Johannes W.M. Hilgenkamp

TEM investigation of YBa2Cu3O7 thin films on SrTiO3 bicrystals

YBa2Cu3O7 films in c-axis orientation on bicrystalline SrTiO3 substrates are investigated by TEM. The films and the substrates are examined in cross-section and in plane view. The grain boundary of the bicrystal substrate contains (110) faceted voids, but is otherwise straight on a nanometer scale. Contrary to this, the film grain boundary is not straight grain boundary can be up to 100 nm for a 100 nm thick film. The deviation from the intended position of the YBCO grain boundary can already occur at the film/substrate interface where it can be as much as ±50 nm.

Four layer monolithic integrated high Tc dc SQUID magnetometer

YBa2Cu3O7−x based monolithic integrated dc SQUID magnetometers, consisting of a dc SQUID integrated with a flux transformer on a single bicrystalline substrate, have been fabricated and characterized. The devices consist of four layers, including two superconducting layers, and first realizations operate up to 73 K. A maximum voltage modulation of 32 μV is observed at 40 K. A field sensitivity of 0.17 pT/√Hz is obtained above 200 Hz at 45 K and 0.49 pT/√Hz at 1 Hz and 65 K.

Multilayer studies and applications in template bi-epitaxial DC SQUIDS

Multilayer deposition for the creation of a well-defined grain boundary, based on different in-plane orientations of c-axis oriented thin YBa/sub 2/O/sub 2/Cu/sub 3/O/sub 7- delta / layers on a single substrate, has been performed on three different kinds of substrates: (1102)-oriented Al/sub 2/O/sub 3/,

Laser scanning imaging and local characterization of superconducting properties in high-Tc thin film multiturn coil

Low-temperature scanning laser microscopy has been used to investigate the spatial variation of the critical temperature Tc and critical current Ic in thin-film high-Tc multilayer structures that include dielectric layers. The method is described and measurements are presented on an YBa2Cu3O7-x-based multiturn coil with SrTiO3 insulating layer. We found that the critical temperature Tc of the YBa2Cu3O7-x top layer, from which the return strip of the coil is formed, is higher than that of the YBa2Cu3O7-x base layer. The critical current of the coil is limited by the quality of the YBa2Cu3O7-x base layer and not by the edges of the crossovers.

Basic elements for photodeposited high Tc thin film devices

Flat films, high quality insulating layers and adequately superconducting via contacts are basic elements for high Tc device fabrication. We studied the influence of the process parameters of laser deposition on the occurrence of droplets and outgrowths in YBaCuO films. The droplet density is minimal when a laser fluence below about 1.0 J cm-2 is used. The outgrowth density decreases with increasing laser pulse rate or decreasing deposition temperature. High quality flat films were obtained with a rate of 10 Hz and at a temperature of 720 °C. Wet chemical etching and etching with an Argon ion source were used for structuring multilayers with SrTiO3 as an insulating layer. Smooth edges were obtained with an argon gun. Bromine and EDTA etching are not adequate techniques for fabricating controllable well-defined edges. Cross-overs, via contacts and coils were prepared.

Monolithic flux transformer-coupled high-T/sub c/ DC SQUID magnetometers

YBa/sub 2/Cu/sub 3/O/sub 7-x/ based monolithic flux transformer-coupled high-T/sub c/ DC SQUID magnetometers operating up to 73 K have been realized. The devices are characterized by high values of the modulation voltage, up to 32 /spl mu/V at 40 K. A minimal white noise level of 0.10 pT//spl radic/Hz was obtained above 200 Hz, and 0.64 pT//spl radic/Hz at 1 Hz and 55 K. The temperature dependence of the modulation voltage, the effective sensing area and the field sensitivity are discussed. Model-calculations have been performed to investigate high frequency resonances in the washer-input coil structure. Methods for damping are considered.<<ETX>>

High T/sub c/ bi-epitaxial junctions and dc SQUIDs structured by focused ion beam etching

Focused ion beam etching has been used to pattern dc SQUIDs into previously characterised template bi-epitaxial grain boundary junctions. Using this technique the screening parameter /spl beta//sub L/ can be optimised for a chosen temperature (in our case 30 K). Electrical characteristics, including noise measurements, are presented. A minimal white noise level of 22 /spl mu//spl phi//sub 0//spl middot/Hz/sup -1/2/ (1.8/spl middot/10/sup -29/ J/spl middot/Hz/sup -1/) has been obtained at 20 K. Using bias current modulation the 1/f noise could almost completely be suppressed down to 1 Hz in the entire temperature range (10-65 K).<<ETX>>

Bi-epitaxial template grain boundaries with different in-plane angles on (100) MgO substrates

Using laser ablation CeO2 and YBa2Cu3O7−d layers have been deposited on (100) oriented MgO substrates for the creation of bi-epitaxial grain boundary weak links. X-Ray Diffraction measurements were used to measure the (in-plane) orientations of the layers. The CeO2 layer has been structured using Ar ion beam etching or CaO lift off. Using these two structuring techniques it is possible to obtain two different grain boundary weak link angles: 18 and 27 degrees. Up till now only 45 degrees weak link angles have been reported in literature, using templates for the creation of these grain boundaries.

Materials aspects of integrated high Tc dc-SQUID magnetometer fabrication

An integrated high Tc dc-SQUID magnetometer is being developed. It has in total 10 layers of five different materials. Various materials aspects of the fabrication process will be discussed, especially the smoothness of the films and the techniques to obtain smooth edges. Cross-overs and superconducting window contacts were fabricated. The critical temperature of the window-contact is 84K (jc(77K)=2·105 A/cm2) and the resistivity of the insulating SrTiO3 layer in the cross-over is 6·105 Ωcm at 77K. The complete coil often shows a small resistive component down to about 50K. Ramp type and bi-epitaxial grain boundary dc-SQUIDs show voltage modulation up to 65K and 79K, respectively. Efforts to fabricate an integrated high Tc dc-SQUID magnetometer will be discussed.

High Tc bi-epitaxial dc SQUIDs structured by focused ion beam etching from single junctions: ßL optimization

Focused ion beam etching has been used to pattern dc SQUIDs into previously characterised template bi-epitaxial grain boundary junctions. In this way, the screening parameter sL can be optimised for a chosen temperature (in our case 30 K). Electrical characteristics, including noise measurements, are presented. A minimal white noise level of 22 µ0·Hz (1.8·10-29 J·Hz-1) has been obtained at 20 K. Using bias current modulation the 1/f noise could be almost completely suppressed down to 1 Hz in the entire temperature range (10?65 K).