Probing Orbital Order in Magnetite with Resonant Elastic X-Ray Scattering

The underlying mechanism driving the metal-insulator transition in magnetite and the presence of charge order in the low-temperature insulating state remain unresolved since Verwey first discovered this transition back in 1939. Standard X-ray diffraction studies are difficult to perform due to twinned domains, and the various studies investigating the bond lengths between Fe+2 and Fe+3 at the octahedral sites have shown at most a charge disproportionality. However, the presence of a Bragg forbidden (001/2) superlattice peak, which is only observable in the insulating state at resonant energies, is a hidden door into magnetite’s charge and orbital states. We have used coherent soft x-rays at the Fe L-edge resonant energy to probe the orbital order at the octahedral Fe sites. An unexpected diffuse scattering ring about the (001/2) superlattice peak that exists in both the insulating and metallic phases was discovered simultaneously. By following the shape and intensity of this diffuse scattering ring in reciprocal space, we can map out the degree of correlated disorder of the Fe ions in both the metallic and insulating phases.