Band structure induced electronic correlations in nickel and iron: van-Hove singularities vs. Earth's core conditions*

Some Bravais lattices have a particular geometry and can slow down the motion of Bloch electrons: a 'pre-localisation' due to band structure properties. Another known source of electronic localisation in solids is the Coulomb repulsion in partially-filled d- or f-orbitals, which leads to the formation of local magnetic moments. The combination of these two effects has been viewed so far as mainly an academic issue. Here we show with ab-initio calculations of unprecedented accuracy and model studies, that their synergy represents instead the underlying physical mechanism in two of the most important ferromagnets: nickel and iron. Furthermore in nickel, the van-Hove singularity is essential for ferromagnetism to appear. Nickel's electron-electron scattering rate is linear in temperature, in violation of the conventional Landau theory of metals. This is true even at Earth's-core conditions, at which iron is instead a good Fermi-liquid. The importance of nickel in models of geomagnetism may therefore be reconsidered.