi-harmonic susceptibility studies, we have investigated the vortex glass-vortex liquid phase transitions in CaKFe4As4 and BaFe2(As0.68P0.32)(2) single crystals. The principle of our method relates the on-set of the third-harmonic susceptibility response with the appearance of a vortex-glass phase in which the dissipation is non-linear. Similar to the high-critical temperature cuprate superconductors, we have shown that even in these iron-based superconductors with significant lower critical temperatures, such phase transition can be treated as a melting in the sense of Lindemann's approach, considering an anisotropic Ginzburg-Landau model. The experimental data are consistent with a temperature-dependent London penetration depth given by a 3D XY fluctuations model. The fitting parameters allowed us to extrapolate the vortex melting lines down to the temperature of liquid hydrogen, and such extrapolation showed that CaKFe4As4 is a very promising superconducting material for high field applications in liquid hydrogen, with a melting field at 20 K of the order of 100 T.

The revolutionary economic potential of micro and nanotechnology is already recognized. Micro and nano-scale technologies that use electrical, magnetic, optical, mechanical, and thermal phenomena separately or in combination are either already in the industrial phase or approaching it. The products of micro and nanotechnology are made of small quantities of material but have high functionality and added value. This paper presents such a product: a system with micro-tweezers for biomedical applications-a micromanipulator with optimized constructive characteristics, including optimal centering, consumption, and minimum size, for handling micro-particles and constructive micro components. The advantage of the proposed structure consists mainly in obtaining a large working area combined with a good working resolution due to the double actuation principle: electromagnetic and piezoelectric.

Among various "families" of iron-based superconductors, the quite recently discovered AeAFe(4)As(4) (where Ae is an alkali-earth metal and A is an alkali metal) has high critical current density, a very high upper critical field, and a low anisotropy, and has recently received much interest for the possibility of high magnetic field applications at the liquid hydrogen temperature. We have performed DC magnetization relaxation and frequency-dependent AC susceptibility measurements on high-quality single crystals of CaKFe4As4 with the aim of determining the pinning potential U*. The temperature dependence of U* displays a clear crossover between elastic creep and plastic creep. At temperatures around 27-28 K, U* has a very high value, up to 1200 K, resulting in an infinitesimally small probability of thermally activated flux jumps. From the dependence of the normalized pinning potential on irreversible magnetization, we have determined the creep exponents in the two creep regimes, which are in complete agreement with theoretical models. The estimation of the pinning potential from multifrequency AC susceptibility measurements was possible only near the critical temperature due to equipment limitations, and the resulting value is very close to the one that resulted from the magnetization relaxation data. Magnetic hysteresis loops revealed a second magnetization peak and very high values of the critical current density.

For high-field power applications of high-temperature superconductors, it became obvious in recent years that nano-engineered artificial pinning centers are needed for increasing the critical current and pinning potential. As opposed to the artificial pinning centers obtained by irradiation with various particles, which is a quite expensive approach, we have studied superconducting samples having self-assembled defects, created during the sample fabrication, that act as effective pinning centers. We introduced a simple, straight-forward method of estimating the frequency-dependent critical current density by using frequency-dependent AC susceptibility measurements, in fixed temperatures and DC magnetic fields, from the positions of the maxima in the dependence of the out-of-phase susceptibility on the amplitude of AC excitation magnetic field. The results are compatible with a model that stipulates a logarithmic dependence of the pinning potential on the probing current. A mathematical derivation allowed us to estimate from the experimental data the pinning potentials in various samples, and in various DC magnetic fields. The resulted values indicate large pinning potentials, leading to very small probability of magnetic flux escaping the pinning wells, hence, leading to very high critical currents in high magnetic fields.

