R. H. Koch

Quantum interference devices made from superconducting oxide thin films

We have fabricated superconducting quantum interference devices (dc SQUID’s) from thin films of the superconducting oxide YBa2Cu3Oy. The devices were made by first lithographically patterning an ion implant mask containing a 40 by 40 μm loop and two 17‐μm‐wide weak links over a ∼1‐μm‐thick oxide film. Ion implantation was then used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID’s. The SQUID’s were operated in the temperature range from 4.2 to 68 K. The superconducting flux quantum was measured to be h/2e in these materials.

Effects of radiation damage in ion‐implanted thin films of metal‐oxide superconductors

The effects of ion implantation into thin films of the superconductor YBa2Cu3Ox have been studied. Using oxygen and arsenic ions, the superconducting transition temperature Tc, the change in room‐temperature electrical properties from conducting to insulating, and the crystalline to amorphous structural transition in the films were studied as a function of ion dose. The deposited energy required to change Tc was found to be 0.2 eV/atom, while 1–2 eV/atom was required to affect the room‐temperature conductivity, and 4 eV/atom to render the film amorphous. This hierarchy of effects is discussed in terms of the damage mechanisms involved.

Flicker (1/f) noise in tunnel junction DC squids

We have measured the spectral density of the 1/f voltage noise in current-biased resistively shunted Josephson tunnel junctions and dc SQUIDs. A theory in which fluctuations in the temperature give rise to fluctuations in the critical current and hence in the voltage predicts the magnitude of the noise quite accurately for junctions with areas of about 2 × 104 µm2, but significantly overestimates the noise for junctions with areas of about 6 µm2. DC SQUIDs fabricated from these two types of junctions exhibit substantially more 1/f voltage noise than would be predicted from a model in which the noise arises from critical current fluctuations in the junctions. This result was confirmed by an experiment involving two different bias current and flux modulation schemes, which demonstrated that the predominant 1/f voltage noise arises not from critical current fluctuations, but from some unknown source that can be regarded as an apparent 1/f flux noise. Measurements on five different configurations of dc SQUIDs fabricated with thin-film tunnel junctions and with widely varying areas, inductances, and junction capacitances show that the spectral density of the 1/f equivalent flux noise is roughly constant, within a factor of three of (10−10/f)ϕ02Hz−1. It is emphasized that 1/f flux noise may not be the predominant source of 1/f noise in SQUIDS fabricated with other technologies.

Reliable single‐target sputtering process for high‐temperature superconducting films and devices

We report a simple, single‐target magnetron sputtering process for films of high‐temperature superconductors involving an off‐axis sputtering geometry. The process lends itself both to film growth with high‐temperature post‐anneals and to low‐temperature in situ film growth. The post‐anneal process routinely yields YBa2Cu3O7−x films on SrTiO3 substrates that are fully superconducting at 86–89 K. Current densities at 77 K range from 104 to 8×105 A/cm2. A single‐level superconducting quantum interference device (dc SQUID), made by photolithographically patterning a low current density film, has a flux noise level at 77 K of 3×10−4 Φ0/(Hz)1/2 at 20 Hz, dominated by low‐frequency noise associated with flux motion in the film.

Batch-fabricated spin-injection magnetic switches

A method is developed for the fabrication of sub-100 nm current-perpendicular spin-valve junctions with low contact resistance. The approach is to use a batch-fabricated trilayer template with the junction features defined by a metal stencil layer and an undercut in the insulator. The spin-valve thin film stack is deposited afterwards into the stencil, with the insulator undercut providing the necessary magnetic isolation. Using this approach, reproducible spin-current-induced magnetic switching is demonstrated for junctions down to 50 nm×100 nm in size.

