Papers by Keyword: Manganite

Abstract: Magnetic, dielectric properties and magnetoelectric effect of ceramics with a composition of 0.9 BaTiO3-0.1 LaMnO3 have been investigated. X-ray diffraction measurements showed that the sintered ceramics are composites containing a small amount of (La, Ba)MnO3 phase in the BaTiO3 matrix. These composites were multiferroic, having ferromagnetic and ferroelectric Curie temperatures of 330 K and 392 K respectively. We found that the composite sintered at 1150 °C exhibits a reduction of spontaneous magnetization as large as 55 % at room temperature when an electric field of 1.4 KV/mm is applied. This reduction is probably ascribed to a change of hole concentration distribution in the precipitated ferromagnetic (La, Ba)MnO3 and the resultant decrease of ferromagnetic Curie temperature.

Abstract: We present extended X-ray absorption fine structure (EXAFS) and anelastic spectroscopy measurements performed on (La0.63Ca0.37)(Mn1-yMy)O3 manganites (M = Cr or Ni and y = 0.03, 0.08). The ferromagnetic metallic phase maintains its long-range character even after Mn substitution with non- Jahn-Teller ions, but both the doping species (Ni or Cr) lower the Curie temperature and broaden the magnetic transition. EXAFS suggests the presence of distorted insulating zones in the framework of phase inhomogeneity. Moreover, a strong dependence of the real part of the complex dynamic Young’s modulus and of the elastic energy-loss coefficient on the Mn substitution is evidenced by the anelastic spectroscopy spectra. These results confirm the correlation among local lattice disorder, dynamic and magnetic degrees of freedom and also evidence the possibility to tune manganites physical properties acting on their chemical composition.

Abstract: The crystal and magnetic structures of (Pr0.55Ca0.45)(Mn1-yCry)O3 (y = 0.00, 0.03, 0.06) have been investigated between 5 and 300 K by means of neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. An orthorhombic-to-monoclinic phase transition occurs on cooling in (Pr0.55Ca0.45)MnO3; at lower temperature charge ordering in the Mn sub-lattice, coupled with orbital ordering, induces an anti-ferromagnetic (AFM) spin ordering within the monoclinic phase. Cr substitution at the Mn site hinders the phase transition, although for the y = 0.03 sample small amounts of the monoclinic phase co-exist at low temperature with the main phase characterized by an orthorhombic structure. In addition Cr favours ferromagnetism (FM) participating to the double exchange mechanism. Cr substitution prevents charge ordering, favouring double exchange and hence FM spin ordering within the Mn sub-lattice. The Curie temperature of the Cr substituted samples is independent on the Cr content.

Abstract: In this work we present results of contact resistance between La2/3Ca1/3MnO3 (LCMO) and Pt system. LCMO epitaxial thin films have been grown by rf sputtering on top of LaAlO3 (LAO) substrate. The contact between the manganite film and the metal have been prepared by exsitu deposition of a 30 nm thick Pt layer on top of the manganite film by e-beam evaporation at room temperature. Different nanostructured contact geometries have been defined by using a focus ion beam system and then transport properties have been tested by means of AFM system working on the current sensing mode with a doped diamond tip. We show that the top-most LCMO layers do play a very relevant role on the transport properties acting as an insulating barrier. AFM measurements in the current sensing mode exhibit typical features of tunneling conduction. Ex-situ annealing in air at high temperature clearly improve the magnetotransport properties of the films reducing the surface insulating barrier. X-ray absorption spectroscopy measurement at the Mn Ledge has been performed to gain a deeper insight into the properties of the top-most LCMO layers.

Abstract: Magnetotransport properties of very high quality fully strained epitaxial thin films of La2/3Ca1/3MnO3 (LCMO), grown on top of SrTiO3 (STO) (001) substrates, are analyzed. As-grown fully strained epitaxial films are ferromagnetic and metallic but exhibit depressed transport and magnetic properties (low TC and MS). Detailed analysis of the structural and magnetotransport properties after high temperature annealing processes, have revealed a progressive structural relaxation, as detected by the variation of the c/a tetragonal distortion, as the annealing temperature rises up. Nevertheless, no changes in the in-plane cell parameters are observed. Simultaneously, the magnetic transition temperature, TC, and saturation magnetisation, MS, substantially increase approaching values similar to that of bulk materials.

Abstract: Investigations of the influence of synthesis procedures and LaSr-manganite stoichiometries on the Curie point and other magnetic characteristics of the material, being the topic of this work, are attractive since they present a necessary primal step in the intended development of nano-magnetic drug carriers based on these materials. In the course of such an investigation, differences in the mechanisms of the formation of desired manganite compounds by following a classical solid-state preparation method, depending on various stoichiometries used, were noticed, with the reaction pathways thereof outlined onward.

Abstract: A comprehensive study of CMR manganite related phases, particularly those that develop under the effects of Ca and Mn-excess co-doping is undertaken. The relationships between phase composition, processing and observed crystalline structure are investigated for co-precipitated powders of composition La1-xCaxMn1+zO3+δ, thermally treated at 800 and 1000 °C in air for Ca contents of x=0.10 and x=0.15 and Mn content from z=0 up to z=0.88. A relevant structural transition from orthorhombic to rhombohedric symmetry is observed as a function of Mn excess near 25% A site vacancies. This solid solution is in equilibrium with the conjugated phases, either LaMn7O12 at 800°C, or Mn3O4 at 1000°C, respectively. Results suggest that at the polymorphic phase transition the solid solution is stable enough to slow down Mn diffusion into the perovskite cell. Ca-doping introduces stability in the perovskite structure and broadens the domain for phase formation reaction. The solid solution limit of the Ca-manganite solid solution is set above 50% Mn excess.