  We study the dynamics of a two-dimensional ensemble of randomly distributed
classical Heisenberg spins with isotropic RKKY and weaker anisotropic
dipole-dipole couplings. Such ensembles may give rise to the flux noise
observed in SQUIDs with a $1/f^{\alpha}$ power spectrum ($\alpha \lesssim 1$).
We solve numerically the Landau-Lifshiftz-Gilbert equations of motion in the
dissipationless limit. We find that Ising type fluctuators, which arise from
spin clustering close to a spin-glass critical behavior with $T_c =0$, give
rise to $1/f^{\alpha}$ noise. Even weak anisotropic interactions lead to a
crossover from the Heisenberg-type criticality to the much stronger Ising-type
criticality. The temperature dependent exponent $\alpha(T) \lesssim 1$ grows
and approaches unity when the temperature is lowered. This mechanism acts in
parallel to the spin diffusion mechanism. Whereas the latter is sensitive to
the device geometry, the spin-clustering mechanism is largely geometry
independent.
