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Effect of halogens in MgO to predict half-metallic ferromagnetism: By first principles calculations
Journal Pre-proof Effect of halogens in MgO to predict half-metallic ferromagnetism: By first principles calculations Nazir Ahmad Teli, M. Mohamed Sheik Sirajuddeen, Ikram Un Nabi Lone PII:
S1293-2558(19)31232-4
DOI:
https://doi.org/10.1016/j.solidstatesciences.2019.106048
Reference:
SSSCIE 106048
To appear in:
Solid State Sciences
Please cite this article as: N.A. Teli, M.M. Sheik Sirajuddeen, I.U. Nabi Lone, Effect of halogens in MgO to predict half-metallic ferromagnetism: By first principles calculations, Solid State Sciences, https:// doi.org/10.1016/j.solidstatesciences.2019.106048. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Masson SAS. All rights reserved.
1
Effect of halogens in MgO to predict half-metallic ferromagnetism: By first principles calculations Nazir Ahmad Telia*, M. Mohamed Sheik Sirajuddeena, Ikram Un Nabi Lonea a-
Department of Physics, B.S Abdur Rahman Crescent Institute of Science and Technology *- Corresponding author email:- [email protected]
Abstract Using the First Principles calculations within the frame work of density functional theory (DFT) the structural, electronic and magnetic properties were determined. Co-doping of halogen atoms in the host MgO with 12.5% concentration was performed and the halfmetallicity property was investigated. It is very interesting that co-doping of halogen atoms in the host MgO plays a vital role in exhibiting the half-metallic nature of the super cells Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875. Energy band gaps were found in spin up (majority) direction at the Fermi level which reveals the halfmetallicity of the compounds. Super cells were found stable in the ferromagnetic phase as compared to the non-magnetic phase. Lattice constants and equilibrium energies were calculated after volume optimization was carried out. In super cells, the halogen atoms which replace the Mg atoms contributes in producing magnetic moment but which replace O atoms having a negative impact on a total spin magnetic moment. Moreover, the contribution to create a total spin magnetic moment in the super cells is due to Mg atoms and O atoms. Keywords: Half-metallic Ferromagnetism (HMF), GGA method, Electronic, Magnetic and Structural properties 1. Introduction Half-metallicity is the property of the materials in which one spin channel behaves as semiconducting/insulator and another one conducting or vice versa. With this property, the materials can be used in the spintronics for manufacturing or fabricating the spin-based materials like spin valves. The half-metallic property was first reported in Heusler alloys by Groot et al [1]. After that, there is enormous research work in the areas of nitrides, oxides, diluted magnetic semiconductors, sulfides etc and have been studied exhibiting half-metallic ferromagnetism (HMF) [2-6]. Half-metallic ferromagnetism was reported on doping MgO with group ( ,
,
) elements of the periodic table [7, 8] and with transition metals [9-12].
So, from the literature survey, it is reported that doping of impurity atoms like metal or non-
2
metal atoms changes the character of host compound (MgO) by exhibiting half-metallic ferromagnetism. It is experimentally reported that Magnesium oxide (MgO) is non- magnetic insulator with band gap equal to 7.8 eV [13,14] and the theoretically reported band gap is equal to 4.5 or 5 eV for MgO [15,16]. A. Khamkongkaeo et al, have found ferromagnetism in magnesium oxide [33]. MgO is a useful oxide with significant implementation in the area of catalysis and electronics [17, 18]. The behavior of Magnesium oxide changes with adding some impurities [19]. The impurities like non-magnetic elements were doped in MgO predicting ferromagnetism [20]. Co-doping of Mn and Nitrogen in MgO is reported to exhibit halfmetallic ferromagnetism. Co-doping has also a great influence on changing the properties of oxides like ZnO doped with Co and Al exhibiting ferromagnetism [21]. Generally, Codoping can be systematic for increasing the dopant solubility, the activation rate and carrier mobility [32]. In this paper, attention is given to co-doping of Halogens in MgO which changes the properties of host compound from insulator to half-metal. It is reported that doping of halogen atoms separately in host compound of MgO predicts metallic character in the super cells as per a recent study [31]. But when halogen atoms are co-doped in MgO, metallic character changes into half-metallicity. It has been found in literature survey that there is no research work carried out in co-doping of halogen in MgO. Half-metallicity is predicted after co-doping of halogens with 12.5% concentration in the host MgO. In this work structural, electronic, half-metallicity and magnetic properties were discussed. 