  Starting from the t-J model, we derive an effective field theory describing
the spin dynamics in the insulating phase of La_{2-x}Sr_xCuO_4, x < 0.055, at
low temperature. Using Monte Carlo simulations, we show that the destruction of
Neel order is driven by the single-hole localization length kappa. A phase
transition at 2% doping is consistent with the value of kappa known from the
variable range hopping conductivity. The static spin structure factor obtained
in our calculations is in perfect agreement with neutron scattering data over
the whole range of doping. We also demonstrate that topological defects (spin
vortex-antivortex pairs) are an intrinsic property of the spin-glass ground
state.
