Paper MI-ThA1
Ferromagnetism and Dopant Ordering in Semiconducting, Epitaxial Ti-doped α-Fe₂O₃ Hematite

Thursday, October 18, 2007, 2:00 pm, Room 619

The classical visualization of a ferromagnetic semiconductor is the random substitution of a fraction of the original atoms within the semiconductor lattice with magnetic atoms, most commonly transition metal ions. An alternate approach which has not garnered much attention until recently is the ordered substitution of non-magnetic metal ions into an otherwise antiferromagnetic semiconductor lattice. Ti-doped α-Fe₂O₃ has been suggested as such a material if Ti(IV) substitutes preferentially in one magnetic sublattice, effectively creating a ferrimagnetic semiconductor. To examine the ordering more fully, we have used oxygen plasma-assisted MBE to grow Ti-doped hematite on α-Al₂O₃(001) for various dopant levels between the endpoints Fe₂O₃ and FeTiO₃. Excellent heteroepitaxy was achieved by first growing a Cr₂O₃ buffer layer to grade the lattice mismatch. Fe was predominantly found to be in the +3 charge state by Fe K-shell XANES and Fe 2p photoemission, except at concentrations nearing x = 0.15. Ti was found to be exclusively in the +4 charge state and to uniformly substitute for Fe(III) in the hematite lattice by Ti K-shell XANES and EXAFS, accompanied by a significant site distortion. The resultant epitaxial films for low dopant concentration are magnetic at room temperature albeit with a fraction (~0.5 μ_B/Ti atom) of the 4 μ_B/Ti saturation magnetization expected if a magnetic ordered phase had nucleated exclusively. DFT predicts that the magnetically ordered and magnetically random structures are nearly iso-energetic which explains the weak normalized moment. We have investigated the atomic structure of the low-doped epitaxial ferromagnetic films using high-resolution TEM and electron diffraction analysis. HRTEM and electron diffraction confirm the lack of long-range chemical ordering of Ti along the [001] direction. HRTEM images show weak but discernable lines in (Ti_xFe_1-x)₂O₃ along the growth direction with an average in-plane periodicity of ~0.94nm. Electron diffraction patterns corroborate this ordering by displaying additional diffraction spots perpendicular to the growth direction. These satellite spots are suggestive of Ti dopant ordering in the basal plane. A proposed atomic model of the dopant ordering including DFT calculations will be discussed in relation to the observed experimental electronic and magnetic properties.