We have investigated the pinning potential of high-quality single crystals of superconducting material CaKFe4As4 having high critical current density and very high upper critical field using both magnetization relaxation measurements and frequency-dependent AC susceptibility. Preliminary studies of the superconducting transition and of the isothermal magnetization loops confirmed the high quality of the samples, while temperature dependence of the AC susceptibility in high magnetic fields show absolutely no dependence on the cooling conditions, hence, no magnetic history. From magnetization relaxation measurements were extracted the values of the normalized pinning potential U*, which reveals a clear crossover between elastic creep and plastic creep. The extremely high values of U*, up to 1200 K around the temperature of 20 K lead to a nearly zero value of the probability of thermally-activated flux jumps at temperatures of interest for high-field applications. The values of the creep exponents in the two creep regimes resulted from the analysis of the magnetization relaxation data are in complete agreement with theoretical models. Pinning potentials were also estimated, near the critical temperature, from AC susceptibility measurements, their values being close to those resulted (at the same temperature and DC field) from the magnetization relaxation data.

The deposition of a ferromagnetic layer can affect the properties of high-temperature superconductors underneath. We investigated the influence of ferromagnetic CaRuO3 on the properties of YBa2Cu3O7-x (YBCO) superconducting thin films when the layers are either in direct contact or separated by a barrier layer of 5 nm SrTiO3. Detailed measurements of the magnetic moment of the superconductor and ferromagnet as a function of temperature and magnetic field have been performed using SQUID magnetometry. Magnetometry and relaxation measurements show that the modification of the superconducting properties of YBCO strongly depends on the interaction with the ferromagnetic layer on top. The barrier layer has a significant impact on both the supercon-ducting properties of the YBCO film and the ferromagnetic ordering of CaRuO3. The physical properties mentioned above were discussed in correlation with the materials' structure determined by XRD analysis.

Almost all proposed configurations and practical achievements based on superconductor/ferromagnet (S/F) heterostrucutres focus on s-wave superconductors. However, several attempts targeted also high temperature superconductors, most of them using manganite ferromagnets LaXMnO3 (X: Ca or Sr) and Y1Ba2Cu3O7-x (YBCO). Here we propose a new ferromagnetic material that can be used with YBCO for the fabrication of S/F hybrid structures. We show that a ferromagnetic order can be induced in a thin layer (similar to 130 nm thickness) of CaRuO3 grown by pulsed laser deposition on epitaxial YBCO film. Detailed magnetic and structural investigations show that the observations of the weak ferromagnetism are consistent with the magnetic order induced by in-plane tensile strain of about 1.7% and the easy-magnetization axis forms an angle of similar to 180o with the layer plane. The value of the Curie temperature T (Curie) estimated using the Curie-Weiss law was 340 K. An unusual temperature dependence of the magnetic moment around the superconducting transition was observed in both field-cooled and zero-field-cooled configurations which is attributed to the paramagnetic Meissner effect.

We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa2Cu3O7 films with artificial pinning centers, with various impurities, various architectures and thickness, and various techniques of nano-scale pinning engineering: substrate decoration, BaZrO3 (BZO) nano-inclusions, and (quasi) multilayer architecture. Here we will present the results regarding vortex matter, dynamics, and pinning, in some of our best samples. Magnetic relaxation studies with magnetic field perpendicular to the film show that in both cases the splayed defects and/or nanoscale inclusions help reduce the dissipation in certain field-temperature range, by inhibiting the detrimental double vortex-kink formation. The response of the vortex system to AC excitation is rather complex but extremely useful for the characterization of the vortex dynamics. We have measured AC susceptibility in various DC fields, with various AC fields and frequencies and found that the effective vortex activation energy U-eff has a logarithmic dependence on the AC-field-induced current density J. Synergetic pinning centers assured a low anisotropy of the critical current for various field orientations, a property desired in coil/solenoid applications.

Effects of proton irradiation on the superconducting properties of sintered MgB2 samples were investigated as a function of proton fluence at fixed energy. All experimental data suggest that critical current densities, irreversibility field and creep activation energy are enhanced because of the complex disorder, which mainly affects the grain boundaries, brought on the crystalline structure by the energetic protons.

