Electronic structures and magnetic properties of La(Zn,TM)AsO from first principles calculations (TM = V, Cr, Mn, Fe, Co and Ni)

Journal Pre-Proof Research paper Electronic structures and magnetic properties of La(Zn,TM)AsO from first principles calculations (TM= V, Cr, Mn, Fe, Co and Ni) Hualong Tao, Linlin Su, Mengxia Wang, Juan Cai, Yan Cui, Zhihua Zhang, Bo Song, Ming He PII: DOI: Reference:

S0009-2614(19)30635-9 https://doi.org/10.1016/j.cplett.2019.136654 CPLETT 136654

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

22 May 2019 31 July 2019 3 August 2019

Please cite this article as: H. Tao, L. Su, M. Wang, J. Cai, Y. Cui, Z. Zhang, B. Song, M. He, Electronic structures and magnetic properties of La(Zn,TM)AsO from first principles calculations (TM= V, Cr, Mn, Fe, Co and Ni), Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136654

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Electronic structures and magnetic properties of La(Zn,TM)AsO from first principles calculations (TM= V, Cr, Mn, Fe, Co and Ni) Hualong Taoa, Linlin Sua, Mengxia Wanga, Juan Caib, Yan Cuia, Zhihua Zhanga, Bo Songc, and Ming He a, ⁎ Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering,

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a

Dalian Jiaotong University, Dalian 116028, China

School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, China

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Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin,

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b

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150080, China

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ABSTRACT

The electronic structures and magnetic properties of TM-doped LaZnAsO (TM=V, Cr, Mn, Fe, Co

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and Ni) were investigated based on first-principles calculations. Except Ni-doping, other systems can exhibit magnetism. The magnetism comes mainly from the d orbitals of TM dopants and the

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As-p orbitals near-neighboring the TM dopants. For the study of the preferred coupling, the Cr, Fe

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and Co-doping configurations are ferromagnetic ground states, while the antiferromagnetism are more stable for V and Mn-doping. The p-d hybridization between TM-3d and As-4p leads to the

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ferromagnetic coupling for the TM-TM pairs, which is the magnetic mechanism of the FM state.

Keywords: LaZnAsO, First-principles, Ferromagnetism, Electronic structure

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Corresponding author. E-mail: [email protected].

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1. Introduction LaZnAsO crystallizes a ZrCuSiAs-type structure with a tetragonal layered structure, which has attracted great interest for the excellent physical properties [1,2]. The LaZnAsO systems with semiconducting phase show a high flexibility due to a large variety of constituent elements [3]. By

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doping other atoms, their basic physical properties can be tuned, opening up an exciting field to

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control the functionalities of these materials [4-7]. It was predicted that the novel dilute magnetic

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semiconductor (DMS) with spin and charge regulation could be obtained by the introduction of magnetic atoms into these nonmagnetic semiconducting ZrCuSiAs-like phases [8-10]. More

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attention had been paid to the new DMS systems in recent years [11,12]. Li et al. [3] studied the control of spin in a La(Mn,Zn)AsO alloy by carrier doping from first-principles calculations with a

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strong-correlated correction. It was found that the carrier doping induced a transition from antiferromagnetic (AFM) semiconductor to ferromagnetic (FM) half metal. Bannikov and

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Ivanovskii [13] investigated the magnetic properties of Mn, Fe and Co doped LaCuSO and

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LaCuSeO systems by FLAPW-GGA calculations. The results showed that a partial substitution of 3d metal atoms for Cu atoms led to a magnetic half-metal, which had 100% spin polarization near

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Fermi level. Bannikov et al. [14] designed a novel magnetic material based on YZnAsO system by

