Jaroslaw Pietosa

Hydrostatic pressure impact on the exchange bias effect as well as specific heat were studied in helical magnets LaxBa1−xFeO3 (x = 0.125, 0.25, 0.33) exhibiting the phase-separated ground state composed of the ferromagnetic and helical antoferromagnetic phases, with the phase ratio varying with doping level x. Under pressure of 10 kbar, the antiferromagnetic component, comprised of helical spin order, becomes stronger for all investigated substitutions. The exchange bias field, HEB, was found to increase under pressure and with increasing cooling field, Hcool, for all investigated substitutions. The enhancement of exchange bias field under pressure is caused by the partial transformation of the ferromagnetic phase into the antiferromagnetic helical spin structure. Specific heat studies revealed an anomaly at 168 K related to the phase transition to the antiferromagnetic state. The size of isolated ferromagnetic clusters embedded in antiferromagnetic matrix was found to decrease from 4.2 nm at ambient pressure to 3.3 nm under the external pressure of 10 kbar.

Second-generation high-temperature superconducting tapes (2G HTS; SuperPower Inc., Glenville, NY, USA) based on GdBCO (GdBa₂Cu₃O_7−δ, where δ denotes oxygen deficiency) were aged at −26.4 °C, +2 °C, and room temperature (RT) to evaluate the degradation of their superconducting properties. HTS tapes stored at RT exhibited a significantly higher deterioration rate compared to those maintained at lower temperatures. Laser-induced breakdown spectroscopy (LIBS) analysis demonstrated a gradual reduction in the effective chemical depth-profiling length over time, indicating a correlation between the degradation mechanism and the reduction in the effective volumetric density of the GdBCO superconducting layer. These findings imply that oxygen diffusion or redistribution processes substantially contribute to the long-term degradation of GdBCO-based HTS tapes.

Magnetic measurements under hydrostatic pressure (P) up to 10 kbar and x-ray diffraction measurements up to 10.7 kbar were performed for a single crystal of Fe_0.975Cu_0.025Te_0.66Se_0.34, which is non-superconducting at ambient pressure. In this compound, we have found pressure-induced recovery of the superconducting state at T_c equal to 13.0 K for P = 10 kbar. We determined the parameters characterizing the superconducting state, including the lower and the upper critical fields, coherence length, and penetration depth, and compared them with those for Fe_0.99Te_0.66Se_0.34. We found that the lower critical field for Fe_0.975Cu_0.025Te_0.66Se_0.34 at 0 K and 10 kbar is comparable to the lower critical field for Fe_0.99Te_0.66Se_0.34 at ambient pressure, while the upper critical field is significantly higher than that for Fe_0.99Te_0.66Se_0.34 at ambient pressure. The estimated increase in superconducting carrier density and effective mass under pressure can be explained if one assumes that applied pressure leads to an increase in structural disorder in the studied material. At 10 kbar, the zero-field critical current density for Fe_0.975Cu_0.025Te_0.66Se_0.34 is four times larger than that for Fe_0.99Te_0.66Se_0.34 at ambient pressure. The x-ray diffraction results indicate that under pressure crystal quality apparently degrades. Comprehensive studies of the impact of pressure on the crystal structure indicate an increasing mosaicity evolution with pressure, suggesting that the pressure-induced superconductivity of Fe_0.975Cu_0.025Te_0.66Se_0.34 originates from inhomogeneities, associated also with the superconductivity in other sulpho-iron seleno-tellurides and anti-PbO-type structures. Obviously, the pressure effect on the crystallographic structure can lead to changes in the electronic structure, which is not excluded.

