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Co-doped rutile TiO2-δ thin films studied by XANES and first principles calculations

The x-ray absorption near-edge structure (XANES) and first-principles calculations were employed to study the local structure and magnetic origin of Co-doped rutile TiO2 thin film. It is found that the experimental Co K-edge XANES can be reproduced by the calculated spectrum of substitutional Co (CoTi) with O vacancy (VO), suggesting the coexistence of CoTi and VO in this system. First-principles total energy calculations further reveal that the O vacancy prefers to reside near the Co ion and substantially facilitates the impurity doping. The electronic structure analysis indicates that the O vacancy can mediate the ferromagnetic interaction between the CoTi ions and thus is responsible for the observed room-temperature ferromagnetism in the Co-doped TiO2-δ thin films.


Introduction
Dilute magnetic semiconductor (DMS) oxides are attracting increasingly interest due to their potential applications in spintronic devices [1]. Following the theoretical predications that room temperature (RT) ferromagnetism might exist in DMSs of wide-band-gap semiconductors like GaN and ZnO [2], transition-metal (TM) doped oxides have been identified as one of the most promising candidates. Especially, a recent discovery of RT ferromagnetism in Co-doped anatase and rutile TiO 2 by Matsumoto et al. [3,4] has motivated intensive studies on the Co:TiO 2 system.
Because of different preparation conditions, some investigators believe that the magnetism of TM:TiO 2 originates from the presence of TM clusters [5,6], while others treat the ferromagnetism as intrinsic [7,8]. On the other hand, the strong interaction between Co ions and O vacancy (V O ) in Codoped TiO 2-δ was found to play an important role in the induction and stability of ferromagnetism [9,10]. Recently, first-principles calculations suggested that the interactions between Co(Fe) ion and V O in the Co(Fe):TiO 2 systems are strongly dependent on their separation distances [11,12]. Unfortunately, the relative spatial distribution and electronic properties of V O and Co ions in Co-doped TiO 2-δ is still an open question.
In this work, x-ray absorption near-edge structure spectroscopy (XANES) and the first-principles calculations were employed to study the local atomic and electronic structures in the Co:TiO 2-δ sample prepared by RT deposition method. The aim is to determine the occupation sites of Co and O vacancy and their correlations with the magnetic interactions between the substituted Co ions. The Co:TiO 2-δ thin film was prepared by RT deposition of Co on TiO 2 substrate, followed by hightemperature annealing in ultra-high vacuum (UHV). About 0.5-nm-thick Co overlayer was deposited on a clean TiO 2 (100) surface at a rate of 1.0 Å/min under ~5.0×10 −9 mbar. In situ annealing was performed at 700 °C for 30 min. X-ray photoelectron spectroscopy (XPS) measurement suggested that the oxidation state of Co ion in the film is +2. Furthermore, XPS depth profile analysis revealed that the effective Co:TiO 2-δ layer on TiO 2 substrate is about 30 nm, and the Co fraction is around 5%. Neither Co metal nor Co-rich rutile cluster was observed in the film. The superconducting quantum interference device (SQUID) magnetometer measurement shows that the saturation magnetic moment was 0.9 μ B /Co, by assuming that all the Co atoms contribute to the magnetization. The Co K-edge XANES spectra were collected at the beamline BL13B1 of Photon Factory (PF) of High Energy Accelerator Research Organization (KEK), Japan and the U7C beamline of National Synchrotron Radiation Laboratory of China.

Computational details
Our spin-polarized density-functional theory calculations were performed using a plane wave basis set with the projector augmented plane-wave (PAW) method [13] as implemented in the Vienna ab initio simulation package [14]. The exchange-correlation interaction was described within the generalized gradient approximation (GGA) [15]. To investigate Co distribution, supercell of 96 atoms (2 × 2 × 2) was used. The lattice constants and the internal parameters were fully relaxed. The energy cutoff was set to 400 eV, and the convergence in energy and force were 10 -4 eV and 10 -2 eV/Å, respectively. We calculate the formation energy to determine the relative stability of different configurations, for which the formation energy is expressed as

