Pis'ma v ZhETF, vol. 101, iss. 12, pp. 897-901
© 2015 June 25
The ferromagnetic origin of Mn-doped Sr3La20sZn2As2:
a first-principles study
L.Hual\ Q.L.Zhu
Nanjing Normal University Taizhou College, 225300 Taizhou, China Submitted 6 May 2015
We have investigated the electronic structure and magnetic properties of Mn-doped Sr3La20sZn2As2 using density functional theory within the generalized gradient approximation (GGA)+U schemes. We have shown that the ground state magnetic structure of Mn-doped Sr3La20sZn2As2 is antiferromagnetic. Zn vacancies and hole-mediated Zener's p—d exchange are responsible for the origin of ferromagnetism.
DOI: 10.7868/S0370274X1512005X
1. Introduction. The dilute magnetic semiconductors (DMS) are nonmagnetic semiconductors doped with magnetic elements, which display ferromagnetic ordering below the Curie temperature (Tc). Mn-doped GaAs is the most investigated DMS and its magnetic properties are well established experimentally [1-3]. In (Ga,Mn)As the Mn substituting for the trivalent Ga cation acts as both an acceptor and a spin source of magnetic moments simultaneously. Moreover, during the fabrication of (Ga,Mn)As thin films, some Mn atoms enter interstitial sites and behave as a double donor, which make it difficult to determine precisely the amount of Mn substituting Ga at ionic sites. Seeking for the new ferromagnetic semiconductor systems with more controllable charge and spin densities might be helpful to understand the general mechanism of ferromagnetism in DMS.
A new type of ferromagnetic DMS that overcomes these difficulties has been discovered recently. Bulk specimens of Li(Zn,Mn)As were successfully fabricated, using excess Li concentrations introduced into hole carriers, while independently making the isovalent substitution of Mn2+ for Zn2+ to achieve local spin doping [4]. Shortly after, Li(Zn,Mn)P with Tc ~ 40 K were also fabricated by doping Mn into the I-II-V direct gap semiconductors LiZnP [5]. LiZnPn (Pn = P, As) can be viewed clS cl derivative of the third family of Fe-based superconductors LiFeAs. Accordingly, the first family of Fe-based superconductor is 1111-type oxypnictides LaFeAs(Oi-a;Fa;). With identical Two-dimensional crystal structure, three 1111 type DMS systems, (La,Ba)(Zn,Mn)AsO with Tc ~ 40K [6], (La,Ca)(Zn,Mn)SbO with Tc ~ 40K [7], (La,Sr)(Cu,Mn)SO with Tc ~ 210K [8] have
^e-mail: pjsd@163.com
been reported. Similarly, two bulk form DMS systems, (Ba,K)(Zn,Mn)2As2 [9] with Tc ~ 180 K and (Ba,K)(Cd,Mn)2As2 [10] with Tc ~ 17 K, have been reported. These two systems are structurally identical to that of 122 type iron pnictides superconductor (Ba,K)Fe2As2. The fourth family of Fe-based superconductors is 11 type FeSei+a, which can be paralleled to the well investigated II-VI DMS, i.e., (Zn,Mn)Se. There are two more families of Fe-based superconductors, namely, 32522 type (Ca3Al2C>5)Fe2As2 and 42622 type Sr4V206Fe2As2. Very recently, 32522 type DMS Sr3La205(Zn,Mn)2As2 with Tc ~ 40 K has been reported [11]. In this paper, we have performed a first-principles density functional theory study on Sr3La2C>5(Zn,Mn)2As2 and discussed its ferromagnetic origin.
2. Computational method. The first-principles calculations were performed by using density functional theory (DFT) method within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) [12], implemented in the Vienna ab initio Simulation Package (VASP) [13]. The strong-correlated correction was considered with GGA+U method [14] to deal with the Mn's 3d electrons. The effective onsite Coulomb interaction parameter (U) and exchange interaction parameter (J) are set to be 4.0 and 1.0 eV for Mn's 3d electrons. These values have been tested and used in the previous experimental and theoretical works [15-17]. For Zn atoms, the strong-correlated correction was not applied as their 3d orbitals are fully occupied. 4s4p5s for Sr, 5p5d6s for La, 2s2p for O, 3dAs for Zn, 3d4s for Mn, and 4s4p for As were treated as valence orbitals in the calculations. The projector augmented wave (PAW) potential [18] and the plane waves cut-off energy of 300eV were used. For Sr3La20sZn2As2 unit cell, a T-centered Monkhorst-Pack [19] A;-point mesh of
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5x5 x 1 was used and the internal atomic coordinates were relaxed until the force was less than 0.01 eV/A. For Mn-doped Sr3La2C>5Zn2As2 supercell, a T-centered Monkhorst-Pack fc-point mesh of 3x3x 1 was used. The criterion for the total energy was set as 10~4eV.
3. Results and discussion. As a parent system, Sr3La205Zn2As2 crystallizes in the tetragonal CagAl205Fe2As2-type crystal structure with the space group IA/mmm, as shown in Fig. 1. The experimen-
Fig. 1. The crystal structure of the Sr3La2C>5Zn2As2 unit cell. Blue, dark yellow, magenta, red and olive spheres represent Sr, La, O, Zn, and As atoms, respectively
Fig. 2.