Magnetic measurements under hydrostatic pressure ( P ) up to 10 kbar and x-ray diffraction measurements up to 10.7 kbar were performed for a single crystal of Fe _0.975 Cu _0.025 Te _0.66 Se _0.34 , which is non-superconducting at ambient pressure. In this compound, we have found pressureinduced recovery of the superconducting state at T _c equal to 13.0 K for P = 10 kbar. We determined the parameters characterizing the superconducting state, including the lower and the upper critical fields, coherence length, and penetration depth, and compared them with those for Fe _0.99 Te _0.66 Se _0.34 . We found that the lower critical field for Fe _0.975 Cu _0.025 Te _0.66 Se _0.34 at 0 K and 10 kbar is comparable to the lower critical field for Fe _0.99 Te _0.66 Se _0.34 at ambient pressure, while the upper critical field is significantly higher than that for Fe _0.99 Te _0.66 Se _0.34 at ambient pressure. The estimated increase in superconducting carrier density and effective mass under pressure can be explained if one assumes that applied pressure leads to an increase in structural disorder in the studied material. At 10 kbar, the zero-field critical current density for Fe _0.975 Cu _0.025 Te _0.66 Se _0.34 is four times larger than that for Fe _0.99 Te _0.66 Se _0.34 at ambient pressure. The x-ray diffraction results indicate that under pressure crystal quality apparently degrades. Comprehensive studies of the impact of pressure on the crystal structure indicate an increasing mosaicity evolution with pressure, suggesting that the pressure-induced superconductivity of Fe _0.975 Cu _0.025 Te _0.66 Se _0.34 originates from inhomogeneities, associated also with the superconductivity in other sulpho-iron selenotellurides and antiPbO-type structures. Obviously, the pressure effect on the crystallographic structure can lead to changes in the electronic structure, which is not excluded.

Fe-Co alloys are the most important soft magnetic materials, which are successfully used for a wide range of applications. In this work, the magnetic properties of lanthanide-substituted (Fe_0.65Co_0.35)_0.95(RE₂O₃)_0.05 (RE = La, Nd, and Sm) nanoparticles, prepared by mechanical alloying, are reported. Our comprehensive studies (X-ray diffraction, Mössbauer spectroscopy, scanning electron microscopy with X-ray energy dispersive spectrometry, SQUID magnetometry and differential scanning calorimetry) have revealed different properties, depending on the dopant type. The RE₂O₃ addition led to a decrease in the crystallite size and to an increase in the internal microstrain. Moreover, because of the high grain fragmentation tendency of RE₂O₃, the cold welding between Fe–Co ductile particles was minimized, indicating a significant decrease in the average particle size. The parent Fe_0.65Co_0.35 alloy is known for its soft ferromagnetism. For the La-substituted sample, the magnetic energy product was significantly lower (0.450 MG·Oe) than for the parent alloy (0.608 MG·Oe), and much higher for the Sm-substituted compound (0.710 MG·Oe). The processing route presented here, seems to be cost-effective for the large-scale production of soft magnetic materials.

The results of colossal magnetoresistance (CMR) properties of La_1-xSr_xMn_yO₃ (LSMO) films grown by the pulsed injection MOCVD technique onto an Al₂O₃ substrate are presented. The grown films with different Sr (0.05 ≤ x ≤ 0.3) and Mn excess (y > 1) concentrations were nanostructured with vertically aligned column-shaped crystallites spread perpendicular to the film plane. It was found that microstructure, resistivity, and magnetoresistive properties of the films strongly depend on the strontium and manganese concentration. All films (including low Sr content) exhibit a metal–insulator transition typical for manganites at a certain temperature, T_m. The T_m vs. Sr content dependence for films with a constant Mn amount has maxima that shift to lower Sr values with the increase in Mn excess in the films. Moreover, the higher the Mn excess concentration in the films, the higher the T_m value obtained. The highest T_m values (270 K) were observed for nanostructured LSMO films with x = 0.17–0.18 and y = 1.15, while the highest low-field magnetoresistance (0.8% at 50 mT) at room temperature (290 K) was achieved for x = 0.3 and y = 1.15. The obtained low-field MR values were relatively high in comparison to those published in the literature results for lanthanum manganite films prepared without additional insulating oxide phases. It can be caused by high Curie temperature (383 K), high saturation magnetization at room temperature (870 emu/cm³), and relatively thin grain boundaries. The obtained results allow to fabricate CMR sensors for low magnetic field measurement at room temperature.