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first-principles calculations. The main finding was that substitutions of Mn, Fe and Co for Zn sites led to the transformations of electronic and magnetic properties of the parent material. Ding et al. [15] reported the synthesis and characterization of a bulk diluted magnetic semiconductor (La1-xBax)(Zn1-xMnx)AsO with a layered crystal structure. The results indicated that together with carrier doping via (La, Ba) substitution, a small amount of Mn substituting for Zn caused the ferromagnetic order with the Curie temperature (TC) up to ~40 K. Ding et al. [16] also studied the 2

JOURNAL PRE-PROOF suppression of TC by Sr doping in diluted ferromagnetic semiconductor (La1-xSrx)(Zn1-yMny)AsO through the solid-state reaction method. Their results showed that 10% Sr doping resulted in a ferromagnetic ordering below TC~30 K, and Sr concentration increasing to 30% heavily suppressed the TC and saturation moments.

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However, there is no systematic study on the electronic structures and magnetic properties of

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La(Zn,TM)AsO (TM: transition metals) to understand the underlying magnetic mechanism. In this

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paper, we calculated the electronic structures and magnetic properties of La(Zn,TM)AsO (TM= V, Cr, Mn, Fe, Co and Ni) by first-principles calculations. The magnetic properties of various

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configurations were carefully examined and the magnetic mechanism was investigated. Our results

2. Computational methods

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provide an expected way to induce ferromagnetism in LaZnAsO system.

The first-principles calculations were performed by using density functional theory (DFT)

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method within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) [17],

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implemented in Cambridge Sequential Total Energy Package (CASTEP) [18]. The plane wave cutoff energy of 450 eV was used. The 2×2×2 Monkhorst-Pack k mesh in the Brillouin zone [19]

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was employed. The energy convergence of 5×10-5 eV/atom was used in present calculations.

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Maximum force on each atom was smaller than 0.03 eV/Å. Maximum stress was 0.05 GPa. Maximum displacement was 10-3 Å. The convergence tests calculation with higher cut-off energy and k-points were also performed, and the results remained unchanged. The ZrCuSiAs-like chalcogenide oxide LaZnAsO has a crystal structure with alternating [ZnAs] and [LaO] layers, where the Zn atoms are arranged on a simple square lattice [20]. The examined LaZnAsO phases adopt a tetragonal structure of space group P4/nmm with lattice 3

JOURNAL PRE-PROOF constants a=b=4.10 Å and c=18.14 Å [21]. The embedded impurities in LaZnAsO were built as a 2×2×1 supercell to explore their magnetic orders, which included 64 atoms, as shown in the Fig. 1. One TM atom substituted one Zn atom, and the doping concentration of Zn was about 6.25 at.%. The magnetic coupling between TM atoms was studied by replacing a pair of Zn atoms with two

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TM atoms, and TM doping concentration was 12.5 at.%. According to some works in the literature

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[22-24], the TM pairs positions had a tendency to be adjacent to each other. Therefore, two TM

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atoms in nearest-neighbor sites were only considered in our study. To ensure accurate results, all doped configurations were optimized before calculating the total energy, electronic structures and

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magnetic properties.

3. Results and Discussions

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3.1 The electronic structure of pure LaZnAsO

Compared with the experimental lattice parameters (a=b=8.19 Å, c=18.14 Å), there is no

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obvious structural distortion of the optimized lattice constants for LaZnAsO (a=b=8.23 Å, c=18.51

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Å). For TM-doped system, the lattice mismatch is small, indicating that there is no obvious structure distortion, as shown in Table 1. The electronic structures, total density of states (TDOS)

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and partial density of states (PDOS) are calculated, as seen in Fig. 2. It can be seen that the states of

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the spin up and spin down are identical around the Fermi level, thus LaZnAsO is a nonmagnetic semiconductor with a band gap about 1 eV, which is smaller than the experimental data (~1.5 eV) due to the well-known underestimation of GGA function. The valence band above -4 eV is mainly contributed by the 4d states of La and As-4s, 4p, and the conduction band above 2 eV is chiefly contributed by the 4d states of La, Zn-3s, As-4p and O-2p. 3.2 The electronic structure and magneic properties of TM doped LaZnAsO 4