The
2x2x1 supercell of
Sr3La.o05 (Zno.875Mno.i25)2 Aso. The configurations of Mn-Mn pair indicate Mnl-Mn2 and Mnl-Mn3. For seeing clearly, we omit the Sr, La, and O atoms
romagnetic (FM) and antiferromagnetic (AFM) state for the first nearest neighboring and the second nearest neighboring configuration.
Table 1
The total energy (eV) of different configuration of ferromagnetic and antiferromagnetic state of Sr3 La2 05(Zno.8T5Mno. 125)2 AS2
Configuration Epm Eafm Eafm-Efm
Mnl—Mn2 -640.1453 -640.3147 -0.1694
Mnl—Mn3 -640.2163 -640.3094 -0.0931
tally measured lattice constant of Sr3La2C>5Zn2As2 (a = 4.2612 A, c = 27.675 A) was used in our calculations [11] and the internal coordinates were optimized. We have used two Mn atoms to substitute two Zn atoms in a 2x2x1 supercell for constructing Sr3La2C>5(Zno.875Mno.i25)2As2, as shown in Fig.2. We have calculated the first nearest neighboring (indicating Mnl-Mn2) and the second nearest neighboring (indicating Mnl-Mn3) pair configuration. Table 1 lists the energy of the Sr3La2O5(Zn0.875Mn0.i25)2As2 with fer-
Our calculations show that the energy of AFM state is lower than FM for Mnl-Mn2 and Mnl-Mn3 configuration. This means that the ground state of Sr3La20s(Zno.875Mno.i25)2As2 should be AFM state. In Fig. 3, we show the spin-polarized DOS Of Sr3La205(Zno.875Mno.l25)2As2 of Mnl-Mn2 AFM configuration. We see that the contributions from the Mn 4s states to the valence bands of Sr3La2 05 (Zno 875M110 125 )2As2 are negligible. So the
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The ferromagnetic origin of Mn-doped SisLaoOsZnoAso
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Fig. 3. The total averaged (black line) density of states of Sr3La2C>5(Zno.875Mno.i25)2As2 per unit cell. The partial density of states of Mnl 3d (red line), 4s (blue line) and Mn2 3d (green line), 4s (blue line) per atom. The energy zero is taken at the Fermi level and indicated by the vertical line
Mn atoms are in the form of cation Mn2+. The Mnl 3d major states are occupied and minor states empty while Mn2 3d states are reverse. This means that the superexchange mechanism [20] leads to the antiferromagnetic coupling between the two Mn atoms. From Fig. 3, we also see that the exchange splitting, due to the Mn 3d electrons localizing and onsite coulomb interaction, is larger than the crystal filed splitting. This leads to a high spin configuration of d5 with the local magnetic moments of Mn atoms with S = 5/2, which is consistent with our calculated moment 4.15yitB- However, the experiments [11] show that Sr3La2O5(Ziii_xMii102As2 (x = 0.05, 0.1, 0.2) is FM state with Tc ~ 40 Iv. Taking into account the appearance of secondary phases of Z113AS2 and S^As for x < 0.1, we speculate that the different magnetic property between experimental samples and our calculated pure Mn-doped SrgLa2 05Zii2As2 may come from the intrinsic defects in the experimental samples. Because the carriers type is not accurately determined by experiments [11], we introduce electron and hole carriers doping into Sr3La2C>5(Ziio.875Miio.i25)2As2, respectively. In Fig. 4, the total energy differences between AFM and FM states are plotted versus the concentration of doped carriers. The positive and negative values are for electron and hole doping, respectively. As shown in the Fig. 4, independent of the Mn-Mn configuration, the AFM state is stable with electron carriers. However, as for hole carriers, increasing the hole doping concentration induces a AFM-FM transition in the Sr3La205(Zno.875Mno.i25)2As2 com-
-1 0 1 Carrier per supercell
Fig. 4. The total energy difference between AFM and FM states with the concentration of doping carriers for the Mnl-Mn2 (black line) and Mnl-Mn3 (red line) configurations
pound. We think that the hole carriers induce the FM state, which has been observed experimentally in the Mn-doped Sr3La205Zii2As2. In order to see the origin of the carrier induced ferromagnetism. The total and partial density of states of Mn 3d, As 4p, and O 2p states are shown in Fig. 5. The DOS in Fig. 5 shows
Fig. 5. The total averaged (black line) density of states of Sr3La2C>5(Zno.875Mno.i25)2As2 and the partial density of states of O ~2p (blue line), As 4p (green line) with hole carriers per unit cell. The partial density of states of Mn 3d (red line) per atom. The energy zero is taken at the Fermi level and indicated by the vertical line
the semi-metallic behavior for the compound at the Fermi level. The hole carriers are introduced into the empty states near the Fermi level, which is mostly composed of the As Ap states. So the delocalized holes have a character of the host states near the top of the valence band with a small admixture of the Mn 3d
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orbital weight. At the same time, the main peak in the partial density of states of the majority spin Mn 3d5 electrons is well below the Fermi lev
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