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First-principles study on ferromagnetism in nitrogen-doped CdO
Physics Letters A 374 (2010) 1889–1892
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
First-principles study on ferromagnetism in nitrogen-doped CdO Chang-wen Zhang ∗ , Pei-ji Wang, Yan Su School of Science, University of Jinan, Jinan, Shandong 250022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 30 January 2010 Received in revised form 15 February 2010 Accepted 17 February 2010 Available online 20 February 2010 Communicated by R. Wu Keywords: Ferromagnetic semiconductor First-principles calculation
a b s t r a c t Using the full-potential linearized augmented plane wave method, we report the electronic structures and magnetism in N-doped CdO system. The results indicate an isolated N atom produces a total magnetic moment of 1.0μ B and introduces spin-polarized 2p states in the band gap. The origin of the magnetic moments is the holes in N 2p band of the N dopant. The half-metallic ferromagnetic properties in N–CdO system is mainly driven by N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) coupling chain through the strong p–p interaction between N and O atoms. © 2010 Elsevier B.V. All rights reserved.
The discovery of oxide-based diluted magnetic semiconductors (DMSs) has given rise to a lot of experimental and theoretical interests [1,2] aiming at finding materials that are ferromagnetic (FM) at room temperature (RT). To date, most of the interesting on DMSs has been focused on transition-metal (TM)-doped semiconductors, such as GaN and ZnO [3,4]. Typically, the Curie temperatures of such systems are below RT, meaning that they are difficult to use in practice. Recently, a number of TM-doped DMSs with the ferromagnetism are reported, such as TM-doped TiO2 [5,6], and In2 O3 [7,8]. However, it is difficult to avoid the presence of TM clustering or secondary phases [9]. These extrinsic magnetic behaviors are undesirable for practical applications and strongly dependent on the preparation methods and conditions. Very recently, unexpected ferromagnetism in the non-magnetic elements doped DMSs [10–16] provide a new opportunity to search new spintronic materials. For instance, both N-doped ZnO and TiO2 were found theoretically to be RT FM materials [11,12, 15]. However, the magnetic couplings between dopants are generally short ranged, and thus high doping concentrations are need. This is very difficult to achieve in experiments due to the high formation energies of dopants. In this Letter, based on first-principles calculations, we report on CdO-based DMS as the possibility of using N as a dopant to produce FM coupling, and demonstrate that a promising long-range RT ferromagnetism could be achieved by N 2p elements doping in N:CdO system. To study the electronic properties and magnetism of N-doped CdO, a 64-atom 2 × 2 × 2 supercell is considered (Fig. 1). For convenience of discussion, we label seven O sites with letters a–g as shown in Fig. 1. We use (i , j ) to denote an N–N pair, in which two
*
Corresponding author. Tel.: +86 0531 82765480; fax: +86 0531 82765480. E-mail address: [email protected] (C.-w. Zhang).
0375-9601/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2010.02.050
Fig. 1. Crystal structure of CdO employed to define various configurations of two N doped CdO. Blue (big) and pink (small) spheres are Cd and O atoms. The positions of O substituted by N are denoted by a–g. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
O atoms are replaced by N at the i and j sites in Fig. 1. The spinpolarized density functional theory calculations are performed using the full-potential linear augmented plane wave method as implemented with the WIEN2k code [17]. Generalized gradient approximation (GGA) [18] is used to treat the exchange correlation
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C.-w. Zhang et al. / Physics Letters A 374 (2010) 1889–1892
Fig. 2. (Left) DOS for single N substituted supercell: TDOS (a), N 2p DOS (b), 2NN O 2p DOS (c), NN Cd 4d DOS (d), and (Right) DOS for two N atoms in FM ordered (a, e) configuration: TDOS (e), N 2p DOS (f), 2NN O 2p DOS (g), NN Cd 4d DOS (h). The vertical line drawn indicates E F position and the majority-spin/minority-spin DOS is shown above/below the abscissa axis.
