Ivanov, Nedko Borissov Max Planck Institute for the Physics of Complex Systems, Max Planck Society;
https://arxiv.org/pdf/cond-mat/0112295.pdf (Preprint)
Ivanov, N. B., Richter, J., & Farnell, D. (2002). Magnetic phases of the mixed-spin J(1)-J(2) Heisenberg model on a square lattice. Physical Review B, 66(1): 014421. doi:10.1103/PhysRevB.66.014421.