Condensed-Matter Physics & Materials Science Seminar

"Electronic structure at NiO / Fe3O4 and CoO / Fe3O4 interfaces"

Presented by Hui-Qiong Wang, Department of Applied Physics, Yale University

Wednesday, February 7, 2007, 1:30 pm — Bldg. 480 conference room

Fe3O4 (magnetite) is a ferrimagnetic metal, whereas NiO and CoO are antiferromagetic insulators; all of these oxides are of significant interest in spintronics. In this work, ultrathin films of NiO (100) and CoO (100) are grown monolayer by monolayer on both bulk single-crystal Fe3O4 (100) and MgO-supported epitaxial Fe3O4 (100) films using oxide molecular beam expitaxy. LEED, RHEED and STM are used to study the structures of the Fe3O4 substrates and overlayers. The concepts of coordinative unsaturation and excess surface charge are used to predict the most stable atomic geometries of both the terrace and step structures on the Fe3O4 substrate, in agreement with experimental measurement. The interfacial electronic properties are characterized by measuring ultraviolet and x-ray photoelectron spectra (UPS, XPS) as from one to several monolayers of NiO or CoO are grown on Fe3O4. The evolution of the density-of-states in the O 2p/Fe 3d/Ni (or Co) 3d band as a function of overlayer thickness observed in UPS is compared to models of the spectra that would be expected based on the bulk compounds, with and without additional interfacial electronic states. The ultrathin (e.g., 1 – 3 ML) films of NiO (100) grown on Fe3O4 (100) show bulk-like UPS spectra, and the Fe3O4/NiO interface appears to be nearly atomically sharp. In contrast, the UPS spectra of ultrathin CoO (100) films exhibit very different valence band structure from that of the bulk, indicating a stronger interfacial interaction between CoO and Fe3O4. Electronic states at the CoO/Fe3O4 interface are identified by comparing measurements to model spectra as a function of CoO film thickness.

Hosted by: Adrian Gozar

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