2. Computational Method The calculations were performed by using the full potential linearized augmented plane wave method and WIEN2k code under the treatment of density functional theory [22-25]. Based on the GGA method the exchange-correlation energy for the interaction of electrons was used [26]. The separation energy was chosen to be -6Ry between the core states. Execution of brillioun zone integration by using the Monkhorst-Pack scheme [27, 28] using 2 ×2×2 k points for the super cells. A unit cell of MgO has rock salt structure with experimental lattice constant 4.21 Å belongs to space group (Fm3m) [29]. In creating super cells the host MgO atoms were replaced by halogen atoms with a co-doping procedure. Cl and Br were co-doped in MgO replacing Mg atom by Cl and O atom by Br to form a supercell Cl0.125Mg0.875Br0.125O0.875. In another Cl0.125Mg0.875I0.125O0.875 super cell, Mg was replaced
3
with Cl but oxygen atom was replaced by I (Iodine). In forming the third Br0.125Mg0.875I0.125O0.875 supercell Mg atom was replaced by Bromine and oxygen atom by Iodine. The concentration of the co-doping atoms in each super cell is 12.5% as the numbers of atoms in each super cell are 16. So, replacing with one atom from 8 atoms gives a percentage of concentration 12.5%. After completion of replacing the atoms, the process of iteration gets started and runs until the energy converges and become lees than 10-4Ry. 3. Results and Discussion 3.1 Volume optimization and Structural properties of undoped and doped compounds Calculation of ground state properties of the compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 were carried out by using WIEN2k code. With volume optimization, the minimum energy and lattice constants were obtained to check the stability of the compounds. The energy verses volume curves were plotted which predicts the phase stability of the compounds in the ferromagnetic phase and are shown in Figure 2. Total Minimum equilibrium energy and lattice constants of the undoped and co-doped compounds are depicted in Table 2 and the atomic positions are shown in Table 3 and 4. The stability can also be obtained by evaluating the cohesive energy of the compounds. If the cohesive energy of the compound is negative then the compound is stable. Cohesive energies were calculated for each compound by using the relation [34, 35] and are shown in Table 1. The calculated negative values of cohesive energies indicate that the co-doped compounds are stable. Cohesive Energy = Total Energy of Compound - Individual Energy of Atoms
(1)
Table 1. Calculated total energy (Eο) of co-doped compounds compounds Individual energies of atoms (Mg, O, Cl, Br, I) and Cohesive energy of co-doped compounds
Super cell
Individual
Cohesive Energy
Total Energy
Energy of atoms
of Supercells
(Ry)
(Ry)
(Ry)
Mg= -400.637207 MgO/super cell
-4411.43513
O= -150.245327
-4.37485
Cl0.125Mg0.875Br0.125O0.875
-9995.26837
Cl= -922.908876
-2.96826
Cl0.125Mg0.875I0.125O0.875
-19019.81424
Br= -5213.213495
-2.67654
Br0.125Mg0.875I0.125O0.875
-23310.18465
I= -14238.051090
-2.74233
4
Table2. Min. Equillibrium Energy, Lattice Constants and Band Gaps of the compounds
Compound
Phase
Min. Eq. Energy
Lattice Constant
Band
(Ry)
(Å)
Gap (eV)
MgO per unit cell
Non-magnetic
-551.4303
4.26
4.5
Cl0.125Mg0.875Br0.125O0.875 Ferromagnetic
-9995.26837
9.21
2.11
Non-magnetic
-9995.25455
9.21
Ferromagnetic
-19019.81424
9.47
Non-magnetic
-19019.79477
9.47
Ferromagnetic
-23310.18465
9.58
Non-magnetic
-23310.17038
9.58
Cl0.125Mg0.875I0.125O0.875 Br0.125Mg0.875I0.125O0.875
Table 3. Atomic positions and symmetry of MgO compound
S.No. Atom 1 Mg 2 O
X 0.00000 0.50000
Y 0.00000 0.50000
Z 0.00000 0.50000
Symmetry 1 1
Table 4. Atomic positions and symmetry of the co-doped compounds Cl0.125Mg0.875Br0.125O0.875 , Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875
S.No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Atom Cl/Br Mg Mg Mg Mg Mg Mg Mg Br/I O O O O O O O
X 0.0000 0.50000 0.25000 0.75000 0.25000 0.75000 0.00000 0.50000 0.25000 0.75000 0.00000 0.50000 0.00000 0.50000 0.25000 0.75000
Y 0.00000 0.00000 0.25000 0.25000 0.00000 0.00000 0.25000 0.25000 0.25000 0.25000 0.00000 0.00000 0.25000 0.25000 0.00000 0.00000
Z 0.00000 0.00000 0.00000 0.00000 0.25000 0.25000 0.25000 0.25000 0.25000 0.25000 0.25000 0.25000 0.00000 0.00000 0.00000 0.00000
