Tunable magnetism in 2D silicon carbide doped with Co and Fe dopants: Ab initio study

Accepted Manuscript Title: Tunable magnetism in 2D silicon carbide doped with Co and Fe dopants: Ab initio study Author: M. Luo Y.H. Shen T.L. Yin PII: DOI: Reference:

S0030-4026(16)31273-6 http://dx.doi.org/doi:10.1016/j.ijleo.2016.10.086 IJLEO 58358

To appear in: Received date: Accepted date:

26-8-2016 25-10-2016

Please cite this article as: M.Luo, Y.H.Shen, T.L.Yin, Tunable magnetism in 2D silicon carbide doped with Co and Fe dopants: Ab initio study, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.10.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

Tunable magnetism in 2D silicon carbide doped with Co and Fe dopants: Ab initio study M. Luoa.*, Y. H. Shenb and T. L. Yinb,c a

Department of Electronic Engineering, Shang Hai Jian Qiao University, Shanghai 201306, P. R. China b Key Laboratory of Polar Materials and Devices, East China Normal University, Shanghai 200241, P. R. China c School of Electronics and Information, Nantong University, Nantong 226019, P. R. China

Abstract The electronic and magnetic properties of Fe- and Co-doped SiC monolayers are investigated by using first-principles method. The magnetism is observed in both two systems. Particular attention is concentrated on the interaction between Fe-Fe and Co-Co atoms in SiC monolayers. It is found that the magnetic properties switch depending on the TM-TM distance, antiferromagntic (AFM), ferromagnetic (FM) and nonmagnetic (NM) states are found in both systems. With increasing distance between two Co or Fe atoms, the doped SiC monolayer undergoes AFM─FM─NM or FM─AFM─NM transitions, respectively. The results show the p–d hybridization mechanism results in its ferromagnetic state. Our studies demonstrate that the Fe- and Co-doped SiC monolayers show tunable electronic and magnetic properties, suitable for applications in electronics and spintronics at nanoscale.

Keywords: SiC; Magnetic properties; First-principles calculation; Tunable magnetism;

*E-mail: [email protected]

1. Introduction Silicon carbide (SiC) has attracted much interest due to its corrosion resistance, large band gap, high mechanical strength, low density, high hardness, high thermal conductivity and low thermal expansion coefficient [1-5]. Similar to graphene, the SiC monolayer (2D-SiC) with a honeycomb structure could be energetically stable [6,7] and SiC Monolayer exhibits a large direct band gap [8]. Several theoretical studies investigated the structure and electronic properties of SiC nano scales [9,10]. Due to its wide band gap (3.2 eV), SiC has potential applications in electronics and optics. Recently a large number of researches have given special attention to the properties of metal doped 2D materials [11-15] and high catalytic activity has been verified. However, few theoretical studies have been focused on the magnetic properties of 3d transition metal (TM) doped SiC monolayers [16,17]. Among a series of TM atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), the Fe- and Co-doped SiC systems have very similar lattice constants but different magnetic behavior. Therefore, it might be interesting to study the magnetic properties in these two systems. In this work, we are concentrated on the interaction between two Fe and two Co atoms in doped SiC monolayers based on first-principles study. The SiC monolayer with one silicon atom substituted by a cobalt or iron atom has large magnetic moments. Then the interaction between Fe-Fe and Co-Co atoms are investigated and the antiferromagntic (AFM), ferromagnetic (FM) and nonmagnetic (NM) states are found in both systems. While the distance between two Co and two Fe atoms is increasing, the SiC monolayer undergoes AFM─FM─NM or FM─AFM─NM transitions, respectively. The results show the p–d hybridization mechanism results in their ferromagnetic state. Such tunable electronic and magnetic properties of SiC monolayer doped with Co and Fe dopants can be applied to nanoelectronic and spintronic. 2. Method

Our calculations are performed first-principles method based on density functional theory (DFT) within the generalized gradient approximations (GGA-PBE) [18] as implemented in the VASP package [19]. The projector augmented-wave (PAW) [20] pseudo potential method is used and the cut off energy is 450 eV. The 4×4×2 k-point grids were used in both 5×5×1 and 10×5×1 supercells. The lattice parameter and bond length of SiC monolayer is 3.094 and 1.786 Å [21], respectively. The separation between two layers is 20 Å. All the calculations are self-consistent and the total energy convergence criterion is set at the value of 10-5 eV. 3. Result and discussions Firstly, we calculate the equilibrium structure of pure SiC monolayer, which is shown in Fig. 1. The band structure of pure SiC monolayer is shown in Fig. 1(c), it has π and π* bands around K point at the Fermi level symmetrically and has a direct band gap of 2.52 eV, which agree with the previous studies [10, 22]. These results verified the reliability of our methods. Then, we study the magnetic properties of Co and Fe atoms doped SiC monolayer in the 5×5×1 supercell. In our calculations, the optimized binding length is 1.794 and 1.791 Å for Co and Fe cases, respectively, which indicate the structures of Co and Fe doped systems are very similar [17]. From our calculations, large total magnetic moments (μtotal) are observed and the values are 2.013 and 2.754 μB for two Co and Fe atoms, respectively. Moreover, the total density of states (DOS) and projected density of states (PDOS) for two systems are shown in Fig. 2. From Figs. 2(a) and 2(b), the spin-up and spin-down density in both systems are nonmagnetic and that is why the systems are nonmagnetic. Next, we come to investigate the interaction between two TM (Co and Fe) atoms in the 10×5×1 SiC monolayer supercell, and several possible configurations of TM dopants are discussed as shown in Fig. 1(b). We use i to mark the dopant TM (0, i) and we position one TM dopant atom in a fixed site [denoted 0 in Fig. 1(b)] and the other TM dopant substitutes a Si atom at one of the marked positions i=1-3. As shown in Table I, it is

found that the ferromagnetic coupling depends on the distance between two TM atoms. In addition, the stability of FM states is determined by the total energy difference for the above

possible

configurations.