JOURNAL PRE-PROOF Moreover, we studied magnetic properties of TM-doped LaZnAsO with 6.25 at.% of substituted TM atom for Zn atom. In order to examine whether the substituted doping is energetically favorable, we calculate the formation energy Ef in TM doped LaZnAsO systems. The formation energy Ef of TM doped LaZnAsO is defined as follows: Ef=ED-EH-μTM+μZn, where ED and

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EH are the total energy of doped and corresponding pristine LaZnAsO, respectively. μTM and μZn are

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the chemical potentials of a doping atoms TM and the replaced atoms Zn, which were given by the

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total energy per each TM and Zn atom in bulk TM and zinc, respectively. The calculated Ef is shown in Table 1 and the formation energy is negative value for all systems. The result indicates

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that the formation of the alloys is exothermic, which confirms the stability of their structure. To study the magnetic properties, the magnetic moment of TM-doped LaZnAsO is calculated. The

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results show that all systems exist magnetic ordering except Ni-doping is not spin-polarized ground states. The magnetism comes mainly from the TM atom with magnetic moments of 2.94 B, 4.28 B,

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4.46 B, 3.22 B and 2.06 B for V, Cr, Mn, Fe and Co-doped LaZnAsO, respectively, as shown in

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Table 1.

To study spin polarization induced by TM dopant, we calculated the total density of states

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(TDOS) and partial density of states (PDOS) of TM doped LaZnAsO, as seen in Fig. 3. For V, Cr,

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Mn, Fe and Co-doped system, as shown in Fig. 3(a)-(e), it is noticed that the Fermi level crosses the spin up (down) states which peaks sharply while it crosses the energy gap of the spin down (up) states. This means that a typical half-metallicity can be expected in the system. Half-metals, which have one conducting spin channel and one semiconducting spin channel, are regarded as ideal spintronic materials to provide 100% spin-polarized [25, 26]. It is also found that the electronic states at Fermi level are mainly contributed by 3d states of TM and 4p states of As, indicating a 5

JOURNAL PRE-PROOF strong p-d hybridization. The hybridization indicates that the magnetic moments are contributed by TM and As atoms. As can be seen from Fig. 3(f), the spin-up states and spin-down states are symmetrical, which implies that Ni-doped LaZnAsO is a nonmagnetic material. These are in good consistent with the results of magnetic moment as shown in Table 1.

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To further investigate the magnetic coupling between the moments induced by dopants, we

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studied the two TM doped LaZnAsO systems, in which two Zn atoms were substituted by dopants

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TM with a 12.5 at.% concentration in a supercell. For each configuration, we considered ferromagnetic and antiferromagnetic coupling of the magnetic moments induced by TM atoms,

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respectively. The magnetization energy ΔE = EAFM - EFM for these configurations is used to indicate relative stability of the FM and AFM ordering. If ΔE > 0, the FM is more stable, or else the AFM is

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more stable. The values of ΔE between FM and AFM are listed in Table 1. The results indicate Cr, Fe and Co-doping configurations are FM ground state, while the AFM are more stable for V and

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Mn-doping. And we find TM doping can result in magnetic coupling, which further enhances the

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magnetic properties in the case with different TM atom introduced. Based on the mean-field theory and Heisenberg model, the TC of the TM-doped LaZnAsO

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systems is calculated by using the relation: kBTC =(2/3C)ΔE, where kB is the Boltzmann constant.

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The ΔE is the different energy between AFM and FM ground state of each system. C is the concentration of dopants [27, 28]. The calculated results of TC are listed in the following Table 1. The TC of the Cr-doped, Fe-doped and Co-doped LaZnAsO systems are 25 K, 37 K and 14 K, respectively. To further present the detailed contributions of the doping atoms to the magnetic properties and the possible coupling between two TM atoms, the spin density distribution is calculated with a 6

JOURNAL PRE-PROOF 0.01 e/Å3 iso-surface, as seen in Fig. 4. The figure clearly indicates the magnetism is mainly contributed by TM atoms and the As atoms near TM atoms, which is coincident with the results of magnetic moment. The hybridization between TM 3d and As 4p orbitals leads to the ferromagnetic coupling for the TM-TM pairs. The FM coupling can be attributed to the p-d interaction [29].

