  We present results of our Monte Carlo simulation of the Ising-O(3) model on
the two-dimensional (2D) and quasi-2D lattices. This model is an effective
classical model for the stacked square-lattice $J_1$-$J_2$ Heisenberg model
where the nearest neighbor ($J_1$) and next-nearest neighbor ($J_2$) couplings
are frustrated and we assume that $J_2$ is dominant. We find an Ising ordered
phase where the O(3) spins remain disordered in a moderate quasi-2D region.
There is a single first order transition for a sufficiently large 3D coupling.
Our results can provide a qualitative explanation on the experiments on
ferropnictides, namely observed sequence and orders of the structural and
magnetic transitions, in terms of the ratio between the inter-layer and
intra-layer coupling.
