J. H. Claassen

A contactless method for measurement of the critical current density and critical temperature of superconducting films

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 ...

Optical detection in thin granular films of Y‐Ba‐Cu‐O at temperatures between 4.2 and 100 K

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.

Magnetic, structural, and transport properties of thin film and single crystal Co2MnSi

The magnetic, structural, and transport properties of the Heusler alloy Co2MnSi are reported for sputtered thin films and a single crystal. X-ray diffraction reveals a phase pure L21 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 (ρ300 K/ρ5 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.

AC losses in a finite Z stack using an anisotropic homogeneous-medium approximation

A finite stack of thin superconducting tapes, all carrying a fixed current I, can be approximated by an anisotropic superconducting bar with critical current density Jc = Ic/2aD, where Ic is the critical current of each tape, 2a is the tape width, and D is the tape-to-tape periodicity. The current density J must obey the constraint , where the tapes lie parallel to the x axis and are stacked along the z axis. We suppose that Jc is independent of field (Bean approximation) and look for a solution to the critical state for arbitrary height 2b of the stack. For c<|x|<a we have J = Jc, and for |x|<c the critical state requires that Bz = 0. We show that this implies in the central region. Setting c as a constant (independent of z) results in field profiles remarkably close to the desired one (Bz = 0 for |x|<c) as long as the aspect ratio b/a is not too small. We evaluate various criteria for choosing c, and we show that the calculated hysteretic losses depend only weakly on how c is chosen. We argue that for small D/a the anisotropic homogeneous-medium approximation gives a reasonably accurate estimate of the ac losses in a finite Z stack. The results for a Z stack can be used to calculate the transport losses in a pancake coil wound with superconducting tape.

NbN/BN granular films—a sensitive, high‐speed detector for pulsed far‐infrared radiation

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.

Epitaxial growth of niobium thin films

Abstract The properties of Nb films grown in a UHV “metal MBE” system are described. The films were characterized by high energy electron diffraction, various X-ray diffraction techniques, as well as transport measurements. It is noted that the epitaxial relationships between (0001), (01 1 0) and (11 2 0) sapphire substrates and the Nb films grown on them represents a three-dimensional registry between the two lattices. Rocking curve widths, superconducting transition temperatures, and residual resistance ratios all are representative of very high quality films. An enhancement of the transition temperature by ∽ 0.3 K occurs in strained textured films grown at ambient temperature on the three orientations mentioned above. Epitaxy is found to occur on the (1 1 02) sapphire face at growth temperatures as low as ambient.

Pulsed laser deposition of high‐quality NbN thin films

Pulsed laser deposition has been used to grow superconducting NbN thin films from niobium targets in a reactive gas atmosphere of N2 (10% H2). The structural and electrical properties of the deposited films have been determined as a function of substrate temperature and crystallographic orientation. Highly textured NbN was deposited on MgO (100). Films deposited on MgO at 600 °C in 60 mTorr gas pressure were characterized by Tc=16.6 K, Jc (4.2 K)=7.1 MA/cm2, and λ(0)=3200 A. Films grown on amorphous fused silica, under the same conditions, were polycrystalline and characterized by Tc=11.3 K and Jc (4.2 K)=1.8 MA/cm2.

Temperature and field dependence of the critical current densities of Y-Ba-Cu-O films

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, Y1Ba3 Cu2O7−x, give lower Jc values that fall off faster in temperature than for stoichiometric compositions.

Optimizing the two-coil mutual inductance measurement of the superconducting penetration depth in thin films

When a pair of coils is positioned on opposite sides of a superconducting thin film, measurement of their mutual inductance may in principle be used to infer the penetration depth λ in the superconductor. We have studied how to optimize this measurement with respect to coil design, and have found that the approach that has been generally used is far from the optimum. Useful simplifications to the expression relating mutual inductance to penetration depth are derived. An analysis of the sources of uncertainty in determining λ is presented. For an optimized coil set, the major source of uncertainty often is uncertainty of the thickness of the film. The sensitivity to changes in λ is also studied; it is shown that this can approach 1 pm for a typical high temperature superconductor sample. Finally, it is shown that the analysis may be extended to normal metal films, with the skin depth playing a role similar to that of the penetration depth in superconductors. Measurement of a high conductivity normal metal ...

Structure and magnetic properties of (Co,Fe)-based nanocrystalline soft magnetic materials

The magnetic and structural properties of as-spun and nanocrystalline (Co0.95Fe0.05)89−xZr7B4Cux alloy ribbons (where x=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.