M. E. Gómez

Antiferromagnetism at the YBa2Cu3O7/La2/3Ca1/3MnO3 interface

The magnetic properties of a series of YBa2Cu3O7−x/La2/3Ca1/3MnO3 (YBCO/LC1/3MO) superlattices grown by dc sputtering at high oxygen pressures (3.5 mbar) show the expected ferromagnetic behavior. However, field-cooled hysteresis loops at a low temperatures show the unexpected existence of exchange bias effect associated with the existence of ferromagnetic/antiferromagnetic (AF) interfaces. The blocking temperature (TB) is found to be thickness dependent and the exchange bias field (HEB) is found to be inversely proportional to the ferromagnetic layer thickness, as expected. The presence of an AF material is probably associated with interface disorder and Mn valence shift toward Mn4+.

Quasiparticle-injection effects in YBa2Cu3O7-δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructures

We have grown YBa2Cu3O7-δ (40 nm)/La1/3Ca2/3MnO3 (2 nm)/La2/3Ca1/3MnO3 (30 nm) junctions for quasiparticle-injection measurements. Ferromagnetic (F), antiferromagnetic (AF), and superconducting (S) thin films were sequentially grown “in situ” using a dc-sputtering technique at 850 °C in pure oxygen at pressures around 3.5 mbar. The temperature dependence of the resistivity of the junction, without any current injection, presented coexistence between magnetism and superconductivity with a metallic insulating transition at 186 K, and zero resistance at 52 K. When current was injected from the F layer into the S layer, superconductivity suppression was obtained at temperatures below 52 K; furthermore, small values of current injection were necessary to change the electrical behavior up to the critical temperature. A negative current gain (G) has been observed and described by the relation G∝ατeff, where τeff is the relaxation time of the quasiparticle. The use of an AF layer as an insulating barrier is also ...

Structure of high-Tc/manganite perovskite superlattices

We study the crystalline and interface structure of RBa2Cu3O7/La1−xAxMnO3 superlattices (R=Y, Gd; A=Ca, Sr) grown on mismatched MgO substrates by means of nondestructive x-ray diffraction. The diffraction patterns are adjusted to a model of the structure which includes interface roughness and interdiffusion. Our results show that these disorder mechanisms cannot be neglected and should be taken into account to understand the physical properties of these superlattices.

Magnetotransport Properties in Epitaxial Exchange-Biased La

Superlattices of ferromagnetic (F) and antiferromagnetic (AF) oxide materials have attracted increased attention given the exchange bias or exchange anisotropy phenomenon. Research on magnetic oxide superlattices has focused toward the technological goals of increasing magnetoresistance in lower applied magnetic fields and more useful temperature ranges for various applications. Superlattices of F-LCMO and AF-LCMO layers were grown on (001)-oriented SrTiO3 substrates. We studied the temperature dependence of magnetotransport properties for a series of [AF-LCMO (7.6 nm)/F-LCMO (tF)]N superlattices as a function of the ferromagnetically doped layer thickness, tF, while the antiferromagnetic (AF) layer thickness, tAF, was kept constant. Magnetotransport measurements at different temperatures were done on a series of [AF(7.6 nm)/F(tF)]N superlattices. The ZFC resistance has a maximum at negative fields, for all multilayers, with descending fields (from 1 to -1 kOe). In all samples a shift along the field axis of the FC loop with respect to that of the ZFC loop is observed. The Hex values determined from these displacements of the FC loops of the magnetoresistance loops increase when temperature decreases and its magnitude also depends on the ferromagnetic layer thickness

scriptstyle_2/3

We grew rhombohedrally distorted BiFeO ₃ and hexagonal YMnO ₃ thin films on Pt/TiO ₂/SiO ₂/Si substrates via RF magnetron sputtering technique in a pure oxygen atmosphere. BiFeO ₃ and YMnO ₃ targets were self-made by the usual solid-state reaction method. We investigated the effects of deposition temperature upon crystalline structure, surface morphology, magnetization, and electrical polarization of BiFeO₃ and YMnO₃ thin films. The crystalline structure was studied by X-ray diffraction, and the topography of film surface was analyzed by atomic force microscopy. We also conducted measurements of ferroelectric and ferromagnetic hysteresis loops to study the electrical and magnetic behavior of the samples. Polarization, as a function of electric field in capacitor structures based on our BiFeO₃ films, shows hysteretic behavior with a coercive field of 54 kV/cm and a remanent polarization of 21 muC/cm²; whereas, YMnO₃ films show hysteretic behavior with coercive field of 2.4 kV/cm, remanent polarization of 1.2 muC/cm², and saturation polarization of 3.5 mu C/cm² . Magnetization measurements of the BiFeO ₃ films evidence weak ferromagnetism, that can be related to the presence of a small quantity of ferromagnetic impurities.

