Y. Bruynseraede

Magnetization reversal in patterned ferromagnetic and exchange-biased nanostructures studied by neutron reflectivity (invited)

We have measured the off-specular polarized neutron reflectivity of periodic arrays of micron-sized rectangular polycrystalline ferromagnetic Co bars and exchange-biased Co∕CoO bars, which were prepared by a combination of electron-beam lithography and evaporation techniques. The intensity of the first-order off-specular neutron satellite reflection was monitored as function of the magnetic field parallel to the long edge of the bars, allowing analysis of the magnetization reversal process using the four spin-polarized scattering cross sections. The neutron data are compared with calculations based on a micromagnetic simulation. The influence of shape anisotropy on the reversal mechanism is demonstrated.

Giant magnetoresistance in Fe/Cr superlattices with very thin Fe layers

Carefully tailored Fe/Cr epitaxial superlattices with extremely thin Fe layers have been grown on MgO(100) by molecular beam epitaxy. The low‐angle x‐ray spectra reveal the presence of sharp interfaces down to an Fe layer thickness of a few monolayers. An [Fe(4.5 A)/Cr(12 A)]50 superlattice shows a 220% magnetoresistance at 1.5 K, and a saturation field of 110 kOe. A further decrease of the Fe layer thickness produces a drastic decrease in the magnetoresistance.

MAGNETIC PROPERTIES OF SUBMICRON CO ISLANDS AND THEIR USE AS ARTIFICIAL PINNING CENTERS

We report on the magnetic properties of elongated submicron magnetic islands and their influence on a superconducting film. The magnetic properties were studied by magnetization hysteresis loop measurements and scanning-force microscopy. In the as-grown state, the islands have a magnetic structure consisting of two antiparallel domains. This stable domain configuration has been directly visualized as a

Photoinduced enhancement of superconductivity

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SUPERCONDUCTIVITY AND GAINT NEGATIVE MAGNETORESISTANCE IN YBA2CU3O7/LA0.67BA0.33MNO3 SUPERLATTICES

-checkerboard pattern by magnetic-force microscopy. In the remanent state, after magnetic saturation along the easy axis, all islands have a single-domain structure with the magnetic moment oriented along the magnetizing field direction. Periodic lattices of these Co islands act as efficient artificial pinning arrays for the flux lines in a superconducting Pb film deposited on top of the Co islands. The influence of the magnetic state of the dots on their pinning efficiency is investigated in these films, before and after the Co dots are magnetized.

Shapiro steps in a superconducting film with an antidot lattice

We show clear and conclusive experimental evidence for the enhancement of superconductivity in GdBa2Cu3Ox and YBa2Cu3Ox thin films by photoexcitation. Upon laser illumination the critical temperature increases while the resistivity of the material decreases. The relaxation back to the original state is very slow, of the order of days at room temperature. The existence of this effect opens the possibility of fabricating weak‐link devices with in situ tunable superconducting characteristics.

Controlled preparation of oxygen deficient YBa2Cu3Ox films

It is shown that the recently discovered giant negative magnetoresistance (GMR) effects in ferromagnetic La1−xMxMnO3 (M=Ca,Sr,Ba) are compatible with the existence of high Tc superconductivity in c‐axis oriented YBa2Cu3O7/La0.67Ba0.33MnO3 superlattices. From magnetoresistivity measurements we deduce that above Tc the giant GMR persists up to room temperature. Below Tc the superlattices show a quasi‐two‐dimensional superconductivity of the YBa2Cu3O7 layers coexisting with magnetism in the La0.67Ba0.33MnO3 layers.

Preparation and structural characterization of thin epitaxial Bi2Sr2CaCu2O8+δ films with Tc in the 90 K range

Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in the voltage-current characteristics of a superconducting film with a square lattice of perforating microholes (antidots)in the presence of radiofrequent radiation. These equidistant steps appear at the second matching field H_2 when the flow of the interstitial vortex lattice in the periodic potential created by the antidots and the vortices trapped by them, is in phase with the applied rf frequency. Therefore, the observation of Shapiro steps clearly reveals the presence of mobile intersitial vortices in superconducting films with regular pinning arrays. The interstitial vortices, moved by the driving current, coexist with immobile vortices strongly pinned at the antidots.

Flux phases and quantized pinning force in superconductor with a periodic lattice of pinning centres.

We describe a simple method based on the oxygen pressure‐temperature phase diagram to prepare oxygen deficient YBa2Cu3Ox thin films. Systematic critical temperature and x‐ray diffraction experiments clearly show that films with different oxygen contents (6.6≤x≤7) are obtained in a controlled, reproducible, and reversible way.

Etching of screw dislocations in YBa2Cu3O7 films with a scanning tunneling microscope

Abstract Thin films of Bi 2 Sr 2 CaCu 2 O 8+δ with T c up to 92 K were prepared by an in situ sputtering method on SrTiO 3 (100) and LaAlO 3 (100) substrates. A strong c -axis orientation of the film growth with low mosaic spread and full epitaxy within the ab -plane were confirmed by X-ray diffraction in Bragg-Brentano and four-circle geometry. Rutherford backscattering and channeling confirm the correct film composition and highly textured growth with a minimum yield of 23%. The surface morphology of the films was examined by STM and SEM studies, revealing a low density of precipitates and the absence of screw dislocations. The depth profile of the composition was investigated by secondary neutrals mass spectroscopy (SNMS).