(color online) Negativity versus Heisenberg coupling parameter j for different values of t with the fixed value of the anisotropy parameters Δ = 1, the single-ion anisotropy parameters d = 1, the biquadratic Heisenberg interaction parameters k = 1, and the external magnetic field (a) h = 0.3, (b) h = 1.0, (c) h = 2.0.