- Email: [email protected]
A first-principle prediction on the magnetism and electronic structure of Ti-doped CoO with two O vacancies
Current Applied Physics 15 (2015) 1549e1555
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
A first-principle prediction on the magnetism and electronic structure of Ti-doped CoO with two O vacancies J. Zhou a, X.C. Wang a, *, G.F. Chen b, B.H. Yang a a
Tianjin Key Laboratory of Film Electronic & Communicate Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin, 300191, China b School of Material Science and Engineering, Hebei University of Technology, Tianjin, 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2015 Received in revised form 7 September 2015 Accepted 8 September 2015 Available online 11 September 2015
The electronic structure and magnetism in Co14Ti2O14 systems are investigated by using the firstprinciples calculations. The system of 2 2 2 Co14Ti2O16 supercell doped with Ti at 9 and 11 position shows a half-metallic character with a high spin polarization. Based on the above system, we remove two O atoms to form two O vacancies. The two O vacancies near Ti have a huge effect on the electronic structure and magnetic properties of Co14Ti2O14 system. When O vacancies locate at 1 and 3 positions, the system shows a half-metallic character. For the O vacancies at 6 and 8 positions, the system shows a semiconducting character. The system with O vacancies at 9 and 11 positions is a typical spin gapless semiconductor. © 2015 Elsevier B.V. All rights reserved.
Keywords: First-principles calculations Electronic structure Magnetism Antiferromagnetic insulator Half-metallic character
1. Introduction Spintronics has developed quickly in recent years [1], where the spin-polarized carriers are necessary for its practical applications. Compared to the conventional electronic devices, the spintronic devices have a faster speed of data processing, high data storage density, nonvolatility of data storage, and low energy consumption [2]. In spintronic materials, the diluted magnetic semiconductors (DMS) are predicted to produce the spin-polarized carriers [1,2]. In order to obtain DMS with high Curie temperature, the wide bandgap semiconductors are chosen as the matrix by doping transition metal element into it, including ZnO [3e6], TiO2 [7e9], In2O3 [10e12], SnO2 [13], CuO [14,15] and HfO2 [16e19]. In the Co-doped ZnO, the defects were observed, which may enhance the magnetic ordering [20]. Gao et al. found that the Zn vacancies give rise to the stable magnetism in ZnO [21]. However, the observed ferromagnetism is weak and the Curie temperature is often below room temperature. Recently, the interesting magnetoresistance has been observed in the magnetic element added disordered oxide semiconductors with large dopant concentrations, such as FeeIn2O3 [22], CoeZnO [23], NieCN [24], CoeTiO2 [23,25], and FeeTieO [26].
* Corresponding author. E-mail address: [email protected] (X.C. Wang). http://dx.doi.org/10.1016/j.cap.2015.09.008 1567-1739/© 2015 Elsevier B.V. All rights reserved.
In our previous work, it is proved that breaking the antiferromagnetic order of wurtzite (WZ) CoO [27,28] can induce the ferromagnetism of CoXO systems, where X is Co vacancies or other 3d transition metal elements. During the fabrication of oxide films, O vacancies can often exist. The effects of Co vacancies have been found [29], and we also study the Co14Ti2O15 systems with single O vacancy, where the half-metallic characteristic is observed as O vacancy locates at a certain position, suggesting that the O vacancies can significantly affect the electronic structure and magnetic properties of Co14Ti2O15 systems. A half-metal is the substance that has density of states at Fermi level for electrons of one spin orientation, but no density of states for those of the opposite orientation, so it acts as a conductor to electrons of one spin orientation, but as an insulator or semiconductor to those of the opposite orientation [30]. Therefore, the half-metal has a spin polarization of 100% [30]. In this paper, for further investigating the effect of O vacancies on the electronic structure of Co14Ti2O15, we calculate the electronic structure and magnetism of the Co14Ti2O14 systems with two O vacancies, so that one can understand the electronic transport and magnetic properties of the CoeTieO systems with O vacancies. It is found that the two O vacancies near Ti have a huge effect on the electronic structure and magnetism of the systems. When O vacancies locate at 1 and 3 positions, the system shows a half-metallic character. The system with O vacancies at 9 and 11 positions is a spin gapless semiconductor.
1550
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
2. Calculation methods We use the first-principles calculations to study the effect of O vacancies in Ti doped CoO supercell. WZ CoO is chosen due to its more stable structure. We use a 2 2 2 WZ CoO supercell with substituting two Ti atoms at 9 and 11 positions (See Fig. 1) because when two Ti atoms at 9 and 11 positions, the system shows a halfmetallic characteristic with a high spin polarization [31]. The lattice constants are a ¼ b ¼ 3.249 Å, c ¼ 5.206 Å [32]. To simulate the defected system, two O atoms are removed, so the system consists of 30 atoms, the concentration of O vacancies is 6.25%. Our firstprinciples calculations are drawn by using the Vienna ab initio simulation package [33] based on the density-functional theory (DFT). A plane-wave energy cutoff of 500 eV with a G-centered 6 6 6 k mesh k-points grids is set, a series of high symmetry points are set in Brillouin zone. The convergence criteria for total energy and force are 105 eV and 0.01 eV/Å. We use the projectoraugmented-wave pseudo potentials (PAW's) to represent the atoms. The electron configurations are described as O (2p2), Co (3d74s2), and Ti (3d24s2). Local density approximation plus U method is used to correct the exchangeecorrelation potential in ferromagnetic system. The U ¼ 6.0 eV is taken for Co d orbitals, U ¼ 3.2 eV and J ¼ 0.9 eV are taken for Ti d orbitals, where the calculated band gap of pure CoO is 1.55 eV and CoO is antiferromagnetic, suggesting that the U is reasonable [29,31]. 3. Result and discussion For comparison, the pure CoO has been calculated in details. The results show that its band gap is 1.55 eV for both spin-up and spin-
Fig. 1. A 2 2 2 supercell of WZ Co14Ti2O16 doped with Ti at 9 and 11 position. The red spheres stand for O atoms, the blue spheres stand for Co atoms, and the brown spheres stand for Co atoms. The numbers marked on the atoms stand for the different positions. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
