- Email: [email protected]
First-principles study on the magnetism of Mn and Co codoped ZnO
Physica B 409 (2013) 5–9
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
First-principles study on the magnetism of Mn and Co codoped ZnO Jianfeng Dai a,b,n, Chouchou Meng b, Qiang Li c a b c
State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou 730050, China School of Science, Lanzhou University of Technology, Lanzhou 730050, China Computational Physics Key Laboratory of Sichuan Province YiBin University, YiBin 644000, PR China
a r t i c l e i n f o
abstract
Article history: Received 15 September 2012 Received in revised form 26 September 2012 Accepted 28 September 2012 Available online 5 October 2012
The ferromagnetic properties of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O are investigated using total energy of plane wave and ultrasoft pseudopotentials method based on the density functional theory (DFT). The results show that Zn0.875Co0.125O and cobalt (Co) and manganese (Mn) codoped ZnObased dilute magnetic semiconductors (DMSs) are of the ferromagnetic order, and the ferromagnetism mostly comes from interaction of spin-polarized oxygen atoms, zinc atoms and transition metals. Meanwhile, it is also found that the magnetic moments are mostly contributed by the spin-polarized oxygen atoms and transition metals. In addition, the electron sharing for the 3d states of transition metals (Co, Mn) and 2p states of O atoms is exacerbated as compared to pure ZnO and Co doped ZnO, helping to obtain p-type ZnO. These results indicate that Co and Mn codoped ZnO are promising magneto-electronic materials and they can be used for nanoscale spintronics device material. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ferromagnetism Dilute magnetic semiconductors Magnetic moment Density functional theory Density of states Spin-polarized
1. Introduction Dilute magnetic semiconductors (DMSs) have attracted a great deal of interest due to their spintronics applications because the charge and spin of the carriers can be simultaneously controlled [1]. In particular, DMS materials are ideal sources of spin-polarized carriers and can easily be integrated with semiconductor devices for semiconducting and magnetic properties [2]. It has been theoretically predicted that wide-band-gap oxide semiconductors doped with transition metal elements are the most promising candidates for DMSs with high Curie temperatures [3]. This prediction has been widely supported by ab initio calculations based on the local density approximation of ferromagnetic ZnO-based semiconductors [4,5]. In those transition metals, Co (3d74s2) or Mn (3d54s2) magnetic ions are the most widely studied dopants in the ZnO host. Furthermore, the Co/Mn doped ZnO DMSs are promising for applications demanding ferromagnetism above room temperature [6,7]. Despite a flurry of work focusing on them, the origin of ferromagnetism is still quite controversial. Investigation has been carried out on the mechanisms of ferromagnetism associated with defects in the theory. The results of first-principles calculations demonstrate that neutral oxygen vacancy in ZnO is nonmagnetic [8], but Zn vacancy does lead to magnetism [9,10]. Chen et al. confirm that Zn vacancy can induce ferromagnetism in ZnO, using first-principles calculations based on
n Corresponding author at: State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou 730050, China. Tel.: þ 86 13919078187. E-mail address: [email protected] (J. Dai).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.09.054
DFT [11]. Experimentally, Hsu et al. have investigated the FM dependence of oxygen vacancies in the Co doped ZnO films by x-ray near edge spectroscopy. It is found that the enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of the oxygen vacancies in ZnO [12]. For transition metal codoped ZnO, the mechanism of ferromagnetism is very complex. Up to now, it has been reported that Co and Mn codoped ZnO produces ferromagnetism [13], but the origin of ferromagnetism is also indistinct. In this paper, the transition metal Co doped ZnO-based DMSs are explored using the first-principles method calculation based on the DFT. The calculations suggest that the dopant is of ferromagnetic order, and the ferromagnetism of dopant is enhanced when there is Mn doping in the system. In order to study the mechanism of ferromagnetism, the partial density of states (DOS) of each atom is calculated. Furthermore, the electron sharing for the 3d states of transition metal (Co, Mn) and 2p states of O atoms is exacerbated. It is very profitable to the formation of p-type ZnO. These results provide a theoretical basis for the experiment.
