Ferromagnetism and structure of epitaxial Cr-doped anatase TiO₂ thin films

Journal Article · Sat Apr 01 00:00:00 EST 2006 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.73.155327· OSTI ID:881934

Kaspar, Tiffany C.; Droubay, Timothy C.; Shutthanandan, V.; Heald, Steve M.; Wang, Chong M.; McCready, David E.; Thevuthasan, Suntharampillai; Bryan, J. D.; Gamelin, Daniel R.; Kellock, A. J.; Toney, Michael F.; Hong, X.; Ahn, Charles; Chambers, Scott A.

The materials and magnetic properties of Cr-doped anatase TiO₂ thin films deposited on LaAlO₃(001) and SrTiO₃(001) substrates by oxygen-plasma-assisted molecular beam epitaxy have been studied in detail to elucidate the origin of ferromagnetic ordering. Cr substitution for Ti in the anatase lattice, with no evidence of Cr interstitials, segregation, or secondary phases, was independently confirmed by transmission electron microscopy (TEM) with energy dispersive x-ray (EDX) spectroscopy, extended x-ray absorption fine structure (EXAFS), and Rutherford backscattering spectrometry (RBS) in the channeling geometry. Epitaxial films deposited at ~0.1 Å/s were found to have a highly defected crystalline structure, as quantified by high resolution x-ray diffraction (XRD). These films were also ferromagnetic at room temperature with a moment of ~0.5 μB/Cr, Curie temperatures in the range of 400 – 700°C, and exhibited shape and in-plane magnetocrystalline anisotropy. However, no free carrier spin polarization was observed by Hall effect measurements, raising questions about the mechanism of magnetism. Films deposited slowly (~0.015 Å/s) possessed a nearly perfect crystalline structure as characterized by XRD. Contrary to expectations, these films exhibited negligible ferromagnetism at all Cr concentrations. Annealing in vacuum to generate additional oxygen defects and free carrier electrons did not significantly increase the ferromagnetic ordering in either fast- or slow-grown films. These results contradict both oxygen-vacancy-derived free-carrier-mediated exchange and F-center-mediated bound magnetic polaron exchange mechanisms, and instead indicate the primary role of extended structural defects in mediating the ferromagnetic ordering in doped anatase films.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 881934
• Report Number(s): PNNL-SA-47923; 6292a; 6292; KP1704020; TRN: US200714%%48
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 73, Issue 15; ISSN 1098-0121
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ANISOTROPY
CURIE POINT
FERROMAGNETISM
FINE STRUCTURE
HALL EFFECT
MAGNETIC PROPERTIES
MOLECULAR BEAM EPITAXY
POLARIZATION
POLARONS
RUTHERFORD BACKSCATTERING SPECTROSCOPY
THIN FILMS
TRANSMISSION ELECTRON MICROSCOPY
X-RAY DIFFRACTION
Environmental Molecular Sciences Laboratory