Elsevier

Physica B: Condensed Matter

Volume 422, 1 August 2013, Pages 28-32
Physica B: Condensed Matter

Possible ferromagnetism in Cd-doped TiO2: A first-principles study

https://doi.org/10.1016/j.physb.2013.04.036Get rights and content

Abstract

The magnetic properties of Cd-doped TiO2 have been investigated by first-principles calculations. It is found that the doped system favors the spin-polarized state and high Curie-temperature ferromagnetism can be expected in it. The ferromagnetism can be attributed to the p-d hybridization between Cd and its surrounded oxygen atoms. Cd atoms do not tend to form clusters in TiO2. The doped system can be favorably synthesized in oxygen-rich condition. Moreover, Ti vacancies are much easier to form than oxygen vacancies in the doped system. We find that oxygen vacancies are harmful to the ferromagnetism of the doped system while Ti vacancies are beneficial to the stability of ferromagnetism.

Introduction

Diluted magnetic semiconductors (DMSs) are very promising for potential applications in spintronics by exploiting both charge and spin degrees of freedom [1], [2], [3], [4]. Much work has been done to obtain ferromagnetic DMSs at room temperature (RT) [5], [6]. Experimentally, ferromagnetism can be obtained by doping magnetic elements into semiconductors. For instance, RT ferromagnetism has been reported in Fe-, Co- or Ni-doped oxide and nitride semiconductors [7], [8], [9], [10], [11], [12]. Theoretically, strong ferromagnetism was also reported in the transition-metal (TM) doped semiconductors of groups II–VI and III–V [13], [14], [15], [16], [17], [18]. However, the clustering of magnetic dopants and formation of secondary phases during precipitation often make the ferromagnetism extrinsic [19], [20], [21], which is detrimental and should be avoided. Alternatively, by introducing light elements into host semiconductors, ferromagnetism without contamination of magnetic elements can be also obtained, which is called d0 ferromagnetism [22]. Theoretical studies predict that holes in the valence band often lead to the band magnetization, and therefore, are crucial to the ferromagnetism of doped systems [23], [24]. Generally, there are two ways to introduce holes into the valence band formed by anion p states of host semiconductors. One is to introduce holes by doping dopants of lower valence or weaker electronegativity into semiconductors. Experimentally, RT ferromagnetism was reported in C- or Li-doped ZnO [25], [26], [27], and C- or Mg-doped SnO2 films [28], [29]. The other is by generating sufficient cation vacancies in the semiconductors. For the latter, dangling bonds are formed in the anions nearest to the cation vacancies, which also introduces holes into the valence band. For instance, ferromagnetism has been also reported in pristine ZnO [30], In2O3 [31], AlN [32] and SnO2 thin films [33], where the cation vacancies are thought to be the origin of the ferromagnetism [34], [35]. However, due to the higher formation energy of vacancies, cation vacancies arehard to form in bulk semiconductors and difficult to control in preparations. Usually, holes are introduced to the valence band by doping TM atoms into semiconductors. For instance, ferromagnetism with high Curie-temperatures has been reported in Cu-doped ZnO [36], AlN [37], TiO2 [38] and SnO2 [39], [40]. However, the origin of ferromagnetism is still unclear. Studies reveal that TM elements do not lead to the magnetization of the doped system directly [41], [42] and the ferromagnetism often relates to the intrinsic vacancies [43], [44]. Whether the ferromagnetism is mediated by anion or cation vacancies in TM-doped semiconductors remains still unclear [45], [46], [47]. As an important wide-band-gap semiconductor, TiO2 attracts much attention recently due to its magnetic and optical properties by doping [48]. First-principles calculations have predicted that ferromagnetism can be obtained in B-, C- or N-doped TiO2 [49], [50], [51], [52]. Experimentally, RT ferromagnetism was reported in magnetic TM-doped TiO2 [7], [20], [21]. However, few studies have been done on the ferromagnetism of nonmagnetic TM-doped TiO2, especially the relations between the ferromagnetism and native vacancies. Therefore, the roles of cation substitution and the mediation of native vacancies in anatase need further investigations.

In this article, we report our investigation on the magnetic properties of Cd-doped TiO2 by first-principles calculations. Both the roles of cation substitution and the mediation of native vacancies are investigated. Our results show that the doped system prefers the spin-polarized state and a strong ferromagnetism can be expected in it. The ferromagnetism can be attributed to the p–d hybridization between Cd and its surrounded atoms. The doped Cd atoms do not tend to form clusters in TiO2. We find that Cd defect has a negative formation energy for higher oxygen chemical potential, which means the doped system can be favorably synthesized in oxygen-rich condition. We also calculate the formation energy of native vacancies in the presence of Cd substitution. We find that oxygen vacancies are much hard to form although they prefer to form on the sites nearest to Cd defect. Oxygen vacancies are harmful to the magnetization of the doped system. However, Ti vacancies can strengthen the magnetic coupling of Cd-doped TiO2 and hence benefit the ferromagnetism. We find that the formation energy of Ti vacancies is largely reduced due to Cd substitutions, which means they may contribute to the ferromagnetism of the doped system considerably.

Section snippets

Calculation details

A 2×2×2 supercell of anatase is built from a unit cell with rutile structure (space group P42/mnm). By substituting one or two Ti sites in the supercell with Cd atoms, the doped structures with impurity concentration 6.25% or 12.5% are generated, respectively. The supercell with possible substituting sites is shown in Fig. 1. We perform the first-principles calculations based on full-potential linear augmented plane wave plus local orbitals methods (LAPW+lo), as implemented in the WIEN2K

Roles of Cd substitution

We perform both spin-polarized and nonmagnetic calculations on the Cd-doped TiO2 with impurity concentration 6.25%. Our results show that the system favors the spin-polarized state, which is 41.7 meV in energy below the nonmagnetic state. A total moment of 1.02 μB is induced in the supercell by doping one Cd, which is less than the moments of Mg-, Zn- or Cd-doped SnO2 [55], [56], [57]. The difference of moments is mainly caused by holes distribution in valence band. It is noticed that the holes

Conclusions

We have investigated the magnetic properties of Cd-doped TiO2 by first-principles calculations. We find that a strong ferromagnetism can be obtained by doping Cd into TiO2 with concentration 12.5%. The ferromagnetism can be attributed to the p–d hybridization between Cd and its surrounded oxygen atoms. Cd atoms do not tend to form clusters in TiO2. The formation energy of Cd defect is negative for higher chemical potential of oxygen, which means it can be favorably synthesized in oxygen-rich

Acknowledgments

This work is supported by National Natural Science Foundation of China (nos. 11247428, 11247210, 10974228, and 11204131). K.-C. Zhang also acknowledges the financial supports from Natural Science Foundation of LiaoNing Province (no. 20121078) and Education Office of LiaoNing Province (no. L201197). Y.-F. Li would like to acknowledge the financial supports from Education Office of Inner Mongolia (no. NJZY11143). Y. Liu acknowledges the supports from Natural Science Foundation of Hebei Province

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