2. Computational methods Total energy density functional calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) program [14] with the generalized gradient approximation (GGA) and plane wave pseudopotential approach. The spin-polarized parameter was used in the calculations. The valence-electron configuration for the O, Zn, Co, and Mn atoms were chosen as 2s22p4, 3d104s2, 3d74s2, and 3d54s2,
6
J. Dai et al. / Physica B 409 (2013) 5–9
respectively. The energy cut-off of 340 eV and the k-point mesh of 3 3 1 were used for all calculations on a 32-atom doped ZnO supercell (2a 2a 2c). Out of 16 zinc ions in the original supercell two ions were substituted with magnetic ions in all systems giving a concentration of 12.5%. In the geometry optimization process, the energy changes as well as the maximum tolerances of the force, ˚ stress, and displacement were set as 1.0 10 5 eV/atom, 0.03 eV/A, ˚ respectively. Thus the results converged well. 0.05 GPa, and 0.001 A,
3. Results and discussion 3.1. Crystal structure and magnetic properties The calculated ZnO bulk lattice constants (a ¼3.281,c¼5.315) are in good agreement with the experimental ones [15,16], which indicate that the calculation model and parameters are
Fig. 1. A 32-atom 2 2 2 supercell with lattice sites labeled, (a) Zn0.875Co0.125O and (b) Zn0.875(Co0.5Mn0.5)0.125O.
reasonable. Based on the unit cell, a supercell (2 2 2) containing 32 atoms is constructed, which is sufficient for studying ferromagnetism [17,18]. To obtain the most favorable structure, two transition metals (Co) are located in sites 17 and 19, as shown in Fig. 1(a). These results show that stabilization is gained when Mn ions move closer to each other. Like Co in Zn0.875Co0.125O, a strong aggregation tendency among Mn ions exists in Zn0.875Mn0.125O [19]. Fig. 1(b) displays the transition metal Co and Mn codoped ZnO supercell. Two Zn ions are substituted in sites 17 and 19 by transition metals Co and Mn, respectively. The total density of states (DOS) for the ZnO, Zn0.875Co0.125O, and Zn0.875(Co0.5Mn0.5)0.125O are shown in Fig. 2(a), (b) and (c), respectively. It is observed that the total DOS between the up and downspin for ZnO is symmetrical. The asymmetrical DOS between the up and down-spin channels near the Fermi level suggests the magnetic properties. Therefore, ZnO has no magnetism. However, DOS for Co and Mn codoped ZnO produces magnetization behavior significantly different from Zn0.875Co0.125O. The DOS of up-spin and down-spin has an excursion, which indicates that the system exhibits magnetic order. On the other hand, the DOS of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O under the Fermi level is calculated using differential and integral calculus. The calculation demonstrates that the electron-number of up-spin is more than that of down-spin. It is suggested that Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O are both ferromagnetic and the ferromagnetism of the latter is stronger than the former. This is in good agreement with recent experimental results [20]. In addition, the DOS of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O stretch across the Fermi level, and the two systems have the properties of metals. Fig. 3 shows the partial DOS of Zn0.875Co0.125O; it is found that the ferromagnetism originates only from the hybridization between Co-3d and O-2p electrons in the vicinity of the Fermi level. The Zn-4s electrons do not contribute to the peak at the Fermi level. Fig. 4 displays the partial DOS of Co and Mn codoped
Fig. 2. Total DOS of (a) ZnO, (b) Zn0.875Co0.125O and (c) Zn0.875(Co0.5Mn0.5)0.125O. The Fermi level is set to 0 eV.
J. Dai et al. / Physica B 409 (2013) 5–9
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Fig. 3. Partial DOS of Zn0.875Co0.125O. (a) 4s state for the Zn atoms, (b) 3d state for the Co atoms and (c) 2p state for the O atoms. The Fermi level is set to 0 eV.
Fig. 4. Partial DOS of Zn0.875(Co0.5Mn0.5)0.125O. (a) 3d state for the Co atoms, (b) 3d state for the Mn atoms, (c) 2p state for the O atoms and (d) 4s state for the Zn atoms. The Fermi level is set to 0 eV. (a) Zn-4s, (b) Co-3d, (c) Mn-3d and (d) O-2p.
