Orbital Engineering of Pulsed Laser Deposited Single-layered Manganite Thin Films

• Main Author: Vafaee Khanjani, Mehran
• Format: Others
• Language: English; en
• Published: 2014
• Online Access: https://tuprints.ulb.tu-darmstadt.de/4191/1/PhD-Thesis-Mehran_Vafaee_Khanjani_B-W.pdf; Vafaee Khanjani, Mehran <http://tuprints.ulb.tu-darmstadt.de/view/person/Vafaee_Khanjani=3AMehran=3A=3A.html> (2014): Orbital Engineering of Pulsed Laser Deposited Single-layered Manganite Thin Films.Darmstadt, Technische Universität, [Ph.D. Thesis]

Single-layered manganite, La1-xSr1+xMnO4, crystallizes in a tetragonal structure in which (La, Sr)O layers separate MnO6 octahedra along the c axis providing 2D MnO2 sheets. Within this structure, anisotropic electrical resistivity was observed where the in-plane resistivity at room temperature is about three orders of magnitude smaller than the out-of-plane counterpart. In perovskite manganites, La1-xSrxMnO3, doping Sr turns insulating LaMnO3 to metallic La0.5Sr0.5MnO3 due to double exchange interaction. On the contrary, the tetragonal La1-xSr1+xMnO4 remains insulating at higher doping levels of Sr, although the resistivity reduces by about one order of magnitude. Herein, the insulating state of LaSrMnO4 endures via doping with Sr up to x=0.5 and a charge-orbital order state forms where the Mn3+ cations show a preferential orbital occupation of d_(3x^2-r^2 )and d_(3y^2-r^2 ). Many investigations of perovskite manganites have shown that preferential orbital occupation and charge localization i.e. charge order, are interrelated with crystal structure. In this context, the key points are the Mn-O bond length, the cooperative rotation of MnO6 octahedra and the Jahn-Teller distortion. These factors define whether the charge-orbital state is favorable or not. Owing to the layered structure of La1-xSr1+xMnO4, tilting the MnO6 octahedra is restricted (no Mn-O-Mn bond along the c axis). In this thesis two doping levels of x=0.0 and x=0.5 were chosen in which the MnO6 octahedra are tetragonally distorted and non-distorted, respectively. The growth of thin films on different single-crystalline substrates allows us to alter the in- and out-of-plane lattice constants; to change the bond length of Mn with apical and planar oxygen atoms. In such a way, for each specific doping level, Mn3+ cations may show different orbital occupation than the one recognized in the bulk or single crystal. Indeed several studies have manifested preferential d_(3z^2-r^2 )or d_(x^2-y^2 )orbitals for Mn3+ cations in La1-xSrxMnO3 films under in-plane compressive or tensile strains, respectively, although there is no preferential orbital occupation for Mn3+ cations in bulk or single crystal of La1-xSrxMnO3. In this study the thin films of La1-xSr1+xMnO4 (x=0.0, 0.5) were deposited on different substrates in order to generate in-plane tensile and compressive strains. The details of the thin film deposition conditions are given in section ‎2.4. The crystal parameters of the films were investigated carefully using x-ray diffraction techniques presented in sections ‎3.4 and ‎3.5. The stoichiometry of the films and oxidation state of Mn cations were investigated using x-ray photoelectron spectroscopy as shown in sections ‎4.3 and ‎4.4. The orbital occupation of Mn cations was studied by means of linearly polarized x-ray absorption spectroscopy where electron density in the valence shell of Mn cations was probed with respect to the crystallographic direction. In this fashion, the orbital occupation along the in- and out-of-plane directions could be inspected. Our findings show that preferential orbital occupation in layered manganites on one the hand depends on the doping level, while on the other hand they are robust against artificially applied strain. The former is expected since the energy band level is a function of the electron density of Mn cations. However, unlike perovskite manganites, the artificial strain does not affect the orbital occupation. Such differentiation between doping levels and artificial strain can, based on our findings, be addressed only in the thin film of layered manganites, as in the Perovskite family members the strain can be compensated by rotation of MnO6 octahedra, a feature which is absent in layered structures. The details of the linearly polarized x-ray absorption spectroscopy measurements are given in section ‎5.