SrTiO3 Bicrystal Substrates Research Articles

High-Tcplanar SQUID gradiometer for eddy current non-destructive evaluation

This paper reports the fabrication and test of a high-T-c SQUID planar gradiometer which is patterned from YBCO thin film deposited on a SrTiO3 bicrystal substrate. The measurement of noise spectrum at 77K shows that the white noise at 200 Hz is about 1 x 10(-4)Phi(0)/root Hz. The minimal magnetic gradient is measured and the results suggest that the minimal magnetic gradient is 94 pT/m. The planar gradiometer is used in non-destructive evaluation (NDE) experiments to detect the artifacts in conducting aluminium plates by performing eddy current testing in an unshielded environment. The effect of the exciting coil dimension on the NDE results is investigated. By mapping out the induced field distribution, flaws about 10mm below the plate surface can be clearly identified.

Properties of (Hg,Re)Ba2CaCu2Oy [100]-tilt grain boundary Josephson junctions

[100]-tilt grain boundary Josephson junctions with misorientation of the c-axis directions have been fabricated using epitaxial (Hg0.9Re0.1)Ba2CaCu2Oy [(Hg,Re)-1212] thin films grown on SrTiO3 bicrystal substrates. Two types of the [100]-tilt junctions, that is, the mountain-type junction and the valley-type junction, of which a-b planes formed a mountain shape and a valley shape along the grain boundaries, respectively, were examined for various misorientation angles (θ). Both the types of junctions showed resistively shunted junction-type current-voltage (I-V) characteristics with relatively high characteristic voltages in a wide temperature range from 4.2to110K. The valley-type junctions exhibited properties superior to those for the mountain-type junctions, such as very low excess current and much homogeneous current distribution, which is attributed to different film growth mode near the grain boundaries. They also exhibited the products of the critical current (Ic) and the normal-state resistance (Rn) as high as 3.0–4.2, 1.0–1.4, and 0.2–0.4mV at 4.2, 77, and 100K, respectively. The misorientation angle dependences of junction parameters for the [100]-tilt junctions were compared with those for conventional [001]-tilt junctions with misorientation of the a-b plane directions. The Rn and the IcRn products for the [100]-tilt junctions were substantially larger than those for the [001]-tilt junctions. Large hysteresis in the magnetic-field dependence of Ic was observed for the applied field larger than a certain threshold value, suggesting flux penetration between the CuO2 planes.

Meandering of the grain boundary and d-wave effects inhigh-Tc bicrystal Josephson junctions

We have produced submicrometre wide bicrystal Josephson junctions on the basis ofYBa2Cu3O7−δ (YBCO)films and investigated their microstructure and superconducting properties. A 300 nm thick buffer layerof SrTiO3 onthe SrTiO3 bicrystal substrates has helped to improve the quality and reproducibility of the junctions.The surface of the films at the grain boundary (GB) was investigated by high-resolutionscanning electron microscopy (HRSEM). This has revealed one unit cell steps on theYBCO film surface and meandering of the GB in the YBCO film. Planar transmissionelectron microscopy investigations of the junction have confirmed that the GB inYBCO had a linear deviation from the underlying GB in the substrate up to∼0.4 µm. The observed spread of the angles of the deviation of the GB in the film from the direction ofthe GB in the substrate on the slot- and other defect-free parts of the GB was below10°. Due to the d-wave symmetry of the order parameter in YBCO thereduction of the curvature of the GB leads to an improvement of thehomogeneity of the current flow and a doubling of the critical currentIc of the junction. Theobserved IcRn product ofsymmetric 20° bicrystaljunctions was ∼400 µV at 77.4 K and ∼1 mV at 63 K. Integration of such junctions in SQUIDs with a square 16 mm multilayer fluxtransformer has allowed us, for the first time, to reach a field resolution for thehigh-Tc magnetometers of ∼3.5 fT Hz−1/2 at 77.4 K.

Local dielectric and strain measurements inYBa2Cu3O7−δ thin films by evanescent microscopy and Raman spectroscopy