Symmetry 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
2.21
1.25
5
-551.414 -551.416
Non-magnetic Curve
-551.418 MgO
Energy (Ry)
-551.420 -551.422 -551.424 -551.426 -551.428 -551.430 -551.432 110 (a)
-9995.230
115
120
125
130
135
140
3
Volume (a.u )
Ferromagnetic Curve Non-magnetic Curve
-9995.235
Energy (Ry)
-9995.240
Cl0.125Mg0.875Br0.125O0.875
-9995.245 -9995.250 -9995.255 -9995.260 -9995.265 -9995.270 1315.5 1316.0 1316.5 1317.0 1317.5 1318.0 1318.5 1319.0 (b)
3
Volume (a.u )
Energy (Ry)
6
-19019.745 -19019.750 -19019.755 -19019.760 -19019.765 -19019.770 -19019.775 -19019.780 -19019.785 -19019.790 -19019.795 -19019.800 -19019.805 -19019.810 -19019.815 1910
Ferromagnetic Curve Non-magnetic Curve Cl0.125Mg0.875I0.125O 0.875
1920
1930
1940
1950
1960
1970
1980
3
(c)
Volume (a.u )
-23310.160
Ferromagnetic Curve Non-magnetic Curve
-23310.162 -23310.164
Br0.125Mg0.875I0.125O0.875
-23310.166 Energy (Ry)
-23310.168 -23310.170 -23310.172 -23310.174 -23310.176 -23310.178 -23310.180 -23310.182 -23310.184 -23310.186 2500 (d)
2510
2520
2530
2540
2550
3
Volume (a.u )
Figure 1. Optimization Curves (a) MgO (b) Cl0.125Mg0.875Br0.125O0.875 (c) Cl0.125Mg0.875I0.125O0.875 (d) Br0.125Mg0.875I0.125O0.875
Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875compounds were replicated in the rocksalt structure with space group Fm3m (225) by replacing the host
7
elements of Mg and O by Cl, Br and I in 12.5% of doping concentration. Three different super cells were obtained by substituting Cl, Br and I atoms in MgO with a good order of codoping. In Cl0.125Mg0.875Br0.125O0.875 super cell, Mg and O atoms were replaced by Cl and Br respectively. The same procedure is followed in supercell Cl0.125Mg0.875I0.125O0.875 but O is replaced by I. In super cell Br0.125Mg0.875I0.125O0.875 the Mg atom is replaced by Br and O by I. Co-doping of the halogen atoms changes the structural properties of the host magnesium oxide. The crystal structures of the unit cell and the resulting super cells were plotted by using the Vesta software [30] are shown in Figure 2.
Figure 2. Crystal Structures of (a) MgO (b) Cl0.125Mg0.875Br0.125O0.875 (c) Cl0.125Mg0.875I0.125O0.875) (d) Br0.125Mg0.875I0.125O0.875
8
Figure 3. Total DOS of (a) MgO
3.2 Electronic Properties 3.2.1 Density of States Super cells were obtained after co-doping of halogens (Cl, Br and I) in host MgO and the electronic properties were studied theoretically. When the individual halogen group elements were separately doped in the host MgO compound, the doped compounds do not exhibit halfmetallic ferromagnetism (HMF) except for fluorine doped compound as per a recent study [31]. Now, in this work co-doping of such halogen atoms in MgO were carried out and all the doped compounds exhibit half-metallic ferromagnetism (HMF). To analyze the electronic properties of the super cells the total DOS and partial DOS of each resulting compounds were plotted. Figure 4 shows the total DOS and partial DOS of the compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875. From the total DOS of respective compounds, half-metallic ferromagnetism was evidenced by the doped compounds. This can be understood by the formation of energy band gap at Fermi Level in spin up (majority) direction which means that the compounds are semiconductors in spin up (majority) direction and metallic in spin down (minority) direction. In the hybridization process, valence bands are contributing in making orbital bonds. It can be seen
from
the
partial
DOS
of
resulting
compounds
Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 as shown in Figure 4 that p-orbitals of Cl, Br, I and O atoms and s-orbital’s of Mg atoms are contributing in hybridization in spin up (majority) direction creating a band gap between the valence and conduction bands at the Fermi level.
9
In first Cl0.125Mg0.875Br0.125O0.875 compound, Cl-3p and Mg-3s bands near Fermi level pushes the O-2p band and Br-4p band in spin up (majority) direction and leaving an energy band gap at Fermi level. In spin-down (minority) direction the Cl-3p and Mg-3s bands cross the Fermi level showing a metallic character and therefore, the compound Cl0.125Mg0.875Br0.125O0.875 is half-metallic. In second compound Cl0.125Mg0.875I0.125O0.875, the bands Cl-3p and Mg-3s bands were pushed by the Cl-3p and I-5p bands and creating a band gap at Fermi level in majority spin direction and the bands Cl-3p and Mg-3s leaving no band gap in minority spin direction which confirms the half-metallic behavior of the compound Cl0.125Mg0.875I0.125O0.875.