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ZnO. It is clear that the Co-3d, Mn-3d, O-2p, and Zn-4s partial DOS are asymmetrical near the Fermi level, and that strong ferromagnetic coupling exists in the O-2p, Mn-3d, Co-3d, and Zn-4s electrons. Comparing Fig. 3 with Fig. 4, the most noticeable change happens in the partial DOS of 4s states of Zn atoms around the Fermi level. For Zn0.875(Co0.5Mn0.5)0.125O system, one can see that the spin-polarized state appears in the Zn-4s state, while the spin-polarized state of Zn atoms in Zn0.875Co0.125O is invisible. Meanwhile, the hybridization between O-2p and Zn-4s electrons is also observed. The interaction causes the O-2p orbital around the Fermi energy to split. The results clarify that Co and Mn codoped ZnO lead to the change of magnetism. This fact reveals that ferromagnetism of the system mainly comes from O-2p, Co-3d, Mn-3d, and Zn-4s electrons. Table 1 displays the magnetic moment of oxygen atoms and transition metals for Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O. It is also obtained that the total DOS for the above two cases are 0.86mB/Co and 2.66mB/Co, respectively. The magnetic moment of Zn0.875Co0.125O is close to the experimental value (0.7mB/Co) [21]. According to the results of calculation, the total magnetic moments of the above two cases are 1.98mB and 7.82mB, respectively. It can be seen that the magnetic moments of oxygen atoms are rather enhanced in the Zn0.875(Co0.5Mn0.5)0.125O system compared with those in the Zn0.875Co0.125O system. The magnetic moment of Mn is as large as 4.62mB, and it takes a percentage of nearly 59.1% in the total magnetic moments. However, the calculation indicates that the Zn-4s state has no contribution to the total magnetic moment directly. The magnetic moments mostly come from the spin- polarized oxygen atoms and transition metals. 3.2. Electronic properties Fig. 5 shows the partial DOS for ZnO, Zn0.875Co0.125O and Zn0.875 (Co0.5Mn0.5)0.125O. The Fermi level is around the middle of the forbidden band, but the results of the calculation clarify that the Fermi level is on top of the valence band [22]. It is observed that the overall shape is similar from the figure. There is a peak near the Fermi level, and the impurity energy level is significantly broadened forming a wider acceptor level than the undoped ZnO. The partial DOS of Zn0.875Co0.125O increased as compared to pure ZnO, because of the 3d states of donor impurity (Co) contributing current carrier. The Fermi level of Co and Mn codoped ZnO deeply enters into valence band at 0.2 eV compared with Zn0.875Co0.125O. The free current carrier of high concentration is produced due to Mn doping in Co distributed ZnO. Therefore, the Co and Mn codoped ZnO exhibits good electrical conductivity, and makes for p-type doping. Furthermore, the most noticeable change occurs in the 3d states of Mn atoms and Co atoms around the Fermi level. The electronnumber of 3d states of Co and Mn codoped ZnO is less than that of
Table 1 Calculated magnetic moments of oxygen atoms and transition metals in Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O. Atom
O1 O2 O3 O5 O6 O7 Co Mn
Moment in Zn0.875Co0.125O (mB)
Moment in Zn0.875(Co0.5Mn0.5)0.125O (mB)
0.08 0 0.04 0.04 0.06 0.04 0.86
0.14 0.06 0.08 0.14 0.06 0.06 2.66 4.62
Fig. 5. Partial DOS of (a) ZnO, (b)Zn0.875Co0.125O and (c) Zn0.875(Co0.5Mn0.5)0.125O. The Fermi level is set to 0 eV.
Co doped ZnO at the bottom of the valence band. The reason is that the electrons of 3d states of Mn atoms and Co atoms have opposite spin. The impurity atoms keep close together and the bound electron wave function of dopants obviously overlaps in high concentration doping. Thereby, the electron sharing for the 3d states of transition metal (Co, Mn) and 2p states of O atoms is exacerbated.
J. Dai et al. / Physica B 409 (2013) 5–9
Meanwhile, the characteristic of the electron transfer is obvious, and the bonding capacity of the electron is enhanced.
4. Conclusion In conclusion, the first-principles method is used to study the magnetism of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O based on the DFT. The results demonstrate that Co doped ZnO-based DMSs are ferromagnetic. For the Zn0.875(Co0.5Mn0.5)0.125O system, the ferromagnetism is enhanced because of Mn doping with the same concentration, and the ferromagnetism mostly originates from the strong coupling of O-2p, Co-3d, Mn-3d, and Zn-4s electrons. Moreover, the spin-polarized oxygen atoms and transition metals contribute to the total magnetic moments. It is also observed that Co and Mn codoped ZnO has good electrical conductivity, and it is very helpful for the formation of ptype ZnO.
Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 50873047) and Foundation of Gansu Education department of China (Grant no. 0903-02).
