Functional Oxide Layers and Superconductors

T. Espenhahn, M. Sparing, A. Berger, K. Nielsch, R. Hühne, Supercond. Sci. Technol. 34 (2021) 125004.

Superconducting magnetic bearings enable friction-free rotation, which is beneficial for different applications. Whereas mechanical losses are drastically reduced, additional hysteretic losses might be generated as for example by a process-related tilt of the field-cooled permanent magnet of the superconducting bearing. We designed a caloric measurement setup with a resolution down to 5 mW in order to study such losses in more detail. Therefore, a field-cooled magnet ring is deliberately tilted up to 3°before it rotates with a speed of up to 5000 rpm above the superconducting ring. The generated losses inside the superconductor lead to an increased evaporation rate in the attached liquid nitrogen bath, which in turn results to a measurable volume flow. The determined losses increase almost linear with speed, whereas an exponential increase was observed for the tilt angle. The results were confirmed by 2D simulations using a two-component model leading to similar dependencies for the hystertic losses on speed and tilt.

B. Magalhaes, S. Engelhardt, C. Molin, S.E. Gebhardt, K. Nielsch, R. Hühne, J. Alloys Com. 891 (2021) 162041.

The basic aim of our study is to investigate the correlation between structural parameters and the electrocaloric effect in lead-free epitaxial Ba_1−xSr_xTiO₃ (BSTO) based thin film architectures. Therefore, BSTO thin films with Sr contents of x = 0 to x = 0.3 were grown on SrRuO₃ buffered SrTiO₃ single crystalline substrates by pulsed laser deposition. Structural characterization verified an epitaxial growth for all Sr contents with an additional tetragonal distortion compared to bulk material. Temperature and frequency- dependent measurements of dielectric properties revealed increased permittivity values for thicker films with broad maxima indicating a diffuse phase transition. The temperature of maximum permittivity decreases with increasing Sr content, whereas polarization measurements indicate a relaxor-like behaviour in particular above room temperature. Adiabatic temperature changes were determined with the indirect method resulting in |ΔT| values of up to 2.9 K for a 680 nm thick BSTO layer with x = 0.3 at an applied electric field of 750 kV cm⁻¹.

S. Holleis, A. Anna Thomas, I.A. Shipulin, R. Hühne, A. Steiger-Thirsfeld, J. Bernardi, M. Eisterer, Supercond. Sci. Technol. 35 (2022) 074001.

Iron-based superconductors are a popular candidate in the search for affordable and simple superconductors for high-field applications. In particular, the relaxed texture requirements fuel hope that films deposited on RABiTS with simple buffer layer architectures could enable cheap coated conductors. We find that a single Yttrium oxide buffer layer can act as a suitable diffusion barrier and epitaxial Fe(Se,Te) thin films were successfully grown by pulsed laser deposition. An analysis of the local current distribution by means of scanning Hall probe microscopy reveals current densities exceeding 1 MA cm⁻², however, granularity still seems to be an issue. Transmission electron microscopy images and analysis by transmission Kikuchi diffraction show that the out-of-plane orientation of underlying Ni-W grains in the substrate has a severe impact on the growth of Fe(Se,Te) films.

S. Holleis, I.A. Shipulin, R. Hühne, J. Bernardi, M. Eisterer, Supercond. Sci Technol. 35 (2022) 104001.

REBCO based coated conductors (CCs) are a viable alternative to conventional superconductors for many applications, therefore the optimization of their current carrying capacity is an ongoing process. A promising route for the increase in performance is the introduction of artificial pinning centers such as BaHfO₃ (BHO) nanoparticles. However, granularity still imposes a substantial performance limitation, especially in REBCO CCs deposited on RABiTS based templates, as the critical current density is severely reduced by moderate misalignment angles of adjacent grains. A combined study of scanning Hall probe microscopy and electron

microscopy of undoped and BHO-doped YBa₂Cu₃O_7-δ(YBCO) films on technical templates shows that BHO-doping leads to a denser microstructure of the superconducting layer and higher global and local critical current densities. The statistical evaluation of local current maps allows for a quantification of the magnetic granularity where a reduction of granularity with increasing film thickness, doping and increasing temperature is found. In particular, the dependence of granularity on the film thickness and enhanced film growth through BHO-doping shows the potential for further optimization of YBCO films on RABiTS based templates.

S. Guo, B. Wang, D. Wolf, A. Lubk, W. Xia, M. Wang, Y. Xiao, J. Cui, D. Pravarthana, Z. Dou, K. Leistner, R.W. Li, R. Hühne, K. Nielsch ACS Nano 17 (2023) 2517-2528.

Colossal magnetoresistance is of great fundamental and technological significance in condensed-matter physics, magnetic memory, and sensing technologies. However, its relatively narrow working temperature window is still a severe obstacle for potential applications due to the nature of the material-inherent phase transition. Here, we realized hierarchical La_0.7Sr_0.3MnO₃ thin films with well-defined (001) and (221) crystallographic orientations by combining substrate modification with conventional thin-film deposition. Microscopic investigations into its magnetic transition through electron holography reveal that the hierarchical microstructure significantly broadens the temperature range of the ferromagnetic−paramagnetic transition, which further widens the response temperature range of the macroscopic colossal magnetoresistance under the scheme of the double-exchange mechanism. Therefore, this work puts forward a method to alter the magnetic transition and thus to extend the magnetoresistance working window by nanoengineering, which might be a promising approach also for other phase-transition-related effects in functional oxides.