Spin dynamics at percolation.

We have carried out an inelastic neutron scattering study of spin relaxation in the dilute, two-dimensional Ising antiferromagnet Rb(2)Co(c)Mg(1-c)F(4) for c very close to the magnetic percolation threshold c(p)=0.593 The dynamical exponent z relating the order paramenter relaxation rate to the magnetic correlation length is found to be 2.39(+0.15)(-0.04). We argue that z has this anomalously high value because of the unusual geometry of percolating networks.