  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.
