M. Sirena

Thickness dependence of the properties of La0.6Sr0.4MnO3 thin films

The film thickness was determined by contact profilometry and x-ray diffraction size effect through Scherrers formula, with agreement within 10%. The chemical composition of the films was determined by energy dispersive x-ray spectroscopy, EDX, and by x-ray photoemission spectroscopy, XPS, and found to be that of the target within the experimental error, 10%. The XPS analysis shows also full surface coverage without pinholes down to a thickness of 5nm. The surface topography was studied with atomic force microscopy, AFM. For the LSMO-MGO series small features of 50nm diameter and rms height of 14nm were observed in thin films (t<30nm), while for thicker films these features have an average size of 300nm and rms height of 35nm. The LSMO-STO films present a sharper size distribution but follow a similar tendency. 3. Results and discussion

Metal-insulator transition induced by postdeposition annealing in low doped manganite films

We studied the transport and magnetic properties of low-doped manganite films after different oxygenation processes. The oxygen content was adjusted by postdeposition annealing at different oxygen pressures and annealing times. For all the samples we observed an increase in the Curie temperature and the remnant magnetization with the oxygen content. In general, for decreasing number of oxygen vacancies, samples under expansive strain become more homogeneous and their electrical resistivity decreases. A metal-insulator transition is induced in highly oxygenated films grown on SrTiO3, probably related to a shift of the mobility edge crossing below the Fermi energy. We found that the oxygenation dynamics depend critically on the strain field induced by the substrates and also on the Sr doping concentration.

Characterization of the insulator barrier and the superconducting transition temperature in GdBa2Cu3O7−δ/BaTiO3 bilayers for application in tunnel junctions

The optimization of the superconducting properties in a bottom electrode and the quality of an insulator barrier are the first steps in the development of superconductor/insulator/superconductor tunnel junctions. Here, we study the quality of a BaTiO3 tunnel barrier deposited on a 16 nm thick GdBa2Cu3O7−δ thin film by using conductive atomic force microscopy. We find that the tunnel current is systematically reduced (for equal applied voltage) by increasing the BaTiO3 barrier thickness between 1.6 and 4 nm. The BaTiO3 layers present an energy barrier of ≈1.2 eV and an attenuation length of 0.35–0.5 nm (depending on the applied voltage). The GdBa2Cu3O7−δ electrode is totally covered by a BaTiO3 thickness above 3 nm. The presence of ferroelectricity was verified by piezoresponse force microscopy for a 4 nm thick BaTiO3 top layer. The superconducting transition temperature of the bilayers is systematically suppressed by increasing the BaTiO3 thickness. This fact can be associated with stress at the interface...

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.

Magnetic after-effect in manganite films

Abstract The time dependence of the magnetic and transport properties on La 0.6 Sr 0.4 MnO 3 films and bulk samples has been studied through magnetization and resistivity measurements. A magnetic after-effect has been observed in all samples. At low temperatures, the low-field magnetization, can be described by the function M(t)=M c +M d exp(− t/τ)+S(H,T )ln( t) . The resistivity increases logarithmically in the same temperature range, indicating the evolution of the sample to a more disordered state. Above a characteristic temperature, this behaviour is reversed and an increase of the magnetization with time is observed. The relaxation parameters depend on the bulk or films character of the samples. In the latter case, a dependence on the film thickness was found. A direct correlation between the time dependence of the resistivity and magnetization curves in manganite compounds was found.