H = J Σ [SˣSˣ + SʸSʸ + ΔSᶻSᶻ] = J Σ [½(S⁺S⁻ + S⁻S⁺) + ΔSᶻSᶻ]. Δ<-1: ferromagnet; -1<Δ<1: XY critical phase; Δ=1: Heisenberg AFM; Δ>1: Ising AFM. Computed via exact diagonalization of N≤16 spins.