potential, and relativistic effects are taken into account. Atomic sphere radii of Cd, O, and N atom are set to 2.2, 1.9 and 1.7 a.u., respectively. The parameter Rkmax is chosen as 7.0 (where R is the smallest muffin-tin radius and kmax is the cut-off for the plane wave) for the convergence parameter for which the calculations stabilize and converge in terms of the desired charge. The potential and charge densities inside the atomic spheres are expanded in lattice harmonics up to L = 10. Following the Monkhorst–Pack scheme [19], the 3 × 3 × 3 k-point meshes are used for the first Brillouin zone integration. To see if the formation of these defects is stable, we first consider a single N substitution by replacing an O atom with N atom (Na ). The energy cost E sub for N to replace O is calculated by using the equation: E sub = E [Cd32 O31 N] − E [Cd32 O32 ] + μO − μN , where E (Cd32 O31 N) is the total energy of the supercell containing the N dopant, E (Cd32 O32 ) is the total energy of the pure host, and μO and μN are the chemical potentials of O and N atoms, respectively. Our calculated value E sub = 1.62 eV. This is a reasonable estimation for us to state that is will be possible to dope CdO with N replacing O atoms in the host. Based on the total energy calculations, the spin-polarized state favors over the non-spin-polarized state by 863 meV. Figs. 2(a)–(d) present the calculated total and partial density of states (DOS) for the N dopant and its nearest-neighboring (NN) Cd and second NN (2NN) O atoms. Strong coupling between 2p orbitals of N, O, and 4d orbitals of Cd near the Fermi level (E F ) can be seen clearly. The 4d orbitals of NN Cd are hybridized with 2p orbitals of dopant N and 2NN O atom. The main feature indicated in the spectra is the hole states located around E F , which are mostly from the N 2p and O 2p orbitals. Relative shallow acceptor levels imply the N:CdO are ionized easily at working temperatures [20]. Also we find that the hybridization between N dopant and its neighboring host atoms results in the splitting of the energy levels near the E F , which shifts the majority-spin states downward and minority-spin states upward to lower the total energy of the system, indicating the ideal case for spin injection applications due to 100% spin-polarized carriers. The impurity bands introduced by the N dopants occupy the energy level above the
Table 1 The relative energy E FM is total energy with respect to the ground state of configurations IV (a, e) for N:CdO. E FM is the total energy difference between the AFM and FM states for each configuration. The optimized N–N distance dN−N and magnetic moments M N on N atom for all the configurations for Cd32 O30 N2 supercell are also given. Configurations
dN−N (Å)
E FM (meV)
E FM (meV)
M N (μ B /N)
I (a, b) II (a, c) III (a, d) IV (a, e) V (a, f ) VI (a, g)
3.6402 5.1480 6.3049 7.2804 8.1397 8.9166
41.5 12.9 67.8 117.8 45.4 67.2
327.8 212.3 69.5 0.0 200.3 238.2
0.52 0.52 0.48 0.54 0.51 0.50
valence band maximum, which shows that the doped N atoms can induce the CdO to p-type semiconductor. Since the majority-spin bands are fully occupied while the minority-spin bands are partially filled, the number of holes in the system is equal to the magnetic moment, 1.0μ B per supercell. The magnetic moments are mainly contributed by the N 2p orbital (0.59μ B /N). The NN Cd atom contributes 0.004μ B , and 2NN O atom provides 0.08μ B . The magnetic moments of the atoms have the same direction, indicating there is FM coupling between the doped N atom and neighboring host atoms, which is similar with the results of N-doped ZnO or TiO2 [12,13]. The spatial spin-density distributions in N:CdO are also shown in Fig. 3(a), where the spin-polarized holes is localized mainly on the N atom and distributed slightly over its four NN Cd atoms and twelve 2NN O atoms. This indicates that magnetic coupling clearly extends to the 2NN O atoms from the N dopant centers. Therefore, anions from the delocalized p orbitals contribute chiefly to the magnetic moment in N:CdO DMS, in consistent with DOS analysis above. To investigate the magnetic coupling between N atoms, several distinct supercell structures with two N substitute for several possible O atoms (Fig. 1), which corresponds to a doping concentration of 6.25%. Table 1 lists the relative energy E FM with respect to the ground state of configurations IV (a, e), the energy difference