Figure 4. Total DOS of (a) Cl0.125Mg0.875Br0.125O0.875 (b) Cl0.125Mg0.875I0.125O0.875 (c) Br0.125Mg0.875I0.125O0.875
In third compound Br0.125Mg0.875I0.125O0.875, the Br-4p and Mg-3s bands impel away the I-5p and O-2p bands making an energy band gap in spin up (majority) direction. This introduces a
10
semiconductor behavior in spin up direction. While the bands in spin down (minority) direction cross the Fermi level and exhibiting metallic character. Therefore, the compound Br0.125Mg0.875I0.125O0.875 is investigated to exhibit the half-metallic characteristics. In this way, the
compounds
Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875,
and
Br0.125Mg0.875I0.125O0.875 are behaving as half-metallic with an energy gap in spin up direction. 3.2.2 Electronic band Structure Figure 5 shows the band structure of the MgO host with a band gap in both spins equal to 4.5 eV measured at Γ point and this gap indicates that the host MgO is an insulator. Co-doping of halogens (Cl, Br), (Cl, I) and (Br, I) in host MgO changes this insulator into half-metallic. The band gaps are clearly visible in spin up (majority) direction at Fermi level of the resulting compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875. These band gaps were measured between the (conduction) lowest bands and (valence) highest bands.
The
calculated
band
gaps
of
the
compounds
Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 are shown in Table 2.
Figure 5. Band structure of MgO
The
electronic
band
structures
of
the
compounds
(Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875are shown in Figure 6. The electronic band structures show indirect band gaps in spin up direction at the Fermi level which were calculated between the highest (L) and lowest (Γ) symmetric points (valence and conduction bands). In these compounds, the band gaps are found in spin up (Majority) direction and no
11
gaps were found in spin down (minority) direction which predicts that the compounds are semiconductors in majority direction and conductors in minority direction. Therefore, the compounds are found to be half-metallic in nature and have the possibility of using in spintronics.
Figure 6. Bandstructures of supercells Cl0.125Mg0.875Br0.125O0.875 , Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875
12
3.3 Magnetic Properties In Table 5, the total spin magnetic moments of the compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875with the contribution from all the atoms is summarized. Co-doping of halogen atoms in the host MgO mostly plays a major role in the origin of magnetism in the super cells. The results distinctly specify the role of the dopants in creating the magnetic moments in the supercells and also the contribution of the host atoms creating spin magnetic moments in entire super cells. O-2p states and dopants (Cl, Br and I) which replaces Mg atom in their corresponding super cells have a good interaction (hybridization) between them and are responsible for increasing the spin magnetic moments in the super cells. The spin magnetic moment of each super cell is almost the same which means that the size of an atom (viz atomic number) has little effect on the total spin magnetic moment of the super cell. So, co-doping induces spin magnetic moment and exhibiting halfmetallicity in the super cells. In each super cell, each doped and undoped atoms are contributing in the total spin magnetic moment. Most probable role in increasing the spin magnetic moment in super cells is of dopant atoms (in place of Mg only) and the dopant atoms (in place of oxygen atom) has a negative impact on the entire magnetic moment of a super cell. Therefore, it specifies that dopants (in place of Mg) are mainly responsible for producing the magnetism in super cells. Moreover, the co-doping is efficient in changing the insulator character of MgO into new material (half-metallic) and producing a spin magnetic moment. Spin polarization is calculated at the Fermi level which is the ratio of DOS of spin up and spin down, (Ps = DOS up – DOS dn / DOS up + DOS dn ) where as Ps is spin polarization. All the co-doped compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875 which were evidenced as half-metallic exhibit 100 per cent spin polarization at the Fermi level. Table 5. Total Spin Magnetic moment of the co-doped super cells
Super cell
Interstit
Mg
O
Cl
Br
I
Total
ial (µB)
(µB)
(µB)
(µB)
(µB)
(µB)
(µB)
Cl0.125Mg0.875Br0.125O0.875
0.2151
0.0044
0.9372
0.8475
-0.0023
….
2.0019
Cl0.125Mg0.875I0.125O0.875
0.2183
0.0049
0.9333
0.8446
….
-0.0017
1.9994
Br0.125Mg0.875I0.125O0.875
0.3966
0.0085
0.9076
….
0.6884
-0.0016
1.9996
13
4. Conclusion Electronic, magnetic and structural properties of these compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 were calculated by using GGA method within the framework of DFT. Volume optimization was performed to determine the stability of the compounds and were found to be stable in the ferromagnetic phase. It has been found that co-doping of halogens in host MgO has contributed towards half-metallicity. Plots of electronic energy band structures and DOS predict the half-metallic nature of the compounds as energy gap is seen in spin up (majority) direction of each compound. These compounds show a 100% spin polarization and are suitable for spintronics application. Halogens (Cl, Br and I) play a pivotal role in the prediction of magnetism in the super cells. Acknowledgments The corresponding author thankfully acknowledges Prof. Peter Blaha and Prof. K. Schwarz of Vienna, Austria for providing WIEN2k code for the computational work done in this paper
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Highlights •
FULL POTENTIAL-LINEARISED AUGMENTED PLANE WAVE METHOD
•
CO-DOPING OF HALOGEN ATOMS IN MgO EXHIBIT HALF-METALLIC FERROMAGNETISM
•
STRUCTURAL PROPERTIES
•
ELECTRONIC PROPERTIES
•
MAGNETIC PROPERTIES
•
APPLICATION IN SPINTRONICS
S1293-2558(19)31232-4
DOI:
https://doi.org/10.1016/j.solidstatesciences.2019.106048
Reference:
SSSCIE 106048
To appear in:
Solid State Sciences
Please cite this article as: N.A. Teli, M.M. Sheik Sirajuddeen, I.U. Nabi Lone, Effect of halogens in MgO to predict half-metallic ferromagnetism: By first principles calculations, Solid State Sciences, https:// doi.org/10.1016/j.solidstatesciences.2019.106048. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Masson SAS. All rights reserved.