down channels, and each Co2þ has three unpaired electrons which offer a magnetic moment of 2.72 mB. The calculated magnetic moment is smaller than the experimental value of 3.38e3.8 mB because there is an orbital moment of about 1 mB that is not included in DFT calculations [34,35]. So, the calculated magnetic moment should be 2.38e2.8 mB. Therefore, our calculations are reliable. The system of Ti-doped CoO at 9 and 11 positions (Co14Ti2O16) shows a half-metallic characteristic [31]. Fig. 2(a) shows the band structure of Ti-doped CoO at 9 and 11 positions. One can find that the spin-up channel goes through Fermi level (EF), but the spin-down channel has a band gap of 0.37 eV. In Fig. 2(b), only the spin-up states exist at EF in the total density of states (TDOS). In the partial density of states (PDOS), Co 3d states overlap with Ti 3d states near EF, suggesting that there is a strong exchange interaction between Co and Ti. The systems with single O defect near Ti are also studied, and when the system with O vacancy at 2 position keeps the half-metallic character, as shown in Fig. 2(c) and 2(d). Firstly, we move the O atoms along the 9e11 direction. Fig. 3(a) shows the results of the Co14Ti2O14 system with O vacancies at 1 and 3 positions. In Fig. 3(a), a band gap of 0.96 eV can be seen in the spin-up channel, but the spin-down channel goes through EF, so that the system shows a half-metallic character. Fig. 3(b) shows the results of the Co14Ti2O14 system with O vacancies at 2 and 4 positions. In this system, both the spin-up and spin-down channels do not go through EF, and the band gap is 0.88 and 0.41 eV respectively. Thus, it is a magnetic semiconductor. EF locates near the valence band (VB) maximum, suggesting that it is p-type semiconductor. In Fig. 3(c), there are band gaps of 0.23 eV in spin-up channel and 0.80 eV in spin-down one for the case with O vacancies at 6 and 8 positions. Due to the small band gap in the spin-up channel, a small energy is required to excite electrons from VB to conductance band (CB), where the excited electrons achieve 100% spin polarization. In Fig. 3(d), the band gap of the spin-up channel is 0.61 eV, and that of spin-down channel is 0.34 eV. It is interesting that EF locates at the VB maximum in the spin-up channel and at the CB minimum for the case with O vacancies at 9 and 1 positions, which is the character of the spin gapless semiconductor. The term “gapless” is used for an energy gap that is smaller than 0.1 eV [36e38]. In spin gapless semiconductors, no energy is required to excite electrons from VB into CB, where the excited electrons will achieve a spin polarization of 100%, which is desirable for spintronic devices. Meanwhile, one can flexibly tune the properties of spin gapless semiconducting materials externally by pressure, electric fields, impurities, etc [36,39]. Fig. 4 shows the TDOS and PDOS of the Co14Ti2O14 systems as discussed above. In Fig. 4(a), one can clearly see that the spin-down states go through EF and there is no states at EF in the spin-up channel, which is a typical half-metallic characteristic with a spin polarization of 100% due to the most contribution from Ti 3d states. Fig. 4(b) shows a magnetic semiconductor because neither spin-up nor spin-down states go through EF. In additional, Ti 3d states appear in the spin-down channel near EF while Co 3d states make the most contribution in VB far from EF. In Fig. 4(c), even though both spin-up and spin-down states do not go through EF, there are states which mainly from Ti 3d states near EF in VB and CB of the spin-up channel. It is easy for the spin-up states to go through EF when energy excites the electrons. In view of Fig. 4(d), there is a sizeable gap in spin-up channel and EF falls within the gap near VB, but in spin-down channel, VB meet CB at EF. Thus, this case can be classified as spin gapless semiconductor. Otherwise, the states near EF in the VB of spin-down channel are offered by Co 3d and Ti 3d states together, but in the CB of spin-down channel are only from Ti 3d states. To further understand the effect of two O vacancies, we move
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
1551
Fig. 2. The calculated electronic band structure, the total density of states (TDOS) and partial density of states (PDOS) of (a) and (b) Co14Ti2O16 doped with Ti at 9 and 11 position, (c) and (d) Co14Ti2O15 doped with Ti at 9 and 11 position with single O vacancy at 2 position.
two O atoms near the Ti at 1 position. Fig. 5 describes the band structures of the systems. In Fig. 5(a), one can see a band gap of 0.49 eV in spin-up channel and 0.61 eV in spin-down channel, it is a p-type semiconductor because EF located near the VB maximum. Fig. 5(b) shows a semiconducting character due to the band gaps of 0.91 and 0.64 eV for the spin-up and spin-down channels, respectively. EF located at the VB maximum, but no states appeared at EF, thus it is a p-type semiconductor. In view of Fig. 5(c), the band gaps of 0.63 eV in the spin-up channel and 0.61 eV in the spin-down channel can be clearly seen. Therefore, it is a magnetic semiconductor. EF located near the VB maximum, suggesting that it is a p-type semiconductor. Fig. 5(d) shows a semiconducting character as well because neither spin-up nor spin-down channel go through EF, where a band gap of 0.47 eV for the spin-up channel and 0.72 eV for the spin-down channel can be observed. It is a p-type semiconductor since EF located near the VB maximum. Fig. 6 shows the TDOS and PDOS for the Co14Ti2O14 systems discussed above. In Fig. 6(a), no states appeared at EF, showing a magnetic semiconductor character. There are two kinds of Co can been seen in PDOS. The Co atom at 1 position do not offer states in the VB maximum, while the other one at 2 position makes contributions by Ti 3d states in spin-up channel. Otherwise, Ti 3d states overlap with Co 3d states due to the strong exchange interaction between Co and O. Fig. 6(b) shows a magnetic semiconducting
character because both spin-up and spin-down channels do not go through EF. The Co also has two types as well as in Fig. 6(a), but in the VB maximum, Ti 3d states become smaller and Co 3d states become larger. Nevertheless, Ti 3d states make the most contribution in the VB maximum of spin-down channel. In Fig. 6(c), it is a magnetic semiconductor because the spin-up and spin-down states do not appeared at EF. The two Co atoms which have been presented in PDOS make contributions to the VB maximum of spin-up channel. The CB minimum mainly from Ti 3d states as a result of the great contribution to the states in CB near EF. In view of Fig. 3(d), neither spin-up nor spin-down states appeared at EF, showing a semiconducting character, one can see that the states near EF are mainly from Ti 3d states. The spin-polarized charge density of the Co14Ti2O14 systems with two O vacancies is shown in Fig. 7. Yellow and cyan isosurfaces represent the positive and negative spin densities (±0.015 e/Å3), respectively. For the appearance of O vacancies, the magnetic moments have been changed by the electron transfer, as shown in Table 1 and Fig. 8, where Fig. 8(a)e(d) are seen under (100) direction and Fig. 8(e)e(h) are seen under (010) direction. In Fig. 7(a), one can clearly see that the spin-polarized charge density of Co at 1 and 3 positions increases due to the O vacancies near the two Co atoms. A part of spin-down electron from Co at 2 position transfer to Ti at 1 position due to the orbital hybridization between the Co
1552
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
Fig. 3. The band structure of Co14Ti2O14 systems with, two O vacancies, (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions.
