P.R. Broussard

Andreev reflection: A new means to determine the spin polarization of ferromagnetic materials

We have developed a simple method to measure the transport spin polarization of ferromagnetic materials. This technique relies on the fact that the Andreev reflection process at the interface between a superconductive and normal is influenced by the spin polarization P of the normal metal. In a very short time we have been able to measure the spin polarization of several metals: NixFe1−x, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2, whose spin polarization ranges from 35% to 90%. Our results compare well with other methods for measuring P.

Observation of a transverse voltage in the mixed state of YBCO thin films

The occurrence of a transverse voltage peak has been observed in YBCO thin films. This phenomenon occurs under conditions of temperature and current in the earths magnetic field such that the films are in the mixed state. The magnitude of the peak was found to scale with both temperature and current, and exhibited a Sin(/spl theta/) orientational dependence measured between the film-substrate and the position of the measurement electrodes on the film surface. These observations are similar to observations made in conventional superconducting thin films such as Nb, NbN, NbCN, and PbBi.<<ETX>>

YBa2Cu3O7−y/Y2BaCuO5 composites: Growth and characterization

We have prepared thin film composites of Y1Ba2Cu3O7−y (‘‘123’’) and Y2Ba1Cu1O5 (‘‘211’’) by off‐axis sputtering from separate targets of the 123 and 211 material. X‐ray diffraction on the films shows the presence of c‐axis oriented 123, and (00L) ordered Y2O3, but no indication of 211 lines. The c‐axis lattice constant does not show any change compared to our pure 123 films. As the volume percentage of 211 material increases, we see a reduction in Tc and Jc, and an increase in the resistivity. The temperature and field dependence of the critical current are different for our composites compared to our pure 123 films.

Laser irradiation effects in YBa2Cu3O7−δ thin films on metallic and single crystal substrates: a micro-Raman study

Abstract In-depth micro-Raman study of YBa 2 Cu 3 O 7−δ thin films deposited on metallic substrates (hastelloy-C, inconel-600 and stainless steel-304) with a TiN buffer layer (YBCOue5f8M) and single crystal LaAlO 3 (YBCOue5f8L) substrate has been carried out in order to investigate the relative phenomenological differences between the behavior of these films on different substrates. A comparative study of films on these substrates revealed that the bandwidths of the prominent oxygen vibrations in YBCOue5f8M are significantly larger than those in the case of YBCOue5f8L and a defect induced, strong and broad feature around 570 cm −1 is always present in the Raman spectra of YBCOue5f8M. The broadening of bands and the presence of the 570 cm −1 band have been attributed to the disorder induced by buffer layer in YBCOue5f8M. Variation of the incident laser power ( P L ) from 1 to 10 mW, or a continuous irradiation were found to induce unusual, irreversible but, dissimilar changes in the Raman spectra of YBCOue5f8M and YBCOue5f8L. A significant change in the intensity distribution of oxygen vibrations and a decrease in the bandwidth of the out-of-phase mode in YBCOue5f8M films have been attributed to an improvement in the crystallinity of YBCOue5f8M films due to oxygen ordering in the CuO 2 planes. Whereas, in YBCOue5f8L, the laser irradiation causes oxygen removal from the CuO chains and formation of impurity phase BaCuO 2 . Distinct laser induced changes in the Raman spectra of YBCOue5f8M and YBCOue5f8L have been explained in terms of photoinduced ordering and local heating effects respectively.