We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2O3 nanoparticles at a static magnetic field H-dc lower than the matching field H-Phi. We proposed a practical formula for the determination of the induced current J during AC susceptibility measurements, which allows us to directly obtain the pinning potential U-c from the characteristics of electric field E versus J. We show that the dynamic critical current J(d) induced at the depinning frequency f(d) can be experimentally obtained by measuring m'(T) and m ''(T) at different frequencies and amplitudes. It was found that the value of f(d) obtained by extrapolation of J to J(d) in the PHz domain is much higher than the frequency reported for ordinary YBa2Cu3O7 thin films as determined by microwave impedance measurements. AC susceptibility, vortex dynamics, pinning energy, depinning frequency, dynamic critical current, microwave impedance measurements

We report on the enhancement of critical current density (J(c)) and the unusual behaviour of its dependence on field orientation in YBa2Cu3O7-x (YBCO) nanostructured films by a combination of substrate decoration with Ag nano-dots, of the incorporation of BaZrO3 (BZO) nano-particles and nano-rods, and of multilayer architecture (a thin SrTiO3 layer separating two 1.5 mu m-thick YBCO layers). SrTiO3 insulating layers were 15, 30 or 45 nm thick. The highest improvement of J(c) in applied magnetic fields along the c-axis and smaller than 1 T occurs in the bi-layer with 30 nm-thick STO, but the influence of STO thickness is small. Our thick nanostructured films show significant improvement of J(c) in the magnetic field along the ab-plane direction. The presence of BZO nano-rods, ab-plane defects and nano particles of BZO and Y2O3 was observed in transmission electron microscopy (TEM) images of the film. The peculiarities of artificial pinning centres revealed in the TEM images of the nanostructured films are used to explain an unusual split of the peak in the J(c) dependence on the magnetic field along the ab-plane of YBCO. Effective pinning potentials in high magnetic fields have rather high values for such thick films.

We investigated the evolution of the DC magnetic hysteresis curves of Nb0.89Ti0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 degrees C in vacuum and cold rolling (similar to 50% thickness reduction). Starting with a rectangular piece (similar to 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak ( where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

The vortex activation energy U-AC in the AC magnetic response of superconductors exhibits a logarithmic variation with the screening current density J (regardless of the pinning structure details), and takes surprisingly high values in the vicinity of the DC irreversibility line, especially at low external DC magnetic fields, as often reported. This is essentially different from the behaviour of the vortex-creep activation energy at long relaxation time scales in DC magnetic measurements, and is not completely understood. We investigated the DC relaxation and the AC response for YBa2Cu3O7 films containing nanorods and nanoparticles, with the DC and AC fields oriented perpendicular to the film surface. It is shown that the large U-AC values in the vicinity of the DC irreversibility line, where the critical-state-related AC signal occurs, are generated by a non-diffusive vortex motion during the AC cycle, with the mean vortex hopping length longer than the average distance between the pinning centres. In these conditions, the smearing of the vortex pinning potential by thermally induced vortex fluctuations is weak, and U-AC mainly results from the strong influence of the pinning-enhanced viscous drag on the vortex hopping process. The logarithmic U-AC(J) dependence is consistent with a high U-AC.

Vortex activation energy U-AC in the critical-state related AC magnetic response of superconductors (appearing in the vicinity of the DC irreversibility line) takes large values, as often reported, which is not yet understood. This behavior is essentially different from that of the vortex-creep activation energy at long relaxation time scales, and may become important for AC applications of superconductors. To elucidate this aspect, we investigated the AC signal of almost decoupled [YBa2Cu3O7](n)/[PrBa2Cu3O7](4) superlattices (with n = 11 or 4 units cells) in perpendicular DC and AC magnetic fields. In these model samples, the length of the hopping vortex segment is fixed by the thickness of superconducting layers and vortices are disentangled, at least at low DC fields. It is shown that the high U-AC values result from the large contribution of the pinning enhanced viscous drag in the conditions of thermally activated, non-diffusive vortex motion at short time scales, where the influence of thermally induced vortex fluctuations on pinning is weak. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

High density bulks (97%-99%) of MgB2 were prepared by spark plasma sintering (SPS) in nitrogen (N-2) atmosphere for different heating rates (10, 20 and 100 degrees C min(-1)) and compared with reference samples processed in vacuum and Ar. N-2 reacts with MgB2 and forms MgB9N along the MgB2 grain boundaries. The high-field critical current density is enhanced for the sample processed in N-2 with a heating rate of 100 degrees C min(-1). At 2-35 K, this sample shows the strongest contribution of the grain boundary pinning (GBP). All samples are in the point pinning (PP) limit and by increasing temperature the GBP contribution decreases.

