Berlin 2008 – wissenschaftliches Programm

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MA: Fachverband Magnetismus

MA 14: Magnetic Half Metals and Oxides

MA 14.2: Vortrag

Dienstag, 26. Februar 2008, 10:45–11:00, EB 202

Origin of magnetism in hematite-ilmenite system from first principles — •Hasan Sadat Nabi and Rossitza Pentcheva — Department of Earth and Environmental Sciences, University of Munich, Theresienstr. 41, 80333 Munich, Germany

The high remanent magnetization measured in exsolutions of the canted antiferromagnet hematite (Fe₂O₃) and room-temperature paramagnet ilmenite (FeTiO₃) has received considerable attention in recent years[1]. To resolve the microscopic origin of magnetism at the interface of hematite and ilmenite, we have performed density functional theory calculations, varying systematically the concentration, distribution, and charge state of Ti (Fe) in a hematite (ilmenite) host. Our investigation shows that including electronic correlation within the LDA+U approach is decisive to obtain the correct magnetic ground state and band gap of the end members, α-Fe³⁺₂O₃ and Fe²⁺Ti⁴⁺O₃. We find that Ti substituting for Fe³⁺ in the hematite host is not inert as commonly assumed but plays an active role in compensating the charge mismatch at the interface and the emergence of magnetism: In a single Ti layer in a hematite host, the preferred charge state is Ti³⁺, Fe³⁺. As soon as a thicker ilmenite-like block forms, the most favorable compensation mechanism is through Ti⁴⁺ and a disproportionation in the Fe contact layer in Fe²⁺, Fe³⁺ giving first theoretical evidence for the lamellar magnetism hypothesis [1]. The substitution of Ti (or Fe) in Fe₂O₃ (FeTiO₃) leads to impurity levels in the band gap and in some cases to half-metallic behavior.

[1] Robinson, P. et al. Nature 418, 517 (2002).