  A model of $CuO_2$ planes weakly doped with partially delocalised holes is
considered. The effect of such a hole on the background AFM spin texture can be
represented by a purely magnetic Hamiltonian $H = - \sum_{(ijk)} (\vec S_i
\cdot \vec S_j \times \vec S_k)^2$, where the summation is over the four
triangles of a single plaquette. We show that this model of randomly
distributed chiral spin defects leads to an in--plane spin correlation length
approximately described by $\xi^{-1} (x,T) = \xi^{-1} (0,T) + \xi^{-1} (x,0)$,
consistent with neutron scattering experiments on $La_{2-x}Sr_xCuO_4$. Further,
this model leads to favourable comparisons with $B_{1g}$ Raman scattering
results for the same cuprate system.
