Structural and magnetic properties of a lamellar manganese oxide

This thesis describes the magnetic and structural phase diagram of the single-layer manganese oxide La1-xSr 1+xMnO4 (0 ≤ x < 0.7) based on single-crystal x-ray and neutron scattering measurements. Single crystals were grown by the traveling-solvent floating-zone method at 18 La/Sr concentrations. The low-temperature phase diagram can be understood by considering the strong coupling of the magnetic and orbital degrees of freedom, and it can be divided into three distinct regions: low ( x < 0.12), intermediate (0.12 ≤ x < 0.45), and high (x ≥ 0.45) doping. LaSrMnO4 ( x = 0) is a spin-2 antiferromagnetic Mott insulator, and its spin-wave spectrum is well-described by linear spin-wave theory for the Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the eg electron concentration (1 - x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Neel order disappears above x = 0.11. The magnetic frustration is closely related to changes in the eg orbital occupations and the associated Jahn-Teller distortions. In the intermediate region, the material exhibits neither long-range superstructural order nor magnetic order. At high doping (x ≥ 0.45), La1-xSr1+ xMnO4 exhibits long-range superstructural order and a complex (CE-type) antiferromagnetic order that is different from that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites. It becomes incommensurate with the lattice above x = 0.50, and the modulation wavevector epsilon depends linearly on the eg electron concentration: epsilon = 2(1 - x). The magnetic order, on the other hand, remains commensurate with the lattice, but loses its long-range coherence upon doping beyond x = 0.5.