1
Effect of halogens in MgO to predict half-metallic ferromagnetism: By first principles calculations Nazir Ahmad Telia*, M. Mohamed Sheik Sirajuddeena, Ikram Un Nabi Lonea a-
Department of Physics, B.S Abdur Rahman Crescent Institute of Science and Technology *- Corresponding author email:- [email protected]
Abstract Using the First Principles calculations within the frame work of density functional theory (DFT) the structural, electronic and magnetic properties were determined. Co-doping of halogen atoms in the host MgO with 12.5% concentration was performed and the halfmetallicity property was investigated. It is very interesting that co-doping of halogen atoms in the host MgO plays a vital role in exhibiting the half-metallic nature of the super cells Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875. Energy band gaps were found in spin up (majority) direction at the Fermi level which reveals the halfmetallicity of the compounds. Super cells were found stable in the ferromagnetic phase as compared to the non-magnetic phase. Lattice constants and equilibrium energies were calculated after volume optimization was carried out. In super cells, the halogen atoms which replace the Mg atoms contributes in producing magnetic moment but which replace O atoms having a negative impact on a total spin magnetic moment. Moreover, the contribution to create a total spin magnetic moment in the super cells is due to Mg atoms and O atoms. Keywords: Half-metallic Ferromagnetism (HMF), GGA method, Electronic, Magnetic and Structural properties 1. Introduction Half-metallicity is the property of the materials in which one spin channel behaves as semiconducting/insulator and another one conducting or vice versa. With this property, the materials can be used in the spintronics for manufacturing or fabricating the spin-based materials like spin valves. The half-metallic property was first reported in Heusler alloys by Groot et al [1]. After that, there is enormous research work in the areas of nitrides, oxides, diluted magnetic semiconductors, sulfides etc and have been studied exhibiting half-metallic ferromagnetism (HMF) [2-6]. Half-metallic ferromagnetism was reported on doping MgO with group ( ,
,
) elements of the periodic table [7, 8] and with transition metals [9-12].
So, from the literature survey, it is reported that doping of impurity atoms like metal or non-
2
metal atoms changes the character of host compound (MgO) by exhibiting half-metallic ferromagnetism. It is experimentally reported that Magnesium oxide (MgO) is non- magnetic insulator with band gap equal to 7.8 eV [13,14] and the theoretically reported band gap is equal to 4.5 or 5 eV for MgO [15,16]. A. Khamkongkaeo et al, have found ferromagnetism in magnesium oxide [33]. MgO is a useful oxide with significant implementation in the area of catalysis and electronics [17, 18]. The behavior of Magnesium oxide changes with adding some impurities [19]. The impurities like non-magnetic elements were doped in MgO predicting ferromagnetism [20]. Co-doping of Mn and Nitrogen in MgO is reported to exhibit halfmetallic ferromagnetism. Co-doping has also a great influence on changing the properties of oxides like ZnO doped with Co and Al exhibiting ferromagnetism [21]. Generally, Codoping can be systematic for increasing the dopant solubility, the activation rate and carrier mobility [32]. In this paper, attention is given to co-doping of Halogens in MgO which changes the properties of host compound from insulator to half-metal. It is reported that doping of halogen atoms separately in host compound of MgO predicts metallic character in the super cells as per a recent study [31]. But when halogen atoms are co-doped in MgO, metallic character changes into half-metallicity. It has been found in literature survey that there is no research work carried out in co-doping of halogen in MgO. Half-metallicity is predicted after co-doping of halogens with 12.5% concentration in the host MgO. In this work structural, electronic, half-metallicity and magnetic properties were discussed. 2. Computational Method The calculations were performed by using the full potential linearized augmented plane wave method and WIEN2k code under the treatment of density functional theory [22-25]. Based on the GGA method the exchange-correlation energy for the interaction of electrons was used [26]. The separation energy was chosen to be -6Ry between the core states. Execution of brillioun zone integration by using the Monkhorst-Pack scheme [27, 28] using 2 ×2×2 k points for the super cells. A unit cell of MgO has rock salt structure with experimental lattice constant 4.21 Å belongs to space group (Fm3m) [29]. In creating super cells the host MgO atoms were replaced by halogen atoms with a co-doping procedure. Cl and Br were co-doped in MgO replacing Mg atom by Cl and O atom by Br to form a supercell Cl0.125Mg0.875Br0.125O0.875. In another Cl0.125Mg0.875I0.125O0.875 super cell, Mg was replaced