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4. Conclusions

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In summary, we have performed first-principles studies on the electronic structures and

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magnetic properties for TM-doped LaZnAsO system (TM=V, Cr, Mn, Fe, Co and Ni). For TM-doping, Ni-doped LaZnAsO is nonmagnetic while the other systems (TM = V, Cr, Mn, Fe and

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Co) are magnetic. The results of ferromagnetic coupling between TM-TM atomic pair confirm that the Cr, Fe and Co-doping configurations are ferromagnetic ground state, while the

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antiferromagnetism are more stable for V and Mn-doping. The hybridization between TM-3d (TM=Cr, Fe and Mn) and As-4p orbitals leads to the ferromagnetic coupling for the TM-TM pairs.

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The p-d hybridization coupling interaction is suggested to be responsible for explaining the

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magnetic mechanism of the ferromagnetic state.

Acknowledgements

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This work was sponsored by National Natural Science Foundation of China under Grant Nos.

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51672057, 51722205 and 51872034. This work was also sponsored by Natural Science Foundation of Liaoning under Grant No. 201602117, the outstanding talents support program by Dalian city under No. 2015R004, and the Supported by LiaoNing Revitalization Talents Program under No. XLYC1807173. This work was also sponsored by Key Projects of Natural Science Foundation of Liaoning and Doctor Start-up Fund of Liaoning under Grant No. 2017052015.

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Table Table 1 The lattice constant, atomic magnetic moment, formation energy, ΔE and Curie temperature TC of TM-doped LaZnAsO

18.51

---

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8.28

18.60

2.94

-6.22

Cr-doping

8.28

18.59

4.28

-8.48

Mn-doping

8.28

18.52

4.46

Fe-doping

8.28

18.56

3.22

Co-doping

8.30

18.63

2.06

Ni-doping

8.27

18.57

0.00

V-doping

---0.69 0.33

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TC (K)

----25

-8.08

-0.73

---

-7.39

0.48

37

-6.93

0.18

14

-5.13

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8.23

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LaZnAsO

ΔE(EAFM-EFM)

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energies (eV)

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moments (B)

c (Å)

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formation

a (Å)

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Atomic magnetic

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Figure captions Figure 1. Schematic illustration of 2×2×1 supercell of LaZnAsO. Figure 2. The total DOS and the corresponding PDOS of LaZnAsO. Figure 3. The TDOS of TM-doped LaZnAsO and PDOS of TM atom, (a)-(f) represent V, Cr, Mn,

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Fe, Co and Ni doped LaZnAsO, respectively.

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Figure 4. The spin density isosurface (isosurface value = 0.01 e/Å3) of TM-doped LaZnAsO, (a)-(f)

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represent V, Cr, Mn, Fe, Co and Ni doped LaZnAsO, respectively. Blue indicates the positive spin

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values and yellow expresses the negative.

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Figure 1. By Tao et al.

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Figure 2. By Tao et al.

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Figure 3. By Tao et al.

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Figure 4. By Tao et al.

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The TM-doped (TM = V, Cr, Mn, Fe and Co) LaZnAsO are magnetic.

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The Cr, Fe and Co-doped LaZnAsO preferred FM states.

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The V and Mn doped LaZnAsO exhibit an AFM ground state.

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TM atoms provided the magnetic moment forming the TM-As-TM chain with the p-d

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hybridization coupling.

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Declaration of interests

✓ The authors declare that they have no known competing financial interests or personal relationships

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that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as

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potential competing interests:

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