9
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Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
First-principles study on the magnetism of Mn and Co codoped ZnO Jianfeng Dai a,b,n, Chouchou Meng b, Qiang Li c a b c
State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou 730050, China School of Science, Lanzhou University of Technology, Lanzhou 730050, China Computational Physics Key Laboratory of Sichuan Province YiBin University, YiBin 644000, PR China
a r t i c l e i n f o
abstract
Article history: Received 15 September 2012 Received in revised form 26 September 2012 Accepted 28 September 2012 Available online 5 October 2012
The ferromagnetic properties of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O are investigated using total energy of plane wave and ultrasoft pseudopotentials method based on the density functional theory (DFT). The results show that Zn0.875Co0.125O and cobalt (Co) and manganese (Mn) codoped ZnObased dilute magnetic semiconductors (DMSs) are of the ferromagnetic order, and the ferromagnetism mostly comes from interaction of spin-polarized oxygen atoms, zinc atoms and transition metals. Meanwhile, it is also found that the magnetic moments are mostly contributed by the spin-polarized oxygen atoms and transition metals. In addition, the electron sharing for the 3d states of transition metals (Co, Mn) and 2p states of O atoms is exacerbated as compared to pure ZnO and Co doped ZnO, helping to obtain p-type ZnO. These results indicate that Co and Mn codoped ZnO are promising magneto-electronic materials and they can be used for nanoscale spintronics device material. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ferromagnetism Dilute magnetic semiconductors Magnetic moment Density functional theory Density of states Spin-polarized
1. Introduction Dilute magnetic semiconductors (DMSs) have attracted a great deal of interest due to their spintronics applications because the charge and spin of the carriers can be simultaneously controlled [1]. In particular, DMS materials are ideal sources of spin-polarized carriers and can easily be integrated with semiconductor devices for semiconducting and magnetic properties [2]. It has been theoretically predicted that wide-band-gap oxide semiconductors doped with transition metal elements are the most promising candidates for DMSs with high Curie temperatures [3]. This prediction has been widely supported by ab initio calculations based on the local density approximation of ferromagnetic ZnO-based semiconductors [4,5]. In those transition metals, Co (3d74s2) or Mn (3d54s2) magnetic ions are the most widely studied dopants in the ZnO host. Furthermore, the Co/Mn doped ZnO DMSs are promising for applications demanding ferromagnetism above room temperature [6,7]. Despite a flurry of work focusing on them, the origin of ferromagnetism is still quite controversial. Investigation has been carried out on the mechanisms of ferromagnetism associated with defects in the theory. The results of first-principles calculations demonstrate that neutral oxygen vacancy in ZnO is nonmagnetic [8], but Zn vacancy does lead to magnetism [9,10]. Chen et al. confirm that Zn vacancy can induce ferromagnetism in ZnO, using first-principles calculations based on
n Corresponding author at: State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou 730050, China. Tel.: þ 86 13919078187. E-mail address: [email protected] (J. Dai).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.09.054
DFT [11]. Experimentally, Hsu et al. have investigated the FM dependence of oxygen vacancies in the Co doped ZnO films by x-ray near edge spectroscopy. It is found that the enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of the oxygen vacancies in ZnO [12]. For transition metal codoped ZnO, the mechanism of ferromagnetism is very complex. Up to now, it has been reported that Co and Mn codoped ZnO produces ferromagnetism [13], but the origin of ferromagnetism is also indistinct. In this paper, the transition metal Co doped ZnO-based DMSs are explored using the first-principles method calculation based on the DFT. The calculations suggest that the dopant is of ferromagnetic order, and the ferromagnetism of dopant is enhanced when there is Mn doping in the system. In order to study the mechanism of ferromagnetism, the partial density of states (DOS) of each atom is calculated. Furthermore, the electron sharing for the 3d states of transition metal (Co, Mn) and 2p states of O atoms is exacerbated. It is very profitable to the formation of p-type ZnO. These results provide a theoretical basis for the experiment.
