Yu.V. Bugoslavsky

Highly aligned, spin polarized thin films of Sr2FeMoO6 by a chemical vapor process

Highly oriented films of Sr2FeMoO6 were fabricated by ultrasonic spray pyrolysis. A combined organic/inorganic solution was developed and the films were deposited on single-crystal LaAlO3 between 600 °C and 900 °C followed by postannealing at 850 °C or 1200 °C in Ar/5% H2. Optimum films showed a metallic resistivity behavior with less than a 0.25% magnetoresistive effect at 50 K, 1 T, indicative of highly quality intra- and intergranular material. At 4.2 K, the lower bound saturation magnetization, Ms was ∼2.5 μB/f.u. and the transport spin polarization was 60±3%.

The microwave surface impedance of MgB2 thin films

In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB2. The powder sample has a Tc = 39 K and the films have Tc = 29 K and 38 K. These samples show different temperature dependences of the field penetration depth. Over a period of six months, the film with Tc = 38 K degraded to a Tc of 35 K. We compare the results on all samples with data obtained elsewhere and discuss the implications as far as is possible at this stage.

Enhanced intergrain magnetoresistance in bulk Sr2FeMoO6 through controlled processing

Enhanced low field magnetoresistance (LFMR) values have been obtained in bulk Sr2FeMoO6 through controlled processing. MR values, R(H)/R(0) of ∼60% [(R(H)−R(H=0)/R(H=0)=40%] were achieved at 50 K, in 1 T, and R(H)/R(0)∼8% [(R(H)−R(H=0)/R(H=0)=92%] at 290 K, in 1 T. For a ∼1 μm starting particle size, the optimum sintering temperature was 1300 °C, 100 °C higher than normally used. The optimum sintering time in Ar/H2 was only 5 h, much shorter than normally used. The magnitude of the LFMR appears to depend strongly on grain boundary oxygen content, controlled through optimization of sintering temperature and time.

Superconducting critical fields and anisotropy of a MgB2 single crystal

Using a double axis vibrating sample magnetometer, we have made detailed magnetic measurements of the lower critical field Hc1 for fields parallel to the two crystallographic directions of MgB2 single crystals. Additionally, using a novel Hall probe magnetometer we have measured high precision magnetization loops, from which we directly determine the upper critical field Hc2 for both field orientations. Our results suggest that Hc1 is much larger than most previous estimates and that consequently the Ginzburg–Landau parameter κ is very low (less than 5). We find the anisotropy parameter γ ~ 2, independent of temperature over the measured range.

Structure of the superconducting gap in MgB2 from point-contact spectroscopy

We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 ? 0.3 K and resistivity of about 20 ?? cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, ?1 = 2.3 ? 0.3 meV and ?2 = 6.2 ? 0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder?Tinkham?Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic coexistence of two values of the gap are discussed.

Microwave surface resistance of MgB2

The microwave power and frequency dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by the breakdown in the ω2 law for poor-quality samples at all temperatures. The performance of MgB2 at 10 GHz and 21 K was compared directly with that of high-quality YBCO films. The surface resistance of MgB2 was found to be approximately three times higher at low microwave power and showed an onset of nonlinearity at microwave surface fields ten times lower than the YBCO film. It is clear that MgB2 films are not yet optimized for microwave applications.

Deconvolution of 2D transport currents from the local magnetic field distribution above flat superconducting samples

Measurements of the magnetic field distribution (magnetic images) above flat superconducting samples provide information on the pattern of current flow within the material. While several deconvolution procedures to calculate the current flow have been proposed, we show here that they do not always give accurate results in situations where the current flow extends to regions that are outside of the magnetic image window, as with transport currents in a conductor. We describe two new procedures which overcome the problem. The most accurate (and most direct) approach is to incorporate magnetic images at different heights above the sample. The second method relates to the more conventional case using only one field image at a finite height above a thin sample (but of finite thickness). The results compare favourably to existing approaches.

Effect of chemical substitution on the electronic properties of highly aligned thin films of Sr2-xAxFeMoO6 (A=Ca, Ba, La; x=0, 0.1)

Highly aligned films of composition Sr1.9A0.1FeMoO6 (A=Ca,Ba,La) and undoped control samples, were fabricated on (100) single-crystal LaAlO3 by ultrasonic spray pyrolysis. Compared to undoped films, the doped films showed reduced saturation magnetization and magnetoresistance and the lanthanum-doped film showed a significant increase (∼40 K) in the Curie temperature. The electrical resistivity was well described by a spin-dependent transport model for all the films. The transport spin polarization has been determined with this model, and independently by point contact Andreev reflection spectroscopy. The doped films do not have improved polarization over the undoped films and the maximum P obtained is of the order of 63%.

Highly disordered intergrowths in Sr2FeMoO6

Two Sr2FeMoO6 samples, synthesized under different conditions and showing very different magnetotransport properties, were analyzed using transmission electron microscopy. The different magnetoresistive behaviors observed in the two samples coincide with the formation of different amounts of striped areas intergrown in the samples. The striped areas are comprised of a disordered and cation deficient phase, SrFe1−x−yMox−zO3−δ. The sample magnetization and magnetoresistance values decrease significantly with an increased presence of the striped phase. The striped phase was promoted by typical reducing formation conditions for Sr2FeMoO6 in Ar-1% H2 gas.

Scanning potentiometry and magnetic imaging of current transport in high-temperature superconductor coated conductors

Second generation Y1Ba2Cu3O7-δ (YBCO) coated conductors promise greater performance than first generation powder-in-tube technology. Along with the development of these materials, advances in characterization techniques have enabled detailed studies of current flow, grain boundaries and homogeneity. We have developed a scanning probe system which can be configured to map either the magnetic field or electric potential at the sample surface (with a local Hall sensor or voltage probe respectively). While the magnetic image can be used to determine the current flow pattern, the potential map allows direct measurement of the local dissipation within the samples.