W.E. Booij

Patterning ferromagnetism in Ni80Fe20 films via Ga+ ion irradiation

We demonstrate that focused Ga+ ion irradiation can comprehensively modify the ferromagnetic properties of Ni80Fe20 thin films. Magneto-optic Kerr effect measurements at room temperature and magnetoresistance measurements at temperatures between 1.5 and 270 K characterized the irradiation effects. Irradiation steadily reduced the films’ room temperature coercivity, and a dose of 1.0×1016 ions/cm2 at 30 keV was found sufficient to cause a loss of ferromagnetism at room temperature in films of thickness up to 15.5 nm. In situ end-point detection and postirradiation atomic force microscopy confirmed that the sputtering which accompanied doses up to 1.0×1016 ions/cm2 did not compromise the protective caps on these Ni80Fe20 films. We therefore conclude that the modification of ferromagnetic properties occurred primarily because of direct Ga+ ion implantation. From these results, we speculate that focused Ga+ ion irradiation could be a convenient tool for the nanoscale patterning of magnetic properties in 3d tr...

DIRECT WRITING OF LOW TC SUPERCONDUCTOR-NORMAL METAL-SUPERCONDUCTOR JUNCTIONS USING A FOCUSED ION BEAM

Using a focused ion beam, we have produced superconductor-normal metal-superconductor junctions by controllably removing a portion of the top layer of a patterned superconductor-normal metal thin film. The high-quality junctions showed Josephson coupling which scaled qualitatively with barrier properties and temperature as expected. The largest product of a junction’s critical current and the normal state resistance measured is 98 μV at 4.2 K. The method has good reproducibility and could be exploited in a number of superconducting electronics applications.

Alternating current Josephson effect in intrinsic Josephson bridges in Tl2Ba2CaCu2O8 thin films

We have performed transport measurements on bridges patterned in misaligned thin films of the superconductor Tl2Ba2CaCu2O8. There is a c-axis component of current flow along the bridge, giving rise to hysteretic Josephson-like current–voltage curves. The temperature dependence of the critical current follows the Ambegaokar–Baratoff theory with IcRN up to 26 mV at 4.2 K. Microwave emission from the Josephson junctions near Tc (≈103 K) has been detected using an X-band detector. We show that 700±15 junctions in the bridge are actively oscillating, confirming that the junctions are “intrinsic” junctions formed by adjacent copper oxide planes in the Tl2Ba2CaCu2O8 crystal structure.

Electron-beam damaged high-temperature superconductor Josephson junctions

Results are presented on the fabrication and characterization of high critical temperature Josephson junctions in thin films of YBa2Cu3O7−δ produced by the process of focused electron-beam irradiation using 350 keV electrons. The junctions so produced have uniform spatial current densities, can be described in terms of the resistive shunted junction model, and their current densities can be tailored for a given operating temperature. The physical properties of the damaged barrier can be described as a superconducting material of either reduced or zero critical temperature (Tc), which has a length of ∼15 nm. The Tc reduction is caused primarily by oxygen Frenkel defects in the Cu–O planes. The large beam currents used in the fabrication of the junctions mean that the extent of the barrier is limited by the incident electron-beam diameter, rather than by scattering within the film. The properties of the barrier can be calculated using a superconductor/normal/superconductor (SNS) junction model with no bound...

Fabrication of in‐plane aligned a‐axis oriented YBa2Cu3O7−x trilayer Josephson junctions

We have fabricated vertical YBa 2Cu3O7−x/PrBa 2Cu3O7−x/YBa 2Cu3O7−x (YBCO/PBCO/YBCO) Josephson junctions using in‐plane aligned a‐axis oriented YBCO multilayers on (100) LaSrGaO4(LSGO). The Tc’s of the device electrodes are typically greater than 80 K. Josephson coupling is observed for barrier thickness up to 800 A. Strong nonlinear I–V characteristics arising from the properties of PBCO are observed for thicker barrier devices. Chip to chip spread in the device characteristics may be intrinsically due to the PBCO barrier.