Fig. 4. The total density of states (TDOS) and the partial density of states (PDOS) of Co14Ti2O14 systems with (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions.
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
1553
Fig. 5. The band structure of Co14Ti2O14 systems with (a) O vacancies at 1 and 2 positions, (b) O vacancies at 1 and 6 positions, (c) O vacancies at 2 and 9 positions, (d) O vacancies at 6 and 9 positions.
Fig. 6. The total density of states (TDOS) and the partial density of states (PDOS) of Co14Ti2O14 systems with (a) O vacancies at 1 and 2 positions, (b) O vacancies at 1 and 6 positions, (c) O vacancies at 2 and 9 positions, (d) O vacancies at 6 and 9 positions.
1554
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
Fig. 7. The spin-polarized charge of Co14Ti2O14 systems with (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions, (e) O vacancies at 1 and 2 positions, (f) O vacancies at 1 and 6 positions, (g) O vacancies at 2 and 9 positions, (h) O vacancies at 6 and 9 positions.
Table 1 The magnetic moment (mB) of Co14Ti2O14 with two O vacancies and each element given in PDOS. Co2þ
System Co14Ti2O16 O defect at O defect at O defect at O defect at O defect at O defect at O defect at O defect at
1 2 6 9 1 1 2 6
and and and and and and and and
3 positions 4 positions 8 positions 11 positions 2 positions 6 positions 9 positions 9 positions
2.70 2.67 2.24 2.66 2.00 2.70 2.69 2.54 2.72
Co2þ
1.95 2.37 2.52 2.60
O2
Ti4þ
Total
0.09 0.02 0.03 0.07 0.12
0.66 0.51 0.25 0.40 0.86 0.41 0.19 1.07 0.90
5.82 5.77 5.74 5.94 2.13 4.09 5.94 4.09 5.84
and Ti, as shown in Fig. 8. At the same time, it makes the Ti show a negative spin with a magnetic moments of 0.51 mB, but the total magnetic moments still match the Co14Ti2O16 system due to the decrease of the Co magnetic moments of about 2.11 mB at 2 position can been clearly seen in Fig. 7(a). Fig. 7(b) shows the similar result, but the effect from neighboring Co atoms to Ti is smaller and the magnetic moment of Ti is 0.25 mB due to the spin-up electrons transfer from Co at 13 position. In Fig. 7(c), the Ti magnetic moment is 0.4 mB, which is smaller than that in Co14Ti2O16 system because there is electron transfer from Ti to the O vacancies, as shown in Fig. 8(c). In additional, it leads to an increase of total magnetic moments (see Table 1). In Fig. 7(d), the spin-polarized charge density of Ti is grown up to 0.86 mB because a large number of spindown electrons are lost, which can be clearly seen in Fig. 8(d). Thus,
Fig. 8. The deformation charge density of Co14Ti2O14 systems with (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions, (e) O vacancies at 1 and 2 positions, (f) O vacancies at 1 and 6 positions, (g) O vacancies at 2 and 9 positions, (h) O vacancies at 6 and 9 positions. The red one stands for getting electrons, and the blue one represents losing electrons. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
the total magnetic moment reduces to 2.13 mB. In Fig. 7(e), one can see that the spin-polarized charge density of Ti at 1 position is smaller than that at 2 position. Fig. 8(e) shows that the spin-up electrons of Ti atoms transfer into Co atom at neighbor positions, which is the reason that the magnetic moment of Co at 2 position reduces to 1.95 mB (see Table 1). The system with O vacancies at 1 and 6 positions shows the largest total magnetic moment of 5.94 mB. The Ti atom has a small negative magnetic moment of 0.19 mB, which is consistent with that in Fig. 7(f) because the spindown electrons transfer (see Fig. 8(f)). The system with O vacancies at 2 and 9 positions has the same total magnetic moment as that with O vacancies at 1 and 2 positions because one O vacancy locates near the Co with negative spin and the other one locates near the spin-up one. For the different distances between the O vacancies and Ti atoms, the transfer electrons between the Co and Ti atoms are not the same. The case (h) has the similar situations with case (f). 4. Conclusion We study the electronic structure and magnetism of Co14Ti2O14 systems with two O vacancies by using first-principles calculations. The system of CoO doped Ti at 9 and 11 positions shows a halfmetallic character. In additional, the two O vacancies will make effect on the systems when it appeared near the Ti atom. The case of O vacancies at 1 and 3 positions shows a half-metallic character with a spin polarization of 100%. For the O vacancies at 6 and 8 positions, the system shows a semiconducting character. The system of O vacancies at 9 and 11 positions is a typical spin gapless semiconductor with a small total magnetic moment. Also, the electron transfer caused by the O vacancies affects the spinpolarized charge density of the atoms at neighboring positions. Acknowledgments This work was supported by the Key Project of Natural Science Foundation of Tianjin City (14JCZDJC37800), and National High Technology Research and Development Program (863 Program) of China (No. 2013AA030801). References [1] I. Zuti c, J. Fabian, S. Sarma, Spintronics: fundamentals and applications, Rev. Mod. Phys. 76 (2004) 323. [2] S. Sarma, Spintronics: a new class of device based on electron spin, rather than on charge, may yield the next generation of microelectronics, Am. Sci. 89 (2001) 516. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Zener model description of ferromagnetism in Zinc-blende magnetic semiconductors, Science 287 (2000) 1019. [4] K. Sato, H. Katayama-Yoshida, Material design for transparent ferromagnets with ZnO-based magnetic semiconductors, Jpn. J. Appl. Phys. 39 (2000) L555. [5] W. Mi, H. Bai, H. Liu, C. Sun, Microstructure, magnetic, and optical properties of sputtered Mn-doped ZnO films with high-temperature ferromagnetism, J. Appl. Phys. 101 (2009) 023904. [6] K. Ueda, H. Tabata, T. Kawai, Magnetic and electric properties of transitionmetal-doped ZnO films, Appl. Phys. Lett. 79 (2001) 988. [7] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma1, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 291 (2001) 854. [8] W. Mi, E. Jiang, H. Bai, High-temperature ferromagnetism observed in facingtarget reactive sputtered MnxTi1xO2, Acta Mater. 56 (2008) 3511.
