The correlation in type-II superconductors between the creep rate S and the Second Magnetization Peak (SMP) phenomenon which produces an increase in J c , as a function of the field (H), has been investigated at different temperatures by starting from the minimum in S(H) and the onset of the SMP phenomenon detected on a FeSe 0.5 Te 0.5 sample. Then the analysis has been extended by considering the entire S(H) curves and comparing our results with those of many other superconducting materials reported in literature. In this way, we find evidence that the flux dynamic mechanisms behind the appearance of the SMP phenomenon in J c (H) are activated at fields well below those where the critical current starts effectively to increase. Moreover, the found universal relation between the minimum in the S(H) and the SMP phenomenon in J c (H) shows that both can be attributed to a sequential crossover between a less effective pinning (losing its effectiveness at low fields) to a more effective pinning (still acting at high fields), regardless of the type-II superconductor taken into consideration.

The relaxation phenomena of a FeSe0.5Te0.5 single crystal sample, with twinned morphology, exhibiting a second magnetization peak effect have been analyzed by means of dc magnetization measurements as a function of magnetic field (H). Using different magnetic field sweep rates for performing the superconducting hysteresis loops m(H), a change in the width of these curves has been observed together with a shift of the second peak field position. A consequent and significant variation of critical current density J(c) has been extracted from the superconducting m(H) curves within the Bean critical state model. In this framework, the ratio between the critical current densities J(c) calculated at 200Oe/s and 5Oe/s has been calculated showing interesting results in terms of transport capabilities. From the J(c)(H) curves at T=10K for different sweep rates, the irreversibility field has been obtained for T=10K and fitted with a power law. The increasing trend of the irreversibility field as a function of sweep rate could suggest the use of this material for power applications where overheating due to vortices movement inside the superconductor has to be limited.

We have performed DC magnetic moment measurements as a function of magnetic field m(H) and time m(t) on a FeSe o.94 and a silver doped FeSe o.94 + 6 wt% Ag. In particular, from the m(H) measurements, the field dependence of the critical current density J c (H) of the samples has been extracted at different temperatures in the framework of the Bean critical state model. Starting from the J c (H) of the samples, we have studied the influence of the silver doping on the temperature dependence of the irreversibility field H irr (T) above which the pinning energy is so reduced that the vortices are free to move. The obtained H irr (T) values have been fitted with the equation H irr (T) = H irr (0) (1-T/T*) n whose results have been compared with the literature. After that, we have analyzed the silver doping effects on the field dependence of the pinning energy U(H), extracted from relaxation measurements at different temperatures and magnetic fields, obtaining that both samples undergo to a pinning regime crossover.

The fundamental harmonic of the AC magnetic susceptibility of a Fe(Se,Te) single crystal iron based superconductor has been studied at different AC field amplitudes and frequencies with and without a superimposed DC magnetic field with the aim of analyzing its superconducting properties that can be also suitable for power applications such as the critical current density and the pinning energy in AC regime. From the analysis of the fundamental Cole-Cole plots a finite influence of flux creep phenomena has been detected. Its study, by means of the Arrhenius plots, evidences thermal activation energy U values as high as 10(3)K, whose dependence on the DC and AC fields, and of the temperature, has been also obtained. The magnetic field dependence shows the existence of the strong pinning regime characterized by plastic deformations of the vortex lattice, whereas the temperature dependence of the critical current density J(c)(T) is consistent with the delta l pinning model. Finally, the irreversibility field values H-irr(T) have been extracted from the J(c)(T) at the different AC frequencies showing very high values if compared with the literature.