Properties of FeTe_0.65Se_0.35 single crystals, with the onset of critical temperature (T_c^onset) at 15.5 K, were modified via hydrogenation performed for 10–90 h, at temperatures ranging from 20 to 250 °C. It was found that the tetragonal matrix became unstable and crystal symmetry lowered for the samples hydrogenated already at 200 °C. However, matrix symmetry was not changed and the crystal was not destroyed after hydrogenation at 250 °C. Bulk T_c^bulk, determined at the middle of the superconducting transition, which is equal to 12–13 K for the as grown FeTe_0.65Se_0.35, rose by more than 1 K after hydrogenation. The critical current density studied in magnetic field up to 70 kOe increased 4–30 times as a consequence of hydrogenation at 200 °C for 10 h. Electron paramagnetic resonance measurements also showed higher values of T_c^bulk for hydrogenated crystals. Thermal diffusion of hydrogen into the crystals causes significant structural changes, leads to degeneration of crystal quality, and significantly alters superconducting properties. After hydrogenation, a strong correlation was noticed between the structural changes and changes in the parameters characterizing the superconducting state.

This article reports on the non-trivial suppression of superconductivity in the Eu_1−xPr_xBCO cuprates. As non-magnetic Eu³⁺ ions are replaced by Pr³⁺ carrying a magnetic moment, spin-related superconductivity loss can be expected. The research shows that the superconductivity disappearance for x > 0.4 results from depletion of the carriers and their localization. The above conclusion was drawn by low-temperature X-ray diffraction analysis showing increased characteristic phonon frequencies with Pr content. This mechanism should promote electron–phonon coupling, at least for acoustic phonons. However, the inverse phenomenon was detected. Namely, there is a gradual deterioration of the optical phonons responsible for vibration of the Cu–O bonds with Pr increasing, as evidenced by Raman spectroscopy. Furthermore, the results of X-ray absorption spectroscopy precisely showed the location of the carriers for Pr-rich specimens. Finally, a schematic diagram for Eu_1−xPr_xBCO is proposed to consolidate the presented research.

Detailed investigations of Ni substituted Fe_0.994Ni_0.007Te_0.66Se_0.34 and unsubstituted Fe_0.99Te_0.66Se_0.34 crystals were performed at ambient pressure and under hydrostatic pressure in order to understand the complex relationship between structure, magnetism, and superconductivity in iron selenides. The aim of the investigations was to determine the structural and superconducting state properties and study their evolution under pressure, up to 11 kbar. A comparison of Fe_0.99Te_0.66Se_0.34, a material with much less developed phase separation, with FeTe_0.5Se_0.5, a material exhibiting pronounced nanoscale phase separation, was also performed. Under ambient pressure, a weakening of superconducting state properties was observed in the Fe_0.994Ni_0.007Te_0.66Se_0.34 crystal, with disorder introduced by Ni substitution, as compared with those in Fe_0.99Te_0.66Se_0.34. For Fe_0.994Ni_0.007Te_0.66Se_0.34, the x-ray diffraction studies have revealed a degradation of crystal quality under applied elevated pressure. Superconducting state properties of the single phase Fe_0.99Te_0.66Se_0.34 crystal, such as upper and lower critical fields, were found to be poorer, at both ambient and hydrostatic pressure, than those observed for FeTe_0.5Se_0.5 crystals exhibiting pronounced nanoscale phase separation. Comprehensive studies of the impact of pressure on crystal structure and on superconducting state properties confirm that enhancement of superconductivity correlates with the appearance of mosaicity under pressure. The above observation leads to the expectation of a pressure-induced increase in inhomogeneity, leading to the enhancement of superconducting properties in other sulpho-iron seleno-tellurides of anti-PbO-type structure.