C.-w. Zhang et al. / Physics Letters A 374 (2010) 1889–1892
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Fig. 3. The spin-density distribution of configuration IV (a, e) for single N dopant (a), and two N atoms in FM coupling (b). The blue (big) and pink (small) balls represent Cd and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
E FM between the AFM and FM states for each configuration. It is found that all the doped configurations are located in FM order. Among all configurations, the IV (a, e) has the lowest total energy and the largest magnitude of E FM (117.8 meV), which is larger than that of Cu-doped ZnO FM materials [21,22] and some other systems, such as N-ZnO [15] and N-TiO2 [16]. More importantly, even when the N–N separation is up to 8.9 Å, the magnitude of E FM is still up to 67.2 meV. While in N-doped ZnO [15], when the two N atoms are separated by 6.136 Å, the energy of FM states is 22 MeV lower than that of the corresponding AFM one. This implies the long-range RT ferromagnetism for N:CdO is quite feasible, which makes N:CdO system superior to the previous observed FM materials [11–16]. The total and partial DOS for the N dopant and its NN Cd and 2NN O atoms in FM ordered IV (a, e) configuration is depicted in Figs. 2(e)–(h). Similar to the single N doping case, hole states arising from the hybridizations between N and its neighboring atoms are also extended to 2NN atoms. The main difference compare to that of single N doping case is that there are more hole states in N 2p and obvious hole states in bridging O 2p and Cd 4d orbitals due to the significant hybridization between the N dopants and neighboring host atoms. The hybridization between the N and neighboring host atoms leads to a formation of N(2p)–Cd(4d)– O(2p)–Cd(4d)–N(2p) coupling chain, which facilitates strong indirect FM coupling between N dopants mediated by the holes. In order to elucidate the origin of ferromagnetism in N-doped CdO, it is instructive to investigate systematically the magnetic coupling between the two dopant atoms. Strong magnetic coupling between N atoms was found to exist even if the separation distance is up to 7.28 Å. Here, both double-exchange and super-exchange interactions cannot account for long-range FM order since the distance between two dopants is very large and the doping concentrations are very low at the same time. Hence, we propose the hybridized N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) chain mechanism through p–d like p–p coupling responsible for the long-range RT FM coupling. Fig. 3(b) shows the spatial spin-density distributions between N dopants in FM order with the distance of 7.28 Å. It is can be seen that the holes localized around the anions between the N atoms are polarized and the spin orientations are parallel to each other. Consequently, these polarized holes mediate the strong coupling between the N atoms. Due to one less valence electrons for N than an O ion, introducing more substituting N atoms into CdO host will lead to more holes in the valence band top. Sufficiently large spin polarized hole forms N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) hybridized chain, in this direc-
