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Presents a study that investigated the microstructure and composition of a YBa[sub2]Cu[sub3]O[sub7-x]/Y[sub2]O[sub3]/YBa[sub2]Cu[sub3]O[sub7-x] trilayer film deposited on manganese oxide. Deposition of the trilayer films; Initial characterization of the films; Interpretation of a cross-section transmission electron microscopy image and the corresponding electron diffraction pattern of the trilayer.

The magnetic and structural properties of as-spun and nanocrystalline (Co[sub 0.95]Fe[sub 0.05])[sub 89-χ] Zr[sub 7]B[sub 4]Cu[sub χ] alloy ribbons (where χ = 0 and 1) are reported. As-spun ribbons were synthesized by melt spinning with subsequent heat treatments used to stabilize the nanocrystalline microstructure. By differential thermal analysis, the onset of primary crystallization was found to occur at 475 and 481 °C for the alloys with and without Cu, respectively. The phase produced by primary crystallization was identified as a face-centered cubic-(Fe,Co) having grains ranging in size from 5 to 17 nm. The nanoscale morphology of the materials provides low hysteretic losses, as evident by the 0.26 Oe coercive field for a sample annealed at 550 °C for 1 h. In addition, the crystalline phase provides a magnetization of 1.57 T. [ABSTRACT FROM AUTHOR]

A toroid with primary and secondary windings is used as a transformer to generate magnetic hysteresis curves. The primary winding is driven by a signal generator, which induces an alternating voltage in the secondary winding. Both input and output voltages are captured using a digital storage oscilloscope and processed to generate and display a hysteresis curve. We show such curves are representative of the material used as the transformer core. Data acquisition and processing steps are presented in a manner suitable for use in an undergraduate laboratory or lecture demonstration. [ABSTRACT FROM AUTHOR]

The core losses of a number of ferrite compositions as well as several metallic ferromagnetic materials have been measured at 78 K. From these measurements we can estimate the power dissipation of a conventional inductor (copper windings, magnetic core) at low temperature. It is found that the magnetic losses generally increase on cooling; this counteracts the reduction in copper losses, and the dissipation would not be much different than its room temperature value. If superconducting windings are substituted for the copper, there remains a tradeoff between losses in the core and in the superconductor. Optimization of this combination will require extensive modeling; it may be that the best solution involves no magnetic core at all. [ABSTRACT FROM AUTHOR]

A new method is described for measuring the critical current density and transition temperature of a superconducting film without making contact to it or modifying it in any way. This technique is particularly well suited for use with high transition temperature oxide films which are notoriously irreproducible and sensitive to patterning. It consists of positioning a flat, multiturn coil near the film surface and driving the coil with an audio frequency sine-wave current. Induced shielding currents flow in the film. We have calculated the radial dependence of the induced currents and show that the induced current density is zero at the coil center, rises to a maximum near the mean radius of the drive coil, and then falls off rapidly as the radius continues to increase. A measurement of the critical current per length can be obtained by monitoring the development of odd harmonic voltage components across the coil as the drive current is increased. We find that this measure of nonlinearity in the coil–film system increases abruptly when the maximum induced current equals the critical current. The critical current obtained by this inductive measurement has been demonstrated to give nearly the same value as that obtained by a transport measurement. Other advantages to this inductive measurement approach include: sensitivity only to intergranular critical current, and an ability to accommodate large substrates. Finally, with only minor modification to the electronics used to measure Jc, the superconducting transition temperature of the sample may be measured as well. [ABSTRACT FROM AUTHOR]

A simple apparatus for liquefying neon is described for use in experiments that require a stable thermal environment at 27 K. This apparatus makes use of a dosed-cycle refrigerator attached to an insulated cold finger inserted into a conventional stainless steel Dewar to liquefy a quantity of neon gas stored previously in a tank. The system is inexpensive and easy to use. Furthermore, the neon is recovered for subsequent reuse upon completion of the experiment. Starting with a Dewar holding a small coil that had not been precooled, and using a refrigerator with rated cooling capacity of 15 W at 26 K, approximately 0.5 l of liquid neon was condensed into the Dewar in ∼4 h. The apparatus has been used to measure the critical currents of high T[sub c] oxide tape coils both with and without the application of an external magnetic field. [ABSTRACT FROM AUTHOR]

The temperature dependent metal-insulator transition in VO2 films displays substantial hysteresis. This is usually attributed to supercooling or superheating in the grains. In the hysteretic region at fixed temperature there is a relaxation of the resistance toward a presumed equilibrium value. We have made careful measurements of this relaxation, and find that it proceeds with a logarithmic dependence on time. If the transformation is thermally activated in individual grains, the log(t) behavior can be explained either by a wide distribution of activation energies U among grains or a dependence of U on the phase of adjacent grains. [ABSTRACT FROM AUTHOR]

