Stephen E. Russek

Production of YBa2Cu3O7-y Superconducting thin films in situ by high-pressure reactive evaporation and rapid thermal annealing

A high‐pressure reactive evaporation process has been used to produce smooth YBa2Cu3O7−y high Tc superconducting films without the necessity of a post‐deposition oven anneal cycle. Augmented in some cases by rapid thermal annealing, the process has yielded films with zero resistance transition temperatures above 80 K, and critical current densities above 106 A/cm2 at 4.2 K on both cubic zirconia and SrTiO3 substrates.

Growth of YBa2Cu3O7 thin films on MgO: the effect of substrate preparation

We discuss the results of a study of the effect of substrate preparation on the microstructure and superconductive properties of YBa2Cu3O7 thin films formed by laser ablation on (001)u2009MgO substrates. Thermal annealing of the substrates is found to be highly effective in producing at fairly low growth temperatures (670u2009°C), epitaxial, c‐axis normal films with good superconductive properties. Alternative surface treatments result in the formation of large angle tilt boundaries and inferior superconductive properties.

Crystallography of YBa 2 Cu 3 O 6+ x thin film-substrate interfaces

The epitactic nature of the growth of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} (YBCO) superconducting thin films on ceramic substrates has been studied using high-resolution electron microscopy (HREM) and selected-area diffraction (SAD) of cross-sectional specimens. The films were grown {ital in} {ital situ} on (001)-oriented MgO and (001)-oriented Y{sub 2}O{sub 3}-stabilized cubic ZrO{sub 2} (YSZ) single-crystal substrates by electron beam evaporation. Both of these materials have large lattice misfits with respect to YBCO. Different orientation relationships were observed for films grown on the two types of substrates. These orientation relationships are shown to provide the best matching of the oxygen sublattices across the substrate-film interfaces. A crystalline intermediate layer, 6 nm thick, between the YBCO film and YSZ substrate was observed by HREM and shown by EDS to be a Ba-enriched phase, possibly barium zirconate formed by a reaction. In contrast, the YBCO--MgO interface was found to be sharp and free of any intermediate layers.

Scaling behavior of YBa2Cu3O7−δ thin‐film weak links

The superconductive weak link properties of microbridges formed in c‐axis normal YBa2Cu3O7−δ polycrystalline thin films containing a variable amount of large angle tilt boundaries have been studied. In the low critical current density limit these weak links have current‐voltage (I‐V) characteristics that are accurately modeled by the resistively shunted junction model. The I‐V’s are found to accurately follow a simple scaling law with the product of the critical current and weak link resistance Rn varying linearly with the weak link conductance.

Hybrid superconducting-magnetic memory device using competing order parameters

In a hybrid superconducting-magnetic device, two order parameters compete, with one type of order suppressing the other. Recent interest in ultra-low-power, high-density cryogenic memories has spurred new efforts to simultaneously exploit superconducting and magnetic properties so as to create novel switching elements having these two competing orders. Here we describe a reconfigurable two-layer magnetic spin valve integrated within a Josephson junction. Our measurements separate the suppression in the superconducting coupling due to the exchange field in the magnetic layers, which causes depairing of the supercurrent, from the suppression due to the stray magnetic field. The exchange field suppression of the superconducting order parameter is a tunable and switchable behaviour that is also scalable to nanometer device dimensions. These devices demonstrate non-volatile, size-independent switching of Josephson coupling, in magnitude as well as phase, and they may enable practical nanoscale superconducting memory devices.

High‐frequency characterization of thin‐film Y‐Ba‐Cu oxide superconducting transmission lines

We report the first measurements of picosecond pulse propagation on transmission lines patterned from YBa2Cu3O7−x films. Distortion‐free propagation of high current density transients is demonstrated. The high‐frequency properties were analyzed by careful study of the relative phase delays of the electrical transients as the temperature of the sample was varied from 1.8 K to Tc. Simulations using Mattis–Bardeen complex conductivities showed good agreement with the measured results. High‐frequency critical current densities in excess of 105 A/cm2 were measured.

Transport properties of high‐angle grain boundary weak links in YBa2Cu3O7 thin films

By patterning ∼1‐μm‐wide microbridges in laser ablated YBa2Cu3O7 films containing high‐angle tilt boundaries, weak links have been isolated with critical currents low enough to avoid self‐screening effects. The current‐voltage characteristics of these high‐angle tilt‐boundary weak links are well described by the resistively shunted junction model, if noise rounding is included. The response of the supercurrent to magnetic field and temperature indicates that the weak links are spatially nonuniform, consisting of relatively small areas of high critical current density, Jc, separated by areas with very low or zero Jc. The response to rf power suggests that the current‐phase relation is nonsinusoidal.

Clean grain boundaries and weak links in high Tc superconducting YBa2Cu3O7−x thin films

It is demonstrated that polycrystalline thin films of high Tc superconducting YBa2Cu3O7−x can be grown with clean grain boundaries, i.e., without a boundary layer of segregation or different phase. In clean stoichiometric samples, angular misorientations of grains may be the origin of weak link behavior. High‐resolution scanning transmission electron microscope images of films grown on ZrO2 and MgO by reactive evaporation, reactive sputtering, and laser ablation show atomic lattice images of clean grain boundaries. X‐ray microanalysis with a 10 A spatial resolution also indicates no composition deviation at the grain boundaries. Grain sizes and epitaxial relations of samples prepared by different methods are characterized.

Epitaxial Y-Ba-Cu-O thin films on MgO deposited by high-pressure reactive magnetron sputtering

Y‐Ba‐Cu‐O thin films have been prepared on MgO(100) substrates by rf reactive magnetron sputtering from a single ceramic target. By adopting high total pressures, typically 280 mTorr, and relatively low substrate temperatures of approximately 650u2009°C, epitaxial films with the relations (001) YBa2Cu3Oy∥(001)MgO and [100]YBa2Cu3Oy∥[100]MgO can be reproducibly obtained. A high degree of epitaxy is confirmed by x‐ray pole figure measurements and ion channeling. These films require a brief rapid thermal oxygen anneal at typically 850u2009°C, to exhibit sharp superconducting transitions with zero resistance around 75 K. Films deposited at higher temperatures above 700u2009°C show transitions as deposited with zero resistance near 80 K. The quality of the transition is correlated with expanded lattice constants with the best transitions occurring in films whose lattice constants approach that of the bulk. The films have high critical current densities of 1–5×106 A/cm2 at 4.2 K. They also show good uniformity and excellen...

Thermal relaxation rates of magnetic nanoparticles in the presence of magnetic fields and spin-transfer effects

We have measured the relaxation time of a thermally unstable ferromagnetic nanoparticle incorporated into a magnetic tunnel junction (MTJ) as a function of applied magnetic field, voltage V (-0.38 V < V < +0.26 V), and temperatures (283 K< T< 363 K) . By analyzing the results within the framework of a modified Neel-Brown formalism we determine the effective attempt time of the nanoparticle and also the bias dependences of the in-plane and out-of-plane spin torques. There is a significant linear modification of the effective temperature with voltage due to the in-plane torque and a significant contribution of a field like torque that is quadratic with voltage. The methods presented here do not require complicated models for device heating or calibration procedures, but instead directly measure how temperature, field, and voltage influence the energy landscape and thermal fluctuations of a two-state system. These results should have significant implications for designs of future nanometer-scale magnetic random access memory elements and provide a straightforward methodology to determine these parameters in other MTJ device structures.