3
with Cl but oxygen atom was replaced by I (Iodine). In forming the third Br0.125Mg0.875I0.125O0.875 supercell Mg atom was replaced by Bromine and oxygen atom by Iodine. The concentration of the co-doping atoms in each super cell is 12.5% as the numbers of atoms in each super cell are 16. So, replacing with one atom from 8 atoms gives a percentage of concentration 12.5%. After completion of replacing the atoms, the process of iteration gets started and runs until the energy converges and become lees than 10-4Ry. 3. Results and Discussion 3.1 Volume optimization and Structural properties of undoped and doped compounds Calculation of ground state properties of the compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 were carried out by using WIEN2k code. With volume optimization, the minimum energy and lattice constants were obtained to check the stability of the compounds. The energy verses volume curves were plotted which predicts the phase stability of the compounds in the ferromagnetic phase and are shown in Figure 2. Total Minimum equilibrium energy and lattice constants of the undoped and co-doped compounds are depicted in Table 2 and the atomic positions are shown in Table 3 and 4. The stability can also be obtained by evaluating the cohesive energy of the compounds. If the cohesive energy of the compound is negative then the compound is stable. Cohesive energies were calculated for each compound by using the relation [34, 35] and are shown in Table 1. The calculated negative values of cohesive energies indicate that the co-doped compounds are stable. Cohesive Energy = Total Energy of Compound - Individual Energy of Atoms
(1)
Table 1. Calculated total energy (Eο) of co-doped compounds compounds Individual energies of atoms (Mg, O, Cl, Br, I) and Cohesive energy of co-doped compounds
Super cell
Individual
Cohesive Energy
Total Energy
Energy of atoms
of Supercells
(Ry)
(Ry)
(Ry)
Mg= -400.637207 MgO/super cell
-4411.43513
O= -150.245327
-4.37485
Cl0.125Mg0.875Br0.125O0.875
-9995.26837
Cl= -922.908876
-2.96826
Cl0.125Mg0.875I0.125O0.875
-19019.81424
Br= -5213.213495
-2.67654
Br0.125Mg0.875I0.125O0.875
-23310.18465
I= -14238.051090
-2.74233
4
Table2. Min. Equillibrium Energy, Lattice Constants and Band Gaps of the compounds
Compound
Phase
Min. Eq. Energy
Lattice Constant
Band
(Ry)
(Å)
Gap (eV)
MgO per unit cell
Non-magnetic
-551.4303
4.26
4.5
Cl0.125Mg0.875Br0.125O0.875 Ferromagnetic
-9995.26837
9.21
2.11
Non-magnetic
-9995.25455
9.21
Ferromagnetic
-19019.81424
9.47
Non-magnetic
-19019.79477
9.47
Ferromagnetic
-23310.18465
9.58
Non-magnetic
-23310.17038
9.58
Cl0.125Mg0.875I0.125O0.875 Br0.125Mg0.875I0.125O0.875
Table 3. Atomic positions and symmetry of MgO compound
S.No. Atom 1 Mg 2 O
X 0.00000 0.50000
Y 0.00000 0.50000
Z 0.00000 0.50000
Symmetry 1 1
Table 4. Atomic positions and symmetry of the co-doped compounds Cl0.125Mg0.875Br0.125O0.875 , Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875
S.No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Atom Cl/Br Mg Mg Mg Mg Mg Mg Mg Br/I O O O O O O O
X 0.0000 0.50000 0.25000 0.75000 0.25000 0.75000 0.00000 0.50000 0.25000 0.75000 0.00000 0.50000 0.00000 0.50000 0.25000 0.75000
Y 0.00000 0.00000 0.25000 0.25000 0.00000 0.00000 0.25000 0.25000 0.25000 0.25000 0.00000 0.00000 0.25000 0.25000 0.00000 0.00000
Z 0.00000 0.00000 0.00000 0.00000 0.25000 0.25000 0.25000 0.25000 0.25000 0.25000 0.25000 0.25000 0.00000 0.00000 0.00000 0.00000
Symmetry 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
2.21
1.25
5
-551.414 -551.416
Non-magnetic Curve
-551.418 MgO
Energy (Ry)
-551.420 -551.422 -551.424 -551.426 -551.428 -551.430 -551.432 110 (a)
-9995.230
115
120
125
130
135
140
3
Volume (a.u )
Ferromagnetic Curve Non-magnetic Curve
-9995.235
Energy (Ry)
-9995.240
Cl0.125Mg0.875Br0.125O0.875
-9995.245 -9995.250 -9995.255 -9995.260 -9995.265 -9995.270 1315.5 1316.0 1316.5 1317.0 1317.5 1318.0 1318.5 1319.0 (b)
3
Volume (a.u )
Energy (Ry)
6
-19019.745 -19019.750 -19019.755 -19019.760 -19019.765 -19019.770 -19019.775 -19019.780 -19019.785 -19019.790 -19019.795 -19019.800 -19019.805 -19019.810 -19019.815 1910
Ferromagnetic Curve Non-magnetic Curve Cl0.125Mg0.875I0.125O 0.875
1920
1930
1940
1950
1960
1970
1980
3
(c)
Volume (a.u )
-23310.160
Ferromagnetic Curve Non-magnetic Curve
-23310.162 -23310.164
Br0.125Mg0.875I0.125O0.875
-23310.166 Energy (Ry)
-23310.168 -23310.170 -23310.172 -23310.174 -23310.176 -23310.178 -23310.180 -23310.182 -23310.184 -23310.186 2500 (d)
2510
2520
2530
2540
2550
3
Volume (a.u )
