W. Saldarriaga

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

The magnetic properties of a series of YBa2Cu3O7−x/La2/3Ca1/3MnO3u200a(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-δu2009(40u2009nm)/La1/3Ca2/3MnO3u2009(2u2009nm)/La2/3Ca1/3MnO3u2009(30u2009nm) 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 850u200a°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.

Evidence of elastic tunneling in planar YBa2Cu3O7−δ∕La1∕3Ca2∕3MnO3∕La2∕3Ca1∕3MnO3 junctions

Evidence of elastic tunneling has been observed in YBa2Cu3O7−δ(S)∕La1∕3Ca2∕3MnO3(AF)∕La2∕3Ca1∕3MnO3(F) junctions. S/AF/F trilayers with thicknesses of ∼100, ∼7, and ∼80nm, respectively, were grown on SrTiO3 (001) substrates. Electrical measurements of the junctions at low temperatures under zero magnetic field display an asymmetry of the I-V and G(V)≡dI∕dV characteristics: for positive voltages, we observed a sharp peak at ∼20meV at 15K; for negative voltages we find at this temperature a peak around −15meV in agreement with the value expected for YBa2Cu3O7−δ. The low values of the conductance G for voltages above the energy gap are discussed.

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...

Nonlinear current-voltage characteristics measured across circular deoxyribonucleic acid (DNA) molecule bundles

We measured current-voltage characteristics of circular deoxyribonucleic acid (DNA) bundle samples embedded in a water mantle lying on a nylon porous membrane between vacuum-evaporated gold electrodes. The nonlinear behavior observed at room temperature on samples of different molecular sizes and variable concentration was interpreted as a long-range conduction mechanism that takes place by means of charge hopping through the network of molecules. A tunneling process acting as an additional transport channel inside single molecules between energetically appropriate bases was inferred. Our data also suggest at low bias a thermionic emission mechanism by means of which the charges move from one electrode, overcoming a potential barrier. Characteristic energies involved in the transport were reproducibly measured.

Electrical and magnetic properties of La2∕3Ca1∕3MnO3∕YBa2Cu3O7−δ∕La2∕3Ca1∕3MnO3 trilayers

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.

Correlation of conductivity and magnetization in epitaxial La2/3Ca1/3MnO3 thin films

We fabricated ∼200 nm thin (001)-oriented films of the ferromagnetic metallic perovskite La2/3Ca1/3MnO3 on single crystal (001)-SrTiO3 substrates by dc-sputtering at high oxygen pressure. The samples feature a Curie temperature TC∼260u2009K and a magnetic moment μ(T→0u2009K)∼3u2009μB per Mn atom. The magnetization loops are nearly square-shaped with a coercive field Hc(5u2009K)=0.03u2009T that decreases linearly in temperature down to TC. At low temperature, the magnetization shows a decrease ΔM∼T2 and the resistivity an increase Δρ∼T2 as is expected for itinerant electron ferromagnets where single particle excitations dominate. As a further indication of the high quality of the samples, the resistivity without external magnetic field peaks right at TC with a maximum value ρ(H=0,T=TC) of only ∼2 mΩu200acm. The magnetoresistance ratio Δρ/ρ0=[ρ(H,T)−ρ(H=0,T)]/ρ(H=0,T) also reaches its maximum value at TC, with Δρ/ρ0(H=6u2009T,T=TC)∼−50%. The complete absence of this colossal magnetoresistance effect in the low-temperature limit is a f...

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.