Longitudinal spin structure factor Szz(0,0,π+q) in (a) the Ising AFM (Néel) phase and (b) the LSDW phase. The system size is 20 × 20 × 128. (c) Transverse spin structure factor Sxy(0,0,π+q) in the TAF phase. The system size is 24 × 24 × 24. (d) Field dependence of the modulated ordering wave vector ΔQ and the magnetization, satisfying the relation in Eq. (3). The model parameters used are J_ab = −0.2 and ε = 0.25, with simulations performed at t = 0.05.