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 investigated the AC magnetic response of a YBa ₂ Cu ₃ O ₇ film with embedded BaZrO ₃ nanorods and Y ₂ O ₃ nanoparticles at a static magnetic field H _dc lower than the matching field H _Φ. 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 YBa ₂ Cu ₃ O ₇ thin films as determined by microwave impedance measurements.

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(As_0.68P_0.32)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.

Among various “families” of iron-based superconductors, the quite recently discovered AeAFe₄As₄ (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.

Among many iron-based superconductors, isovalently substituted BaFe₂(As_1−xP_x)₂ displays, for x ≈ 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.

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.