2. Computational methods Total energy density functional calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) program [14] with the generalized gradient approximation (GGA) and plane wave pseudopotential approach. The spin-polarized parameter was used in the calculations. The valence-electron configuration for the O, Zn, Co, and Mn atoms were chosen as 2s22p4, 3d104s2, 3d74s2, and 3d54s2,
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J. Dai et al. / Physica B 409 (2013) 5–9
respectively. The energy cut-off of 340 eV and the k-point mesh of 3 3 1 were used for all calculations on a 32-atom doped ZnO supercell (2a 2a 2c). Out of 16 zinc ions in the original supercell two ions were substituted with magnetic ions in all systems giving a concentration of 12.5%. In the geometry optimization process, the energy changes as well as the maximum tolerances of the force, ˚ stress, and displacement were set as 1.0 10 5 eV/atom, 0.03 eV/A, ˚ respectively. Thus the results converged well. 0.05 GPa, and 0.001 A,
3. Results and discussion 3.1. Crystal structure and magnetic properties The calculated ZnO bulk lattice constants (a ¼3.281,c¼5.315) are in good agreement with the experimental ones [15,16], which indicate that the calculation model and parameters are
Fig. 1. A 32-atom 2 2 2 supercell with lattice sites labeled, (a) Zn0.875Co0.125O and (b) Zn0.875(Co0.5Mn0.5)0.125O.
reasonable. Based on the unit cell, a supercell (2 2 2) containing 32 atoms is constructed, which is sufficient for studying ferromagnetism [17,18]. To obtain the most favorable structure, two transition metals (Co) are located in sites 17 and 19, as shown in Fig. 1(a). These results show that stabilization is gained when Mn ions move closer to each other. Like Co in Zn0.875Co0.125O, a strong aggregation tendency among Mn ions exists in Zn0.875Mn0.125O [19]. Fig. 1(b) displays the transition metal Co and Mn codoped ZnO supercell. Two Zn ions are substituted in sites 17 and 19 by transition metals Co and Mn, respectively. The total density of states (DOS) for the ZnO, Zn0.875Co0.125O, and Zn0.875(Co0.5Mn0.5)0.125O are shown in Fig. 2(a), (b) and (c), respectively. It is observed that the total DOS between the up and downspin for ZnO is symmetrical. The asymmetrical DOS between the up and down-spin channels near the Fermi level suggests the magnetic properties. Therefore, ZnO has no magnetism. However, DOS for Co and Mn codoped ZnO produces magnetization behavior significantly different from Zn0.875Co0.125O. The DOS of up-spin and down-spin has an excursion, which indicates that the system exhibits magnetic order. On the other hand, the DOS of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O under the Fermi level is calculated using differential and integral calculus. The calculation demonstrates that the electron-number of up-spin is more than that of down-spin. It is suggested that Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O are both ferromagnetic and the ferromagnetism of the latter is stronger than the former. This is in good agreement with recent experimental results [20]. In addition, the DOS of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O stretch across the Fermi level, and the two systems have the properties of metals. Fig. 3 shows the partial DOS of Zn0.875Co0.125O; it is found that the ferromagnetism originates only from the hybridization between Co-3d and O-2p electrons in the vicinity of the Fermi level. The Zn-4s electrons do not contribute to the peak at the Fermi level. Fig. 4 displays the partial DOS of Co and Mn codoped
Fig. 2. Total DOS of (a) ZnO, (b) Zn0.875Co0.125O and (c) Zn0.875(Co0.5Mn0.5)0.125O. The Fermi level is set to 0 eV.
J. Dai et al. / Physica B 409 (2013) 5–9
7
Fig. 3. Partial DOS of Zn0.875Co0.125O. (a) 4s state for the Zn atoms, (b) 3d state for the Co atoms and (c) 2p state for the O atoms. The Fermi level is set to 0 eV.
Fig. 4. Partial DOS of Zn0.875(Co0.5Mn0.5)0.125O. (a) 3d state for the Co atoms, (b) 3d state for the Mn atoms, (c) 2p state for the O atoms and (d) 4s state for the Zn atoms. The Fermi level is set to 0 eV. (a) Zn-4s, (b) Co-3d, (c) Mn-3d and (d) O-2p.
