Magnetite: a search for the half-metallic state

Abstract

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.

Scanning Tunneling Microscopy Study of an Altered Fe₃O₄(001) Thin Films Surface by Hydrogen Adsorption

“…Although they are also observed in the proximity of surface defects such as steps edges, this paper discusses bright protrusions other than those. Previous STM studies of a freshly prepared Fe 3 O 4 (001) surface have also observed them [5,8,21,27,28]. As a result of XPS measurements of the surface, we have observed no peaks derived from impurities such as CO and no peak shifts for O and Fe (data not shown).…”

“…1(b). STM experiments have demonstrated that Fe B rows of a clean Fe 3 O 4 (001) surface show Jahn-Teller distorted wave-like structures [5,8,22,23,29]. However, STM measurements of a hydrogen-adsorbed Fe 3 O 4 (001) surface have revealed that the configuration of surface atomic rows turns to straight upon hydrogen adsorption [13].…”

“…The electronic and electron spin states for surface Fe cations differ from ones for bulk Fe cations since there is no Fe atoms to be bonded above the surface Fe cations. In fact, theoretical and experimental electron spin polarization measurements of a clean Fe 3 O 4 (001) surface have shown much lower values than expected [5][6][7][8][9][10][11][12].…”

CVD synthesis of polycrystalline magnetite thin films: structural, magnetic and magnetotransport properties

“…Because of its abovementioned intrinsic characteristics, magnetite is one of the most interesting materials for application as ferromagnetic (oxide) layer into spintronic devices [4]. Even though very high P values (-80 %) have been observed in epitaxial magnetite films [5], the possibility to obtain full spin polarization in this material is still disputed [4,6]. The large number of recent publications on this topic underlines the high interest in this material [5][6][7][8][9][10][11][12][13][14][15][16], but the practical achievement of (the expected) high performances in functional spintronic devices like magnetic tunnel junctions (MTJ) incorporating Fe 3 O 4 has not been reached yet [9][10][11]13].…”

Anomalous anisotropic magnetoresistance in epitaxialFe3O4thin films on MgO(001)

“…Hence, it is viewed as a potentially interesting material for spintronic applications. 4,5 The Verwey transition in Fe 3 O 4 leads to a change in conductivity by two orders of magnitude across the transition temperature and is believed to be associated with a order-disorder transition from a charge-ordered state of B-site Fe ions (Fe +2,+3 , for octahedral Fe-ions) at low temperatures to a disordered state (Fe +2.5 at all B-sites) at higher temperatures. Despite decades of activity, the underlying physics related to this transition is not yet completely clear.…”