Figure 1. Optimization Curves (a) MgO (b) Cl0.125Mg0.875Br0.125O0.875 (c) Cl0.125Mg0.875I0.125O0.875 (d) Br0.125Mg0.875I0.125O0.875
Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875compounds were replicated in the rocksalt structure with space group Fm3m (225) by replacing the host
7
elements of Mg and O by Cl, Br and I in 12.5% of doping concentration. Three different super cells were obtained by substituting Cl, Br and I atoms in MgO with a good order of codoping. In Cl0.125Mg0.875Br0.125O0.875 super cell, Mg and O atoms were replaced by Cl and Br respectively. The same procedure is followed in supercell Cl0.125Mg0.875I0.125O0.875 but O is replaced by I. In super cell Br0.125Mg0.875I0.125O0.875 the Mg atom is replaced by Br and O by I. Co-doping of the halogen atoms changes the structural properties of the host magnesium oxide. The crystal structures of the unit cell and the resulting super cells were plotted by using the Vesta software [30] are shown in Figure 2.
Figure 2. Crystal Structures of (a) MgO (b) Cl0.125Mg0.875Br0.125O0.875 (c) Cl0.125Mg0.875I0.125O0.875) (d) Br0.125Mg0.875I0.125O0.875
8
Figure 3. Total DOS of (a) MgO
3.2 Electronic Properties 3.2.1 Density of States Super cells were obtained after co-doping of halogens (Cl, Br and I) in host MgO and the electronic properties were studied theoretically. When the individual halogen group elements were separately doped in the host MgO compound, the doped compounds do not exhibit halfmetallic ferromagnetism (HMF) except for fluorine doped compound as per a recent study [31]. Now, in this work co-doping of such halogen atoms in MgO were carried out and all the doped compounds exhibit half-metallic ferromagnetism (HMF). To analyze the electronic properties of the super cells the total DOS and partial DOS of each resulting compounds were plotted. Figure 4 shows the total DOS and partial DOS of the compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875. From the total DOS of respective compounds, half-metallic ferromagnetism was evidenced by the doped compounds. This can be understood by the formation of energy band gap at Fermi Level in spin up (majority) direction which means that the compounds are semiconductors in spin up (majority) direction and metallic in spin down (minority) direction. In the hybridization process, valence bands are contributing in making orbital bonds. It can be seen
from
the
partial
DOS
of
resulting
compounds
Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 as shown in Figure 4 that p-orbitals of Cl, Br, I and O atoms and s-orbital’s of Mg atoms are contributing in hybridization in spin up (majority) direction creating a band gap between the valence and conduction bands at the Fermi level.
9
In first Cl0.125Mg0.875Br0.125O0.875 compound, Cl-3p and Mg-3s bands near Fermi level pushes the O-2p band and Br-4p band in spin up (majority) direction and leaving an energy band gap at Fermi level. In spin-down (minority) direction the Cl-3p and Mg-3s bands cross the Fermi level showing a metallic character and therefore, the compound Cl0.125Mg0.875Br0.125O0.875 is half-metallic. In second compound Cl0.125Mg0.875I0.125O0.875, the bands Cl-3p and Mg-3s bands were pushed by the Cl-3p and I-5p bands and creating a band gap at Fermi level in majority spin direction and the bands Cl-3p and Mg-3s leaving no band gap in minority spin direction which confirms the half-metallic behavior of the compound Cl0.125Mg0.875I0.125O0.875.
Figure 4. Total DOS of (a) Cl0.125Mg0.875Br0.125O0.875 (b) Cl0.125Mg0.875I0.125O0.875 (c) Br0.125Mg0.875I0.125O0.875
In third compound Br0.125Mg0.875I0.125O0.875, the Br-4p and Mg-3s bands impel away the I-5p and O-2p bands making an energy band gap in spin up (majority) direction. This introduces a
10
semiconductor behavior in spin up direction. While the bands in spin down (minority) direction cross the Fermi level and exhibiting metallic character. Therefore, the compound Br0.125Mg0.875I0.125O0.875 is investigated to exhibit the half-metallic characteristics. In this way, the
compounds
Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875,
and
Br0.125Mg0.875I0.125O0.875 are behaving as half-metallic with an energy gap in spin up direction. 3.2.2 Electronic band Structure Figure 5 shows the band structure of the MgO host with a band gap in both spins equal to 4.5 eV measured at Γ point and this gap indicates that the host MgO is an insulator. Co-doping of halogens (Cl, Br), (Cl, I) and (Br, I) in host MgO changes this insulator into half-metallic. The band gaps are clearly visible in spin up (majority) direction at Fermi level of the resulting compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875. These band gaps were measured between the (conduction) lowest bands and (valence) highest bands.