The near-field evanescent microwave microscope is based on a coaxial transmissionline resonator with a silver-plated tungsten tip protruding through an end-wallaperture. The sensor is used to measure local dielectric properties of thin-filmYBa2Cu3O7−δ deposited onthree different SrTiO3 bi-crystal substrates having mismatch grain boundary angles of3°,6°,and 12°. The measurements in the superconducting state are below critical temperature atT = 79.4 K. The dielectric property of the superconductor within the near field of thetip frustrates the electric field and measurably changes the transmission line’sresonant frequency. The shift of the resonator’s frequency is measured as afunction of tip–sample separation and associated changes in quality factor(ΔQ) image scans of the thin film are presented. A quantitative relationship between the real andimaginary parts of the local dielectric constant and the frequency shift using the method ofimages is established. The comparison between experimental data and theory based on thismethod is given and discussed. Raman measurements of the intergranular strain within theYBa2Cu3O7−δ thin filmdeposited on each SrTiO3 substrate in the region of the bi-crystal junction showed excellent correlation between grainboundary mismatch and induced grain boundary strain. Compressive strains normal to thea axis (i.e. tensilestrains normal to the b axis) were detected across the grain boundary. The magnitude of induced strain as well asits spread away from the grain boundary increased as the mismatch angle increased.

Midgap state-based π-junctions for digital applications

Midgap state-based YBa2Cu3O7−x π-junctions have been fabricated by a focused ion-beam system using 45° symmetric [001] tilt SrTiO3 bicrystal substrates. Measurements, performed by inserting the junctions in a superconducting loop (as a double-phase sensitive test), show both an unconventional nonmonotonic temperature dependence of the Josephson current, with a local minimum at a crossover temperature T* and, around T*, a half flux quantum shift in the critical current versus magnetic field modulations, clear signs of a 0–π crossover with temperature. Such results demonstrate that conventional 45° symmetric grain boundary junctions may have potential for applications, from digital circuits to quantum computing.

The microstructure of La 0.67 Sr 0.33 MnO 3 grain boundaries on SrTiO 3 bicrystal substrates

The aim of this study is to characterize the grain boundary microstructure of artificial La0.67Sr0.33MnO3 (LSMO) grain boundaries obtained by epitaxial growth on SrTiO3 (STO) bicrystal substrates. The films were deposited on STO bicrystal substrates using pulsed laser deposition. The substrates were (001) oriented and the boundaries were symmetrical [001]-tilt boundaries with nominal tilt angle 24° and 37° respectively. The microstructures were characterized using a transmission electron microscope. The thickness of the LSMO films was 32 and 37 nm respectively. The films were relatively smooth and the interface between the substrate and the films was sharp without any evidence of chemical reaction or interdiffusion.

The microstructure of La0.67Sr0.33MnO3 grain boundaries on SrTiO3 bicrystal substrates

We report measurements of the electrical and magnetic properties of bilayers of YBa2Cu3O7 (YBCO) and La2/3Ca1/3MnO3 (LCMO) deposited on SrTiO3 substrates by Pulsed Laser Deposition. We find distinct differences in the properties depending on the order of the deposition, with the lower layer always being degraded. In particular, the YBCO has a Tc suppressed to ∼ 60 K when it is covered by LCMO. We argue that the effect is not due to an electronic interaction, but results from a reduction in the oxygen content of the YBCO by the LCMO.

High-Tc dc-SQUID gradiometers in flip-chip configuration

We describe a new design of a gradiometric flip-chip antenna, which is inductively coupled to a dc-SQUID gradiometer. Both components are patterned out of thin films of the high-Tc superconductor YBa2Cu3O7−x (YBCO). For the flip-chip antenna, a 40 mm × 10 mm SrTiO3 single crystalline substrate is used, while the gradiometer sensors are prepared on 10 mm × 10 mm SrTiO3 bicrystal substrates. Special attention is paid to the inductive coupling between the flip-chip antenna and the read-out gradiometer antenna. We investigate different designs of coupling loops in order to optimize the coupling inductance between both components of the sensor. With optimized coupling the sensor achieves a field-gradient resolution of 12 fT cm−1 Hz−1/2 in the white noise region and of 310 fT cm−1 Hz−1/2 at 1 Hz in the unshielded laboratory environment.

Magnetic domain structure and lattice distortions in manganite films under tensile strain

We performed detailed studies of the magnetic domain structure and strain effects in epitaxial La1−xSrxMnO3(001) films grown on SrTiO3(001) bicrystal substrates by combining magnetic force microscopy, x-ray diffraction, and classical magnetometry. We show that, in addition to magnetic domain walls which nucleate at grain boundaries, 180° magnetic domain walls not associated with structural defects form along the 〈100〉 direction. The size of the magnetic domains are of the order of tens of microns with the magnetic easy axes along the 〈100〉 direction in the plane of the film. Spin reorientation occurs at the grain boundaries, which we attribute to the different strain state compared to the grain interior.

