Ab initio calculations of incommensurate antiferromagnetic spin fluctuations in hcp iron under pressure

We present ab initio calculations of the static paramagnetic spin susceptibility for hcp iron at finite temperatures and for a range of pressures. The dominant magnetic fluctuations in hcp Fe are found to be incommensurate antiferromagnetic, characterized by the wave vector ${\mathbf{q}}_{\mathrm{inc}}=(0.56,0.22,0).$ We show that ${\mathbf{q}}_{\mathrm{inc}}$ is linked to a Fermi-surface nesting feature. For the lowest pressure $\ensuremath{\sim}16\mathrm{GPa}$ at which hcp Fe forms, we find that these modes become unstable below a N\'eel temperature ${(T}_{N})$ of 69 K. ${T}_{N}$ rapidly diminishes with increasing pressure. We therefore predict that hcp Fe will be found to have an incommensurate spin-density-wave-ordered state over a small pressure range starting with the onset of hcp phase. We note the coincidence with the superconductivity recently found in this material.