Abstract
It is demonstrated that the usual perturbation approach is not applicable when considering systems with strong inter-electron correlations (with Hubbard sub-bands). In particular, the expansion in the electron occupation numbers in the low-density limit yields contributions to the ground-state energy which diverge at U to infinity , with U being the Hubbard intra-site repulsion. The dynamic magnetic susceptibility in the Hubbard model with U to infinity is calculated self-consistently using many-electron Hubbard operators. The saturation magnetisation, spin-wave spectrum, Curie temperature and the paramagnetic susceptibility containing the Curie-Weiss contribution are obtained. The results are used to interpret the properties of the system Fe1-xCoxS2. The method developed is generalised in order to consider the narrow-band s-d exchange model.
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