(Georgia Institute of Technology, 1998)
Wang, Z. L. (Zhong Lin); Yin, Jinsong; Berta, Yolande; Zhang, Jiming
Magnetic oxides of (La,A)MnO₃ and (La,A)CoO₃ have two typical structural characteristics:
cations with mixed valences and oxygen vacancies, which are required to balance the charge
introduced by cation doping. The consequences introduced by each can be different, resulting in
different properties. It is important to quantitatively determine the percentage of charges balanced
by each, but this analysis is rather difficult particularly for thin films. This paper has demonstrated
that electron energy-loss spectroscopy (EELS) can be an effective technique for analyzing Mn and
Co magnetic oxides with the use of intensity ratio of white lines, leading to a new technique for
quantifying oxygen vacancies in functional and smart materials.
(Georgia Institute of Technology, 1997-06-23)
Wang, Z. L. (Zhong Lin); Yin, Jinsong; Jiang, Yongdong; Zhang, Jiming
Using the white line intensities, electron energy-loss spectroscopy in a transmission electron microscope has been employed to characterize the valence conversion and oxygen vacancies in La₁₋ₓCaₓMnO₃₋[subscript y]. For a nominal doping composition x = 0.33, the ratio of Mn³⁺ to Mn⁴ ⁺ is determined to be more than 0.25 but less than 0.5, and the content of oxygen vacancy y is no more than 0.065 (equivalent to 2.2 at. % of the oxygen content). At y[subscript max] = 0.065, 60% of the residual charge introduced by Ca doping is balanced by the conversion of Mn³⁺ to Mn⁴ ⁺and 40% by oxygen vacancy.
(Georgia Institute of Technology, 1996-07-01)
Wang, Z. L. (Zhong Lin); Zhang, Jiming
La₁₋ₓSrₓCoO₃ (LSCO) has recently been found to exhibit large magnetoresistance (MR) effect. In this paper, microstructures of LSCO thin films, which may provide important clues for understanding the unusual MR characteristics in these intrinsic magnetic oxides, were determined by transmission electron microscopy. An ordered, anisotropic perovskite-type structure n-LSCO was observed and its atomic model was determined. The ordered structure has a tetragonal cell with a La-Co-Sr-Co-[001] layered atom distribution along the c axis. This structure is intrinsic for the LSCO system and is resulted directly from the lattice substitution between La and Sr. The entire film is composed of [001], [010], and [100] anisotropic domains of sizes in the order of 30–200 nm. Microstructures of (011)- and (001)-type domain boundaries were determined. Although n-La₀.₅Sr₀.₅CoO₃ is the dominant structure, a tetragonal phase n-La₀.₃₃Sr₀.₆₇CoO₃ has also been identified, which is produced by both the lattice substitution between La and Sr and the variation of local chemical composition. These structures could play a key role in determining the MR properties of the materials which are believed due to the intrinsic coupled spin scattering of Co atoms in different atomic layers.
(Georgia Institute of Technology, 1996-05-27)
Dai, Z. R.; Wang, Z. L. (Zhong Lin); Duan, X. F.; Zhang, Jiming
Interface microstructures of BaTiO₃/LaAlO₃ grown by metalorganic chemical vapor deposition(MOCVD) are studied using high-resolution transmission electron microscopy (HRTEM). Interface dislocations in BaTiO₃/LaAlO₃ have been shown to be directly linked up with the 90° domain boundaries in BaTiO₃. This association is a result of strain relief due to a phase transformation when cooled down from the growth temperature. The Burgers vector of the interface dislocations is 010.