The AC magnetic response of multi-domain FeSe crystals has been explored by means of fundamental and third harmonic AC magnetic susceptibility (ACMS) analysis. Our previous studies have revealed a complex morphology which especially modifies the mixed state properties. The effects were expressed by an additional 'pseudo' peak feature of the magnetic hysteresis and in the present study we analyze its vortex dynamics nature and irreversibility behavior in the context of ACMS. We find that the effect is detectable mainly with the sensitive third harmonic component and especially at high AC field frequencies. From the analysis, the temperature dependence of the 'pseudo' peak effect at different DC fields ranges was determined, confirming the previously established superconductor-normal metal-superconductor type links between the domains in the crystal. The irreversibility line, the most important parameter for high power applications, shows a typical glass/liquid transition in the vortex matter. In addition we have observed surface barrier effects and vortex avalanche activity, the behavior of which appears to be influenced by the superconducting and magnetic nature coexistence and morphology. The presented results show the effective application of the harmonic AC magnetic susceptibility technique for a versatile analysis of complex nonlinear phenomena in materials with a sophisticated AC magnetic response.

We report the analysis of the magnetic response detected on the cuprate superconductor Nd1.85Ce0.15CuO4. In particular the magnetic behavior of the sample has been studied by means of DC magnetization measurements as a function of the temperature (T) and DC magnetic field (H). The superconducting critical temperature T-c has been obtained by analyzing the m(T) curve performed in Zero Field Cooling-Field Cooling conditions. Moreover, the m(T) curve shows the presence of a magnetic background for temperatures above T-c. By considering the superconducting m(H) hysteresis loop at different temperatures, it can be noted that the width of the curves appears narrow corresponding to a weak superconductivity. This is confirmed by the field dependence of the critical current densities J(c) extracted from the superconducting hysteresis loops m(H) at different temperatures within the Bean critical state model. In fact, at the lowest measurement temperature, J(c) is close to zero already at low magnetic fields. Nevertheless, by means of the temperature dependence of J(c), the sample shows a strong pinning behavior that can open perspectives for future improvement in the fabrication of this material.

The iron based superconductor FeSe0.5Te0.5 has been investigated by means of DC magnetic measurements as a function of magnetic field (H). By considering the superconducting m(H) hysteresis loops at different temperatures, the sample shows a strong superconducting signal together with the presence of a peak effect that causes an anomalous increase in the field dependence of the critical current density Jc(H). The presence of the peak effect has been studied by means of the Jc(T) obtained at different magnetic fields starting from the Jc(H) curves. The analysis of the Jc(T) curves shows that the peak effect is due to a crossover from a weak pinning regime to a strong pinning regime.

The measurements of DC magnetization M as a function of magnetic field (H) and time (t) have been performed in order to study the superconducting and pinning properties of a Fe(Se, Te) iron based superconductor fabricated by means of the Bridgman technique. By performing the superconducting hysteresis loops M(H) at different temperatures in the case of perpendicular and parallel field, the critical current density Jc (H) has been extracted in the framework of the Bean critical state model for both configurations. The Jc (H) curves have shown the presence of the second magnetization peak effect that causes an anomalous increase in the field dependence of the critical current density. In order to obtain the Jc anisotropy of the sample, we have performed the ratio between perpendicular and parallel critical current density values [Formula: see text] and compared its values with the literature ones. The information regarding the pinning energy U have been extracted by means of the relaxation of the irreversible magnetization M(t) in the case H∣∣c. In particular, performing relaxation measurements at different temperatures and magnetic fields, the temperature dependence of the pinning energy U(T) at different magnetic fields has been obtained showing an anomalous temperature scaling of the curves. The presence of a maximum in the U(T) curves suggests a pinning crossover at a given field and temperature H cr(T). The H cr(T) values have been fitted with the equation H cr(T) = H cr(0) (1 - T/T*) n whose results confirm the correlation between the elastic/plastic crossover and the end of the peak effect phenomenon.

The possible evidence of a peak effect in the temperature dependent magnetization and in the field dependent critical current density has been found for a FeSeTe superconducting granular sample in presence of an ac field. The existence of the peak effect has been confirmed by the numerical calculation of the inter- and intragranular magnetizations in presence of a demagnetization field with components at the third harmonic frequency generated by the sample magnetization. This suggests that the peak effect could be related to the flux dynamics inside the sample through the demagnetization field third harmonic.