Temperature dependence of magnetoresistance and nonlinear conductance of the bicrystal grain boundary in epitaxial La0.67Ba0.33MnO3 thin films

We investigated conduction through an artificial grain-boundary junction made in La0.67Ba0.33MnO3 thin films, deposited on a 36.7° SrTiO3 bicrystal substrate using a laser ablation technique. The grain boundary exhibits substantial magnetoresistance at low temperatures and also shows nonlinear I–V characteristics. Analysis of temperature dependence of the dynamic conductance allows us to identify three carrier transport mechanisms across the grain boundary. These mechanisms exist in parallel, and at a given temperature one mechanism may dominate. Particularly, at higher temperatures (T>175 K) the transport across the grain boundary involves spin–flip scattering, which we establish leads to decrease of the bicrystal grain-boundary contribution in magnetoresistance. At lower temperature (4.2–45 K), tunneling through a disordered oxide at the grain boundary dominates, whereas in the temperature range from 100 to 175 K, carrier transport is dominated by inelastic tunneling via pairs of manganese atoms.

Local mapping of strain at grain boundaries in colossal magnetoresistive films using x-ray microdiffraction

Using x-ray submicrobeam, we spatially mapped the strain in epitaxial La1−xSrxMnO3 films grown on SrTiO3(001) bicrystal substrates. Our results show that there is an elastic strain gradient at the artificial grain boundary, which decays over a length scale of ∼1 μm. The tensile strain at the interior of the grain—due to the lattice mismatch between La1−xSrxMnO3 and SrTiO3—relaxes as the film nears the grain boundary, yielding a grain boundary lattice constant which approaches the value of that in bulk La1−xSrxMnO3.

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La0.67Ba0.33MnO3 (LBMO) thin film is deposited on a 36.7°C SrTiO3 bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.

Grain boundary faceting in YBa2Cu3O7–x bicrystal thin films on SrTiO3 substrates

The structure of [001] tilt boundaries in YBa2Cu3O7–x (YBCO) thin films deposited on [001] tilt SrTiO3 (STO) bicrystal substrates has been characterized by transmission electron microscopy (TEM). These boundaries are (100)/(210), (310)/(510), (410)/(310), (510)/(210), (210)/(410), and (210)/(310), with corresponding misorientation angles of 26°, 29°, 32°, 37°, 40°, and 44°. It was found that the YBCO film boundaries were meandering along the relatively straight substrate boundaries. High-resolution lattice images indicated that the microscopic meandering of the film boundary essentially consisted of many straight segments of facets at the atomic scale. On the basis of the observed facets, three competing factors controlling the formation of facets are discussed. First, the boundary plane is defined by Miller indices (hk0) in both crystals with sufficiently small h, k (i.e., h, k ≤ 5) and sufficiently large effective interplanar spacing (i.e., deff &gt; 0.06 nm). Second, the closure failure defined by the difference between the local misorientation from the design misorientation is small, i.e., less than 2°. Third, the deviation of a local facet plane is observed to be less than 30° from the design boundary plane. Higher values of deffs are observed to give tolerance to higher deviation angles.

Dependence on film thickness of grain boundary low-field magnetoresistance in thin films of La0.7Ca0.3MnO3

The magnetoresistance of grain boundaries in the perovskite manganites is being studied, both in polycrystalline materials, and thin films grown on bicrystal substrates, because of interest in low-field applications. In this article we show that epitaxial films grown on SrTiO3 bicrystal substrates of 45° misorientation show magnetoresistance behavior which is strongly dependent on the thickness of the film. Thin films, e.g., 40 nm, can show a large low-field magnetoresistance at low temperatures, with very sharp switching between distinct high and low resistance states for fields applied in plane and parallel to the boundary. Thicker films show a more complex behavior of resistance as a function of field, and the dependence on the angle between the applied field and the grain boundary is altered. These changes in magnetoresistance behavior are linked to the variation in morphology of the films. Thin films are coherently strained, due to the mismatch with the substrate, and very smooth. Thicker films relax, with the formation of defects, and hence different micromagnetic behavior.

Near-field scanning optical microscopy studies of electronic and photonic materials and devices

Probing optical properties of materials and optical characterization of crystallographic defects at the nanometer scale have been inaccessible until recently due to the diffraction limit of light. With the invention of near-field scanning optical microscopy (NSOM), resolution at the 50–100 nm level using visible or near infrared light is now practical. In addition to describing the NSOM technique, this review focuses on the application of NSOM to the characterization of electronic and photonic materials and devices, with particular emphasis on defects. The unique capability of NSOM to simultaneously measure surface topography and local optoelectronic properties, thereby eliminating the need to perform cross correlation analysis on results obtained using different techniques, is particularly useful in this area. Several examples are discussed. By performing near-field photocurrent (NPC) measurements, NSOM is used to probe electrical activities associated with individual threading dislocations and dislocation networks in strain relaxed, compositionally graded GeSi films. The non-destructive nature of NSOM helps elucidate how microstructural defects in the SrTiO3 bicrystal substrates affect YBa2Cu3O7 film growth and GBJJ performance. Characterization of III–V and II–VI semiconductors, guantum dots grown by strain epitaxy, laser diodes, waveguides, and photonic crystals is also included. The advantages and disadvantages of NSOM in each application will be outlined. Throughout the review, emphasis is placed on how NSOM complements existing materials characterization techniques, as well as how quantitative results can be obtained from NSOM measurements.

