Magnetic field penetration into YBa₂Cu₃O₇ thin film deposited on a cleaved MgO substrate

Abstract

A thin film of YBa2Cu30, was deposited by laser ablation onto the (001) surface of a cleaved MgO substrate. The magnetic field penetration into the thin film was photographed using a magneto-optic apparatus. The field penetrates into the sample along specific directions that we identify with the cleavage steps that are more than 0.25 pm high.Une couche mince de YBa2Cu30, a C t t dCposCe par ablation au laser sur la surface (001) d'un substrat de MgO. La penetration du champ magnCtique dans cet Cchantillon a CtC photographiCe en utilisant un montage magnCto-optique. Le champ pCnbtre le long de directions spCcifiques que nous avons identifiCes comme ttant des pas de clivage d'une hauteur supirieure i 0.25 pm.Can. J. Phys. 69, 786 (1991) The applications of the new oxide superconductors in the field of electronics will occur in thin films (1). Many techniques of film fabrication have proven useful and in all of them, the interaction between the film and the substrate is critical. Single crystal substrates, with interatomic spacing that match the superconducting oxide lattice, have been used to grow polycrystalline or single-crystal films with the c axis oriented normal to the plane so that the highly conducting a-b plane is parallel to the substrate surface. In the polycrystalline oriented films, the grain boundaries are weak links and in single-crystal films, twin boundaries separating regions in which the a and b axes alternate directions also constitute weak links. These naturally occumng weak links have been used to produce Josephson junctions (2) but the properties of the superconducting film have also been deliberately weakened by reducing the thickness of the superconducting film or altering its composition (doping with non-superconducting compound or removal of oxygen) over a microscopic region. Conventional photolithography, laser annealing, or ion-beam milling are a few methods that have been used to pattern these films to produce electronic devices (2).To obtain effective devices, the channels patterned in these films should show uniform properties but Frenkel et al. (3) have observed that the critical current density, Jc, in a 25 pm wide channel of YBa2Cu307 on SrTiO, is not uniform. Imaging Jc with a low-temperature scanning electron microscope (LTSEM), they were able to show that some of these inhomogeneities could be correlated with scratches in the substrate visible on SEM pictures. They suggested that the reduced J, resulted from the current travelling along the c axis on the walls of the scratch.The discontinuity in film properties that occurs at steps in the substrate has been used previously to manufacture electronic devices using low-temperature superconductors (4, 5 ) . Similar fabrication methods are now being suggested for hightemperature superconductors. In this letter, we show directly how field penetration occurs in superconducting oxide films at cleavage steps on a MgO substrate.The YBa2Cu307 films was deposited onto a cleaved (001)-oriented MgO single crystal, 11 x 12 mm2, using the p...