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Abstract
The uniaxially anisotropic 3D Heisenberg antiferromagnet model in an external field has been attracted great interest during recent decades, in particular, in its structures and phase transition behaviors in an applied magnetic field. It has been known that it displays an antiferromagnet (AF) phase(where spins from two sublattices tend to be aligned in opposite directions along the z axis) in low field and temperature and a spin-flop (SF) phase (where spins from two sublattices have the same z components but opposite xy components) and paramagnetic (P) phase (where spins are aligned in the same z direction). The meeting point of three phases is the multicritical point, however, the nature of the multicritical point has been controversial. It has been suspected that there exists a biconical structure between the AF and SP phase, and, the bicritical point is substituted with a tetracritical point. We study the phase transition near the multicritical point using Monte Carlo simulationto detect any evidence of the biconical phase. In addition, phase transitions near critical point are discussed. Results are compared to previous findings.