The occurrence of a second magnetization peak (SMP) on the dc magnetization curves of superconducting single crystals with random quenched disorder in increasing external magnetic field H is quite common. Many models for this intriguing phenomenon have been proposed so far, but its origin is still under strong debate. Optimally doped and overdoped La2-xSrxCuO4 single crystals (x >= 0.15) exhibit an SMP over a large temperature interval, up to very close to the critical temperature. By decreasing doping, it was found that with H oriented along the c axis the SMP completely disappears in the doping domain of well developed static charge and spin stripes (x similar to 1/8), reappearing for x <= 0.10. This behaviour follows the instability of the Bragg vortex glass generated by the static stripe order (as revealed by small-angle neutron scattering experiments), which is confirmed by our magnetization relaxation measurements. If H is along the (a, b) planes, the SMP is observed for all investigated specimens. The presented analysis supports the scenario in which the SMP is generated by the pinning-induced disordering of the low-field Bragg vortex glass in the dynamic conditions of dc magnetic measurements. (C) 2015 Elsevier B.V. All rights reserved.

AC susceptibility and DC magnetization relaxation measurements have been performed for YBa2Cu3O7 films containing BaZrO3 nanorods preferentially oriented along the c axis, with the external magnetic field perpendicular to the film surface. It was found that the effective vortex activation energy U (e) supplied by the standard analysis of the frequency dependent AC magnetic response remains very high even in the vicinity of the irreversibility line indicated by the DC relaxation. We show that the large U (e) values appearing in the AC signal are related to the pinning enhanced viscous drag and significant vortex velocities.

Vortex pinning on natural and artificial defects is essential for large scale applications of superconducting materials. One of the most promising solutions for the creation of efficient pinning structures is to combine the strong pinning supplied by columnar defects (with the radius of the order of the superconducting coherence length) and the presence of random quenched disorder, which inhibits the detrimental vortex kink formation. A strong pinning is revealed by high values of the vortex activation energy in the magnetic relaxation process. We present a critical analysis of the interpretation of the relaxation data at long- and short time scales, by extracting the so called normalized vortex-creep activation energy. This allowed us to find the actual temperature interval for the characteristic vortex excitations in YBa2Cu3O7 films with embedded BaZrO3 nanorods 9preferentially oriented along the c axis), and to unambiguously determine the characteristic vortex pinning energy. The observed drastic change of magnetic relaxation at short time scales (attained in standard AC measurements) is attributed to a large contribution of the pinning enhanced viscosity to the vortex hopping activation energy.

We investigated the temperature (T) variation of the normalized magnetization relaxation rate S and of the corresponding normalized vortex-creep activation energy U* = T/S for YBa2Cu3O7 films containing BaZrO3 nanorods, with the external magnetic field H oriented perpendicular to the film surface. It was found that by increasing the film thickness and using nanodot decorated substrates the high-T S(T) maximum appearing at low H is substituted by a minimum in S(T). As revealed by the analysis of the current density dependence of U*, this behaviour is due to the inhibition of vortex excitations involving double vortex-kinks and superkinks formation in the investigated thick films, owing to the large nanorod splay and pinning energy dispersion.

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB2Cu3O7 (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T (dp) very close to the critical temperature T (c) (T (dp)similar to 0.95T (c)), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T (dp) (similar to 0.5T (c)). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO3 (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T (c) does not support a low T (dp). The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T (dp)) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T (dp) close to T (c). By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.

We present an investigation of the field dependence of the pinning force of dense MgB2-based superconducting composites as obtained by spark plasma sintering with different ingredients which are designated to enhance the pinning force F. Generally, the latter quantity, scaled to the maximum pinning force F-max, obeys a scaling law as a function of the scaled field h when the irreversibility field B-irr is considered as scaling field. The scaled function is described in terms of a generalized scaled function h(p)(1-h)(q). However, in our samples the scaling is absent. In addition, the peak of the scaled function shifts to higher reduced fields h when the temperature increases. Depending on the level of doping and the nature of the nanosized particles used to built the superconducting composites, we found that the scaled force can be depicted either with a combination of generalized scaled functions with different exponents or with a single function but multiplied with an envelope, usually an exponential factor. The former dependence is present in the samples with a high amount of nanoparticles and mirrors two types of pinning of close weights. The presence of the exponential factor in the latter dependence is attributed to the combined effect of the intrinsic anisotropy of MgB2 and the random orientation of the grains relative to the magnetic field.