Effect of surface layers on ferromagnetic resonance in thin Fe films: Ni, Co, Si, and YBa2Cu3O7−δ

The results of ferromagnetic resonance experiments on 10 nm polycrystalline films of Fe with very thin surface layers of Ni and Co, using Si and a high temperature superconductor (YBCO) as substrates is reported herein. An earlier fmr investigation of Fe films on YBCO showed strongly temperature dependent linewidths and resonance fields but only small effects in Fe on Si. In this work, the resonance of Fe on Si is found to be significantly narrower when the Fe film has thin (0.5 to 1.0 nm) Ni surface layers, compared to uncoated Fe; linewidths are as narrow as 15 Oe at 9 GHz. However, when YBCO substrates are used, the iron resonance properties are nearly the same as in the case where no Ni was used. This indicates that the effects seen in earlier work were not the result of an oxide layer on the Fe but may indicate a magnetically active surface layer in the YBCO. Co surface layers have the effect of producing strongly temperature dependent anisotropies and linewidths on all substrates used, with the stro...

Resistivity anomaly in nonmagnetic metals with ferromagnetic insulator proximity layers

Epitaxial bilayers and multilayers were prepared using the ferromagnetic insulator EuO and the metals V and Ag. For metal films of thickness 10 nm or less, there is a sharp change in the slope of the resistivity versus temperature, which occurs at 86 K for EuO/V specimens and at 50 K for EuO/Ag specimens. Soft x‐ray magnetic circular dichroism measurements showed a clear dichroism for Eu but none for V. Therefore, the dR/dT discontinuity is attributed to a change in spin‐dependent surface scattering at the insulator/metal interface caused by the magnetization of the surface layer of Eu++ ions. The difference in temperature between the dR/dT discontinuities between the two metals (and from the bulk EuO Curie temperature of 70 K) is tentatively attributed to the influence of the Ruderman–Kittel–Kasuya–Yosida (RKKY) mechanism.

Structural inhomogeneities in thin epitaxial films of YBa2Cu3O7−δ and their effects on superconducting properties

Abstract Structural inhomogeneities in thin single-crystal films of YBa 2 Cu 3 O 7− δ are characterized using high resolution X-ray diffraction. Results of these measurements show a distribution in the c -axis lattice parameter and variations in the integrated intensities of (00l) diffraction peaks. Both effects can be interpreted as due to a variance in the oxygen content. Films prepared under identical growth conditions, when subjected to different post-annealing treatments showed varying structural and superconducting properties. A film with 7− δ close to 7.0 had a transition temperature (T c ) of 91 K, a width ( Δ T c ) of 0.6 K, and a critical current density (J c ) of 6.5×10 6 A/cm 2 , whereas a film in which the oxygen content varied between 6.86 and 6.96 had a T c of 87 K, a Δ T c of 0.5 K, and a J c of 8.17×10 6 A/cm 2 . For each of the samples measured, the full width at half maximum (FWHM) of the (006) diffraction peak ranged between 340 and 2100 arc-seconds and was dependent on the method of preparation and the substrate. The J c s of these films were not sensitive to the quality of the film, as determined from the FWHM of the (006) peak, and the oxygen content, as determined from the (005) to (006) integrated intensity ratio. High resolution X-ray topographs showed uniform microstructure of the films, indicating that oxygen distribution is finely dispersed in the film.

Microscopic x-ray characterization of inhomogeneities in YBCO crystals with sharp superconducting transitions

Abstract High resolution x-ray diffraction measurements of YBCO single crystals with optimal transport properties reveal gross structural inhomogeneities, such as domains of different oxygen content and high dislocation densities. Thin epitaxial YBCO films show similar structural inhomogeneities.

Measurement of the transport spin-polarization of oxides using Point Contact Andreev Reflection (PCAR)

A new method for determining the transport spin-polarization, point contact tunneling from a low temperature superconductor into a ferromagnet, is used to determine the spin polarization of various ferromagnetic thin films and crystals. The Andreev reflection process and its utility in measurements of spin-polarization are described. Preliminary results for the spin polarization of several oxide materials are presented.

Conduction across the interface between a superconductor and a spin polarized metal

We have studied Andreev reflection in point contacts formed between several low T/sub c/ superconductors and various spin polarized normal metals. We have also developed a theoretical model for the phenomenon. There is good accordance between theory and experiment.