E. Baca

Depression of the superconducting critical temperature and finite-size scaling relation in YBa 2 Cu 3 O 7-δ /La 2/3 Ca 1/3 MnO 3 bilayers

High-quality YBa2Cu3O7-δ/La2/3Ca 1/3MnO3 (YBCO/LCMO) bilayers were fabricated on (001)-oriented SrTiO3 (STO) substrates by dc-sputtering technique. Bottom layer was always LCMO since it grows better on STO than on YBCO. The thickness of the ferromagnetic layer varied between 5 and 35 monolayers (~2-13nm) and that of the top YBCO was fixed at 10 monolayers (~12nm). The transport properties of the YBCO layers as well as the magnetic properties of the LCMO counterparts were studied as a function of the LCMO layer thickness. A sizeable depression of the Curie temperature (TC) of the LCMO layers from the bulk to lower temperatures is observed when decreasing their layer thickness dLCMO, which might be ascribed to intrinsic dimensionality effects or strain-induced phenomena. On the contrary, the superconducting critical temperature of the YBCO layer TS displays a sudden strong decrease at a critical LCMO thickness of ~12nm. Since no dramatic change of the structural and morphological quality of the YBCO top layers with increasing dLCMO is observed, the suppression of TS of the YBCO layers should take place via proximity effect due to the increasing magnetization strength in the LCMO layer. However, extrinsic factors like interface strain, interdiffusion of cations between YBCO and LCMO or injection of spin-polarized carriers from the magnetic into the superconducting layer could also play an important role or even to be directly responsible for the observed depressed TS.

Magnetic anisotropic behavior in epitaxial La2/3Ca1/3MnO3/YBa2Cu3O7-δ/ La2/3Ca1/3MnO3 and La2/3Ca1/3MnO3/La1/3Ca2/3MnO3/YBa2Cu3O7-δ trilayered structures

The temperature dependence of the magnetization along with magnetization hysteresis loops were measured on dc sputtered La2∕3Ca1∕3MnO3[ferromagnetic(F)]∕YBa2Cu3O7−δ[superconductor(S)]∕La2∕3Ca1∕3MnO3(F) and La2∕3Ca1∕3MnO3(F)∕La1∕3Ca2∕3MnO3[antiferromagnetic(AF)]∕YBa2Cu3O7δ(S) trilayered structures. The magnetic fields were applied both parallel (in-plane) and perpendicular (out-of-plane) to the film plane. Magnetization hysteresis loops, recorded at 5K, shown a clear central peak around zero field when the magnetic field was applied perpendicular to the film plane and a typical ferromagnetic behavior when applied parallel. Additionally, the dependence of the magnetization on the temperature, performed under the zero-field-cooling (ZFC) and field-cooling (FC) regimens, evidenced the active presence of the diamagnetic and ferromagnetic phases both in the F∕S∕F and F∕AF∕S trilayers. Thus, the diamagnetic phase was clearly observed under the ZFC regimen, whereas a possible paramagnetic Meissner effect was obse...

Effect of Y-substitution on the electrical properties of epitaxial Bi2Sr2Ca1−xYxCu2O8+δ thin films

Abstract We have performed a study on the preparation conditions and electrical transport properties of epitaxial Bi 2 Sr 2 Ca 1−x Y x Cu 2 O 8+δ thin films. The deposition of the films was carried out “ in situ ” by a high oxygen pressure dc-sputtering technique on (100) SrTiO 3 substrates. The structural properties were determined by x-ray diffraction. By increasing the yttrium content the temperature dependence of resistivity change from a metallic to an insulating behavior. For x ΔT C and dϱ n / dT as function of the x value has been analized. For x ≥0.50, the samples showed a semiconductor behavior what correspond to a variable-range-hopping conduction between localized states in according to a two-dimensional system given by σ = σ 0 exp (− T 0 T ) 1 3 . Ultra-thin Bi 2 Sr 2 Ca 1−x Y x Cu 2 O 8+δ films (for x=1) have been used as artificial barriers in tunneling heterostructures with Bi 2 Sr 2 CaCu 2 O 8+δ superconducting electrodes.

Deposición y caracterización de recubrimientos de Nitruro de Boro producidos por pulverización catódica

Boron nitride (BN) is an important technological material with thermal, electrical, and mechanical properties that approach those of diamond. BN may be formed in three main crystalline forms: cubic (c-BN), wurtzite (w-BN), and hexagonal (h-BN). c-BN is the most attractive BN structure due to an unique set of properties. Three factors are common to all successful PVD c-BN experiments: bombardment of the growing film by energetic ions (generally 100-500 eV). an elevated substrate temperature (300-400 °C optimum), and the achievement of the correct film stoichiometry. In this work the literature was reviewed on the physical vapour deposition and characterization of c-BN thin films, placing special emphasis on the substrate bias (d.c. or r.f.) as a dominant effect on the c-BN content onto films. BN thin films were prepared using d.c and r.f. negative substrate bias. Threshold bias for formation of c-BN phase was identified for both cases. Films were characterized by Fourier Transformed Infrared Spectroscopy (FTIR) used for phase identification. Atomic Force Microscope (AFM) and Scaning Electronic Microscope (SEM) for morphological structural characterization. Characterization of thin films obtained is according to layer structural models proposed in the literature.