tion the strong p–p interaction effectively leading to a stronger coupling between impurity and carrier spin orientations, as confirmed with spin-density distributions in Fig. 3(b). The spatially extended p states of the host and the impurity are able to extend the p–p interaction and spin alignment to the longer range along the way of N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) chain, and thus to cause long-range FM interaction in N:CdO system. Therefore, the N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) chain interaction is the key to stabilize the long-range RT FM order in the N:CdO system. In summary, we have demonstrated that N:CdO system is in FM ground state and calculated magnetization energy of N:CdO is found to be larger than some of the known RT DMS, which implies the RT ferromagnetism for N:CdO can be expected. The FM exchange interaction between N dopants is activated through holes induced by N doping via an N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) coupling chain in the large N–N separations, which plays the major role in forming the long-range FM order in N:CdO system. Our calculated results are more interesting and then open to the future experimental verification. Acknowledgements This work was supported by Foundation for Young Scientist in Shandong Province (Grant No. BS2009CL012), and the School Scientific Research Foundation of Jinan University (Grant No. XKY0716). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
D.D. Awschalom, M.E. Flatté, Nature Phys. 3 (2007) 153. ´ J. Fabian, S.D. Sarma, Rev. Mod. Phys. 76 (2004) 323. I. Žutic, C. Liu, F. Yun, H. Morko, J. Mater. Sci. Mater. Electron. 16 (2005) 555. M.H.F. Sluiter, Y. Kawazoe, P. Sharma, A. Inoue, A.R. Raju, C. Rout, U.V. Waghmare, Phys. Rev. Lett. 94 (2005) 187204. N.H. Hong, J. Sakai, W. Prellier, Phys. Rev. B 70 (2004) 195204. N.H. Hong, J. Sakai, N.T. Huong, V. Brize, Appl. Phys. Lett. 87 (2005) 102505. A. Gupta, H. Cao, K. Parekh, K.V. Raoa, A.R. Raju, U.V. Waghmare, J. Appl. Phys. 101 (2007), 09N513. C. Liu, F. Yun, H. Morko, J. Mater. Sci.: Mater. Electron. 16 (2005) 555. S. Zhou, K. Potzger, J. von Borany, R. Grötzschel, W. Skorupa, M. Helm, J. Fassbender, Phys. Rev. B 77 (2008) 035209. S. Talapatra, P.G. Ganesan, T. Kim, R. Vajtai, M. Huang, M. Shima, G. Ramanath, D. Srivastava, S.C. Deevi, P.M. Ajayan, Phys. Rev. Lett. 95 (2005) 097201. J.G. Tao, L.X. Guan, J.S. Pan, C.H.A. Huan, L. Wang, J.L. Kuo, Z. Zhang, J.W. Chai, S.J. Wang, Appl. Phys. Lett. 95 (2009) 062505. H. Pan, J.B. Yi, L. Shen, R.Q. Wu, J.H. Yang, J.Y. Lin, Y.P. Feng, J. Ding, L.H. Van, J.H. Yin, Phys. Rev. Lett. 99 (2007) 127201. H. Peng, H.J. Xiang, S.-H. Wei, S.-S. Li, J.-B. Xia, J. Li, Phys. Rev. Lett. 102 (2009) 017201.
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[14] C.W. Zhang, S.S. Yan, Appl. Phys. Lett. 95 (2009) 232108. [15] L. Shen, R.Q. Wu, H. Pan, G.W. Peng, M. Yang, Z.D. Sha, Y.P. Feng, Phys. Rev. B 78 (2008) 073306. [16] K. Yang, Y. Dai, B. Huang, M.-H. Whangbo, Appl. Phys. Lett. 93 (2008) 132507. [17] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka, J. Luitz, WIEN2k, An augmented-plane-wave + local orbitals program for calculating crystal properties,
Karlheinz Schwarz, Techn Wien, Austria, 2001. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. H.J. Monkhorst, J. Pack, Phys. Rev. B 13 (1976) 5188. Y.F. Yan, S.H. Wei, Phys. Status Solidi B 245 (2008) 641. D.B. Buchholz, R.P.H. Chang, J.H. Song, J.B. Ketterson, Appl. Phys. Lett. 87 (2005) 082504. [22] L.H. Ye, A.J. Freeman, B. Delley, Phys. Rev. B 73 (2006) 033203. [18] [19] [20] [21]
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
First-principles study on ferromagnetism in nitrogen-doped CdO Chang-wen Zhang ∗ , Pei-ji Wang, Yan Su School of Science, University of Jinan, Jinan, Shandong 250022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 30 January 2010 Received in revised form 15 February 2010 Accepted 17 February 2010 Available online 20 February 2010 Communicated by R. Wu Keywords: Ferromagnetic semiconductor First-principles calculation
a b s t r a c t Using the full-potential linearized augmented plane wave method, we report the electronic structures and magnetism in N-doped CdO system. The results indicate an isolated N atom produces a total magnetic moment of 1.0μ B and introduces spin-polarized 2p states in the band gap. The origin of the magnetic moments is the holes in N 2p band of the N dopant. The half-metallic ferromagnetic properties in N–CdO system is mainly driven by N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) coupling chain through the strong p–p interaction between N and O atoms. © 2010 Elsevier B.V. All rights reserved.
