Orbital Ordering Induced Antiferromagnetic Spin Fluctuations in the   Ferromagnetic State of 50 % Hole-doped Manganites: Pr{1/2}Sr{1/2}MnO3 and   Nd{1/2}Sr{1/2}MnO3
  We report inelastic neutron scattering results on the spin dynamics in the 50 % hole-doped manganites, Pr{1/2}Sr{1/2}MnO3 and Nd{1/2}Sr{1/2}MnO3. In the paramagnetic phase, these systems exhibit a ridge-type diffuse scattering, indicating a two-dimensional ferromagnetic (FM) correlation. With decreasing temperature, the systems enter the FM state, but simultaneously they develop A-type antiferromagnetic spin fluctuations. The spin wave dispersion in the FM state is strongly anisotropic. These behaviors can be consistently interpreted from a viewpoint of the underlying orbital ordering. 
H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara and Y.   Tokura