The
calculated
band
gaps
of
the
compounds
Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 are shown in Table 2.
Figure 5. Band structure of MgO
The
electronic
band
structures
of
the
compounds
(Cl0.125Mg0.875Br0.125O0.875,
Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875are shown in Figure 6. The electronic band structures show indirect band gaps in spin up direction at the Fermi level which were calculated between the highest (L) and lowest (Γ) symmetric points (valence and conduction bands). In these compounds, the band gaps are found in spin up (Majority) direction and no
11
gaps were found in spin down (minority) direction which predicts that the compounds are semiconductors in majority direction and conductors in minority direction. Therefore, the compounds are found to be half-metallic in nature and have the possibility of using in spintronics.
Figure 6. Bandstructures of supercells Cl0.125Mg0.875Br0.125O0.875 , Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875
12
3.3 Magnetic Properties In Table 5, the total spin magnetic moments of the compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875with the contribution from all the atoms is summarized. Co-doping of halogen atoms in the host MgO mostly plays a major role in the origin of magnetism in the super cells. The results distinctly specify the role of the dopants in creating the magnetic moments in the supercells and also the contribution of the host atoms creating spin magnetic moments in entire super cells. O-2p states and dopants (Cl, Br and I) which replaces Mg atom in their corresponding super cells have a good interaction (hybridization) between them and are responsible for increasing the spin magnetic moments in the super cells. The spin magnetic moment of each super cell is almost the same which means that the size of an atom (viz atomic number) has little effect on the total spin magnetic moment of the super cell. So, co-doping induces spin magnetic moment and exhibiting halfmetallicity in the super cells. In each super cell, each doped and undoped atoms are contributing in the total spin magnetic moment. Most probable role in increasing the spin magnetic moment in super cells is of dopant atoms (in place of Mg only) and the dopant atoms (in place of oxygen atom) has a negative impact on the entire magnetic moment of a super cell. Therefore, it specifies that dopants (in place of Mg) are mainly responsible for producing the magnetism in super cells. Moreover, the co-doping is efficient in changing the insulator character of MgO into new material (half-metallic) and producing a spin magnetic moment. Spin polarization is calculated at the Fermi level which is the ratio of DOS of spin up and spin down, (Ps = DOS up – DOS dn / DOS up + DOS dn ) where as Ps is spin polarization. All the co-doped compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875, and Br0.125Mg0.875I0.125O0.875 which were evidenced as half-metallic exhibit 100 per cent spin polarization at the Fermi level. Table 5. Total Spin Magnetic moment of the co-doped super cells
Super cell
Interstit
Mg
O
Cl
Br
I
Total
ial (µB)
(µB)
(µB)
(µB)
(µB)
(µB)
(µB)
Cl0.125Mg0.875Br0.125O0.875
0.2151
0.0044
0.9372
0.8475
-0.0023
….
2.0019
Cl0.125Mg0.875I0.125O0.875
0.2183
0.0049
0.9333
0.8446
….
-0.0017
1.9994
Br0.125Mg0.875I0.125O0.875
0.3966
0.0085
0.9076
….
0.6884
-0.0016
1.9996
13
4. Conclusion Electronic, magnetic and structural properties of these compounds Cl0.125Mg0.875Br0.125O0.875, Cl0.125Mg0.875I0.125O0.875 and Br0.125Mg0.875I0.125O0.875 were calculated by using GGA method within the framework of DFT. Volume optimization was performed to determine the stability of the compounds and were found to be stable in the ferromagnetic phase. It has been found that co-doping of halogens in host MgO has contributed towards half-metallicity. Plots of electronic energy band structures and DOS predict the half-metallic nature of the compounds as energy gap is seen in spin up (majority) direction of each compound. These compounds show a 100% spin polarization and are suitable for spintronics application. Halogens (Cl, Br and I) play a pivotal role in the prediction of magnetism in the super cells. Acknowledgments The corresponding author thankfully acknowledges Prof. Peter Blaha and Prof. K. Schwarz of Vienna, Austria for providing WIEN2k code for the computational work done in this paper
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Highlights •
FULL POTENTIAL-LINEARISED AUGMENTED PLANE WAVE METHOD
•
CO-DOPING OF HALOGEN ATOMS IN MgO EXHIBIT HALF-METALLIC FERROMAGNETISM
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STRUCTURAL PROPERTIES
•
ELECTRONIC PROPERTIES
•
MAGNETIC PROPERTIES
•
APPLICATION IN SPINTRONICS