Search for the elusive magnetic state of hexagonal iron: The antiferromagnetic Fe71Ru29 hcp alloy

The magnetic states of iron and their dependence on crystal structure represent an important case study for the physics of magnetism and its role in fundamental and applied science, including geophysical sciences. hcp iron is the most elusive structure as it exists only at high pressure but, at the same time, it is expected to be stable up to very high temperature. Exploring the magnetic state of pure Fe at high pressure is difficult and no conclusive results have been obtained. Simple binary alloys where the hexagonal phase of Fe is stabilized, offer a more controllable alternative to investigate iron magnetism. We carried out a neutron diffraction experiment on hcp Fe71Ru29 disordered alloy as a function of temperature. Fe in the hexagonal lattice of this specific alloy results to be antiferromagnetically aligned with a rather complex structure and a small magnetic moment. The temperature dependence suggests a Néel temperature TN=124±10 K, a value consistent with the low magnetic moment of 1.04 ± 0.10 μB obtained from the diffraction data that also suggest a non-commensurate magnetic structure with magnetic moments probably aligned along the c axis. The present data provide evidence for magnetic ordering in hcp Fe and support the theoretical description of magnetism of pure Fe at high pressure.