1555
[9] W. Mi, E. Jiang, H. Bai, Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films, J. Magn. Magn. Mater. 321 (2009) 2472. [10] Y. Yoo, Q. Xue, H. Lee, S. Cheng, X. Xiang, G. Dionne, Bulk synthesis and hightemperature ferromagnetism of (In1xFex)2O3s with Cu co-doping, Appl. Phys. Lett. 86 (2005) 042506. [11] J. He, S. Xu, Y. Yoo, Q. Xue, H. Lee, S. Cheng, X. Xiang, G. Dionne, I. Takeuchi, Room temperature ferromagnetic n-type semiconductor in (In1xFex)2O3s, App. Phys. Lett. 86 (2005) 052503. [12] J. Philip, A. Punnoose, B. Kim, K. Reddy, S. Layne, J. Holmes, B. Satpati, P. LeClair, T. Santos, J. Moodera, Carrier-controlled ferromagnetism in transparent oxide semiconductors, Nat. Mater. 298 (2006) 304. [13] J. Coey, A. Douvalis, C. Fitzgerald, M. Venkatesan, Ferromagnetism in Fe-doped SnO2 thin films, Appl. Phys. Lett. 84 (2004) 1332. [14] S. Yang, T. Li, B. Gu, Y. Du, H. Sung, S. Hung, C. Wong, A. Pakhomov, Ferromagnetism in Mn-doped CuO, Appl. Phys. Lett. 83 (2003) 3746. [15] W. Mi, H. Bai, Q. Zhang, B. Zhang, X. Zhang, Ferromagnetism in reactive sputtered Cu0.96Fe0.04O1d nanocrystalline films evidenced by anomalous Hall Effect, Appl. Phys. Express 4 (2011) 043001. [16] M. Venkatesan, C. Fitzgerald, J. Coey, Thin films: unexpected magnetism in a dielectric oxide, Nature 430 (2004) 630. [17] C. Pemmaraju, S. Sanvito, Ferromagnetism driven by intrinsic Point defects in HfO2, Phys. Rev. Lett. 94 (2005) 217205. [18] J. Coey, M. Venkatesan, P. Stamenov, C.B. Fitzgerald, L.S. Dorneles, Magnetism in hafnium dioxide, Phys. Rev. B 72 (2005) 024450. [19] Y. Chang, Y. Soo1, W. Lee, M. Huang, Y. Lee, S. Weng, W. Sun, M. Hong, J. Kwo1, S. Lee, J. Ablett, C. Kao, Observation of room temperature ferromagnetic behavior in cluster-free, Co doped HfO2 films, Appl. Phys. Lett. 91 (2007) 082504. [20] S. Yan, C. Ren, X. Wang, Y. Xin, Z. Zhou, L. Mei, M. Ren, Y. Chen, Y. Liu, H. Garmestani, Ferromagnetism and magnetoresistance of CoeZnO inhomogeneous magnetic semiconductors, Appl. Phys. Lett. 84 (2004) 2376. [21] H. Gao, J. Xia, Effect of Li-doping on the magnetic properties of ZnO with Zn vacancies, J. Appl. Phys. 111 (2012) 093902. [22] L. Zhang, B. Huang, Y. Liu, L. Zhang, R. Zhang, Giant magnetoresistance in FeIn2O3 granular films, J. Magn. Magn. Mater. 261 (2003) 257. [23] S. Yan, J. Liu, L. Mei, Y. Tian, H. Song, Y. Chen, G. Liu, Spin-dependent variable range hopping and magnetoresistance in Ti1xCoxO2 and Zn1xCoxO magnetic semiconductor films, J. Phys. Condens. Mat. 18 (2006) 20469. [24] X. Wang, W. Mi, E. Jiang, H. Bai, Large magnetoresistance observed in facingtarget sputtered Ni-doped CNx amorphous composite films, Acta Mater. 55 (2007) 3547. [25] H. Song, L. Mei, Y. Tian, X. Ma, J. Liu, Y. Wang, Z. Zhang, Microstructure, ferromagnetism, and magnetic transport of Ti1xCoxO2 amorphous magnetic semiconductor, J. Appl. Phys. 90 (2006) 123903. [26] X. Wang, Y. Bai, W. Mi, Negative magnetoresistance in reactive sputtered nonuniform amorphous FexTi1xOd films, Mater. Res. Bull. 48 (2013) 3449. [27] J. Coey, d0 ferromagnetism, Solid State Sci. 7 (2005) 660. [28] J. Coey, M. Venkatesan, C. Fitzgerald, Donor impurity band exchange in dilute ferromagnetic oxides, Nat. Mater. 4 (2005) 173. [29] M. Yin, X. Wang, W. Mi, G. Chen, B. Yang, A first-principles prediction on the magnetism in CoO with Co and O Vacancies, J. Alloy. Compd. 610 (2014) 422. [30] J. Coey, M. Venkatesan, Half-metallic ferromagnetism: example of CrO2, J. Appl. Phys. 91 (2001) 8345. [31] M. Yin, X. Wang, W. Mi, Y. Ding, G. Chen, B. Yang, Magnetism and electronic structure in Zn and Ti doped CoO: a first-principles study, Comp. Mater. Sci. 93 (2014) 193. [32] K. An, N. Lee, J. Park, S. Kim, Y. Hwang, J. Park, J. Kim, J. Park, M. Han, J. Yu, T. Hyeon, Synthesis, characterization, and self-assembly of pencil-shaped CoO nanorods, J. Am. Chem. Soc. 128 (2006) 9753. [33] G. Kresse, J. Furthmuller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169. [34] W. Roth, Magnetic structures of MnO, FeO, CoO, and NiO, Phys. Rev. 110 (1958) 1333. [35] R. Hanafin, T. Archer, S. Sanvito, Magnetism of wurtzite CoO nanoclusters, Phys. Rev. B 81 (2010) 054441. [36] X. Wang, Proposal for a new class of materials: spin gapless semiconductors, Phys. Rev. Lett. 100 (2008) 156404. [37] N. Feng, W. Mi, Y. Cheng, Z. Guo, U. Schwingenschlogl, H. Bai, First principles prediction of the magnetic properties of Fe-X6 (X ¼ S, C, N, O, F) doped monolayer MoS2, Sci. Rep. 4 (2014) 3987. [38] X. Hu, W. Zhang, L. Sun, A. Krasheninnikov, Gold-embedded zigzag graphene nanoribbons as spin gapless semiconductors, Phys. Rev. B 86 (2012) 195418. [39] A. Ramasubramaniam, Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides, Phys. Rev. B 86 (2012) 125409.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
A first-principle prediction on the magnetism and electronic structure of Ti-doped CoO with two O vacancies J. Zhou a, X.C. Wang a, *, G.F. Chen b, B.H. Yang a a
Tianjin Key Laboratory of Film Electronic & Communicate Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin, 300191, China b School of Material Science and Engineering, Hebei University of Technology, Tianjin, 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2015 Received in revised form 7 September 2015 Accepted 8 September 2015 Available online 11 September 2015
The electronic structure and magnetism in Co14Ti2O14 systems are investigated by using the firstprinciples calculations. The system of 2 2 2 Co14Ti2O16 supercell doped with Ti at 9 and 11 position shows a half-metallic character with a high spin polarization. Based on the above system, we remove two O atoms to form two O vacancies. The two O vacancies near Ti have a huge effect on the electronic structure and magnetic properties of Co14Ti2O14 system. When O vacancies locate at 1 and 3 positions, the system shows a half-metallic character. For the O vacancies at 6 and 8 positions, the system shows a semiconducting character. The system with O vacancies at 9 and 11 positions is a typical spin gapless semiconductor. © 2015 Elsevier B.V. All rights reserved.
