Spin fluctuations in γ-Fe and inFe3Pt Invar from local-density-functional calculations

The electronic structure and the ensuing magnetic properties of γ-Fe and ${\mathrm{Fe}}_3$Pt Invar are determined using the local approximation to spin-density-functional theory and the augmented-spherical-wave method. A noncollinear constrained-moment procedure is described and employed to obtain the total energy as a function of the volume and the magnetic moment arranged both in collinear and noncollinear spiral structures. This, on a microscopic basis, establishes the connection of the volume with longitudinal and transverse spin fluctuations. We determine the thermal expansion coefficients of γ-Fe and ${\mathrm{Fe}}_3$Pt and give an explanation of their anomalous magnetovolume properties.