  We report a neutron scattering study of the dynamic spin correlations in
Rb$_2$MnF$_4$, a two-dimensional spin-5/2 antiferromagnet. By tuning an
external magnetic field to the value for the spin-flop line, we reduce the
effective spin anisotropy to essentially zero, thereby obtaining a nearly ideal
two-dimensional isotropic antiferromagnet. From the shape of the quasielastic
peak as a function of temperature, we demonstrate dynamic scaling for this
system and find a value for the dynamical exponent $z$. We compare these
results to theoretical predictions for the dynamic behavior of the
two-dimensional Heisenberg model, in which deviations from $z=1$ provide a
measure of the corrections to scaling.