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ZnO. It is clear that the Co-3d, Mn-3d, O-2p, and Zn-4s partial DOS are asymmetrical near the Fermi level, and that strong ferromagnetic coupling exists in the O-2p, Mn-3d, Co-3d, and Zn-4s electrons. Comparing Fig. 3 with Fig. 4, the most noticeable change happens in the partial DOS of 4s states of Zn atoms around the Fermi level. For Zn0.875(Co0.5Mn0.5)0.125O system, one can see that the spin-polarized state appears in the Zn-4s state, while the spin-polarized state of Zn atoms in Zn0.875Co0.125O is invisible. Meanwhile, the hybridization between O-2p and Zn-4s electrons is also observed. The interaction causes the O-2p orbital around the Fermi energy to split. The results clarify that Co and Mn codoped ZnO lead to the change of magnetism. This fact reveals that ferromagnetism of the system mainly comes from O-2p, Co-3d, Mn-3d, and Zn-4s electrons. Table 1 displays the magnetic moment of oxygen atoms and transition metals for Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O. It is also obtained that the total DOS for the above two cases are 0.86mB/Co and 2.66mB/Co, respectively. The magnetic moment of Zn0.875Co0.125O is close to the experimental value (0.7mB/Co) [21]. According to the results of calculation, the total magnetic moments of the above two cases are 1.98mB and 7.82mB, respectively. It can be seen that the magnetic moments of oxygen atoms are rather enhanced in the Zn0.875(Co0.5Mn0.5)0.125O system compared with those in the Zn0.875Co0.125O system. The magnetic moment of Mn is as large as 4.62mB, and it takes a percentage of nearly 59.1% in the total magnetic moments. However, the calculation indicates that the Zn-4s state has no contribution to the total magnetic moment directly. The magnetic moments mostly come from the spin- polarized oxygen atoms and transition metals. 3.2. Electronic properties Fig. 5 shows the partial DOS for ZnO, Zn0.875Co0.125O and Zn0.875 (Co0.5Mn0.5)0.125O. The Fermi level is around the middle of the forbidden band, but the results of the calculation clarify that the Fermi level is on top of the valence band [22]. It is observed that the overall shape is similar from the figure. There is a peak near the Fermi level, and the impurity energy level is significantly broadened forming a wider acceptor level than the undoped ZnO. The partial DOS of Zn0.875Co0.125O increased as compared to pure ZnO, because of the 3d states of donor impurity (Co) contributing current carrier. The Fermi level of Co and Mn codoped ZnO deeply enters into valence band at 0.2 eV compared with Zn0.875Co0.125O. The free current carrier of high concentration is produced due to Mn doping in Co distributed ZnO. Therefore, the Co and Mn codoped ZnO exhibits good electrical conductivity, and makes for p-type doping. Furthermore, the most noticeable change occurs in the 3d states of Mn atoms and Co atoms around the Fermi level. The electronnumber of 3d states of Co and Mn codoped ZnO is less than that of
Table 1 Calculated magnetic moments of oxygen atoms and transition metals in Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O. Atom
O1 O2 O3 O5 O6 O7 Co Mn
Moment in Zn0.875Co0.125O (mB)
Moment in Zn0.875(Co0.5Mn0.5)0.125O (mB)
0.08 0 0.04 0.04 0.06 0.04 0.86
0.14 0.06 0.08 0.14 0.06 0.06 2.66 4.62
Fig. 5. Partial DOS of (a) ZnO, (b)Zn0.875Co0.125O and (c) Zn0.875(Co0.5Mn0.5)0.125O. The Fermi level is set to 0 eV.
Co doped ZnO at the bottom of the valence band. The reason is that the electrons of 3d states of Mn atoms and Co atoms have opposite spin. The impurity atoms keep close together and the bound electron wave function of dopants obviously overlaps in high concentration doping. Thereby, the electron sharing for the 3d states of transition metal (Co, Mn) and 2p states of O atoms is exacerbated.
J. Dai et al. / Physica B 409 (2013) 5–9
Meanwhile, the characteristic of the electron transfer is obvious, and the bonding capacity of the electron is enhanced.
4. Conclusion In conclusion, the first-principles method is used to study the magnetism of Zn0.875Co0.125O and Zn0.875(Co0.5Mn0.5)0.125O based on the DFT. The results demonstrate that Co doped ZnO-based DMSs are ferromagnetic. For the Zn0.875(Co0.5Mn0.5)0.125O system, the ferromagnetism is enhanced because of Mn doping with the same concentration, and the ferromagnetism mostly originates from the strong coupling of O-2p, Co-3d, Mn-3d, and Zn-4s electrons. Moreover, the spin-polarized oxygen atoms and transition metals contribute to the total magnetic moments. It is also observed that Co and Mn codoped ZnO has good electrical conductivity, and it is very helpful for the formation of ptype ZnO.
Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 50873047) and Foundation of Gansu Education department of China (Grant no. 0903-02).
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