Keywords: First-principles calculations Electronic structure Magnetism Antiferromagnetic insulator Half-metallic character
1. Introduction Spintronics has developed quickly in recent years [1], where the spin-polarized carriers are necessary for its practical applications. Compared to the conventional electronic devices, the spintronic devices have a faster speed of data processing, high data storage density, nonvolatility of data storage, and low energy consumption [2]. In spintronic materials, the diluted magnetic semiconductors (DMS) are predicted to produce the spin-polarized carriers [1,2]. In order to obtain DMS with high Curie temperature, the wide bandgap semiconductors are chosen as the matrix by doping transition metal element into it, including ZnO [3e6], TiO2 [7e9], In2O3 [10e12], SnO2 [13], CuO [14,15] and HfO2 [16e19]. In the Co-doped ZnO, the defects were observed, which may enhance the magnetic ordering [20]. Gao et al. found that the Zn vacancies give rise to the stable magnetism in ZnO [21]. However, the observed ferromagnetism is weak and the Curie temperature is often below room temperature. Recently, the interesting magnetoresistance has been observed in the magnetic element added disordered oxide semiconductors with large dopant concentrations, such as FeeIn2O3 [22], CoeZnO [23], NieCN [24], CoeTiO2 [23,25], and FeeTieO [26].
* Corresponding author. E-mail address: [email protected] (X.C. Wang). http://dx.doi.org/10.1016/j.cap.2015.09.008 1567-1739/© 2015 Elsevier B.V. All rights reserved.
In our previous work, it is proved that breaking the antiferromagnetic order of wurtzite (WZ) CoO [27,28] can induce the ferromagnetism of CoXO systems, where X is Co vacancies or other 3d transition metal elements. During the fabrication of oxide films, O vacancies can often exist. The effects of Co vacancies have been found [29], and we also study the Co14Ti2O15 systems with single O vacancy, where the half-metallic characteristic is observed as O vacancy locates at a certain position, suggesting that the O vacancies can significantly affect the electronic structure and magnetic properties of Co14Ti2O15 systems. A half-metal is the substance that has density of states at Fermi level for electrons of one spin orientation, but no density of states for those of the opposite orientation, so it acts as a conductor to electrons of one spin orientation, but as an insulator or semiconductor to those of the opposite orientation [30]. Therefore, the half-metal has a spin polarization of 100% [30]. In this paper, for further investigating the effect of O vacancies on the electronic structure of Co14Ti2O15, we calculate the electronic structure and magnetism of the Co14Ti2O14 systems with two O vacancies, so that one can understand the electronic transport and magnetic properties of the CoeTieO systems with O vacancies. It is found that the two O vacancies near Ti have a huge effect on the electronic structure and magnetism of the systems. When O vacancies locate at 1 and 3 positions, the system shows a half-metallic character. The system with O vacancies at 9 and 11 positions is a spin gapless semiconductor.
1550
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
2. Calculation methods We use the first-principles calculations to study the effect of O vacancies in Ti doped CoO supercell. WZ CoO is chosen due to its more stable structure. We use a 2 2 2 WZ CoO supercell with substituting two Ti atoms at 9 and 11 positions (See Fig. 1) because when two Ti atoms at 9 and 11 positions, the system shows a halfmetallic characteristic with a high spin polarization [31]. The lattice constants are a ¼ b ¼ 3.249 Å, c ¼ 5.206 Å [32]. To simulate the defected system, two O atoms are removed, so the system consists of 30 atoms, the concentration of O vacancies is 6.25%. Our firstprinciples calculations are drawn by using the Vienna ab initio simulation package [33] based on the density-functional theory (DFT). A plane-wave energy cutoff of 500 eV with a G-centered 6 6 6 k mesh k-points grids is set, a series of high symmetry points are set in Brillouin zone. The convergence criteria for total energy and force are 105 eV and 0.01 eV/Å. We use the projectoraugmented-wave pseudo potentials (PAW's) to represent the atoms. The electron configurations are described as O (2p2), Co (3d74s2), and Ti (3d24s2). Local density approximation plus U method is used to correct the exchangeecorrelation potential in ferromagnetic system. The U ¼ 6.0 eV is taken for Co d orbitals, U ¼ 3.2 eV and J ¼ 0.9 eV are taken for Ti d orbitals, where the calculated band gap of pure CoO is 1.55 eV and CoO is antiferromagnetic, suggesting that the U is reasonable [29,31]. 3. Result and discussion For comparison, the pure CoO has been calculated in details. The results show that its band gap is 1.55 eV for both spin-up and spin-
Fig. 1. A 2 2 2 supercell of WZ Co14Ti2O16 doped with Ti at 9 and 11 position. The red spheres stand for O atoms, the blue spheres stand for Co atoms, and the brown spheres stand for Co atoms. The numbers marked on the atoms stand for the different positions. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
down channels, and each Co2þ has three unpaired electrons which offer a magnetic moment of 2.72 mB. The calculated magnetic moment is smaller than the experimental value of 3.38e3.8 mB because there is an orbital moment of about 1 mB that is not included in DFT calculations [34,35]. So, the calculated magnetic moment should be 2.38e2.8 mB. Therefore, our calculations are reliable. The system of Ti-doped CoO at 9 and 11 positions (Co14Ti2O16) shows a half-metallic characteristic [31]. Fig. 2(a) shows the band structure of Ti-doped CoO at 9 and 11 positions. One can find that the spin-up channel goes through Fermi level (EF), but the spin-down channel has a band gap of 0.37 eV. In Fig. 2(b), only the spin-up states exist at EF in the total density of states (TDOS). In the partial density of states (PDOS), Co 3d states overlap with Ti 3d states near EF, suggesting that there is a strong exchange interaction between Co and Ti. The systems with single O defect near Ti are also studied, and when the system with O vacancy at 2 position keeps the half-metallic character, as shown in Fig. 2(c) and 2(d). Firstly, we move the O atoms along the 9e11 direction. Fig. 3(a) shows the results of the Co14Ti2O14 system with O vacancies at 1 and 3 positions. In Fig. 3(a), a