Spin-fluctuation theory beyond Gaussian approximation

TL;DR

This paper extends the spin-fluctuation theory beyond the Gaussian approximation to model continuous second-order phase transitions, improving the understanding of magnetic properties in Fe-Ni Invar.

Contribution

It introduces high-order terms in the free energy expansion to eliminate the jump phase transition, enabling a more accurate continuous transition description.

Findings

Eliminates the jump phase transition in spin-fluctuation theory.

Renormalizes mean field and spin susceptibility through higher-order terms.

Applied to Fe-Ni Invar, improving magnetic property calculations.

Abstract

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.