Browsing by Author "Carbone, Gerardina"

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    Structural and magnetic studies of strained thin films of La 2/3 Ca 1/3 MnO 3
    (2004) Carbone, Gerardina; Helmuth, Dosch (Prof. Dr. Rer. Nat.)
    Mn-compounds with formula A1-xBxMnO3 have been widely studied in the past 50 years because of their interesting physics. The recent discovery of phenomena of Colossal Magneto Resistance (CMR) and dense granular magneto-resistance in ferromagnetic thin films (for x~1/3) has renewed the interest towards them, as materials suitable for applications in electronic devices. This work focuses on La1-xCaxMnO3 in the x=1/3 phase (LCMO). LCMO is ferromagnetic metallic below its Curie temperature (TC ~ 280K) and presents the phenomenon of CMR. The main scope of this thesis has been the accurate structural and magnetic study of strained thin films of LCMO epitaxially grown on SrTiO3 Due to the mismatch between the crystal lattice of the two compounds mechanical strain is imposed to the film structure, which can be responsible for changes of the physical properties of the material. The interest of this research is the analysis of the strain-induced modifications to the structure of LCMO epitaxial thin films and their effect on magnetic and transport properties. The films studied, with thickness in the range 50A-600A, have been grown by Pulsed Laser Ablation by Dr.J.Klein, at the University of Cologne-Germany. their TC, of the order of 130K, is lower than the bulk, does not show thickness dependence. Structural analyses on these films have been performed with x-ray diffraction (XRD) using synchrotron radiation. The measurements have been carried out at the beamline BM28 at the ESRF (European Synchrotron Radiation Facility) in Grenoble -France, and at the MPI Surface Scattering Beamline at ANKA in Karlsruhe - Germany. Grazing Incidence Diffraction (GID) measurements has allowed the observation of a particular microstructure of the films, consisting of twin domains of similar size periodically arranged in the plane of the growth. The signature of this periodic microstructure, is the presence of broad superlattice (SL) peaks around in-plane Bragg peaks. No SL peaks have been found around purely out-of-plane peaks for the thin film (100A). For thicker films, instead, a broad peak has been observed together with the sharp Bragg component of [0 0L]-type peaks. Such broad component can also be ascribed to a poorly correlated out-of-plane periodicity. Such twin domain modulation (TDM) is the structural response of the film to the strain induced by the substrate. In this special case of epitaxial growth, the TDM creates an average structure which helps reducing the mismatch between film and substrate. A more detailed description of the model is found in the PhD thesis of U. Gebhardt. In our proposed model, the size of the twin domains is statistically distributed around an average value L0 with a standard deviation DeltaL0.For the 100A film the distribution of twin domains, has an average domain size of LO = 32u.c. and a distribution width of DeltaL0 = 16 u.c. For higher thickness, as for the 400A film, 3 different in-plane TD distributions are needed to reproduce the data (: L01 = 32u.c. and DeltaL01 = 14u.c, L02 = 64u.c and DeltaL02 = 24u.c, L03 = 79u.c and DeltaL03 = 49u.c.) This can be justified with a model of increased disorder in the domain distribution in the direction of the growth. The presence of the broad peak found at [0 0L]-type positions has been explained, within the same model, as due to an out-of-plane TD modulation due to the presence of a monoclinic distortion of the LCMO unit cell CTR measurements confirm the presence, in thick films, of a form of strain-relaxation along the direction of the growth, consisting in a variation of the c-spacing both at the surface of the films and at the substrate- film interface. Magnetic measurements have been performed both with both SQUID and polarized neutron reflectivity (PNR) techniques. The magnetisation of the samples does not saturate completely up to the maximum value of external field applied H = 6Tesla. This effect can be ascribed either to a canting of the magnetic moments, or to the presence of magnetic domains. Furthermore, the value of the saturation magnetisation of the films, calculated at the maximum field measured decreases with increasing thickness of the samples, following an exponential law. These results have been confirmed by PNR measurements, performed at EVA@ILL (Grenoble - France). For one of the thick samples, 400A, PNR give evidence of a strong gradient in the magnetic profile of the film. The average magnetic moment found is maximum at the interface, which decreases exponentially along the direction of the growth. These peculiar magnetic properties, together with the model of change in distribution of twin domains and variation of c-axis, can be attributed to an increase of disorder in the domains, which reflects in the disorder in magnetic domains. All the measurements converge in a model of epitaxial growth, achieved by twin domain periodicity, with a distribution of domain sharper at the interface and broader at the surface.