band gap of 0.96 eV can be seen in the spin-up channel, but the spin-down channel goes through EF, so that the system shows a half-metallic character. Fig. 3(b) shows the results of the Co14Ti2O14 system with O vacancies at 2 and 4 positions. In this system, both the spin-up and spin-down channels do not go through EF, and the band gap is 0.88 and 0.41 eV respectively. Thus, it is a magnetic semiconductor. EF locates near the valence band (VB) maximum, suggesting that it is p-type semiconductor. In Fig. 3(c), there are band gaps of 0.23 eV in spin-up channel and 0.80 eV in spin-down one for the case with O vacancies at 6 and 8 positions. Due to the small band gap in the spin-up channel, a small energy is required to excite electrons from VB to conductance band (CB), where the excited electrons achieve 100% spin polarization. In Fig. 3(d), the band gap of the spin-up channel is 0.61 eV, and that of spin-down channel is 0.34 eV. It is interesting that EF locates at the VB maximum in the spin-up channel and at the CB minimum for the case with O vacancies at 9 and 1 positions, which is the character of the spin gapless semiconductor. The term “gapless” is used for an energy gap that is smaller than 0.1 eV [36e38]. In spin gapless semiconductors, no energy is required to excite electrons from VB into CB, where the excited electrons will achieve a spin polarization of 100%, which is desirable for spintronic devices. Meanwhile, one can flexibly tune the properties of spin gapless semiconducting materials externally by pressure, electric fields, impurities, etc [36,39]. Fig. 4 shows the TDOS and PDOS of the Co14Ti2O14 systems as discussed above. In Fig. 4(a), one can clearly see that the spin-down states go through EF and there is no states at EF in the spin-up channel, which is a typical half-metallic characteristic with a spin polarization of 100% due to the most contribution from Ti 3d states. Fig. 4(b) shows a magnetic semiconductor because neither spin-up nor spin-down states go through EF. In additional, Ti 3d states appear in the spin-down channel near EF while Co 3d states make the most contribution in VB far from EF. In Fig. 4(c), even though both spin-up and spin-down states do not go through EF, there are states which mainly from Ti 3d states near EF in VB and CB of the spin-up channel. It is easy for the spin-up states to go through EF when energy excites the electrons. In view of Fig. 4(d), there is a sizeable gap in spin-up channel and EF falls within the gap near VB, but in spin-down channel, VB meet CB at EF. Thus, this case can be classified as spin gapless semiconductor. Otherwise, the states near EF in the VB of spin-down channel are offered by Co 3d and Ti 3d states together, but in the CB of spin-down channel are only from Ti 3d states. To further understand the effect of two O vacancies, we move
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
1551
Fig. 2. The calculated electronic band structure, the total density of states (TDOS) and partial density of states (PDOS) of (a) and (b) Co14Ti2O16 doped with Ti at 9 and 11 position, (c) and (d) Co14Ti2O15 doped with Ti at 9 and 11 position with single O vacancy at 2 position.
two O atoms near the Ti at 1 position. Fig. 5 describes the band structures of the systems. In Fig. 5(a), one can see a band gap of 0.49 eV in spin-up channel and 0.61 eV in spin-down channel, it is a p-type semiconductor because EF located near the VB maximum. Fig. 5(b) shows a semiconducting character due to the band gaps of 0.91 and 0.64 eV for the spin-up and spin-down channels, respectively. EF located at the VB maximum, but no states appeared at EF, thus it is a p-type semiconductor. In view of Fig. 5(c), the band gaps of 0.63 eV in the spin-up channel and 0.61 eV in the spin-down channel can be clearly seen. Therefore, it is a magnetic semiconductor. EF located near the VB maximum, suggesting that it is a p-type semiconductor. Fig. 5(d) shows a semiconducting character as well because neither spin-up nor spin-down channel go through EF, where a band gap of 0.47 eV for the spin-up channel and 0.72 eV for the spin-down channel can be observed. It is a p-type semiconductor since EF located near the VB maximum. Fig. 6 shows the TDOS and PDOS for the Co14Ti2O14 systems discussed above. In Fig. 6(a), no states appeared at EF, showing a magnetic semiconductor character. There are two kinds of Co can been seen in PDOS. The Co atom at 1 position do not offer states in the VB maximum, while the other one at 2 position makes contributions by Ti 3d states in spin-up channel. Otherwise, Ti 3d states overlap with Co 3d states due to the strong exchange interaction between Co and O. Fig. 6(b) shows a magnetic semiconducting
character because both spin-up and spin-down channels do not go through EF. The Co also has two types as well as in Fig. 6(a), but in the VB maximum, Ti 3d states become smaller and Co 3d states become larger. Nevertheless, Ti 3d states make the most contribution in the VB maximum of spin-down channel. In Fig. 6(c), it is a magnetic semiconductor because the spin-up and spin-down states do not appeared at EF. The two Co atoms which have been presented in PDOS make contributions to the VB maximum of spin-up channel. The CB minimum mainly from Ti 3d states as a result of the great contribution to the states in CB near EF. In view of Fig. 3(d), neither spin-up nor spin-down states appeared at EF, showing a semiconducting character, one can see that the states near EF are mainly from Ti 3d states. The spin-polarized charge density of the Co14Ti2O14 systems with two O vacancies is shown in Fig. 7. Yellow and cyan isosurfaces represent the positive and negative spin densities (±0.015 e/Å3), respectively. For the appearance of O vacancies, the magnetic moments have been changed by the electron transfer, as shown in Table 1 and Fig. 8, where Fig. 8(a)e(d) are seen under (100) direction and Fig. 8(e)e(h) are seen under (010) direction. In Fig. 7(a), one can clearly see that the spin-polarized charge density of Co at 1 and 3 positions increases due to the O vacancies near the two Co atoms. A part of spin-down electron from Co at 2 position transfer to Ti at 1 position due to the orbital hybridization between the Co
1552
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
Fig. 3. The band structure of Co14Ti2O14 systems with, two O vacancies, (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions.