An approach to estimate the hysteretic loss in a coil wound with superconducting tape is presented. The loss depends on magnetic fields in the superconductor at peak coil current. For a given total current, the fields depend on how the current is distributed across the tape width. The loss is calculated as a function of a single parameter characterizing a range of plausible current distributions. The minimum should represent an upper bound to the actual loss. Results are presented for a specific pancake coil example, both isolated and incorporated as part of an assembly of stacked, identical coils. [ABSTRACT FROM AUTHOR]

Superconducting films are expected to contribute nonlinear effects in microwave circuits due to a current dependence of penetration depth λ. Data from a two-coil mutual inductance measurement can be used to extract the coefficient J[sub 0] representing a dependence of the form λsup2;(J) = λ²(0)[1 + (J/J[sub 0])²]. A method of extracting J[sub 0], using a measurement of third-harmonic generation, is compared to one previously reported. The method is significantly easier to use and probably more accurate. Measurements on a number of YBa[sub 2]Cu[sub 3]O[sub 7-x] films at 78 K have shown no sign of a nonquadratic nonlinearity, and give values of J[sub 0] as high as ∼100 MA/cm². [ABSTRACT FROM AUTHOR]

A direct measurement of strain in magnetostrictive materials yields a meaningful result only if the sample remains saturated and the magnetization is rotated between directions parallel and perpendicular to the measurement direction. A simple way to accomplish this is to use small permanent magnets mounted on a rotating shaft. Within the constraints of the limited available space for a sample, we found it most convenient to use a strain gage to detect small changes in length. With commonly available laboratory equipment we were able to achieve a sensitivity ∼0.03 ppm. There is a loading effect that reduces the apparent magnetostriction if the sample is of comparable size to the strain gage. We showed that for a typical nanocrystalline ribbon sample this correction could be experimentally estimated and was of order 20%. [ABSTRACT FROM AUTHOR]

The magnetic, structural, and transport properties of the Heusler alloy Co[sub 2]MnSi are reported for sputtered thin films and a single crystal. X-ray diffraction reveals a phase pure L2[sub 1] structure for all films grown between 573 and 773 K. Films grown at 773 K display a four-fold decrease in the resistivity relative to those grown at lower temperatures and a corresponding 30% increase in the residual resistivity ratio (ρ[sub 300 K]/ρ[sub 5 K]). We show that the higher growth temperature results in lattice constants, room temperature resistivities, and magnetic properties that are comparable to that of the bulk single crystal. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Characterization results are given for a set of TiNx films grown on sapphire substrates at temperatures between 140 and 850 °C. A relationship between resistivity and spectral reflectivity is established, with highest reflectivities and lowest resistivities observed for the highest substrate temperatures. It is found that grain orientation within the films is random except at the highest deposition temperature where a preferred 100 orientation is indicated. Measurement of the superconducting transition temperature, lattice parameter, and microcrystalline texture of each film has been made and correlated with the stoichiometry, electrical, and optical properties. [ABSTRACT FROM AUTHOR]

We have prepared thin films of Y1Ba2Cu3O7-x using a coevaporation technique and measured their critical current densities as a function of both temperature and field using a transport technique. For a stoichiometric composition we find that Jc at 4.2 K in zero field is 1.04×106 A/cm2 and does not drop below 105 A/cm2 until T>60 K. Fitting the data near Tc shows that Jc follows a power law of (1-t), with t being the reduced temperature. In an applied field perpendicular to the film’s surface, Jc also drops slowly, and up to 90 kOe Jc>105 A/cm2 for T=4.2 K. Measurements for an off-stoichiometric film, Y1Ba3Cu2O7-x, give lower Jc values that fall off faster in temperature than for stoichiometric compositions. [ABSTRACT FROM AUTHOR]

Thin superconducting, granular films of NbN/BN are investigated as photodetectors for pulsed, far-infrared radiation. These films may be thought of as a random network of Josephson junctions. The absorption of photons causes the film resistance to increase. This is thought to occur by destroying the phase coherence of the Cooper pair wave function among the junctions. This resistance change can be used as a basis for a photodetector. It is found that the NbN/BN thin-film detectors have response times faster than 1 ns, and sensitivities of about 0.7 V/W when terminated to 50 Ω, at wavelengths of about 0.50 mm. [ABSTRACT FROM AUTHOR]

It is demonstrated here that granular films of Y-Ba-Cu-O may serve as optical detectors, operating at wavelengths from the visible to the far infrared, at temperatures well above that of liquid helium. Preliminary measurements using a blackbody source show that an upper bound of the minimum detectable power is 1 μW. The response time as determined by a pulsed far-infrared source is of the order 20 ns. Methods to improve the sensitivity will be discussed. [ABSTRACT FROM AUTHOR]

We have studied the transport properties of disordered WSi films near the metal/insulator transition (MIT) and we have also reviewed the data for several other disordered materials near their MIT. In all cases, we found the presence of enhanced superconductivity. We constructed a superconductivity "phase diagram" (i.e., T(c) versus sigma) for each system, which reveals a striking correlation: In all cases, T(c) values are significantly enhanced only for samples whose conductivities lie within a narrow range on the metallic side of, and moderately near, the MIT. We present a heuristic model to explain this phenomenon.