Tl2Ba2CaCu2Ox Bicrystal Junction DC-SQUID Magnetometers Operating in Unshielded Environment

We fabricated direct-current superconducting quantum interference device magnetometers with single layer epitaxial Tl2Ba2CaCu2Ox films on 36.8° SrTiO3 bicrystal substrates. The white flux noise and the field-flux transformation coefficient of the devices are 1.5 × 10-5 Φ0/√Hz and 40 nT/Φ0, respectively. The magnetometers can work in unshielded environment.

Characterization of Josephson Junctions for the Elaboration of High-Tc SQUIDS

In this work we report the fabrication of superconductor loops, formed by an array of two Josephson junctions (JJ) in parallel. The JJ are based on YBa2Cu3O7–x epitaxial thin films deposited on SrTiO3 bicrystal substrates with misorientation angles of 24° and 36.8°. The epitaxial, c-axis oriented YBa2Cu3O7–x films were grown “in situ” by a high oxygen pressure dc sputtering technique. For the fabrication of the superconductor loop a photolithographic method and a non-aqueous chemical etching were used. The superconducting properties of the superconductor loops were analyzed by measuring the current–voltage characteristics. The magnetic field dependence of the critical current Ic(B) and the I–V curves under microwave irradiation showed weak-link Josephson behavior. The dependence of Ic(B) shows an interference pattern due to the phase differences across the both junctions.

A high-temperature superconducting delta-sigma modulator based on a multilayer technology with bicrystal Josephson junctions

We have designed, fabricated and successfully tested a first-order delta-sigma modulator using a high-temperature superconducting multilayer technology with bicrystal Josephson junctions. The circuit has been fabricated on a SrTiO3 bicrystal substrate. The YBa2Cu3O7/SrTiO3/YBa2Cu3O7 trilayer was fabricated by laser deposition. The bottom layer served as a superconducting groundplane. The Josephson junctions were formed at the bicrystal line in the upper layer. The integrator resistance has been made from a Pd/Au thin film. The circuit consists of a dc-SFQ converter, a Josephson transmission line, a comparator, an L/R integrator and an output stage. The correct operation of the modulator has been tested using dc measurements. The linearity of the modulator was studied by measuring the harmonic distortions of a 19.5 kHz sine wave input signal. From the recorded spectrum, a minimum resolution of at least 5 bits can be estimated. This accuracy was limited by the noise of the preamplifier. The correct operation of the current feedback loop was demonstrated by cutting the feedback inductance.

Highly sensitive magnetometers based on YBa2Cu3O7 Josephson junction arrays

The dependence of the critical current of a Josephson junction on the magnetic flux in the junction area can be used for the sensitive detection of external magnetic fields. In contrast to superconducting quantum interference devices, also the measurement of absolute magnetic fields is possible. To increase the transfer function ∂V/∂B, we use serial arrays of up to 105 Josephson junctions between flux-concentrating areas on 24° SrTiO3 bicrystal substrates. We investigate the scaling properties of the critical current Ic, the normal state resistance Rn and the flux density noise SB in dependence on the number of Josephson junctions in the serial array. By the use of a magnetic field modulation scheme, the 1/f noise from critical current fluctuations in the junctions can be suppressed. At 77 K, we achieve a white noise level of SB=1.2 pT/Hz for a 105-junction array.

Tunneling-like magnetoresistance in bicrystal La0.8Sr0.2MnO3−δ thin films

Epitaxial ferromagnetic La0.8Sr0.2MnO3−δ films have been sputtered on SrTiO3 bicrystal substrates. Etched patterns crossing the bicrystal grain boundary are compared with identical patterns not crossing it. The films were annealed at different conditions and their magnetoresistance measured as a function of temperature T and of in plane magnetic field H strength and direction. Annealing at 900 °C was found to modify the grain boundary and to increase its magnetoresistance. For H=±80 Oe parallel to the grain boundary and T=32 K narrow magnetoresistance peaks of 60% height are measured. They are interpreted in the frame of an in plane magnetotunneling structure.