The discovery of oxide-based diluted magnetic semiconductors (DMSs) has given rise to a lot of experimental and theoretical interests [1,2] aiming at finding materials that are ferromagnetic (FM) at room temperature (RT). To date, most of the interesting on DMSs has been focused on transition-metal (TM)-doped semiconductors, such as GaN and ZnO [3,4]. Typically, the Curie temperatures of such systems are below RT, meaning that they are difficult to use in practice. Recently, a number of TM-doped DMSs with the ferromagnetism are reported, such as TM-doped TiO2 [5,6], and In2 O3 [7,8]. However, it is difficult to avoid the presence of TM clustering or secondary phases [9]. These extrinsic magnetic behaviors are undesirable for practical applications and strongly dependent on the preparation methods and conditions. Very recently, unexpected ferromagnetism in the non-magnetic elements doped DMSs [10–16] provide a new opportunity to search new spintronic materials. For instance, both N-doped ZnO and TiO2 were found theoretically to be RT FM materials [11,12, 15]. However, the magnetic couplings between dopants are generally short ranged, and thus high doping concentrations are need. This is very difficult to achieve in experiments due to the high formation energies of dopants. In this Letter, based on first-principles calculations, we report on CdO-based DMS as the possibility of using N as a dopant to produce FM coupling, and demonstrate that a promising long-range RT ferromagnetism could be achieved by N 2p elements doping in N:CdO system. To study the electronic properties and magnetism of N-doped CdO, a 64-atom 2 × 2 × 2 supercell is considered (Fig. 1). For convenience of discussion, we label seven O sites with letters a–g as shown in Fig. 1. We use (i , j ) to denote an N–N pair, in which two
*
Corresponding author. Tel.: +86 0531 82765480; fax: +86 0531 82765480. E-mail address: [email protected] (C.-w. Zhang).
0375-9601/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2010.02.050
Fig. 1. Crystal structure of CdO employed to define various configurations of two N doped CdO. Blue (big) and pink (small) spheres are Cd and O atoms. The positions of O substituted by N are denoted by a–g. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
O atoms are replaced by N at the i and j sites in Fig. 1. The spinpolarized density functional theory calculations are performed using the full-potential linear augmented plane wave method as implemented with the WIEN2k code [17]. Generalized gradient approximation (GGA) [18] is used to treat the exchange correlation
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C.-w. Zhang et al. / Physics Letters A 374 (2010) 1889–1892
Fig. 2. (Left) DOS for single N substituted supercell: TDOS (a), N 2p DOS (b), 2NN O 2p DOS (c), NN Cd 4d DOS (d), and (Right) DOS for two N atoms in FM ordered (a, e) configuration: TDOS (e), N 2p DOS (f), 2NN O 2p DOS (g), NN Cd 4d DOS (h). The vertical line drawn indicates E F position and the majority-spin/minority-spin DOS is shown above/below the abscissa axis.
