Magnetism in iron and nickel

The fluctuating local band theory of itinerant electron ferromagnetism in nickel and iron is investigated with the use of first-principles numerical calculations. In this theory the excitations predominantly responsible for the phase transition are fluctuations in the direction of local magnetization. The free energy in the presence of a fluctuation is evaluated numerically in the approximation that this direction changes in time and space slowly enough to justify the use of the static approximation and second-order perturbation theory. The energies and wave functions used to incorporate the band and wavevector dependence of the relevant interaction matrix elements were obtained by Slater-Koster fits to earlier ab initio self-consistent energy bands. Results for nickel and iron are obtained in terms of an effective classical Heisenberg exchange. This is compared with other theoretical calculations and available experimental data. From the numerical results, it is concluded that both quantum effects (the time dependence of the exchange field) and local-field effects are important to account for the transition temperature $T_C$.