Abstract
We include an additional diagram in the bare electron-electron vertex within the generalised random phase approximation (GRPA) for the t-J model. The equation for the effective electron-electron vertex becomes more complicated but is solved exactly for the simple cubic lattice. The dynamical magnetic susceptibility obtained is the same as in GRPA but with all quantities renormalised. Instability of the paramagnetic phase with respect to the formation of ferromagnetic or antiferromagnetic ordering is studied. Formulae for Curie and Neel temperatures are obtained. The additional diagram leads to an extension of the antiferromagnetic region and a reduction in the ferromagnetic region.
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