Magnetite: a search for the half-metallic state

Abstract

We present a detailed study of the spin-dependent electronic structure of thinepitaxial magnetite films of different crystallographic orientations. Using spin- andangle-resolved photoelectron spectroscopy at room temperature, we determine for epitaxialFe3O4(111) films a maximum spinpolarization value of −(80 ± 5)% near EF. The spin-resolved photoelectron spectra for binding energies between 1.5 eV andEF 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 highsymmetry directions, thus providing evidence for the half-metallic ferromagnetic state ofFe3O4 in the [111] direction.In the case of the Fe3O4(100) surface, both the spin-resolved photoelectron spectroscopy experiments and the DFT density ofstates give evidence for a half-metal to metal transition: the measured spin polarization of about−(55 ± 10)% atEF and the theoreticalvalue of −40% are significantlylower than the −100% predicted by local spin density approximation (LSDA) calculations for the bulk magnetite crystal as well as the−(80 ± 5)% obtainedfor the Fe3O4(111) films. The experimental findings were corroborated by DFT calculations as due to a surfacereconstruction leading to the electronic states in the majority-spin band gap and thus tothe reduced spin polarization.