Shane A. Cybart

Reversible electric control of exchange bias in a multiferroic field-effect device

Electric-field control of magnetization has many potential applications in magnetic memory storage, sensors and spintronics. One approach to obtain this control is through multiferroic materials. Instead of using direct coupling between ferroelectric and ferromagnetic order parameters in a single-phase multiferroic material, which only shows a weak magnetoelectric effect, a unique method using indirect coupling through an intermediate antiferromagnetic order parameter can be used. In this article, we demonstrate electrical control of exchange bias using a field-effect device employing multiferroic (ferroelectric/antiferromagnetic) BiFeO(3) as the dielectric and ferromagnetic La(0.7)Sr(0.3)MnO(3) as the conducting channel; we can reversibly switch between two distinct exchange-bias states by switching the ferroelectric polarization of BiFeO(3). This is an important step towards controlling magnetization with electric fields, which may enable a new class of electrically controllable spintronic devices and provide a new basis for producing electrically controllable spin-polarized currents.

Planar MgB2 Josephson junctions and series arrays via nanolithography and ion damage

We have fabricated planar thin-film MgB2 Josephson junctions and 20-junction series arrays using 200-keV ion implantation and electron-beam lithography. Resistively shunted junction I-V characteristics were observed in the temperature range of 34–38K. The ac Josephson effect was observed and flat giant Shapiro steps in arrays suggest good junction uniformity with a small spread in junction parameters. The temperature dependence of the critical current suggests that the nature of the interface between the superconductor and normal region can be described using a soft boundary proximity effect coupling model. We believe that the higher operating temperature and close spacing of these junctions make them promising candidates for quantum voltage standards and other devices.

Series array of incommensurate superconducting quantum interference devices from YBa2Cu3O7-δ ion damage Josephson junctions

We have fabricated a series array of 280 superconducting quantum interference devices (SQUIDs) using YBa2Cu3O7−δ thin film ion damage Josephson junctions. The SQUID loop areas were tapered exponentially so that the response of the current-biased array to magnetic field is a single voltage spike at zero field. We fitted the current-voltage characteristics of the array to a model in which we summed the voltages across the SQUIDs assuming a resistively shunted junction model with a normal distribution of SQUID critical currents. At 75 K the standard deviation of these critical currents was 12%.

Planar MgB2 superconductor-normal metal-superconductor Josephson junctions fabricated using epitaxial MgB2∕TiB2 bilayers

We have fabricated planar superconductor-normal metal-superconductor MgB2 Josephson junctions using TiB2 as the barrier material. On a bilayer film of MgB2∕TiB2 on SiC substrate, MgB2 was locally removed to create a gap of ⩽50nm, separating the MgB2 film into two electrodes. A Josephson coupling is established through the TiB2 film across the gap by proximity effect. The junctions exhibit resistively shunted junction like current-voltage characteristics up to 31K. An ac Josephson effect was observed and the behavior of the Shapiro steps are in good agreement with theoretical predictions. The magnetic field modulation of the critical current also agrees with the thin film planar junction behavior.

Nano Josephson superconducting tunnel junctions in YBa2Cu3O7–δ directly patterned with a focused helium ion beam

Since the discovery of the high-transition-temperature superconductors (HTSs), researchers have explored many methods to fabricate superconducting tunnel junctions from these materials for basic science purposes and applications. HTS circuits operating at liquid-nitrogen temperatures (∼77 K) would significantly reduce power requirements in comparison with those fabricated from conventional superconductors. The difficulty is that the superconducting coherence length is very short and anisotropic in these materials, typically ∼2 nm in the a-b plane and ∼0.2 nm along the c axis. The electrical properties of Josephson junctions are therefore sensitive to chemical variations and structural defects on atomic length scales. To make multiple uniform HTS junctions, control at the atomic level is required. In this Letter we demonstrate all-HTS Josephson superconducting tunnel junctions created by using a 500-pm-diameter focused beam of helium ions to directly write tunnel barriers into YBa2Cu3O(7-δ) (YBCO) thin films. We demonstrate the ability to control the barrier properties continuously from conducting to insulating by varying the irradiation dose. This technique could provide a reliable and reproducible pathway for scaling up quantum-mechanical circuits operating at liquid-nitrogen temperatures, as well as an avenue to conduct novel planar superconducting tunnelling studies for basic science.

