Magnetic fluctuations and effective magnetic moments in \gamma-iron due to electronic structure peculiarities

Abstract

Applying the local density and dynamical mean field approximations to paramagnetic \gamma-iron we revisit the problem of theoretical description of magnetic properties in a wide temperature range. We show that contrary to \alpha-iron, the frequency dependence of the electronic self-energy has a quasiparticle form for both, t_{2g} and e_g states. In the temperature range T=1200-1500 K, where \gamma-iron exist 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. At the same time, at low temperatures \gamma-iron (which is realized in precipitates) is better described in terms of itinerant picture. In particular, the nesting features of the Fermi surfaces yield maximum of the static magnetic susceptibility at the incommensurate wave vector q_{max} belonging the direction q_X-q_W (q_X=(2\pi/a)(1,0,0),q_W=(2\pi/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 q_X-q_L-q_K, where q_L=(2\pi/a)(1/2,1/2,1/2), q_K=(2\pi/a)(3/4,3/4,0). From the analysis of the uniform magnetic susceptibility we find that contrary to \alpha-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 non-linearity 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.