Results and discussion
It is suggested that in the Co-doped TiO 2 films some native defects, for example, V O and Co interstitials (Co I ) are located at sites neighboring the substitutional Co ions. To study the local structures around Co in Co-doped TiO 2 , we performed XANES calculations with FEFF8.4 code [17] at Co K-edge for three representative model clusters: the substitutional Co Ti , interstitial Co I , and Co Ti +V O . Figure 1 displays the Co K-edge XANES spectra of Co:TiO 2 and Co metal, along with the simulated XANES spectra for the above four model structures. First of all, the existence of metallic Co in the Co:TiO 2 sample can be easily excluded, since its XANES spectral shapes and edge positions are significantly different from those of the Co:TiO 2 . Four peaks a, b, c and d can be observed in the Co K-edge XANES for the Co:TiO 2 film. From Fig. 1, we can find that these near edge features of Co:TiO 2 film can only be reproduced by FEFF8.4 calculation for the Co Ti +V O model. When O vacancy is located beyond the nearest-neighbor of Co, the calculated XANES resembles that of the Co Ti model (not shown in Fig. 1). These results have confirmed that the Co ions were substitutionally incorporated into the TiO 2 lattice, and suggest that oxygen vacancy prefers to stay near the Co impurity. Table 1 lists the calculated formation energies and magnetic moments for the cases of Co Ti and three Co Ti +V O models with the first-, second-, and third-neighbor distances. It can be seen that the formation energy for one Co Ti ion is 4.58 eV. And the presence of one Vo decreases the formation energy of Co Ti by about 3 eV, and thus significantly facilitates the Co substitution in the TiO 2 lattice. Furthermore, it is found that the formation energy increases with the Co Ti -Vo distance, confirming that the O vacancy favors to  Now, we explore the effects of the Vo on the magnetic interaction and electronic structure of the Co:TiO 2 . Figure 2 shows the calculated densities of states (DOSs) for three representative configurations including the Co Ti , V O , and Co Ti +V O cases. For reference, the DOS for pure TiO 2 is also calculated [shown in Fig. 2(a)], which is in good agreement with that reported by others [18]. In the case of a Co atom substituted at the Ti site [ Fig. 2(b)], the Co 3d levels are split into a doubly degenerate e g band and a triply degenerate t 2g band due to the octahedral symmetry of Co atoms. The t 2 g↑ state is fully occupied, while the t 2g ↓ and e g bands remain only partly occupied. Correspondingly, the calculated Co K-edge XANES for the Co Ti model exhibits two pre-edge peaks which are similar to the Ti K-edge XANES, as indicated by arrows in Fig. 1. These two peaks arise from the hybridization between the unoccupied Co 3d (t 2g ↓ and e g ) and O 2p states. As seen from Fig. 2(c), the O vacancy induces a donor state a 1 within the gap. This state sufficiently overlaps with the bottom of the conduction band, and therefore, the states induced by O vacancy are delocalized over the entire crystal, consistent with the result for oxygen-deficient TiO 2 [19]. When an O vacancy neighboring a Co ion is introduced, the e g ↑ and t 2g ↑ bands are mixed due to the reduced crystal field in the pyramidal structure composed of five oxygen anions [ Fig. 2(d)]. Moreover, Fig. 2(d) shows that the t 2g bands and e g ↑ band are basically occupied. This indicates that Co Ti adopts a low-spin configuration (t 3 2g ↑e 2 g ↑,t 3 2g ↓). Subsequently, the hybridization between the unoccupied Co 3d e g and O 2p states gives rise to the preedge peak a in the Co K-edge XANES spectrum. Moreover, Fig. 2(d) exhibits that this spin-split donor impurity band is hybridized evidently with the Co Ti 3d states. This suggests the existence of an exchange interaction between the delocalized states and Co ions within the carrier orbit. According to the resonant magnetic polaron (RMP) model [20,21], the delocalized band serves as an effective resonant magnetic polaron and can activate a long-ranged ferromagnetic interaction among the doped Co ions. As a result, the ferromagnetic alignment between the Co ions in the Co:TiO 2-δ system is realized and thus enhances the saturated magnetic moment from 0.88 to 1.00 μ B /Co (Table 1).

Conclusions
In summary, by using XANES and first-principles calculations, we have studied the local atomic structure and magnetic origin of Co-doped TiO 2-δ film with RT ferromagnetism. Co K-edge XANES analysis together with total energy calculations revealed the formation of Co Ti 2+ -Vo complex in the TiO 2 matrix. Based on our first-principles calculations, it is clarified that the impurity states induced by O vacancy are delocalized over the entire crystal, and the strong hybridization between the V Oinduced spin-split donor impurity band and Co 3d state might be responsible for the ferromagnetism origin of the Co:TiO 2-δ thin film.