Origin of magnetic moments in defective ${\text{TiO}}_2$ single crystals

In this Brief Report we show that ferromagnetism can be induced in pure ${\text{TiO}}_2$ single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman scattering, and electron-spin resonance spectroscopy, a defect complex, i.e., ${\text{Ti}}^{3+}$ ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated ${\text{TiO}}_2$. This kind of defect complex results in a local $\left({\text{TiO}}_{6-x}\right)$ stretching Raman mode. We elucidate that ${\text{Ti}}^{3+}$ ions with one unpaired $3d$ electron provide the local magnetic moments.

Figure 1

(Color online) (a) Magnetic moments measured at 300 K as a function of magnetic field before and after irradiation. Inset: the low-field part of the loop for sample 5E15. (b) Magnetic moments at 300 K for samples with an equal sample size and different ion fluences. The linear paramagnetic background has been subtracted.

Figure 2

(Color online) (a) XRD pattern for O-irradiated ${\text{TiO}}_2$. The peaks (marked by arrows) in the left side of the ${\text{TiO}}_2\left(330\right)$ peak evidence the irradiation-generated strain in ${\text{TiO}}_2$. (b) Raman spectra of ${\text{TiO}}_2$ single crystals after oxygen ion irradiation with different fluences. The spectra are shifted vertically for better visualization. A clear mode at the range of $B_{2g}$ (as indicated by the dashed arrow) has been observed after irradiation.

Figure 3

ESR signals observed in sample 5E15 at 5 K. The applied field is along the [001] axis. The main ESR peak accompanied by a hyperfine interaction pattern has a $g$ factor of 1.9408.

Figure 4

(Color online) (a) Angular-dependent resonance field of the ESR spectrum: it is split into two lines upon field orientation changing from the [001] to the [110] axes. (b)–(e) Comparison between simulated (solid lines) and measured (open circles) ESR data. (b) and (d): with field along the [110] direction without (b) and with (d) considering hyperfine interaction. (c) and (e): with field along the [001] direction without (c) and with (e) considering hyperfine interaction.