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Martin Greven

Structural and Magnetic Properties of the Single-Layer Manganese Oxide La1-xSr1+xMnO4
S. Larochelle, A. Mehta, L. Lu, P. K. Mang, O. P. Vajk, N. Kaneko, J. W. Lynn, L. Zhou, and M. Greven , Phys. Rev. B 71, 024435 (2005)

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Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La1−xSr1+xMnO4(0≤x<0.7) . Single crystals were grown by the 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 an antiferromagnetic Mott insulator, and its spin-wave spectrum is well described by linear spin-wave theory for the spin-2 square-lattice 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 Néel order disappears above xc=0.115(10) . The magnetic frustration is closely related to changes in the eg orbital occupancies and the associated Jahn-Teller distortions. In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural “nanopatches” begin to form above x∼0.25 . At high doping (x≥0.45) , the ground state of La1−xSr1+xMnO4 exhibits long-range superstructural order and a complex antiferromagnetic order, which differs from that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites, and for x=0.50 we conclude that it is of B2mm symmetry. For x>0.50 , the superstructural order becomes incommensurate with the lattice, with a modulation wave vector ϵ that depends linearly on the eg electron concentration: ϵ=2(1−x) . On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x=0.50 .