Planar thin film YBa2Cu3O7−δ Josephson junction pairs and arrays via nanolithography and ion damage

We have fabricated in-plane high-Tc Josephson junction pairs and series arrays using our established nanolithography and ion damage process. Junctions in a pair showed nearly identical electrical properties. The ten-junction array exhibited current–voltage characteristics that can be described by the resistively shunted junction model at 78K. Flat giant Shapiro steps were observed in both cases. We believe that the ion-damaged high-Tc superconducting Josephson junction is a good candidate to form large numbers of junctions in series arrays that can function above 77K for quantum voltage standards and other applications.

Comparison of measurements and simulations of series-parallel incommensurate area superconducting quantum interference device arrays fabricated from YBa2Cu3O7−δ ion damage Josephson junctions

We have fabricated series-parallel (two-dimensional) arrays of incommensurate superconducting quantum interference devices (SQUIDs) using YBa2Cu3O7−δ thin film ion damage Josephson junctions. The arrays initially consisted of a grid of Josephson junctions with 28 junctions in parallel and 565 junctions in series, for a total of 15 255 SQUIDs. The 28 junctions in the parallel direction were sequentially decreased by removing them with photolithography and ion milling to allow comparisons of voltage–magnetic field (V–B) characteristics for different parallel dimensions and area distributions. Comparisons of measurements for these different configurations reveal that the maximum voltage modulation with magnetic field is significantly reduced by both the self inductances of the SQUIDs and the mutual inductances between them. Based on these results, we develop a computer simulation model from first principles which simultaneously solves the differential equations of the junctions in the array while considering...

Large voltage modulation in magnetic field sensors from two-dimensional arrays of Y-Ba-Cu-O nano Josephson junctions

We have fabricated and tested two-dimensional arrays of YBa2Cu3O7−δ superconducting quantum interference devices. The arrays contain over 36 000 nano Josephson junctions fabricated from ion irradiation of YBa2Cu3O7−δ through narrow slits in a resist-mask that was patterned with electron beam lithography and reactive ion etching. Measurements of current-biased arrays in magnetic field exhibit large voltage modulations as high as 30 mV.

YBa2Cu3O7−δ superconducting quantum interference devices with metallic to insulating barriers written with a focused helium ion beam

In this work, we demonstrate the ability to fabricate superconducting quantum interference devices (SQUIDs) by directly writing Josephson junctions into the plane of YBa2Cu3O7−δ thin films with a focused helium ion beam. This technique allows for the control of the Josephson barrier transport properties through the single parameter, ion dose. SQUIDs written with a dose of 4 × 1016 ions/cm2 had metallic barrier junctions that exhibited nearly ideal electrical transport characteristics at 50 K and a flux noise of 20 μΦ0/Hz at 10 Hz. At higher irradiation doses, the SQUIDs had insulating barrier Josephson junctions with a quasi particle energy gap edge at 20 meV.

Simulation of Series Arrays of Superconducting Quantum Interference Devices

The voltage as a function of applied magnetic field (V-B) was calculated for arrays of superconducting quantum interference devices (SQUIDs) connected in series. Comparisons were made between arrays of equal area SQUIDs and superconducting quantum interference filters (SQIFs). The areas for the SQIFs were varied exponentially, so that the V-B had a sharp minimum at zero field. We used equations for the dc SQUID based on resistively shunted junctions, with typical parameters for YBa2Cu3O7 - δ ion damage Josephson junctions. The maximum transfer coefficient of the central minimum VB = (∂V/∂B)max of the SQIF decreases as the area range increases. We find that the equal area array is more robust to the effects of non-uniform junction critical currents than the SQIF, for the junction parameters and SQUID area distributions chosen. Furthermore, we find that slight variations (~5%) to the area due to fabrication irregularities have little effect on the central minimum of V-B for either device.