  Inelastic neutron scattering is applied to study the role of magnetism in
stabilizing the charge ordered state in \emph{R}$_{1/3}$Sr$_{2/3}$FeO$_{3}$
(\emph{R}SFO) (\emph{R} = La, Pr, and Nd). The ratio of the ferromagnetic
exchange energy (J$_F$) and antiferromagnetic exchange energy (J$_{AF}$),
|J$_F$ /J$_{AF}|, is a key indicator of the stability of the charge ordered and
antiferromagnetic ordered state. This ratio is obtained from the spin wave
spectrum by inelastic neutron scattering and is sufficiently large to suggest
that the magnetic exchange energy alone can stabilize the charge ordered state
in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ and Pr$_{1/3}$Sr$_{2/3}$FeO$_{3}$. The
exchange ratio decreases from La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ to
Nd1$_{1/3}$Sr$_{2/3}$FeO$_{3}$ which indicates a gradual destabilization of the
magnetic exchange mechanism for charge ordering in correspondence with the
observed reduction in the ordering temperature.