potential, and relativistic effects are taken into account. Atomic sphere radii of Cd, O, and N atom are set to 2.2, 1.9 and 1.7 a.u., respectively. The parameter Rkmax is chosen as 7.0 (where R is the smallest muffin-tin radius and kmax is the cut-off for the plane wave) for the convergence parameter for which the calculations stabilize and converge in terms of the desired charge. The potential and charge densities inside the atomic spheres are expanded in lattice harmonics up to L = 10. Following the Monkhorst–Pack scheme [19], the 3 × 3 × 3 k-point meshes are used for the first Brillouin zone integration. To see if the formation of these defects is stable, we first consider a single N substitution by replacing an O atom with N atom (Na ). The energy cost E sub for N to replace O is calculated by using the equation: E sub = E [Cd32 O31 N] − E [Cd32 O32 ] + μO − μN , where E (Cd32 O31 N) is the total energy of the supercell containing the N dopant, E (Cd32 O32 ) is the total energy of the pure host, and μO and μN are the chemical potentials of O and N atoms, respectively. Our calculated value E sub = 1.62 eV. This is a reasonable estimation for us to state that is will be possible to dope CdO with N replacing O atoms in the host. Based on the total energy calculations, the spin-polarized state favors over the non-spin-polarized state by 863 meV. Figs. 2(a)–(d) present the calculated total and partial density of states (DOS) for the N dopant and its nearest-neighboring (NN) Cd and second NN (2NN) O atoms. Strong coupling between 2p orbitals of N, O, and 4d orbitals of Cd near the Fermi level (E F ) can be seen clearly. The 4d orbitals of NN Cd are hybridized with 2p orbitals of dopant N and 2NN O atom. The main feature indicated in the spectra is the hole states located around E F , which are mostly from the N 2p and O 2p orbitals. Relative shallow acceptor levels imply the N:CdO are ionized easily at working temperatures [20]. Also we find that the hybridization between N dopant and its neighboring host atoms results in the splitting of the energy levels near the E F , which shifts the majority-spin states downward and minority-spin states upward to lower the total energy of the system, indicating the ideal case for spin injection applications due to 100% spin-polarized carriers. The impurity bands introduced by the N dopants occupy the energy level above the
Table 1 The relative energy E FM is total energy with respect to the ground state of configurations IV (a, e) for N:CdO. E FM is the total energy difference between the AFM and FM states for each configuration. The optimized N–N distance dN−N and magnetic moments M N on N atom for all the configurations for Cd32 O30 N2 supercell are also given. Configurations
dN−N (Å)
E FM (meV)
E FM (meV)
M N (μ B /N)
I (a, b) II (a, c) III (a, d) IV (a, e) V (a, f ) VI (a, g)
3.6402 5.1480 6.3049 7.2804 8.1397 8.9166
41.5 12.9 67.8 117.8 45.4 67.2
327.8 212.3 69.5 0.0 200.3 238.2
0.52 0.52 0.48 0.54 0.51 0.50
valence band maximum, which shows that the doped N atoms can induce the CdO to p-type semiconductor. Since the majority-spin bands are fully occupied while the minority-spin bands are partially filled, the number of holes in the system is equal to the magnetic moment, 1.0μ B per supercell. The magnetic moments are mainly contributed by the N 2p orbital (0.59μ B /N). The NN Cd atom contributes 0.004μ B , and 2NN O atom provides 0.08μ B . The magnetic moments of the atoms have the same direction, indicating there is FM coupling between the doped N atom and neighboring host atoms, which is similar with the results of N-doped ZnO or TiO2 [12,13]. The spatial spin-density distributions in N:CdO are also shown in Fig. 3(a), where the spin-polarized holes is localized mainly on the N atom and distributed slightly over its four NN Cd atoms and twelve 2NN O atoms. This indicates that magnetic coupling clearly extends to the 2NN O atoms from the N dopant centers. Therefore, anions from the delocalized p orbitals contribute chiefly to the magnetic moment in N:CdO DMS, in consistent with DOS analysis above. To investigate the magnetic coupling between N atoms, several distinct supercell structures with two N substitute for several possible O atoms (Fig. 1), which corresponds to a doping concentration of 6.25%. Table 1 lists the relative energy E FM with respect to the ground state of configurations IV (a, e), the energy difference
C.-w. Zhang et al. / Physics Letters A 374 (2010) 1889–1892
1891