The

total

energy

difference

is

estimated

as E  EFM  EAFM , where EFM and EAFM are the total energies of two TM atoms aligned ferromagnetically (FM) and antiferromagnetically (AFM), respectively. For the nearest Co-Co distance, E is 31 meV which indicates the interaction between two Co atoms is AFM. However, the situation changes with increasing Co-Co distance; the AFM state disappears and the FM state appears. For the cases of (0,2) and (0,3) in the 10×5×1 supercell, E are about -60 and -29 meV, respectively. For Fe doped system, E changes from -23 to 19 meV showing FM─AFM transitions. Moreover, the (0,1) case has the lowest total energies for both two systems, which indicates the nearest substitutions should be the most stable candidates for the 2D silicon carbide In order to under the FM interaction between the two Co and Fe dopants, the TDOS and PDOS are shown in Fig. 4 and Fig. 5, respectively. From Fig. 4(b) and Fig. 5, the d state of Co (Fe) overlaps with that of p state of the C atom. In the minority spin channel, the p state of the connecting C atoms contributes significantly to the unoccupied states, which indicate a strong hybridization between Co (Fe) and its neighboring C atoms. The polarized spins in C p states couple with the d localized spins of Co (Fe), showing a p-d like interaction chain, which results in an indirect FM interaction between Co (Fe) and C atoms. Because of the p-d interaction, the minority spin states are lifted upward together with the downward shift of the majority spin states, which lower the total energy of the system [23]. Thus, the p-d hybridization is responsible for FM in Co- and Fe-doped SiC monolayers. For another, the AFM states in Co-doped system, as shown in Fig. 4(a), the spin-up and spin-down is coincide and there is no splitting between spin-up and spin-down d bands, so the ground state is AFM with null spin polarization [29]. For the AFM state of Fe-doped system, we consider the FM interaction between the Fe atoms is

mediated by the Fe-induced polarization of the valence electron spins, which exhibit an antiferromagnetic coupling between two Fe dopants [30]. It is noteworthy that a much larger Co-Co or Fe-Fe distance is considered in the 10×5×1 supercell. For the cases of (0,4) and (0,5) shown in Fig. 1(b), the total energy difference E is negligible. The results indicate the Co- and Fe-doped SiC monolayers exhibit nonmagnetic (NM) behavior, which simultaneously means the interactions between two TM dopants undergo AFM─FM─NM and FM─AFM─NM transitions in Co- and Fe-doped SiC monolayers, respectively. The uniform magnetic properties render the Co- and Fe-doped SiC monolayers a valuable material. More detailed studies need to be done in the future.

4. Conclusion We use the density function theory to study on the interaction between two Fe and Co atoms in SiC monolayers. Tunable magnetic behavior is found in both two systems. The interaction between two Fe atoms or two Co atoms depends on the distance. With increasing distance between two Co or Fe atoms, the doped SiC monolayer undergoes AFM─FM─NM or FM─AFM─NM transitions, respectively. The results show the p–d hybridization mechanism results in their ferromagnetic states. Our studies indicate that the tunable electronic and magnetic properties of SiC monolayer doped with Co and Fe dopants could have potential applications in nanoelectronic and spintronic.

Acknowledgments The work is supported by the Shanghai Committee of Science and Technology, China (Grant No. ZHT. K1507). We also thank the Supercomputer Center of ECNU for using the Dawn 5000A supercomputer.

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Fig. 1. Schematic structure showing (a) side view of pristine SiC monolayer; (b) the hexagonal structure viewed along the c axis, and the configurations in two TM atoms-doped SiC monolayer in the 10×5×1 supercell; (c) band structure of pristine SiC monolayer.

Fig. 2. Spin-polarized DOS of: (a) Fe-doped and (b) Co-doped SiC monolayer in the 5×5×1 supercell. The Fermi level is marked by blue dashed line.

Fig. 3. (Color online) The total energy difference (∆E) vs TM-TM distance. Inside dashed circle is nonmagnetic (NM) state.

Fig. 4. (Color online) (a) DOS for two AFM coupling Co atoms of (0,1); (b) DOS for two FM coupling Co atoms of (0,2).

Fig. 5. (Color online) DOS of (0, 1): (a) two Fe and (b) its neighboring five C atoms. The Fermi level is marked by blue dashed line.

Table I. Optimized TM-TM distance (dTM-TM), the total magnetic moments (Mtot), the total energies (Etot), the energy differences between FM and AFM (ΔE) and the configurations of TM atoms are as shown in Fig. 1(b). Structure

Si48Fe2C50

Si48Co2C50

-

1 2 3 4 5

dTM-TM (Å) 3.050 6.156 9.292 12.381 15.430

Mtot (μB) 5.62 -

Etot (eV) 676.838 676.166 676.135 675.521 675.245

ΔE (eV) -0.023 0.033 0.019 0.002 0.001

1 2 3 4 5

3.106 6.211 9.300 12.382 15.435

4.877 5.630 -

674.903 673.775 673.642 673.247 673.019

0.031 -0.060 -0.029 -0.001 -0.001

Configuration