We investigate the spin dynamics of ${\mathrm{SrCr}}_{9p}{\mathrm{Ga}}_{12-9p}{\mathrm{O}}_{19}$ for $p$ below and above the percolation threshold $p_c$ using muon spin relaxation. Our major findings are as follows: (i) At $T\to 0$ the relaxation rate is $T$ independent and $\propto p^3$, (ii) the slowing down of spin fluctuation is activated with an energy $U$, which is also a linear function of $p^3$ and $\underset{p\to 0}{limop}U=8\mathrm{}\mathrm{K}$; 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.

• Received 12 July 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.3450