Effect of single-ion uniaxial anisotropy on the phase diagrams of magneticsystem in the presence of internal spin fluctuation

Basing on the two-spin-per-site Heisenberg model, the effect of single-ion uniaxial anisotropy on the phase diagrams of magnetic system in the presenc e of internal spin fluctuation has been investigated by use of the mean fie ld theory. It was found that single-ion uniaxial anisotropy has important e ffect on the phase digrams. In the ferromagnetic case (J(3) > 0) the positi ve single-ion uniaxial anisotropies (D) suppress the internal spin fluctuat ion and raise the phase trasition temperature, and negative single-ion unia xial anisotropies (D) increase the internal spin fluctuation and reduce the phase trasition temperature. In the antiferromagnetic case (J(3) < 0), the re exist two critical values J(c1) and J(c2) (\J(c2)\, < \J(c1)\) in the po sitive D values. In the \J(3)\ < \J(c2)\ range intra-spin exchange coupling prevails inter-spin exchange coupling, the positive D values suppress the internal spin fluctuation and raise the phase transition temperature. In th e \J(3)\ > \J(c1)\ range the two sub-spins behave as a rigid spin and the p ositive D values make the reduction of the phase transition temperature. We also observe that the larger D values make the range of internal spin fluc tuation to move towards the larger \J(3)\ range. (C) 1999 Elsevier Science B.V. All rights reserved.