Competition of 3d/4f orbitals due to competing conductivity and ferromagnetism in Fe/CoAs layers in Eu(Fe_0.89Co_0.11) ₂As₂

The spin-polarized electron momentum density along the [100] direction of Eu(Fe_0.89Co_0.11)₂As₂ was measured using Magnetic Compton Scattering (MCS). Measurements were performed at different temperatures and applied external magnetic fields. These results provide the first direct insight into the orbital interactions that are responsible for the competing conductivity and ferromagnetism (FM) in this iron pnictide system. The orbital-wise decompositions of the density distribution show that at low applied magnetic field (0.1 T), the spin-polarized average Fe/Co-3d character is enhanced between 15 K and 19 K with a maximum at 17 K, the temperature at which the resistivity shows a local maximum, followed by re-entrant SC (superconducting)-like character. This clearly shows the dominance of the ferromagnetic character over conducting character in this temperature region where the resistivity shows a maximum. At higher applied magnetic field, we do not see this enhancement and subsequent collapse of the Fe/Co-3d character as the temperature decreases, consistent with the disappearance of the re-entrant SC-like feature at higher applied magnetic field. These data support a model in which the spin-polarization of the Fe/Co-3d orbitals is enhanced by the ferromagnetic Eu-ions, leading to the suppression of conductivity at ∼17 K (for 0.1 T), while at lower temperature the conductivity recovers and the Fe/Co-3d spin-polarization collapses.