Superconducting bulks of MgB2 were obtained by an ex-situ two-temperature route applied to spark plasma sintering (SPS). Processing of samples was performed at lower temperatures than previously reported. Samples produced by the two-temperature route show a higher morphological uniformity, a higher density (above 98%), a higher Vickers hardness, and undesirable stronger microscale flux jumps, as indicated by magnetic relaxation measurements when compared to a sample obtained by the one-temperature route (95.3% relative density). At the same time, all sintered samples show approximately constant crystallite size, critical current density, irreversibility field, critical temperature, weight fraction of impurity phases (MgB4 and MgO), and the amount of carbon accidentally introduced during SPS processing. (C) 2012 Elsevier B.V. All rights reserved.

The relaxation of the irreversible magnetic moment m(t) in YBa(2)Cu(3)O(7) (YBCO) films was investigated as a function of temperature T and the external magnetic field H along the c axis applied in zero-field cooling conditions, for the determination of vortex creep parameters. The data analysis was performed using the T and current density dependence of the normalized vortex creep activation energy, or by the fit of the m(t) data with the well known interpolation formula in the framework of the general vortex creep equation. It was found that (i) even for specimens with strong static pinning the characteristic pinning energy remains small in the low-T range, where the vortex creep appearing in standard magnetization measurements is elastic, (ii) the observed strong increase of the time scale for creep and the decrease of the creep exponent with increasing H beyond the theoretical predictions can be attributed to the crossover elastic-creep-plastic-creep generated by the macroscopic currents induced in the specimen during magnetization measurements, and (iii) the creep parameters extracted with the interpolation formula are highly affected by the m(t) data registered at short t, which may lead to unphysical creep parameter values.

We investigated the relaxation of the irreversible magnetization in a series of 200 nm thick YBa2Cu3O7/PrBa2Cu3O7 [(YBCO)(n)/(PrBCO)(m)] superlattices, where the thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept to m = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thickness n of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standard zero-field-cooling dc magnetization relaxation data obtained in the low temperature T region with the applied magnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at low T-plastic vortex creep at high T, generated by the T dependent macroscopic currents induced in the sample during measurements. For thin superlattices (n < 20) the creep crossover temperature T-cr proportional to n, and T-cr decreases linearly with increasing ln(H) for a fixed n. This crossover represents an alternative to the elastic vortex glass behavior reported for superlattices, as well as to 'quantum vortex creep' at unexpectedly high T inferred for thin films. We also discuss the absence of an increase of the magnetically determined critical current density with decreasing YBCO thickness in our superlattices, which apparently contradicts the collective pinning theories.

We measured the critical current density J(c) of optimally doped YBa(2)Cu(3)O(7) (- delta) thin films obtained by dc sputtering on (100) oriented SrTiO(3) substrates and by pulsed laser deposition (PLD) on (100) oriented MgO substrates with SrTiO(3) buffers. J(c) at various temperatures T was determined from the magnetization curves registered with a vibrating sample magnetometer for an external magnetic field H (oriented along the c axis) up to 90 kOe. It was found that J(c) of the films obtained by PLD usually overcomes the J(c) values of the sputtered films in the whole investigated (H, T) domain.

The relaxation of the irreversible magnetization in optimally doped YBCO films with natural and artificial pinning centres was measured in zero-field cooling conditions using SQUID magnetometry. The external magnetic field H was oriented along the c axis. An appropriate method for the determination of the characteristic vortex pinning energy from the normalized vortex-creep activation energy is discussed. This is based on the existence of a crossover elastic (collective) vortex creep at low temperatures T - plastic vortex creep at high T, caused by the T dependent macroscopic currents induced in the sample during magnetization measurements.

