E. Baca

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 (LCMO6 u.c./YBCO5 u.c.)15 bilayer superlattice, and a depressed value of the saturation magnetization of 20 emu cm−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.

Critical exponents of inhomogeneous ferromagnets

A method for the analysis of temperature and field dependent magnetization data near the Curie temperature TC is devised, which allows the reliable determination of critical exponents even in strongly inhomogeneous ferromagnets. It is shown, both experimentally and theoretically, that the field-induced broadening of the ferromagnetic phase transition follows a power law with a characteristic critical exponent β⋅δ, where β and δ are the conventional temperature and field dependent critical exponents of the magnetization. Experimental results on inhomogeneous La2/3Ca1/3MnO3 films yield critical exponents β=0.3682±0.0033 and δ=4.76±0.32, even though the intrinsic TC distribution width is larger than the field-induced effect.

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+.

Interface Effects in Perovskite Superlattices

The effect of interface disorder in perovskite superlattices, either with the substrate or between layers dominates the physics of the material, even when the lattice parameter of the component materials differs in less than 1%. Unexpected behavior emerges, like exchange bias in a system where no antiferromagnetic material has been included in the superlattice design.

Study of surface morphology in epitaxial thin films of HoBa2Cu3O7−δ

Abstract A morphological study on the surface of epitaxial thin films of HoBa 2 Cu 3 O 7− δ was realized by atomic force microscopy. The films were grown using a high-pressure sputtering technique. Chemical etching process with a non-aqueous solution of Br–ethanol was used to modify the thickness of the films. The progress of morphology with thickness variation and the effect of the chemical etching on the film structure are discussed.

Epitaxial growth and tunneling properties of YBa2Cu3O7−x/PrBa2Cu3O7−x/YBa2Cu3O7−x trilayer structures

Abstract Using a multitarget high oxygen pressure sputtering system, we have produced in situ high quality epitaxial YBa 2 Cu 3 O 7−x / PrBa 2 Cu 3 O 7−x /YBa 2 Cu 3 O 7−x trilayer structures on SrTiO 3 (001) substrates with PrBa 2 Cu 3 O 7−x barrier thicknesses ranging from 10 to 30 nm. The structural properties were determined by x-ray diffraction. Rutherford Back Scattering (RBS) and Transmission Electron Microscopy (TEM) analysis, which showed an epitaxial growth of the trilayers with sharp and clean interfaces. I–V and dI/dV vs V measurements showed a clear SIS quasiparticle tunneling behavior with well developed peaks at ± 45m V indicating the presence of energy gap structures at ±2Δ and flat background conductances at high bias. Ratios of the conductance at zero bias, G(0), to the normal conductance at 150 mV, G(150), of about 10 % are observed. These characteristics can be related to the very good interfaces and superconducting properties observed in our heterostructures.

SURFACE TOPOGRAPHY ON EPITAXIAL HoBa2Cu3O7-δ AND Bi2Sr2CaCu2O8+δ THIN FILMS

Surface topography on epitaxial HoBa2Cu3O7-δ (HBCO) and Bi2Sr2CaCu2O8+δ (BSCCO) thin films analysis by using the atomic force microscopy (AFM) technique was carried out. The films were deposited in situ on SrTiO3 substrates with thicknesses ranging from 100 to 300 nm by a high-pressure sputtering process. Chemical etching with a nonaqueous solution of Br-ethanol was used to modify the surface of the samples. HBCO films showed spiral grains, while BSCCO samples exhibited a terraced growth. Etching with Br-ethanol has different effects on HBCO and BSCCO. In HBCO, etching produces clean surfaces with light changes on the surface morphology, whereas in BSCCO it conduces to strong changes in the structure and the roughness of the film surface.

Properties of Bi-2212/Bi-22Y2 step-stack Josephson junctions

We have fabricated step-stack Josephson junctions based on high quality epitaxial BSCCO (2212 phase) thin films, deposited on Y-doped bismuth (Bi-22Y2) steps. Bi-2212 and Bi-22Y2 films were grown by a high oxygen pressure dc-sputtering technique. The structural characteristics have been analyzed by X-ray, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry (RBS). Bi-22Y2 steps between 100 and 300 nm high were patterned by photolithography and non-aqueous chemical etching. Junctions were characterized by current-voltage (I-V) measurements under magnetic fields and microwave irradiation. I-V curves have shown a clearly hysteretic weak-link Josephson behavior at different temperatures. The temperature dependence of the critical current in these step-stack junctions has also been analyzed.