Ab initiofull-potential study of the structural and magnetic phase stability of iron

Abstract

We have investigated the magnetic and structural phase diagram of iron employing the full potential linear augmented plane-wave method within the generalized gradient approximation. Therefore, total-energy calculations have been performed together with investigations with varying $c/a$ ratio to check the phase stability. This study focuses on the structural and magnetic properties relevant to Invar and anti-Invar and structural phase transitions occurring in these materials. We show that the properties of antiferromagnetic fcc iron can be understood by collinear full potential calculations. In order to do this, the antiferromagnetic structure has been distorted by short-range ferromagnetic nearest-neighbor coupling. From this we can conclude that the classical low-spin behavior can be replaced by antiferromagnetic ordering. Additionally, the thermal properties of iron, especially the free-energy and thermal-expansion coefficient $\ensuremath{\alpha}(T)$ have been analyzed, which is important for the understanding of the anti-Invar effect. The free energy and $\ensuremath{\alpha}(T)$ were estimated from a Debye scheme for which ab initio results were given as input. Besides the more common cubic phases we have investigated hcp Fe at large volumes in view of its magnetic structure.