Fig. 3. The spin-density distribution of configuration IV (a, e) for single N dopant (a), and two N atoms in FM coupling (b). The blue (big) and pink (small) balls represent Cd and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
E FM between the AFM and FM states for each configuration. It is found that all the doped configurations are located in FM order. Among all configurations, the IV (a, e) has the lowest total energy and the largest magnitude of E FM (117.8 meV), which is larger than that of Cu-doped ZnO FM materials [21,22] and some other systems, such as N-ZnO [15] and N-TiO2 [16]. More importantly, even when the N–N separation is up to 8.9 Å, the magnitude of E FM is still up to 67.2 meV. While in N-doped ZnO [15], when the two N atoms are separated by 6.136 Å, the energy of FM states is 22 MeV lower than that of the corresponding AFM one. This implies the long-range RT ferromagnetism for N:CdO is quite feasible, which makes N:CdO system superior to the previous observed FM materials [11–16]. The total and partial DOS for the N dopant and its NN Cd and 2NN O atoms in FM ordered IV (a, e) configuration is depicted in Figs. 2(e)–(h). Similar to the single N doping case, hole states arising from the hybridizations between N and its neighboring atoms are also extended to 2NN atoms. The main difference compare to that of single N doping case is that there are more hole states in N 2p and obvious hole states in bridging O 2p and Cd 4d orbitals due to the significant hybridization between the N dopants and neighboring host atoms. The hybridization between the N and neighboring host atoms leads to a formation of N(2p)–Cd(4d)– O(2p)–Cd(4d)–N(2p) coupling chain, which facilitates strong indirect FM coupling between N dopants mediated by the holes. In order to elucidate the origin of ferromagnetism in N-doped CdO, it is instructive to investigate systematically the magnetic coupling between the two dopant atoms. Strong magnetic coupling between N atoms was found to exist even if the separation distance is up to 7.28 Å. Here, both double-exchange and super-exchange interactions cannot account for long-range FM order since the distance between two dopants is very large and the doping concentrations are very low at the same time. Hence, we propose the hybridized N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) chain mechanism through p–d like p–p coupling responsible for the long-range RT FM coupling. Fig. 3(b) shows the spatial spin-density distributions between N dopants in FM order with the distance of 7.28 Å. It is can be seen that the holes localized around the anions between the N atoms are polarized and the spin orientations are parallel to each other. Consequently, these polarized holes mediate the strong coupling between the N atoms. Due to one less valence electrons for N than an O ion, introducing more substituting N atoms into CdO host will lead to more holes in the valence band top. Sufficiently large spin polarized hole forms N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) hybridized chain, in this direc-
tion the strong p–p interaction effectively leading to a stronger coupling between impurity and carrier spin orientations, as confirmed with spin-density distributions in Fig. 3(b). The spatially extended p states of the host and the impurity are able to extend the p–p interaction and spin alignment to the longer range along the way of N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) chain, and thus to cause long-range FM interaction in N:CdO system. Therefore, the N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) chain interaction is the key to stabilize the long-range RT FM order in the N:CdO system. In summary, we have demonstrated that N:CdO system is in FM ground state and calculated magnetization energy of N:CdO is found to be larger than some of the known RT DMS, which implies the RT ferromagnetism for N:CdO can be expected. The FM exchange interaction between N dopants is activated through holes induced by N doping via an N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) coupling chain in the large N–N separations, which plays the major role in forming the long-range FM order in N:CdO system. Our calculated results are more interesting and then open to the future experimental verification. Acknowledgements This work was supported by Foundation for Young Scientist in Shandong Province (Grant No. BS2009CL012), and the School Scientific Research Foundation of Jinan University (Grant No. XKY0716). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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