Magnetism of surfaces by spin polarized photoemission

Magnetite: a search for the half-metallic state

We present a detailed study of the spin-dependent electronic structure of thin epitaxial magnetite films of different crystallographic orientations. Using spin- and angle-resolved photoelectron spectroscopy at room temperature, we determine for epitaxial Fe(3)O(4)(111) films a maximum spin polarization value of -(80 ± 5)% near E(F). The spin-resolved photoelectron spectra for binding energies between 1.5 eV and E(F) show good agreement with the spin-split band structure from density functional theory (DFT) calculations which predict an overall energy gap in the spin-up electron bands in high symmetry directions, thus providing evidence for the half-metallic ferromagnetic state of Fe(3)O(4) in the [111] direction. In the case of the Fe(3)O(4)(100) surface, both the spin-resolved photoelectron spectroscopy experiments and the DFT density of states give evidence for a half-metal to metal transition: the measured spin polarization of about -(55 ± 10)% at E(F) and the theoretical value of -40% are significantly lower than the -100% predicted by local spin density approximation (LSDA) calculations for the bulk magnetite crystal as well as the -(80 ± 5)% obtained for the Fe(3)O(4)(111) films. The experimental findings were corroborated by DFT calculations as due to a surface reconstruction leading to the electronic states in the majority-spin band gap and thus to the reduced spin polarization.