Ion beam amorphization of YBa2Cu3Ox

The microstructure of ion‐implanted thin films of the superconductor YBa2Cu3Ox has been investigated by transmission electron microscopy. The superconducting properties of the films were dominated by large pancake‐shaped grains of YBa2Cu3Ox with their c axis perpendicular to the substrate. Other grains of YBa2Cu3Ox whose c axis was parallel to the substrate formed spherulites. Irradiation with 500 keV O+ ions caused amorphous zones to appear on the grain boundaries between the pancake grains, which initially were free of amorphous or second phases. At higher dose a continuous amorphous layer 150 A thick was formed. However, the interior of the grains showed no irradiation‐induced microstructural features until they became amorphous at a dose of 3×1014 ions/cm2. The appearance of the amorphous layer on the grain boundaries at low doses accounts for the reduction in the superconducting transition temperature observed in these films.The microstructure of ion-implanted thin films of the superconductor YBa/sub 2/Cu/sub 3/O/sub x/ has been investigated by transmission electron microscopy. The superconducting properties of the films were dominated by large pancake-shaped grains of YBa/sub 2/Cu/sub 3/O/sub x/ with their c axis perpendicular to the substrate. Other grains of YBa/sub 2/Cu/sub 3/O/sub x/ whose c axis was parallel to the substrate formed spherulites. Irradiation with 500 keV O/sup +/ ions caused amorphous zones to appear on the grain boundaries between the pancake grains, which initially were free of amorphous or second phases. At higher dose a continuous amorphous layer 150 A thick was formed. However, the interior of the grains showed no irradiation-induced microstructural features until they became amorphous at a dose of 3 x 10/sup 14/ ions/cm/sup 2/. The appearance of the amorphous layer on the grain boundaries at low doses accounts for the reduction in the superconducting transition temperature observed in these films.

All high Tc edge junctions and SQUIDs

We present the first observations of superconducting quantum interference in multilevel, all high Tc, lithographically patterned edge junction structures. The current‐voltage characteristics are nonhysteretic and have well‐defined critical currents. The dynamic resistance is independent of current above the critical current. These devices show periodic sensitivity to magnetic fields and low levels of magnetic hysteresis up to temperatures around 60 K.

Low-noise thin-film TlBaCaCuO dc SQUIDs operated at 77 K

We have made a series of single‐level dc superconducting quantum interference devices (SQUIDs) from 4‐μm‐thick TlBaCaCuO films with large grain sizes and operated them in liquid nitrogen. Although device characteristics could not be precisely controlled, some devices had white‐noise levels that approached thermally limited noise above ∼1000 Hz. In addition, devices with 5 and 80 pH loop inductances had 1/ f noise levels at 10 Hz of 2×10−29 and 5×10−29 J/Hz, respectively. The noise levels at these frequencies are comparable to commercial rf SQUIDs operating in liquid helium, but the hysteresis of the voltage‐flux characteristic of the high Tc SQUIDs remains large.

Three SQUID gradiometer

We have invented a three superconducting quantum interference device (SQUID) gradiometer (TSG) that uses three SQUID magnetometers and a novel feedback method to measure magnetic field gradients. One SQUID, designated the reference SQUID, operates normally except that its feedback loop output is directed to all three SQUIDs through identical nonsuperconducting coils around each SQUID. The feedback loops for the remaining two SQUIDs, the sensor SQUIDs, measure the differences between the magnetic field at the reference SQUID location and those at the sensor SQUID locations. The voltage difference between the two sensor SQUID outputs divided by the gradiometer base line, the distance between the sensor SQUIDs, represents the average magnetic field gradient. We have measured gradient sensitivities of 10−12 and 10−10 T/m√Hz for TSGs made from bare low‐Tc and high‐Tc SQUIDs. An advantage of a TSG is that a sensitive gradiometer, free of hysteresis error, can be made using relatively small substrates.

Low‐frequency noise in low 1/f noise dc SQUID’s

We demonstrate that the low‐frequency noise in our edge junction dc superconducting quantum interference devices, with a basic 1/f flux noise of 2×10−12 Φ20/Hz at 1 Hz, can all be accounted for in terms of junction critical current fluctuations. A novel modulation readout scheme is able to cancel the effect of junction critical current fluctuations and reduce our total noise to 1×10−12 Φ20/Hz at 0.1 Hz, a level that is three times lower than the lowest flux noise ever previously reported at this frequency.