The ferromagnetic transition temperature ( T c ) of a weak ferromagnetic thin film Cu 0.38 Ni 0.62 has been estimated by using an evolution of the Belov, Goriaga, and Arrott (BGA) model. In fact, the typically used measurements of the zero-field-cooled-field-cooled magnetic moment ( m ) as a function of temperature ( T ), the magnetic hysteresis loops ( m ( H )) as a function of field, and the temperature dependence of the remanent magnetic moment, performed on our sample, did not allow us to individuate precisely the value of T c , mainly due to the influence of thermal effects and noise on the weak ferromagnetic response of the sample, resulting in a large uncertainty in the estimation of T c . On the other hand, also the commonly used method of determining the T c by means of a linear extrapolation of the high-field region of m 2 as a function of the normalized magnetic field ( H / m ) at different temperatures (Arrott curves), working though for our analyzed material, could not completely fulfill the basic assumptions of the BGA model in general. For these reasons, we have considered the analysis of the derivatives of the Arrott curves, starting from observing that the T c of the sample corresponds to the temperature where the curvature of the Arrott curves at low fields inverts. In this way, the sensitivity in the determination of the Curie point from the inductive measurements of the magnetic moment is improved while the correct assumptions of the Arrott model are also fully respected.

We present the fabrication of FeSe 0.5 Te 0.5 polycrystalline samples by self-flux method, showing the presence of the peak-effect in the vortex lattice configuration. To improve the performances at high magnetic fields for this iron-based superconductor of the 11-family, the two-step solid-state reaction process has been successfully modified by increasing the temperature of the heat treatment and by slowing down the cooling rate. The magnetic field-temperature phase diagram has been investigated by magneto-resistance, magnetization, and heat capacity measurements in applied magnetic fields up to 18 T. The magnetization curves exhibit an enhancement of the peak-effect whose position shifts by varying the temperature, following a similar dependence than that previously reported in high-temperature superconducting materials such as YBa 2 Cu 3 O 7-δ . The presence of the peak-effect can be correlated to the sample manufacture, since by tuning a proper heat treatment it becomes observable ever more in the magnetic field-temperature phase diagram. This fabrication route paves the way to a systematic increase in the critical current density thus becoming relevant for applications.

The correlation between the appearance of a peak effect in the critical current of a superconducting material and the presence of twin boundaries, involved in a crossover between different pinning regimes, is investigated by means of dc magnetic measurements on a FeSe0.5Te0.5 crystal. In particular, by analyzing the temperature dependence of the critical current density Jc(T) for different magnetic fields H, a crossover from a weak pinning regime to a strong pinning regime has been revealed. The analysis shows that this crossover can be ascribed to the presence of twin boundary defects inside the sample, and can be associated to the onset of the peak effect and interpreted as the start of the vortex dynamic processes responsible for the increase of Jc with the field. On the basis of the information extracted by our analysis, a plausible dynamic scenario involving the contribution of the different pinning regimes depending on the applied field has been described, and the relative H(T) vortex phase diagram has been determined. Moreover, in our description, the peak in the Jc(H) curve corresponds to the end of the processes leading to the peak effect and it is confirmed to be related to the transition from an elastic to a plastic deformation regime in the vortex lattice.

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 J(c) 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 H-c2, points out that the Bridgman method is most attractive for exploiting superconducting and transport properties in view of applications. Published by AIP Publishing.

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 J(c)(H, T) extracted from the superconducting hysteresis loops at different temperatures within the Bean critical state model. Moreover, the combined analysis of the J(c)(T) and of the pinning force F-p(H/H-irr) 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 T-c materials.

The first harmonics of the magnetic response, of inter- and intragranular volume fractions of a FeSe0.5Te0.5 granular superconducting sample in an alternate magnetic field, have been numerically extracted from the measured magnetization of the whole system by using a model which considers the effects of the fundamental demagnetization fields generated by the magnetization itself. This allowed us to obtain the temperature and field behaviors of the inter- and intragranular ac losses during an ac cycle. These parameters have been found sensitive to the existence of the third harmonic components of the demagnetization field produced by the corresponding magnetization harmonics of the sample. This confirms that the effects of the magnetic response third harmonic components on the sample magnetization, through the demagnetization field, should be taken into account for a more detailed analysis of the measured ac magnetic curves.

