J.-P. Krumme

New magneto‐optic memory concept based on compensation wall domains

A new concept of thermomagnetic localized reversible magneto‐optic storage is proposed based on the application of compensation wall domains (CWD). Individual CWDs of 12 × 12‐μm dimensions in an LPE film of composition Y2.34Gd0.52Yb0.14Fe3.74Ga1.26 O12 with 3‐μm thickness have been switched by laser flashes of 10−2 erg/μm2 energy in 5 μs at 5145‐A wavelength using a stationary field of 41 Oe normal to the film plane.

Thermomagnetic Recording in Thin Garnet Layers

Thermomagnetic flux reversal has been studied in single‐crystal layers of Y3Ga1.1Fe3.9O12 with 5‐μm thickness, prepared by liquid‐phase epitaxy (LPE) on a (111) Gd3Ga5O12 wafer. By structuring the layers in a checkerboard pattern of squares with 10‐μm periodicity and 2‐μm width and depth of the surface grooves, arbitrary stable domain patterns have been obtained. Individual squares have been switched magnetically using heat pulses from a laser beam of 20‐mW intensity at 5145‐A wavelength with fields as low as 13 Oe in case of a nearly demagnetized domain distribution. The beam energy for reversing the magnetization of a square is below 1 erg.

A multi-layer process for the fabrication of HTSC flux transformers and SQUIDS

A multi-layer technology, based on YBaCuO as the superconducting material and SrTiO3 as the insulating material, is described. Patterning is performed by photolithography and Ar-ion-beam etching under fiat incidence. Using a resist bake-out prior to the etching, step angles in the patterned lower film of less than 20 degrees are obtained. Superconducting 10-turn thin-film coils have been fabricated with transition temperatures of up to 83 K and critical current densities at 77 K of 2*105 A cm-2. Furthermore we have fabricated a thin-film flux transformer and combined it in flip-chip configuration with a low-noise YBCO step-edge DC SQUID. We measured a magnetic field resolution of the complete magnetometer of 200 fT Hz-1/2 at 1 Hz, dominated by the SQUID noise itself.

Integrated high-T/sub c/ SQUID magnetometer

We have fabricated an integrated SQUID magnetometer based on Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ step-edge Josephson junctions. A tight coupling between the input coil and the SQUID washer has been achieved with a coupling coefficient K of 0.9. Noise measurements on our SQUIDs showed a nearly frequency independent flux noise of 50 /spl mu//spl Phi//sub 0///spl radic/Hz at 77 K. 1/f-noise contributions at lower temperatures were always completely suppressed by bias reversal. From our measurements of the field sensitivity and the flux noise we can estimate a field resolution of the magnetometer of 60 fT//spl radic/Hz down to 1 Hz.<<ETX>>

YBa2Cu3O7 ramp‐type junctions and superconducting quantum interference devices with an ultrathin barrier of NdGaO3

We have fabricated ramp‐type Josephson junctions and SQUIDs (superconducting quantum interference devices) using an ultrathin barrier layer of NdGaO3 as weak contact between the YBa2Cu3O7 electrodes. The junctions operate up to 82 K, exhibiting current‐voltage characteristics of the resistively‐shunted‐junction type. A normal‐state resistance of up to 30 Ω at a junction width of 5 μm and a characteristic voltage in excess of 100 μV at 77 K are obtained. We find hysteretic behavior below 70 K and an intrinsic junction capacitance of about 0.15 μF/cm2 at 4 K. SQUIDs show a transfer function δ V/δ Φ of 50 μV/Φ0 at an inductance of 40 pH at 77 K. Thus we observe excellent Josephson properties at a thickness of the NdGaO3 barrier of 2 nm.

High- SQUID magnetometers for biomagnetic measurements

We have designed and fabricated three types of high- SQUID (superconducting quantum interference device) magnetometers based on step-edge Josephson junctions using three different concepts of coupling magnetic flux into the SQUID: (i) a single pickup loop galvanically coupled to the SQUID, (ii) a flux transformer inductively coupled to the SQUID and (iii) a multiloop pickup loop used directly as the SQUID inductance. On a substrate we achieved an effective flux capture area of and for the inductively coupled and multiloop devices, respectively. Due to the low white noise levels of for the inductively coupled magnetometer and for the multiloop device high quality magnetocardiograms were recorded inside a magnetically shielded room without signal averaging.

Multilevel devices of YBa/sub 2/Cu/sub 3/O/sub 7/ with NdGaO/sub 3/ barrier

We have fabricated YBa/sub 2/Cu/sub 3/O/sub 7/ ramp-type junctions incorporating a barrier layer of NdGaO/sub 3/ with a nominal thickness of 2 nm. The junctions exhibit pronounced Josephson effects and operate up to 82 K. The characteristics are well described within the resistively shunted junction model. We observe large hysteresis parameters /spl beta//sub c/ even at elevated temperatures. The output voltage of a high-T/sub c/ dc SQUID is found to benefit from the intrinsic junction capacitance.<<ETX>>