  We investigate the spin dynamics of SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ for p
below and above the percolation threshold p_c using muon spin relaxation. Our
major findings are: (I) At T->0 the relaxation rate is T independent and
proportional to p^3, (II) the slowing down of spin fluctuation is activated
with an energy U which is also a linear function of p^3 and lim_{p-> 0}U = 8 K;
this energy scale could stem only from a single ion anisotropy, and (III) the p
dependence of the dynamical properties is identical below and above p_c,
indicating that they are controlled by local excitation.
