Abstract
Using time-of-flight and triple-axis inelastic neutron spectroscopy,
we determine spin wave excitations throughout the Brillouin zone for
ferromagnetic manganites La$_{1-x}$Ca$_x$MnO$_3$ ($x=0.25,0.3$) in
their low temperature metallic states. While spin wave excitations
in the long wavelength limit (spin stiffness $D$) have similar
values for both compounds, the excitations near the Brillouin zone
boundary of La$_{0.7}$Ca$_{0.3}$MnO$_3$ are considerable softened in
all symmetry directions compared to that of
La$_{0.75}$Ca$_{0.25}$MnO$_3$. A Heisenberg model with the nearest
neighbor and the fourth neighbor exchange interactions can describe
the overall dispersion curves fairly well. We compare the data with
various theoretical models describing the spin excitations of
ferromagnetic manganites.