Fig. 4. The total density of states (TDOS) and the partial density of states (PDOS) of Co14Ti2O14 systems with (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions.
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
1553
Fig. 5. The band structure of Co14Ti2O14 systems with (a) O vacancies at 1 and 2 positions, (b) O vacancies at 1 and 6 positions, (c) O vacancies at 2 and 9 positions, (d) O vacancies at 6 and 9 positions.
Fig. 6. The total density of states (TDOS) and the partial density of states (PDOS) of Co14Ti2O14 systems with (a) O vacancies at 1 and 2 positions, (b) O vacancies at 1 and 6 positions, (c) O vacancies at 2 and 9 positions, (d) O vacancies at 6 and 9 positions.
1554
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
Fig. 7. The spin-polarized charge of Co14Ti2O14 systems with (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions, (e) O vacancies at 1 and 2 positions, (f) O vacancies at 1 and 6 positions, (g) O vacancies at 2 and 9 positions, (h) O vacancies at 6 and 9 positions.
Table 1 The magnetic moment (mB) of Co14Ti2O14 with two O vacancies and each element given in PDOS. Co2þ
System Co14Ti2O16 O defect at O defect at O defect at O defect at O defect at O defect at O defect at O defect at
1 2 6 9 1 1 2 6
and and and and and and and and
3 positions 4 positions 8 positions 11 positions 2 positions 6 positions 9 positions 9 positions
2.70 2.67 2.24 2.66 2.00 2.70 2.69 2.54 2.72
Co2þ
1.95 2.37 2.52 2.60
O2
Ti4þ
Total
0.09 0.02 0.03 0.07 0.12
0.66 0.51 0.25 0.40 0.86 0.41 0.19 1.07 0.90
5.82 5.77 5.74 5.94 2.13 4.09 5.94 4.09 5.84
and Ti, as shown in Fig. 8. At the same time, it makes the Ti show a negative spin with a magnetic moments of 0.51 mB, but the total magnetic moments still match the Co14Ti2O16 system due to the decrease of the Co magnetic moments of about 2.11 mB at 2 position can been clearly seen in Fig. 7(a). Fig. 7(b) shows the similar result, but the effect from neighboring Co atoms to Ti is smaller and the magnetic moment of Ti is 0.25 mB due to the spin-up electrons transfer from Co at 13 position. In Fig. 7(c), the Ti magnetic moment is 0.4 mB, which is smaller than that in Co14Ti2O16 system because there is electron transfer from Ti to the O vacancies, as shown in Fig. 8(c). In additional, it leads to an increase of total magnetic moments (see Table 1). In Fig. 7(d), the spin-polarized charge density of Ti is grown up to 0.86 mB because a large number of spindown electrons are lost, which can be clearly seen in Fig. 8(d). Thus,
Fig. 8. The deformation charge density of Co14Ti2O14 systems with (a) O vacancies at 1 and 3 positions, (b) O vacancies at 2 and 4 positions, (c) O vacancies at 6 and 8 positions, (d) O vacancies at 9 and 1 positions, (e) O vacancies at 1 and 2 positions, (f) O vacancies at 1 and 6 positions, (g) O vacancies at 2 and 9 positions, (h) O vacancies at 6 and 9 positions. The red one stands for getting electrons, and the blue one represents losing electrons. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
J. Zhou et al. / Current Applied Physics 15 (2015) 1549e1555
the total magnetic moment reduces to 2.13 mB. In Fig. 7(e), one can see that the spin-polarized charge density of Ti at 1 position is smaller than that at 2 position. Fig. 8(e) shows that the spin-up electrons of Ti atoms transfer into Co atom at neighbor positions, which is the reason that the magnetic moment of Co at 2 position reduces to 1.95 mB (see Table 1). The system with O vacancies at 1 and 6 positions shows the largest total magnetic moment of 5.94 mB. The Ti atom has a small negative magnetic moment of 0.19 mB, which is consistent with that in Fig. 7(f) because the spindown electrons transfer (see Fig. 8(f)). The system with O vacancies at 2 and 9 positions has the same total magnetic moment as that with O vacancies at 1 and 2 positions because one O vacancy locates near the Co with negative spin and the other one locates near the spin-up one. For the different distances between the O vacancies and Ti atoms, the transfer electrons between the Co and Ti atoms are not the same. The case (h) has the similar situations with case (f). 4. Conclusion We study the electronic structure and magnetism of Co14Ti2O14 systems with two O vacancies by using first-principles calculations. The system of CoO doped Ti at 9 and 11 positions shows a halfmetallic character. In additional, the two O vacancies will make effect on the systems when it appeared near the Ti atom. The case of O vacancies at 1 and 3 positions shows a half-metallic character with a spin polarization of 100%. For the O vacancies at 6 and 8 positions, the system shows a semiconducting character. The system of O vacancies at 9 and 11 positions is a typical spin gapless semiconductor with a small total magnetic moment. Also, the electron transfer caused by the O vacancies affects the spinpolarized charge density of the atoms at neighboring positions. Acknowledgments This work was supported by the Key Project of Natural Science Foundation of Tianjin City (14JCZDJC37800), and National High Technology Research and Development Program (863 Program) of China (No. 2013AA030801). References [1] I. Zuti c, J. Fabian, S. Sarma, Spintronics: fundamentals and applications, Rev. Mod. Phys. 76 (2004) 323. [2] S. Sarma, Spintronics: a new class of device based on electron spin, rather than on charge, may yield the next generation of microelectronics, Am. Sci. 89 (2001) 516. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Zener model description of ferromagnetism in Zinc-blende magnetic semiconductors, Science 287 (2000) 1019. [4] K. Sato, H. Katayama-Yoshida, Material design for transparent ferromagnets with ZnO-based magnetic semiconductors, Jpn. J. Appl. Phys. 39 (2000) L555. [5] W. Mi, H. Bai, H. Liu, C. Sun, Microstructure, magnetic, and optical properties of sputtered Mn-doped ZnO films with high-temperature ferromagnetism, J. Appl. Phys. 101 (2009) 023904. [6] K. Ueda, H. Tabata, T. Kawai, Magnetic and electric properties of transitionmetal-doped ZnO films, Appl. Phys. Lett. 79 (2001) 988. [7] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma1, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 291 (2001) 854. [8] W. Mi, E. Jiang, H. Bai, High-temperature ferromagnetism observed in facingtarget reactive sputtered MnxTi1xO2, Acta Mater. 56 (2008) 3511.
