Magnetic fluctuations and effective magnetic moments inγ-iron due to electronic structure peculiarities

Applying the local density and dynamical mean field approximations to paramagnetic γ-iron we revisit the problem of the theoretical description of its magnetic properties in a wide temperature range. We show that contrary to α-iron, the frequency dependence of the electronic self-energy has a quasiparticle form for both t2g and eg states. In the temperature range $T=1200$--1500 K, where γ-iron exists in nature, this substance can be nevertheless characterized by temperature-dependent effective local moments, which yield relatively narrow peaks in the real part of the local magnetic susceptibility as a function of frequency. At the same time, at low temperatures γ-iron (which is realized in precipitates) is better described in terms of the itinerant picture. In particular, the nesting features of the Fermi surfaces yield the maximum of the static magnetic susceptibility at the incommensurate wave vector qₘₐₓ belonging in the direction qX-qW (qX≡(2π/a)(1,0,0),qW≡(2π/a)(1,1/2,0), a is a lattice parameter) in agreement with the experimental data. This state is found, however, to compete closely with the states characterized by magnetic wave vectors along the directions qX-qL-qK, where qL≡(2π/a)(1/2,1/2,1/2), qK≡(2π/a)(3/4,3/4,0). From the analysis of the uniform magnetic susceptibility we find that contrary to α-iron, the Curie-Weiss law is not fulfilled in a broad temperature range, although the inverse susceptibility is nearly linear in the moderate-temperature region (1200--1500 K). The nonlinearity of the inverse uniform magnetic susceptibility in a broader temperature range is due to the density of states peak located close to the Fermi level. The effective exchange integrals in the paramagnetic phase are estimated on the base of momentum-dependent susceptibility.