Effects of oxygen vacancy on the magnetic properties of Mn(II)-doped anatase TiO₂

Highlights

• ${\text{V}}_{\text{O}}^{\text{2+}}$ nearest to the Mn(II)_Ti ion is more likely to be formed.

• The presence of ${\text{V}}_{\text{O}}^{\text{+}}$ increases the magnetic moment of the doping system.

• The doping system with ${\text{V}}_{\text{O}}^{\text{2+}}$ is more inclined to ferromagnetic coupling.

• The introduction of ${\text{V}}_{\text{O}}^{\text{2+}}$ dose not change the magnetic moment of the doping system.

Abstract

We studied the magnetism of Mn(II)-doped anatase TiO₂ (a-TiO₂) with and without oxygen vacancy (V_O) by Heyd-Scuseria-Ernzerhof hybrid functional method. Introduction of Mn(II) dopants causes the nonmagnetic a-TiO₂ to have a magnetic moment, which is in direct proportion to the limited doped concentrations. The type of preferential magnetic coupling in Ti_1-δMn(II)_δO₂ depends upon the Mn-Mn distance. Introduction of ${\text{V}}_{\text{O}}^{\text{+}}$ increases the local magnetic moment in Ti_1-δMn(II)_δO₂, and the antiferromagnetism of the system is further enhanced. In contrast, the local magnetic moment of Ti_1-δMn(II)_δO₂ with ${\text{V}}_{\text{O}}^{\text{2+}}$ does not change, but the ferromagnetic coupling becomes more stable.

Introduction

As a promising wide-band gap semiconductor, titanium dioxide (TiO₂) has received much attention due to its outstanding performance in the photonic, electronic, and magnetic applications [1], [2], [3], [4]. Many investigations found that doping with 3d transition metal (TM) induce the magnetic moment in TiO₂ [5], [6], [7], [8], [9], [10], [11], even if the origin of the magnetism in 3d TM-doped TiO₂ remains confusing. For instance, some different results were observed in Mn-doped TiO₂. Bhattacharyya et al. [5] reported that the room temperature (RT) ferromagnetism behavior of Ti_1-xMn_xO₂ nanocrystals is dependent on the Mn doping concentration. Hong et al. [6] demonstrated that Mn dopant did not play an important role in Mn-doped TiO₂ thin films. When the dopant concentration is below 5%, the Mn atom enhances the magnetic moment, whereas the Mn doping destroys the ferromagnetism of the host if the dopant content increases to larger than 10% [6]. Moreover, some researches showed that the undoped TiO₂ thin films can also exhibit RT ferromagnetism. Vacuum annealing enhances the RT ferromagnetic signal, and the subsequent annealing in air can reduce it, which illustrates that the observed RT ferromagnetism in undoped TiO₂ thin films is connected with oxygen vacancies [7].

On the other hand, several different results [8], [9] have been reported by someone who use the First-principles to investigate the magnetism of TiO₂. Errico et al. [8] investigated the ferromagnetism of Mn-, Fe-, Co–, and Ni-doped TiO₂ by first-principles calculations, and found that the spin-polarization mainly occurs at the impurity sites, and the magnetic moments of the impurities are independent of the impurity concentration. Li et al. [9] proposed that O divacancy, Ti single vacancy and divacancy could introduce the local magnetic moments in undoped TiO₂.

The recent research by Ahmed confirmed that the interactions between manganese ions and oxygen vacancies could cause the RT ferromagnetism in Mn-doped TiO₂ [10]. However, Duhalde et al. [11] prepared Ti_0.9TM_0.1O_2-δ (TM = Mn, Fe, Co, Ni, Cu) thin films by pulsed laser deposition, and found that Ti_0.9Mn_0.1O_2-δ film has no ferromagnetism signal. Up to now, no rigorous theoretical model can determine the roles of Mn(II) ions in Ti_1-xMn(II)_xO₂ system, and the correlation between magnetism and oxygen vacancies remains controversial. Therefore, it is necessary to give a detailed description of the magnetic properties of Mn(II)-doped TnO₂ and understand reasonably the effects of V_O on the magnetism of this doping system. In addition, compared with the traditional DFT calculation, the hybrid function could get more accurate results [12]. The calculated parameter testing is also one of the work in this paper.