Anisotropic Heisenberg Ferromagnet with Biquadratic Exchange Interaction

For the anisotropic Heisenberg ferromagnet of S = 1 with the bilinear exchange interaction J and the biquadratic exchange interaction J 1 , the magnetization and the Curie temperature T C have been caluculated by making use of the pair model approximation. It is pointed out that the biquadratic exchange interaction J 1 gives different effects on the magnetization of the Heisenberg, the anisotropic Heisenberg and the Ising ferromagnets especially in the negative region of J 1 ≤ -J. Furthermore, the dependence of the Curie temperature T C on the biquadratic exchange interaction J 1 turns out to show an anormalous behavior for the Heisenberg ferromagnet in the range of J 1 ≤-2.35 J.