Ca

The magnetic and electrical properties of ferromagentic∕superconducting∕ferromagnetic trilayers as a function of the thickness of the superconducting (SC) layer (between 5 and 15 monolayers) were measured. La2∕3Ca1∕3MnO3∕YBa2Cu3O7−δ∕La2∕3Ca1∕3MnO3 trilayered films were fabricated in situ on (001) SrTiO3 single-crystal substrates using dc sputtering with high oxygen pressures (3.5mbar). The crystalline quality of the trilayers was analyzed by x-ray diffraction. Transport and magnetic measurements revealed a weak dependence of the ferromagnet’s (FMs) Curie temperature TFMC and the FM magnetization on the thickness of the SC layer, and a strong dependence of the superconducting transition temperature TC.

scriptstyle_1/3

Abstract We have prepared Bi 2 Sr 2 CaCu 2 O 8−δ /Bi 2 Sr 2 YCu 2 O 8−δ /Bi 2 Sr 2 CaCu 2 O 8−δ trilayer structures, using a “in situ” DC-sputtering process at high oxygen pressures on (001) SrTiO 3 substrates. Bi 2 Sr 2 CaCu 2 O 8−δ films were used for superconducting electrodes and semiconductor-like Bi 2 Sr 2 YCu 2 O 8−δ films with thickness down 5 nm were used as barrier layers. Both materials have the same crystalline structures and allowed the epitaxial growth of the heterstructure as shown by X-ray, RBS measurements and cross section TEM analysis. The superconducting properties were examined by resistance, AC-susceptibility and current-voltage characteristics as function of the temperature. For junctions with areas of 120×120μm 2 defined by photolitography a RSJ behavior was found at temperatures below T c .

MnO

In this work, we report a study of the temperature dependence of magnetotransport properties for a series of [AF-LCMO (7.6nm)/F-LCMO (tF)]N superlattices. Superlattices of F-LCMO and AF-LCMO layers were grown on (001)-oriented SrTiO3 substrates via a high-pressure dc sputtering process. The modulation period have been experimentally derived from X-ray diffraction measurements, confirming that for all superlattices actual layer thicknesses were within 10% of the nominal values. A high resolution transmission electron microscopy micrograph of a superlattice showing sharp interfaces with interfacial roughness value around 0.38 nm. Atomic force microscopy (AFM) images of the 190 nm-total thickness multilayer show surface roughness below 10 nm. Several properties associated with CMR and exchange bias effects have been studied in F-LCMO/AF-LCMO multilayers.

scriptstyle_3

A dynamic scaling and kinetic roughening study was done on digitized atomic force microscope (AFM) images of Pb(Zr0.52,Ti0.48)O3 (PZT) thin films. The films were grown on Si(001) and Nb - SrTiO3(001) (STNO) substrates via rf-sputtering technique at high oxygen pressures at substrate temperatures of 600 oC by varying the deposition time and keeping other growth parameters fixed. By using a specific self-designed algorithm, we can extract from digitized 512-pixel resolution AFM-images, quantitative values of roughness parameters, i.e., interface width (s(l)), lateral correlation length (x||) and, roughness exponent (a). Herein, we report on the sputter-time deposition dependence of the parameters describing roughness for both kinds of substrates. We report a-values for different time depositions (between 15 and 60 minutes) close to 0.55 for Si substrates and 0.63 for Nb - SrTiO3 substrates, indicating that the surface becomes more correlated in STNO substrates. The a-values are associated to the Lai-Das-Sarma-Villain model.

/La

Abstract Using a dc sputtering method at high oxygen pressures we have deposited epitaxial Ag-doped YBa 2 Cu 3 O 7−x thin films on SrTiO 3 bicrystals with missorientation angle of 36.8°. A 15 % Ag-doped YBa 2 Cu 3 O 7−x sintered target was used to sputter the films. Critical current of 4–5 10 6 A/cm 2 at 77 K were measured in doped films. When compared with 1 × 10 6 A/cm 2 for undoped films, indicated an improvement of the superconducting properties by Ag doping. 200–300 nm thick Ag-doped films were patterned across the bicrystal line to form 5–20 μm-wide junctions. Current-voltage characteristics of the grain boundary junctions at temperatures from 10 K to T C showed a resistively shunted junction (RSJ) behavior with I C R N values of 160–170 μV at 77 K which are higher than the measured values for undoped junctions.