HEISENBERG SPIN MODEL WITH SINGLE-ELECTRON AND DOUBLE-ELECTRON EXCHANGE

This paper presents a discussion about a spin model arising out of sin gle- and double-electron exchange processes. The Hamiltonian which con sists of the usual Heisenberg exchange, biquadratic exchange and three -atom coupling is studied using the Green function equation-of-motion method within the random-phase approximation. The effects of biquadrat ic exchange and three-atom coupling on the magnetization curves, quadr upolar ordering and the transition temperature T(C) are studied. It is shown that the magnetization changes from second to first order at T( C) depending on the strengths of the biquadratic and three-atom coupli ng. It is also seen that T(C) becomes equal to zero at certain values of the parameters. The results are compared with those of molecular-fi eld theory.