Effect of external pressure on the Fe magnetic moment in undoped LaFeAsO from density functional theory: Proximity to a magnetic instability

We investigate the effect of external pressure on the Fe magnetic moment in undoped LaFeAsO within the framework of density-functional theory and show that this system is close to a magnetic instability. The Fe moment is found to drop by nearly a factor of 3 within a pressure range of $\pm 5\text{GPa}$ around the calculated equilibrium volume. While the Fe moments show an unusually strong sensitivity to the spin arrangement (type of antiferromagnetic structure), the low-temperature structural distortion is found to have only a minor influence on them. Analysis of the Fermi-surface topology and nesting features shows that these properties change very little up to pressures of at least 10 GPa. We discuss the magnetic instability in terms of the itinerancy of this system.

Figure 1

(Color online) Crystal structure of LaFeAsO and spin arrangement of the three antiferromagnetic structures (a) AF1, (b) AF2, and (c) AF3 considered in the calculations. The unit cell for AF3 is doubled along the $c$ direction.

Figure 2

(Color online) Left: Fe moment as a function of volume (top: LSDA; bottom: GGA) with $c/a$, $z_{\text{As}}$, and $z_{\text{La}}$ fixed to their experimental ambient pressure values for different types of AF spin arrangement (see text). Dashed vertical lines denote the calculated LSDA equilibrium volume, the experimental volume, and the calculated GGA equilibrium volume (from left to right). Right: corresponding total energies.

Figure 3

Fe moment as a function of pressure for the AF3 spin structure with optimized structural parameters. The inset shows the variation in $c/a$, $V$, $z_{\text{As}}$, and $z_{\text{La}}$ with pressure.

Figure 4

(Color online) Cuts through the FS of LaFeAsO for two different volumes (top: $V=141.9{\text{Å}}^3$; bottom: $V=120{\text{Å}}^3$). Cuts perpendicular to the $c$ axis in the $\Gamma$ plane (left) and in the $Z$ plane (right) are shown. To visualize nesting, the same cuts shifted by $\mathbf{Q}=\left(\pi ,\pi ,0\right)$ are also drawn (light color). Similar nesting is observed throughout $\Gamma \text{−}Z$.