The temperature T variation of the normalized magnetization relaxation rate S in high-temperature superconductors (HTS) with strong vortex pinning exhibits a maximum in the low-T range. This was reported for various HTS, and the origin of the faster relaxation at low T appearing in standard magnetization relaxation measurements was usually related to specific pinning properties of the investigated specimens. Since the observed behaviour seems to be characteristic to all HTS with enhanced pinning (generated by random and/or correlated disorder), we show that the S(T) maximum can be explained in terms of classic collective vortex creep. The influence of thermo-magnetic instabilities in the low-T range is also evidenced. The collective (elastic) creep regime is generated by the T dependent macroscopic currents induced in the sample during standard magnetization measurements. (C) 2010 Elsevier B.V. All rights reserved.

We investigated the influence of fullerene (C-60) addition on the superconducting parameters of MgB2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5 wt% C-60 addition leads to the reduction of the critical temperature by a few K, whereas the magnetically determined critical current density increases by approximately one order of magnitude in an applied magnetic field H = 70 kOe at temperature T = 10 K. This indicates substitution of B by C in C-60 added MgB2 processed by SPS, contrary to some data reported in the literature obtained using conventional methods. For our relatively large specimens, the occurrence of macroscale flux jumps was observed over a wide (H, T) domain. The relaxation of the irreversible magnetization suggests that the macroscale flux jumps transform into microscale flux jumps with increasing H and T

The transport properties of the gamma-irradiated CrO2-polymer composites were investigated. The resistance R is strongly current I dependent, except at high temperature where it displays a semiconductor-like temperature dependence for all currents. At low currents, I <= 10 mu A, as the temperature decreases far below the ferromagnetic transition, the resistance decreases first, then reaches a minimum, and at last increases again toward a second peak. At even lower temperatures, the second peak is followed by a metallic-like temperature dependence of R that ends at a cusp point marking the metal-insulator transition. The increase of the current shifts the cusp toward lower temperatures and the first minima toward higher temperatures. The resistance increases with the increase in current for all currents in the range 0.2 <= I = 50 mu A, the resistance increases monotonically with the decrease in temperature in the whole temperature range but obeys different laws at low and high temperatures. An explanation attempt in terms of spin transport, disorder, and thermal effects is proposed. (C) 2009 Elsevier B.V. All rights reserved.

We present the fabrication and transport properties of a series of composites made of La2/3Sr1/3MnO3 and acrylamide-based copolymers. The most important result is the very narrow transition, of only 27 K, displayed by the peak that appears around the metal-insulator transition of the composites made with poly(acrylamide-vinylacetate). Although the amount of polymer is rather low, different copolymers change drastically the electric transport characteristics. Copyright (C) 2009

We investigated the transport properties of a series of composites made of La2/3Sr1/3MnO3 and different copolymers. The temperature dependence of the electrical resistance of the samples is strongly dependent on the nature of the polymers. We found four types of temperature dependences of the electrical resistance: i) simple current dependence of resistivity for poly(methyl-methacrylate-co-styrene) copolymers, ii) single peaked characteristics for poly(acryl amide-co-vinyl acetate), iii) multipeaked dependence found in poly(methyl-methacrylate-co-butadiene) and star type polysiloxane-g-styrene, and iv) current independent characteristics visible in the samples made with linear polysiloxane-based composites.

The relaxation of the irreversible magnetization of MgB2 bulk samples obtained by electric-field assisted sintering was investigated using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. We observed a crossover plastic creep at high temperatures T-elastic creep at low T, described by H alpha T-2 in the low T range, which appears to be caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can easily be explained through the occurrence of micro flux jumps, leading to a finite magnetization relaxation rate in the low-T limit. (C) 2008 Elsevier B.V. All rights reserved.

Well-compacted MgB2 specimens with the density higher than 90 % of the theoretical value were obtained by electric-field assisted sintering. This method assures a good grain connectivity, which leads to the appearance of efficient pinning centres at the grain boundaries. We measured the DC magnetization curves and the relaxation of the irreversible magnetization using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. The critical current density is of the order of 1010 A/m(2) at H = 20 kOe and T = 10 K. A crossover plastic creep at high temperatures T - elastic creep at low T described by H proportional to T-2 in the low T - high H domain was observed. This is caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can be explained through the occurrence of micro flux jumps, which seem to be responsible for the finite magnetization relaxation rate in the low-T limit. The relaxation of the irreversible magnetization allowed us the precise determination of the characteristic pinning energy barrier.