Structural and magnetic properties of the single-layer manganese oxide La_1−xSr_1+xMnO₄

Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La_1−xSr_1+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 e_g 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 x_c=0.115(10). The magnetic frustration is closely related to changes in the e_g 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 La_1−xSr_1+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 e_g 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.