Michael R. Koblischka

Analysis of Pinning in NdBa2Cu3O7-δ Superconductors

Volume pinning forces, F p , are measured in a wide temperature (5 ≤ T ≤ 92 K) and field range (0 ≤ μ0 H a ≤ 9 T) on different NdBa2Cu3O7-δ samples. Above 60 K a good scaling of F p , versus the reduced field, h = H a /H irr can be established. The scaled pinning curves are compared to several theoretical predictions. Experimental evidence for strong pinning at extended (Vpin ~ ξ2 d) defects is given. These defects are ascribed to spatial composition fluctuations found in light rare earth superconductors, providing a scatter of the transition temperature, T c . Such a pinning echanism is especially important for applications of high-T c superconductors operating at 77 K.

Flux pinning in ternary (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy melt-processed superconductors

The flux pinning characteristics of ternary melt-processed (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG) superconductors are studied in the temperature range 60⩽T⩽90 K. NEG samples exhibit a strongly developed peak effect in the dependence of the critical current densities on the external field, Ha. The scaling of the pinning forces versus the reduced field h=Ha/Hirr (where Hirr denotes the irreversibility field) yields a peak at h0=0.5 which is an indication of pinning provided by a spatial variation of the transition temperature. The presence of a weaker superconducting second phase is demonstrated by means of field cooling and warming experiments in fields up to 7 T. Furthermore, we discuss the possible effect of the magnetic moments of Gd and Nd on the flux pinning.

Enhancement of Jc by 211 particles in ternary (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy melt-processed superconductors

(Nd0.33Eu0.33Gd0.33)2BaCuO5 (NEG-211) particles are added to ternary (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG) bulk, melt-processed superconductors. By means of transmission electron and polarized light microscopy, we found that two types of 211 particles are formed; large NEG-211 and submicron-sized particles containing mostly Gd in the rare earth site are formed which can act as effective flux pinning sites. The critical current density at zero field increases monotonously as a function of the NEG-211 concentration, but the secondary peak vanishes above 40 mol % 211 addition. As a result, a critical current density of 68 000 A cm−2 at 77 K and 2.5 T applied field is achieved with at 40 mol % 211 additions.

Nanoscopic netted structure of compositional modulation in (Sm0.33Eu0.33Gd0.33)Ba2Cu3O7−δ superconductors

We report a nanoscaled crisscross network in high-quality melt-processed (Sm0.33Eu0.33Gd0.33)Ba2Cu3O7−δ superconductors investigated with atomic force microscopy (AFM) and transmission electron microscopy. In the ten-micrometer scale, such a net was unveiled as a consequence of crossing annular stripes originating from ordering compositional modulation. The AFM topographic images further displayed that this compositional stripe result in a surface modulation with a few-nanometer wavelength and roughness. The forming mechanism of this compositional stripe and its role as strong δTc-type pinning defects were discussed in the context of rare earth∕Ba oscillation around the stoichiometric ratio and the spatial fluctuation of local Tc values.

Improvements of the lateral resolution of the MFM technique

We report on the preparation of high-resolution magnetic force microscopy (MFM) tips using the electron beam deposition method for MFM measurements on soft magnetic samples. Electron beam lithography using a scanning electron microscope was used to define small particles of magnetic material at the very end of a commercial scanning microscope tip to achieve maximum lateral resolution with low magnetic moment. Several approaches were tried out and demonstrated on permalloy thin films. The achieved resolution (down to 20 nm) is clearly proven by the fact that more details of the magnetic structure can be observed. However, it turned out that no standard exists in order to reliably determine the resolution of the MFM measurements; the development of such a standard will be a very important new task.

High frequency magnetic force microscopy-imaging of harddisk write heads

A high-frequency magnetic force microscope (HF-MFM) is built up for the observation of the high-frequency stray fields of harddisk write heads. We demonstrate the performance of the system using harddisk write heads from different manufacturers. We show that the use of advanced high-aspect ratio MFM cantilevers yields HF-MFM images with clearly improved spatial resolution. Furthermore, we present results obtained using MFM cantilevers with a magnetic coating of Ba3Co2Fe24O41 (BCFO).

Application of electron backscatter diffraction to bulk high-Tc superconductors

We have successfully obtained electron backscatter diffraction (EBSD) Kikuchi patterns on various bulk high-Tc superconductors (polycrystalline KClO3-doped and undoped YBa2Cu3O7−δ (YBCO), single crystalline YBCO and NdBa2Cu3O7−y) and on YBCO and NdBCO thin films. We have established a sample polishing procedure to obtain intense Kikuchi patterns without an additional carbon coating of the ceramic superconductors. We present Kikuchi patterns, pole figures and orientation maps of the samples. On the polycrystalline KClO3-doped YBCO samples we find two major orientations (001) and (100) normal to the sample surface, while the undoped sample does not exhibit any orientation maxima, which is ascribed to the effect of a liquid phase due to the KClO3-additive during the preparation process. Furthermore, we discuss the future possibilities for the application of the EBSD technique for growing larger, bulk melt-textured samples for applications.

Low magnetic relaxation in a single crystal and a melt processed sample

Induced current densities and magnetic relaxation rates were measured over a wide temperature and field range on a single crystal and a melt processed sample of using a SQUID magnetometer. The temperature dependence of the magnetic relaxation rate shows a pronounced minimum at elevated temperatures. This leads to very high values of the effective pinning energy and for the single crystal and the melt processed sample respectively, for an applied magnetic field . Furthermore, these samples show a well developed fishtail effect, leading to a current density . This demonstrates that the fishtail effect is closely related to the low relaxation rates observed here.

Comparison of different approaches to modelling the fishtail shape in RE-123 bulk superconductors

We analyse and compare properties of different models proposed so far for modelling the shapes of FB and JB . curves measured in RE-Ba Cu O RE-123, REs rare earth samples. The formulas following from the discussed 23 7 yd models allow for a direct fit of experimental data in various representations. The models divide in two categories, according to the pinning potential used, namely the power-law and the logarithmic potential. We show that none of the pinning regimes . proposed for low-T superconductors accounts for the fishtail effect FE observed in high- T materials. The conventional c c expressions allow a good fit of high-T experimental data, however, the resulting fitting parameters are much higher than c those predicted by the theory. The logarithmic pinning potential justified in high- T materials by both magnetic and transport c

Interactions of Y2BaCuO5 particles and the YBCO matrix within melt-textured YBCO samples studied by means of electron backscatter diffraction

By means of automated electron backscatter diffraction (EBSD) analysis, we studied the local orientations of embedded Y2BaCuO5(211) particles within melt-textured YBa2Cu3Ox (YBCO) samples. On both components, we obtained high-quality Kikuchi patterns, thus allowing the automated mapping of the crystal orientations and a two-phase analysis of the samples. Investigations were performed on a variety of melt-textured YBCO samples. In melt-textured YBCO with (001) orientation, we find that the embedded 211 particles do not have any preferred orientation. The EBSD maps also reveal that at certain orientations of the 211 particles the YBCO growth is not altered. From the obtained EBSD mappings, we can conclude that the formation of small 211 particles will not disturb the YBCO matrix growth, whereas the presence of large 211 particles causes severe changes in the YBCO growth, leading to the formation of subgrains. The EBSD results are presented in the form of local orientation maps and local pole figures.