Triplet ground state of the neutral oxygen-vacancy donor in rutileTiO2

Abstract

Electron paramagnetic resonance (EPR) is used to investigate the triplet ($S$ = 1) ground state of the neutral oxygen vacancy in bulk rutile ${\mathrm{TiO}}_{2}$ crystals. This shallow donor consists of an oxygen vacancy with two nearest-neighbor, exchange-coupled Ti${}^{3+}$ ions located along the [001] direction and equidistant from the vacancy. The spins of the two trapped electrons, one at each Ti${}^{3+}$ ion, align parallel to give the $S$ = 1 state. These neutral oxygen vacancies are formed near 25 K in as-grown oxidized ${\mathrm{TiO}}_{2}$ 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 Ti${}^{3+}$ ions are equivalent; i.e., they have equal hyperfine $A$ matrices. The $A$ matrix for each Ti${}^{3+}$ ion in the neutral $S$ = 1 oxygen vacancy is approximately half of the $A$ matrix reported earlier for the one Ti${}^{3+}$ 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 ${\mathrm{TiO}}_{2}$ 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.