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(Se -x 1 Te 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, U H , 0 ( ) whereas the activation energy as a function of temperature, U T , ( ) 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.
The magnetic behavior of an iron-based FeSe crystal sample has been studied by means of dc magnetization measurements as a function of the temperature (T), the dc magnetic field (H) and the time (t). The M(T) curves show a discrepancy in the determination of the onset of the critical temperature T C with respect to what is observed in the superconducting M(H) measurements obtained by subtracting the ferromagnetic background from the curves measured at various temperatures. By using magnetic relaxation measurements M(t), the correct value of T C has been obtained. Moreover, the superconducting M(H) loops show the presence of a noisy signal up to an anomalous 'peak effect' only found for positive and negative increasing fields. These features have been analyzed by fitting the temperature dependence of the critical current density J c (T), extracted from the M(H) loops, with the help of the J c (T) dependencies governing an S-N-S junction network. This analysis has allowed us to interpret the behavior found in the M(H) loops and to obtain the value of the intrinsic critical current density J 0 which is not influenced by the presence of the junctions.
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 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 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 Hcr(T). The Hcr(T) values have been fitted with the equation Hcr(T) = Hcr(0) (1 − T/T*)n whose results confirm the correlation between the elastic/plastic crossover and the end of the peak effect phenomenon.
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 Jc(H,T) extracted from the superconducting hysteresis loops at different temperatures within the Bean critical state model. Moreover, the combined analysis of the Jc(T) and of the pinning force Fp(H/Hirr) 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 Tc materials.
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