1555
[9] W. Mi, E. Jiang, H. Bai, Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films, J. Magn. Magn. Mater. 321 (2009) 2472. [10] Y. Yoo, Q. Xue, H. Lee, S. Cheng, X. Xiang, G. Dionne, Bulk synthesis and hightemperature ferromagnetism of (In1xFex)2O3s with Cu co-doping, Appl. Phys. Lett. 86 (2005) 042506. [11] J. He, S. Xu, Y. Yoo, Q. Xue, H. Lee, S. Cheng, X. Xiang, G. Dionne, I. Takeuchi, Room temperature ferromagnetic n-type semiconductor in (In1xFex)2O3s, App. Phys. Lett. 86 (2005) 052503. [12] J. Philip, A. Punnoose, B. Kim, K. Reddy, S. Layne, J. Holmes, B. Satpati, P. LeClair, T. Santos, J. Moodera, Carrier-controlled ferromagnetism in transparent oxide semiconductors, Nat. Mater. 298 (2006) 304. [13] J. Coey, A. Douvalis, C. Fitzgerald, M. Venkatesan, Ferromagnetism in Fe-doped SnO2 thin films, Appl. Phys. Lett. 84 (2004) 1332. [14] S. Yang, T. Li, B. Gu, Y. Du, H. Sung, S. Hung, C. Wong, A. Pakhomov, Ferromagnetism in Mn-doped CuO, Appl. Phys. Lett. 83 (2003) 3746. [15] W. Mi, H. Bai, Q. Zhang, B. Zhang, X. Zhang, Ferromagnetism in reactive sputtered Cu0.96Fe0.04O1d nanocrystalline films evidenced by anomalous Hall Effect, Appl. Phys. Express 4 (2011) 043001. [16] M. Venkatesan, C. Fitzgerald, J. Coey, Thin films: unexpected magnetism in a dielectric oxide, Nature 430 (2004) 630. [17] C. Pemmaraju, S. Sanvito, Ferromagnetism driven by intrinsic Point defects in HfO2, Phys. Rev. Lett. 94 (2005) 217205. [18] J. Coey, M. Venkatesan, P. Stamenov, C.B. Fitzgerald, L.S. Dorneles, Magnetism in hafnium dioxide, Phys. Rev. B 72 (2005) 024450. [19] Y. Chang, Y. Soo1, W. Lee, M. Huang, Y. Lee, S. Weng, W. Sun, M. Hong, J. Kwo1, S. Lee, J. Ablett, C. Kao, Observation of room temperature ferromagnetic behavior in cluster-free, Co doped HfO2 films, Appl. Phys. Lett. 91 (2007) 082504. [20] S. Yan, C. Ren, X. Wang, Y. Xin, Z. Zhou, L. Mei, M. Ren, Y. Chen, Y. Liu, H. Garmestani, Ferromagnetism and magnetoresistance of CoeZnO inhomogeneous magnetic semiconductors, Appl. Phys. Lett. 84 (2004) 2376. [21] H. Gao, J. Xia, Effect of Li-doping on the magnetic properties of ZnO with Zn vacancies, J. Appl. Phys. 111 (2012) 093902. [22] L. Zhang, B. Huang, Y. Liu, L. Zhang, R. Zhang, Giant magnetoresistance in FeIn2O3 granular films, J. Magn. Magn. Mater. 261 (2003) 257. [23] S. Yan, J. Liu, L. Mei, Y. Tian, H. Song, Y. Chen, G. Liu, Spin-dependent variable range hopping and magnetoresistance in Ti1xCoxO2 and Zn1xCoxO magnetic semiconductor films, J. Phys. Condens. Mat. 18 (2006) 20469. [24] X. Wang, W. Mi, E. Jiang, H. Bai, Large magnetoresistance observed in facingtarget sputtered Ni-doped CNx amorphous composite films, Acta Mater. 55 (2007) 3547. [25] H. Song, L. Mei, Y. Tian, X. Ma, J. Liu, Y. Wang, Z. Zhang, Microstructure, ferromagnetism, and magnetic transport of Ti1xCoxO2 amorphous magnetic semiconductor, J. Appl. Phys. 90 (2006) 123903. [26] X. Wang, Y. Bai, W. Mi, Negative magnetoresistance in reactive sputtered nonuniform amorphous FexTi1xOd films, Mater. Res. Bull. 48 (2013) 3449. [27] J. Coey, d0 ferromagnetism, Solid State Sci. 7 (2005) 660. [28] J. Coey, M. Venkatesan, C. Fitzgerald, Donor impurity band exchange in dilute ferromagnetic oxides, Nat. Mater. 4 (2005) 173. [29] M. Yin, X. Wang, W. Mi, G. Chen, B. Yang, A first-principles prediction on the magnetism in CoO with Co and O Vacancies, J. Alloy. Compd. 610 (2014) 422. [30] J. Coey, M. Venkatesan, Half-metallic ferromagnetism: example of CrO2, J. Appl. Phys. 91 (2001) 8345. [31] M. Yin, X. Wang, W. Mi, Y. Ding, G. Chen, B. Yang, Magnetism and electronic structure in Zn and Ti doped CoO: a first-principles study, Comp. Mater. Sci. 93 (2014) 193. [32] K. An, N. Lee, J. Park, S. Kim, Y. Hwang, J. Park, J. Kim, J. Park, M. Han, J. Yu, T. Hyeon, Synthesis, characterization, and self-assembly of pencil-shaped CoO nanorods, J. Am. Chem. Soc. 128 (2006) 9753. [33] G. Kresse, J. Furthmuller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169. [34] W. Roth, Magnetic structures of MnO, FeO, CoO, and NiO, Phys. Rev. 110 (1958) 1333. [35] R. Hanafin, T. Archer, S. Sanvito, Magnetism of wurtzite CoO nanoclusters, Phys. Rev. B 81 (2010) 054441. [36] X. Wang, Proposal for a new class of materials: spin gapless semiconductors, Phys. Rev. Lett. 100 (2008) 156404. [37] N. Feng, W. Mi, Y. Cheng, Z. Guo, U. Schwingenschlogl, H. Bai, First principles prediction of the magnetic properties of Fe-X6 (X ¼ S, C, N, O, F) doped monolayer MoS2, Sci. Rep. 4 (2014) 3987. [38] X. Hu, W. Zhang, L. Sun, A. Krasheninnikov, Gold-embedded zigzag graphene nanoribbons as spin gapless semiconductors, Phys. Rev. B 86 (2012) 195418. [39] A. Ramasubramaniam, Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides, Phys. Rev. B 86 (2012) 125409.