Abstract
This paper presents a discussion about a spin model arising out of singleand double-electron exchange processes. The Hamiltonian which consists 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 biquadratic exchange and three-atom coupling on the magnetization curves, quadrupolar ordering and the transition temperature TC are studied. It is shown that the magnetization changes from second to first order at TC depending on the strengths of the biquadratic and three-atom coupling. It is also seen that TC becomes equal to zero at certain values of the parameters. The results are compared with those of molecular-field theory.
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