A characteristic feature of the Gaussian approximation in the
functional-integral approach to the spin-fluctuation theory is the jump phase
transition to the paramagnetic state. We eliminate the jump and obtain a
continuous second-order phase transition by taking into account high-order
terms in the expansion of the free energy in powers of the fluctuating exchange
field. The third-order term of the free energy renormalizes the mean field, and
fourth-order term, responsible for the interaction of the fluctuations,
renormalizes the spin susceptibility. The extended theory is applied to the
calculation of magnetic properties of Fe-Ni Invar.Comment: 20 pages, 4 figure