Vortex dynamics is strongly connected with the mechanisms responsible for the photon detection of superconducting devices. Indeed, the local suppression of superconductivity by photon absorption may trigger vortex nucleation and motion effects, which can make the superconducting state unstable. In addition, scaling down the thickness of the superconducting films and/or the width of the bridge geometry can strongly influence the transport properties of superconducting films, e.g. affecting its critical current as well as its switching current into the normal state. Understanding such instability can boost the performances of those superconducting devices based on nanowire geometries. We present an experimental study on the resistive switching in NbN and NbTiN ultra-thin films with a thickness of few nanometers. Despite both films were patterned with the same microbridge geometry, the two superconducting materials show different behaviors at very low applied magnetic fields. A comparison with other low temperature superconducting materials outlines the influence of geometry effects on the superconducting transport properties of these materials particularly useful for devices applications.

The stability against quench is one of the main issues to be pursued in a superconducting material, which should be able to perform at very high levels of current densities. Here we focus on the connection between the critical current I-c and the quenching current I* associated to the so-called Flux-Flow Instability phenomenon, which sets-in as an abrupt transition from the flux flow state to the normal state. To this purpose, we analyze several current-voltage characteristics of three types of Iron-Based thin films, acquired at different temperature and applied magnetic field values. For these samples, we discuss the impact of a possible coexistence of intrinsic electronic mechanisms and extrinsic thermal effects on the quenching current dependence upon the applied magnetic field. The differences between the quenching current and the critical current are also reported in the case of predominant intrinsic mechanisms. Carrying out a comparison with the HTS case, we suggest, which material can be the best tradeoff between maximum operating temperature, higher upper critical field, and stability under high current bias.

A model has been developed to determine the effective ac magnetic response of magnetic systems, taking into account the demagnetization effects arising from the sample geometry which determine the out-of-phase components of the applied fundamental frequency and higher harmonic components. Indeed, demagnetization fields and their intermodulation can significantly affect the ac magnetic response. This approach provides a system of self-consistent linear equations relating the magnetic response to the external magnetic field by means of nonlinear magnetic susceptibility. The model is extended to the magnetic response of granular systems in terms of the contributions of the individual grains and of the whole sample in the presence of demagnetization effects of the whole sample and of the grains on a macroscopic scale. In particular, our model is applied to a granular superconducting system. The comparison between the performed numerical simulations and the experimental data shows that the demagnetization fields of the single grains and of the whole sample, and their intermodulation, are relevant if magnetic measurements are used to extract detailed information about the analyzed material.

The melilite-type oxides are potential targets for exploring interesting magnetic and electronic properties as well as multiferroicity and magnetoelectric effects. Polycrystalline samples of Ba2Cu1−xMnxGe2O7 have been synthesized by solid state reaction method. The morphology and chemical composition of the samples have been investigated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). By using powder X-ray diffraction, the phase composition of the synthesized compounds and the evolution of their crystallographic axes as a function of the doping have been systematically studied. The synthesis of the polycrystalline compounds reported in this work is a prerequisite for the growth of high quality single crystals of mixed melilite-type oxides essential for the investigations of the complex magnetic phase diagram of these non-centrosymmetric systems.

High-temperature superconductivity in cuprate materials is achieved by hole or electron doping of the parent Mott insulator. In this paper, electron-doped Nd2−xCexCuO4±δ films with different content of cerium have been grown on (010) SrTiO3(TiO2) substrates through a dc sputtering technique. Since the normal conducting films can be obtained by controlling the oxygen content inside the crystalline structure by means of appropriate annealing procedures (that allow a reduction process), different as-grown nonsuperconducting films have been obtained. The samples have been thermally treated ex-situ at different temperatures and atmospheres in order to investigate the effects of different content of oxygen on the films properties. The results of the composition analysis, of the structural characterization and of the electric transport measurements on samples with a Ce fraction of 0 and 0.15 will be discussed.