Where J is the exchange coupling between spins S i and S j , and A and Λ Λ are on-site and inter-site magnetic anisotropies, respectively. Setting A → → ± ± ∞ ∞ recovered the 2D Ising model and the XY model. (positive sign for n = 1 and negative for n = 2 ), while A ≈ ≈ 0 and Λ Λ ≈ ≈ 0 recovers the Heisenberg model ( n = 3 ). Along with the idealized models described above, the spin Hamiltonian can be used for most experimental setups, and it can also model dipole-dipole interactions by renormalization of the parameter A . However, sometimes including further neighbours or using different exchange coupling, such as antisymmetric exchange, is required.