(a) Field dependence of the magnetization of the model in Eq. (4) with an FM interchain coupling (magenta line) J_ab = −0.2, ε = 0.25, compared to that with an AFM interchain coupling (blue line) J_ab = 0.2, ε = 0.25. Both curves are obtained at t = 0.05 with the system size 20 × 20 × 128. (b) The phase diagram in the h–t plane with FM (magenta) and AFM (blue, adapted from Ref. [24]) interchain couplings, |J_ab| = 0.2, ε = 0.25.