Nathascia Lampis

Rietveld refinements of the paraelectric and ferroelectric structures of PbFe0.5Ta0.5O3

The aristotype and the two ettotypes of the complex perovskite PbFe0.5Nb0.5O3 have been refined by the Rietveld method from neutron and x-ray powder diffraction data. To avoid errors due to sample off-stoichiometry or inhomogeneity, only powders obtained by grinding single crystals were analysed. At 523 K the cubic phase (space group Pmm), which is known to be stable for T > 376 K, has the lattice parameter ac = 4.010(1) A and is characterized by disorder of the lead and oxygen atoms, as in most Pb-based complex perovskites. At 363 K the intermediate phase, stable for 355 K < T < 376 K, is confirmed to be tetragonal, space group P4mm, with lattice parameters at = 4.007(1) A and ct = 4.013(1) A. The best refinement for the low-temperature structure (T < 355 K) is obtained, both at 250 and 80 K, in monoclinic symmetry, space group Cm. The monoclinic cell at 250 K has parameters am = 5.674(1) A, bm = 5.663(1) A, cm = 4.013(1) A and = 89.84°(2).

Growth and characterization of stable SrO-terminated SrTiO3 surfaces

A simple technique for the growth of SrO-terminated SrTiO3 surfaces is reported. High quality SrTiO3 epitaxial films were grown by reflection high energy electron diffraction assisted pulsed laser deposition on suitably prepared NdGaO3 (110) substrates. The surface properties, analyzed within a growth/characterization multichamber ultrahigh vacuum system by photoemission spectroscopy performed on the core-level spectra of Sr and Ti, low energy electron diffraction-, scanning tunneling-, and atomic force microscopy, are fully consistent with a single Sr oxide termination. The availability of such high quality SrO-terminated SrTiO3 surfaces is of major importance for the controlled growth of oxide epilayers and interfaces.

In situ investigation of the early stage of TiO2 epitaxy on (001) SrTiO3

We report on a systematic study of the growth of epitaxial TiO(2) films deposited by pulsed laser deposition on Ti-terminated SrTiO(3) (001) single crystals. By using in situ reflection high energy electron diffraction, low energy electron diffraction, x-ray photoemission spectroscopy, and scanning probe microscopy, we show that the stabilization of the anatase (001) phase is preceded by the growth of a few nanometers thick pseudomorphic Sr(x)TiO(2+y) (x, y < 1) intermediate layer. The data demonstrate that the formation of this intermediate phase is related to the activation of a long range Sr migration from the SrTiO(3) substrate into the film. Our results enrich the phase diagram of the Sr-Ti-O system under epitaxial strain opening a route for the study of the electronic and dielectric properties of the reported Sr-deficient SrTiO(3) phase.

Charge density waves enhance the electronic noise of manganites

The transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin films in the temperature range from room temperature to 160 K are reported. It is shown that both the broadband 1/f noise properties and the dependence of resistance on electric field are consistent with the idea of a collective electrical transport, as in the classical model of sliding charge density waves. On the other hand, the observations cannot be reconciled with standard models of charge ordering and charge melting. Methodologically, it is proposed to consider noise-spectra analysis as a unique tool for the identification of the transport mechanism in such highly correlated systems. On the basis of the results, the electrical transport is envisaged as one of the most effective ways to understand the nature of the insulating, charge-modulated ground states in manganites.

Magnetic properties of pseudomorphic epitaxial films of Pr0.7Ca0.3MnO3 under different biaxial tensile stresses

In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn atom along the easy axis. On the base of high resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviours are discussed in terms of the crystallographic constraints imposed by the epitaxy of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the tensile strain on the film c-axis, lying within the substrate plane, favours the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.

RF-sputtering deposition OF Nd1-xSrxCoO3 oriented thin films

In this paper we reported, to the best of our knowledge, the first deposition of highly oriented thin films (with thickness of about 90 nm) of NdCoO3 and Nd0.8Sr0.2CoO3 cobaltites on single-crystalline STO and LAO substrates. Our investigation has shown that highly oriented single phase thin films of NCO and NSCO can be successfully deposited by means of rf-sputtering if the substrates is heated at high temperatures (700C); lower substrate temperature has shown to lead to multi-phase materials with a low crystallinity degree . LAO substrate showed to give origin to a prefect match of the out-of-plane lattice constant of the NSCO target material.