The phase diagrams of spin-1 Heisenberg antiferromagnets with ferromagnetic next-nearest-neighbor (NNN) interactions, single-site uniaxial anisotropy, and a magnetic field along the easy axis are studied. If we treat uniaxial anisotropy exactly and exchange interactions in the mean-field approximation, novel multicritical behavior is obtained at the coexistence of paramagnetic, spin-flop, and antiferromagnetic phases. New higher-order critical points are found which combine bicritical, tricritical, critical-end-point, and triple-point properties. Occurring at well-defined values of anisotropy and NNN interactions, the multicritical points merge at a new multicritical point in the four-dimensional parameter space of temperature, magnetic field, NNN interactions, and anisotropy. Appropriate phase diagrams as well as topology of the various multicritical points involved are discussed.

• Received 10 February 1986

DOI:https://doi.org/10.1103/PhysRevB.34.6428