Spin dynamical properties and orbital states of the layered perovskite La2-2xSr1+2xMn2O7 (0.3<x0.5) -: art. no. 064414

Low-temperature spin dynamics of the double-layered perovskite La2-2xSr1+2xMn2O7 (LSM0327) was systematically studied in a wide hole concentration range (0.3 less than or equal to x < 0.5). The spin-wave dispersion, which is almost perfectly two-dimensional, has two branches due to a coupling between layers within a (double-layer. Each branch exhibits a characteristic intensity oscillation along the out-of-plane direction. We found that the in-plane spin stiffness constant and the gap between the two branches strongly depend on x. By fitting to calculated dispersion relations and cross sections assuming a Heisenberg model, we have obtained the in-plane (J(\)). intra-bilayer (J(perpendicular to)) and inter-bilayer (J') exchange interactions at each x. At x = 0.30, J(parallel to) = -4 meV and J(perpendicular to) = -5 meV, namely almost isotropic and ferromagnetic. Upon increasing x, J(perpendicular to) rapidly approaches zero while \J(parallel to)\ increases slightly, indicating an enhancement of the planar magnetic anisotropy. At x = 0.48, J(parallel to) reaches -9 meV, while J(perpendicular to) turns to + 1 meV, indicating an antiferromagnetic interaction. Such a drastic and change of the exchange interactions can be ascribed to the change of the relative stability of the d(x2-y2) and d(3z2-r2) orbital states upon doping. However, a simple linear combination of the two states results it an orbital state with an orthorhombic symmetry, which is inconsistent with the I4/mmm tetragonal symmetry Of the crystal structure. We thus propose that an orbital liquid state realizes in LSMO327, where the charge distribution symmetry is kept tetragonal around each Mn site. Orbital liquid states are formulated in a theoretical model which takes into account strong electron correlations. The calculated results satisfactorily explain the systematic changes of the exchange interactions in LSMO327 observed in die experiments.