Triplet ground state of the neutral oxygen-vacancy donor in rutile TiO2

A. T. Brant, E. M. Golden, N. C. Giles, Shan Yang (杨山), M. A. R. Sarker, S. Watauchi, M. Nagao, I. Tanaka, D. A. Tryk, A. Manivannan, and L. E. Halliburton
Phys. Rev. B 89, 115206 – Published 28 March 2014

Abstract

Electron paramagnetic resonance (EPR) is used to investigate the triplet (S = 1) ground state of the neutral oxygen vacancy in bulk rutile TiO2 crystals. This shallow donor consists of an oxygen vacancy with two nearest-neighbor, exchange-coupled Ti3+ ions located along the [001] direction and equidistant from the vacancy. The spins of the two trapped electrons, one at each Ti3+ ion, align parallel to give the S = 1 state. These neutral oxygen vacancies are formed near 25 K in as-grown oxidized TiO2 crystals by illuminating with sub-band-gap 442 nm laser light. The angular dependence of the EPR spectra provides the principal values and axes for the g and D matrices. Observations of the Ti and Ti hyperfine lines when the magnetic field is along high-symmetry directions show that the two Ti3+ ions are equivalent; i.e., they have equal hyperfine A matrices. The A matrix for each Ti3+ ion in the neutral S = 1 oxygen vacancy is approximately half of the A matrix reported earlier for the one Ti3+ ion in the singly ionized S = 1/2 oxygen vacancy [Brant et al., J. Appl. Phys. 114, 113702 (2013)]. The neutral oxygen vacancies are thermally unstable above 25 K. They release an electron to the conduction band with an activation energy near 63 meV and convert to singly ionized S = 1/2 oxygen vacancies. When undoped TiO2 is sufficiently oxygen deficient (i.e., reduced), this combination of conduction band electrons and singly ionized oxygen vacancies may result in carrier-mediated ferromagnetism at room temperature.

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  • Received 28 December 2013
  • Revised 28 February 2014

DOI:https://doi.org/10.1103/PhysRevB.89.115206

©2014 American Physical Society

Authors & Affiliations

A. T. Brant1,*, E. M. Golden1, N. C. Giles1,†, Shan Yang (杨山)2,‡, M. A. R. Sarker3,§, S. Watauchi3, M. Nagao3, I. Tanaka3, D. A. Tryk4, A. Manivannan5, and L. E. Halliburton2

  • 1Department of Engineering Physics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433, USA
  • 2Department of Physics, West Virginia University, Morgantown, West Virginia 26505, USA
  • 3Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu Yamanashi, 400-8511, Japan
  • 4Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu, Yamanashi, 400-0021, Japan
  • 5National Energy Technology Laboratory, Morgantown, West Virginia 26507, USA

  • *Present address: Nokomis Inc., 310 5th Street, Charleroi, Pennsylvania 15022, USA.
  • Corresponding author: nancy.giles@afit.edu
  • Present address: Mechanical and Aerospace Engineering Department, Case Western Reserve University, Cleveland, Ohio 44106, USA.
  • §Permanent address: Department of Physics, University of Rajshahi, Rajshahi-6205, Bangladesh.

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Vol. 89, Iss. 11 — 15 March 2014

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