The Curie Temperature of the Two-Dimensional Quadratic Ising Ferromagnet with Mixed Spins of <i>S</i>=1/2 and <i>S</i>=1

Abstract

For the two-dimensional quadratic Ising ferromagnet with mixed spins of S =1/2 and S =1, the Curie temperature T c has been calculated with the use of Bethe-Peierls theory. Assuming the bilinear exchange interactions J 1 between nearest neighbors (n.n.) of the different spins, J 2 and J 3 between two kinds of next nearest neighbors (n.n.n.) of the same spins, and the biquadratic exchange J 1 ′ between n.n. and J 2 ′ between n.n.n. of spins S =1, the behaviors of T c as a function of these exchange parameters are investigated. The ferromagnetic order is shown to be unstable when the ratio J 1 ′ / J 1 amounts to -1.85. It is also pointed out that J 1 ′ and J 2 ′ give a qualitatively different effect on T c . Furthermore, it turns out that the exchange interaction J 2 between n.n.n. of spins S =1 gives more significant effect on T c than J 3 between n.n.n. of spins S =1/2.