Six new phase diagrams, including a novel multicritical topology and two new ordered phases, high-entropy ferrimagnetic and antiquadrupolar, are found in the spin-1 Ising model with only nearest-neighbor interactions, for negative biquadratic couplings. Thus, the global phase diagram of this simple spin system includes nine distinct topologies and three ordered phases. It is indicated that these results, obtained by mean-field theory, are applicable to three-dimensional systems.

• Received 13 June 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.1027