J. E. Villegas

Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3/YBa2Cu3O7-δ superlattices

We study the interplay between magnetism and superconductivity in high-quality YBa_(2)Cu_(3)O_(7)(YBCO)/La_(0.7)Ca_(0.3)MnO_(3)(LCMO) superlattices. We find evidence for the YBCO superconductivity depression in the presence of the LCMO layers. We show that due to its short coherence length, superconductivity survives in the YBCO down to a much smaller thickness in the presence of the magnetic layer than in low Tc superconductors. We also find that for a fixed thickness of the superconducting layer, superconductivity is depressed over a thickness interval of the magnetic layer in the 100 nm range. This is a much longer length scale than that predicted by the theory of ferromagnetic/superconducting proximity effect.

Superconductivity depression in ultrathin YBa2Cu3O7−δ layers in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices

We report on the depression of the superconducting critical temperature of ultrathin YBa2Cu3O7 (YBCO) layers, when their thickness is reduced in the presence of La0.7Ca0.3MnO3 (LCMO) magnetic layers in [LCMO (15 unit cells)/YBCO (N unit cells)] superlattices. The thickness of the manganite layer is kept at 15 unit cells and the YBCO thickness is varied between N=12 and N=1 unit cells. The structural analysis, using x-ray diffraction and electron microscopy, shows sharp interfaces with little structural disorder. While a critical temperature, TC=85u2009K, is found for 12 YBCO unit cells, superconductivity is completely suppressed for YBCO layer thickness below 3 unit cells. The possible interaction between superconductivity and magnetism is investigated.

Magnetism and superconductivity in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices

We report on the magnetic and superconducting properties of La0.7Ca0.3MnO3/YBa2Cu3O7 (LCMO/YBCO) superlattices. For a constant LCMO layer thickness of 6 unit cells (u.c.), resistance and susceptibility measurements show superconductivity for YBCO layer thickness in excess of 4 unit cells. The critical temperature increases with YBCO thickness, and a Tc of 58 K is found for a YBCO thickness of 10 unit cells. Magnetization measurements show a ferromagnetic transition at 100 K in a (LCMO6u200au.c./YBCO5u200au.c.)15u200abilayer superlattice, and a depressed value of the saturation magnetization of 20 emuu200acm−3. These results are discussed in terms of interface disorder (analyzed by x-ray diffraction and transmission electron microscopy) and of the possible interaction between magnetism and superconductivity.

Mixed-state properties of superconducting Nb/Ni superlattices

Superlattices of ðNb10 nm=NitÞ 8 (1 6 t 6 5 nm) have been grown on Si(1 0 0) by magnetron sputtering. Samples show textured growth of Nb(1 1 0) and Ni(1 1 1), with negligible interdiffusion and interface roughness below 0.3 nm. The critical superconducting temperature (Tcs) of these superlattices decreases as the Ni thickness (tNi) increases. Ferromagnetic order in Ni layers is suppressed for tNi < 2:2 nm. We have explored the behavior of these superlattices in the mixed state by measuring the upper critical fields. Critical fields in perpendicular (Hc2?) and parallel (Hc2k) configurations show moderate superlattice-induced anisotropy values, which increase as tNi increases. From Hc2ðT Þ measurements, we have found that superconducting superlattices behave like a three-dimensional system. This dimensional behavior is discussed in terms of coupling of Nb through Ni layers. 2001 Elsevier Science B.V. All rights reserved.

Superconducting and structural properties of Nb/Ni multilayers

Multilayers of Nb/Ni have been fabricated by magnetron sputtering technique. Diffraction patterns show a good multilayer structure. The Ni layers induce superconducting critical temperatures lower than the expected taking into account only metallic proximity effect. The upper critical magnetic fields measurements allow us to extract a superconducting coherence length perpendicular to the layers of the order of 27 A.

Magnetic instabilities along the superconducting phase boundary of Nb∕Ni multilayers

We report vibrating reed and superconducting quantum interference device magnetometer data that exhibit prominent dips or oscillations of the superconducting (SC) onset temperature, ΔTC(H)≈0.01–0.7K, for a [Nb(23nm)∕Ni(5nm)]5 multilayer (ML) in dc magnetic fields applied nearly parallel to the ML plane. The vibrating reed data exhibit reproducible structures below TC that may reflect multiple SC transitions, but they are sensitive to ac field amplitude and dc field orientation. This striking behavior poses challenges for theoretical and experimental investigations of interfaces between SC and ferromagnetic layers that involve magnetic pair breaking effects, “pi phase shifts” of the SC order parameter, and exotic (“LOFF”) pairing states. Alternatively, the anomalies may mark dynamical instabilities within a confined, strongly anisotropic Abrikosov vortex lattice.

Experimental adiabatic vortex ratchet effect in Nb films with asymmetric pinning trap

Nb films grown on top of an array of asymmetric pinning centers show a vortex ratchet effect. A net flow of vortices is induced when the vortex lattice is driven by fluctuating forces on an array of pinning centers without reflection symmetry. This effect occurs in the adiabatic regime and it could be mimiced only by reversible DC driven forces.

Experimental Vortex Ratchet Effect in Nanostructured Superconductors

Superconducting Nb thin films were grown on different arrays of triangle‐shape metallic islands. The vortex lattice dynamics could be strongly modified by these asymmetric vortex traps. These asymmetric pinning potentials lead to a rectification effect on the vortex motion: Injecting an ac supercurrent on the sample yields a net dc vortex flow. This vortex ratchet effect is adiabatic and reversible: The effect is frequency independent and the polarity of the dc voltage output could be tuned by the applied magnetic fields and the input ac currents.

Vortex motion channeling effects in Nb with mesoscopic arrays of Ni lines

Ordered arrays of submicrometric Ni lines have been fabricated in sputtered Nb films. Magnetotransport R(H) and (I, V) curves were measured close to the critical temperature using a cross-shape bridge that allows us to apply current in two directions: parallel or perpendicular to the lines. The experimental results show anisotropic vortex motion with clear channeling effects. In R(H) data, magnetic features appear but they are absent in the (I, V) curves.

Fabrication of 2D, 1D and 0D ordered metallic nanostructures

Abstract Fabrication and structural characterization of periodic structures in the submicrometric range are ones of the most interesting topics in the nanotechnology field. In this work we report on the fabrication of 2-dimensional periodic nanostructures (superlattices) by magnetron sputtering and 1-dimensional (lines) and 0-dimensional (dots) periodic nanostructures by electron beam lithography and etching techniques. The interplay between the characteristic sample length scale and the length scale which governs the physics, could strongly modify the physical properties of the system. As one typical example we will present the superconducting properties of Nb films in three systems: (a) Nb/Ni multilayers (2D), (b) periodic lines in Nb films (1D) and (c) periodic Ni dots (0D) in Nb films.