Measurement of the dynamic error rate of a high temperature superconductor rapid single flux quantum comparator

The application of high temperature superconductor (HTS) Josephson junctions in digital rapid single flux quantum circuits requires a careful study of the influence of thermal noise on the bit error rate (BER). We have determined experimentally, for the first time, the BER of a HTS rapid single flux quantum circuit. A comparator, formed by two Josephson junctions, was integrated in a Josephson transmission line ring oscillator, allowing us to perform high speed testing of the comparator at GHz frequencies. For fabrication, focused-electron-beam-irradiated junctions have been used because of their small parameter spread and excellent alignment possibilities. A BER of less than 10−11 was obtained at 39 K.

Electrical properties of electron and ion beam irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

An electron beam with sufficient energy can be used to create a high quality Josephson junction in a single layer of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The number of junctions thus produced is severely limited by the serial nature of the technique. An alternative method to create similar high quality Josephson junctions without such a serious throughput limitation is possibly the combination of high resolution masking and ion irradiation. For this reason we have studied the electrical properties of both electron and proton irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// in some detail. It was found that the resistivity of electron beam irradiated barriers of intermediate length (200 nm) are strongly influenced by a proximity effect when the irradiated material has a finite T/sub c/. At higher electron doses the superconducting properties are fully suppressed and the electrical behaviour is dominated by a Variable Range Hopping mechanism.

Capacitance as a probe of high angle grain boundary transport in oxide superconductors

We report a series of studies of grain boundary (GB) capacitance for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films grown on SrTiO/sub 3/ (STO) bicrystal substrates. By varying the film thickness and the width of the track containing the GB, we find that the substrate makes no contribution to the capacitance measured using Fiske resonances or hysteresis in most cases. This is due to the frequency dependence of the dielectric properties of SrTiO/sub 3/. We have also found that GB capacitance per unit area c/sub GB/ correlates with the resistance-area product R/sub n/A. For our own GBs and GBs reported in the literature the data is is consistent with c/sub GB//spl prop/(R/sub n/A)/sup -1/. We attribute this to variations in GB barrier properties, which reduce the active area, whilst maintaining locally the transport mechanism as tunneling.

Nanoscale SNS junction fabrication in superconductor-normal metal bilayers

We have developed a reliable and versatile technique for fabricating SNS junctions in a superconductor-normal metal bilayer using a focused ion beam microscope (FIB) in conjunction with an in-situ resistance measurement technique. This technique offers a simple method for creating multi-junction devices (SQUIDs, 3-terminal devices, arrays) with high integration densities. In this paper we discuss recent results from devices created in Nb-Cu tracks by cutting 50 nm trenches in the top Nb layer to weaken the superconducting coupling. Cuts of depths between 60 and 100% of the Nb thickness yield reproducible junctions with current voltage (I(V)) characteristics in accordance with the resistively-shunted-junction (RSJ) model, characteristic voltage I/sub C/R/sub N//spl sim/50 /spl mu/V at 4.2 K and excellent microwave response. A thorough study has been carried out of the effect on device parameters of varying the Cu layer thickness (0-175 nm). In addition transmission electron microscopy (TEM) studies have been carried out on the device structure. A two-channel model of device operation has been developed and related to the results of I/sup C/R/sub N/(T) measurements (down to 350 mK) on selected devices.

2D Monte Carlo simulation of proton implantation of superconducting YBa2Cu3O7−δ thin films through high aspect ratio Nb masks

Abstract Perturbation of proton beam damage profile due to sidewall interactions in very high aspect ratio implant masks has been studied using Monte Carlo simulations. The model structure is composed of amorphous Nb metal mask, crystalline high temperature superconducting YBa 2 Cu 3 O 7− δ (YBCO) thin film, and amorphous LaAlO 3 substrate. The simulation results reveal the existence of enhanced proton beam penetration in target materials due to sidewall interactions.