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We compare the temperature and field dependence of the critical current densities of high- $T_c$ superconductor materials intended for various bulk applications such as trapped-field magnets. This comprises bulk samples of YBa $_2$ Cu $_3$ O $_x$ (YBCO), MgB $_2$ , and iron-based materials, including also various versions of the YBCO compound such as melt-textured ones, infiltration-growth processed ones, and YBCO foams. Critical current densities and flux pinning forces were obtained from magnetization loops measured using Quantum Design SQUID and physical property measurement system (PPMS) systems with applied magnetic fields of up to $\pm$ 9 T. The obtained data are compared to each other with respect of the optimal cooling temperature possible using modern cryocoolers. Furthermore, we plot the temperature dependencies of the critical current densities versus the normalized temperature $t=T/T_c$ . This enables a direct judgement of the performance of the material in the trapped-field applications.

International audience; The FeSe compound is the simplest high-temperature superconductor (HTSc) possible, and relatively cheap, not containing any rare-earth material. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing FeSe using solid-state sintering yields samples exhibiting strong ferromagnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above Tc. Due to the complicated phase diagram, the samples are a mixture of several phases, the superconducting beta-FeSe, and the non-superconducting delta-FeSe and gamma-FeSe. Furthermore, antiferromagnetic Fe7Se8 and ferromagnetic alpha-Fe may be contained, depending directly on the Se loss during the sintering process. Here, we show MHLs measured up to ±7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T ) measurements. We also performed a thorough analysis of the microstructures in order to establish a relation between microstructure and the resulting sample properties.

International audience; Various MgB2 thin films and single crystals were found in the literature to exhibit a sharp, narrow peak at low fields in the volume pinning force, Fp(H)-diagrams. The origin of this peak is associated with a steep drop of the current density when applying external magnetic fields and is ascribed to sample purity. We show here that bulk MgB2 prepared by spark-plasma sintering also shows the sharp, narrow peak in Fp. The peak is also seen in the volume pinning force scaling, Fp/Fp,max vs h = H/H irr. Furthermore, polycrystalline bulk MgB2 samples prepared close to the optimum reaction temperature reveal this peak effect as well, but other samples of the series show a regular scaling behavior. The combination of magnetization data with data from electric transport measurements on the same samples demonstrates the origin of this peak effect. On increasing preparation temperature, the pinning force scaling changes from grain boundary pinning to point pinning and the grain connectivity gets worse. Hence, the sharp, low-field peak in Fp vanishes. Therefore, the occurrence of the peak effect in Fp gives important information on the grain coupling in the MgB2 samples.

International audience; The current flow and the flux pinning properties on struts of superconducting YBa2Cu3Ox (YBCO) foams are analyzed in detail in the temperature range 60 K < T < Tc. For this purpose, magnetization loops were measured taken from various positions of a 5 × 2 × 2 cm3 large foam sample prepared at RWTH Aachen. From these data, the critical current densities, jc, and the flux pinning forces, Fp = jc × B, were calculated and pinning force scaling diagrams Fp/Fp,max vs. h = Ha/Hirr were established. The scaling in the temperature range 60 K < T < 90 K was found to be well developed for all samples with peak positions, h0, above 0.4, which is an indication of δTc-pinning. From microstructure analysis of the foam struts employing the electron backscatter diffraction technique (EBSD) [1] it is known that the struts are fully converted into the 123-phase with embedded 211 particles being relatively small. These small 211 particles and their specific distribution within the samples play an important role to explain the observations. The variation of Fp depending on the position of the struts in the original sample directly reveals the changes of the texture as found employing EBSD.[1] M.R. Koblischka, A. Koblischka-Veneva, E. S. Reddy, G. Schmitz, J. Adv. Ceram. 3, 317 (2014).