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Author's personal copy ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) L25–L27
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Letter to the Editor
Magnetic structure of double perovskites Ca2MWO6 (M=Co, Ni): A first principles study Alo Dutta n, T.P. Sinha Department of Physics, Bose Institute, 93/1 Acharya Prafulla Chandra Road, Kolkata—700009, India
a r t i c l e in fo
abstract
Article history: Received 5 October 2009 Received in revised form 6 January 2010 Available online 20 January 2010
First principles calculations have been performed to study the electronic and magnetic structures of double perovskites Ca2MWO6 (M= Co, Ni) using full potential linearized augmented plane wave method. The density of states and spin magnetic moments are calculated and we have examined the valence states of Co, Ni and W ions. The results predict the half-metallic ground state of Ca2CoWO6 and the insulating nature of Ca2NiWO6. & 2010 Elsevier B.V. All rights reserved.
Keywords: FPLAPW Magnetic structure Density of state Magnetic moment
1. Introduction Double perovskite, a broad class of compounds, has been studied since the 1960s [1–3]. It has the general formula A2B0 B00 O6, where A can be an alkaline earth metal such as barium, strontium, calcium, or a lanthanide, and B0 and B00 are usually transition metals. Each transition metal site is surrounded by an oxygen octahedron, and the A atoms are situated in holes produced by eight adjacent oxygen octahedral. More than 300 compounds with double perovskite structure have been synthesized [4–11]. These materials have attracted intense research activities in many applied and fundamental areas of solid state science and advanced materials research due to exotic properties such as high temperature superconductivity [12], colossal magnetoresistance [13], half-metallicity [14] and magnetodielectricity [15]. In particular, much effort has been devoted in the past years to study the ferromagnetic double perovskites of formula A2FeMoO6 (A =Ba, Sr, Ca) initiated by the work of Kobayashi et al. [16], where a convincing half-metallic behaviour in Sr2FeMoO6 is observed. The calculated electronic structure of this compound shows a gap in the up spin channel, while for the down spin channel one observes strongly hybridized Fe 3d(t2g), Mo 4d(t2g), and O 2p states at the Fermi level. Das et al. [17] have studied the electronic structure and dielectric anomaly in ferromagnetic insulator La2NiMnO6 by first principles density functional calculation. The existence of very soft infrared active phonons that couple strongly with spins at the Ni and Mn sites has been predicted.
n
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[email protected] (A. Dutta).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.01.019
In spite of many research in this field, there are many open issues of fundamental importance concerning the electronic and magnetic structures of double perovskite compounds. In this work, we have reported the magnetic structure of Ca2MWO6 (M= Co, Ni) having monoclinic (P21/n) structure [18] by first principles calculations using full potential linearized augmented plane wave method. Due to the small size of the Ca2 + cation the perovskite structure is fairly distorted, which may force the Co(Ni)O6 and WO6 octahedra to tilt in order to optimize the Ca–O bond distances. The Co(Ni)O6 and the WO6 octahedra are fully ordered and alternating along the three directions of the crystal, in such a way that each Co(Ni)O6 octahedron is linked to six WO6 octahedra. The calculation of electronic structure for these complex perovskite oxides can be seen as a many body problem of ions and electrons.
2. Calculation method The density of states of the compounds is obtained by the full potential linearized augmented plane wave method, in the framework of density functional theory (DFT) [19] as implemented in Wien 2k code [20]. The exchange and correlation effects are treated by using the generalized gradient approximation (GGA) [21]. We have performed our calculations using the experimentally determined structure and the atomic positions [18]. Spin polarization has been taken into account. We have used 42k points in the irreducible Brillouin zone, and the muffin-tin radii for Ca, Co, Ni, W and O are 2.2, 2.0, 2.0, 2.0 and 1.6 a.u., respectively. The density plane cut-off Rkmax is 7.0, where kmax is the plane wave cut-off and R is the smallest of all atomic sphere
Author's personal copy ARTICLE IN PRESS L26
A. Dutta, T.P. Sinha / Journal of Magnetism and Magnetic Materials 322 (2010) L25–L27
radii. The self-consistency is better than 0.001 e/a.u.3 for charge density and spin density, and the stability is better than 0.01 mRy for the total energy per cell.
24 16 8 0 −8 −16 −24 0.4
3. Results and discussion
24 16 8 0 −8 −16 −24
Ca2CoWO6
0.3
Ca-p
DOS (Sates /eV cell)
−0.3
Co-d
0.5
O-p
t2g
W-d
eg eg
t2g
0.0 −0.5 -15
-10
0.0 −0.2 −0.4 6 4 2 0 −2 −4 −6 4
t2g
Ni-d
eg eg
t2g W-d
2
t2g
eg
t2g
eg
0 −2 −4 1.0
O-p
0.5 0.0 −0.5 −1.0 -20
-15
-10
-5
0
5
10
Energy (eV) Fig. 2. Total and partial densities of states of Ca-p, Ni-d, W-d and O-p for Ca2NiWO6.
-5 0 Energy (eV)
Components
Ca2CoWO6
Ca2NiWO6
Ca Co Ni W O1 O2 O3
0.00036 2.54357
0.00098
Total
t2g e g t2g eg
-20
Ca-p
Table 1 Magnetic moment of atoms in compounds under study.
0.0
4 2 0 2 4 4 2 0 −2 −4
0.2
DOS (Sates /eV cell)
The spin polarized total density of states (DOS) per formula unit of Ca2CoWO6 along with the partial density of states (PDOS) of Ca-p, Co-d, W-d and O-p is shown in Fig. 1. The half-metallic character is reflected in the DOS for Ca2CoWO6 where the up spin channel maintains a band gap of size 1.8 eV and down spin channel shows conducting behaviour. The occupied part of the bands near the Fermi level in the down spin channel is mainly composed of Co d character, which hybridizes with the oxygen p states. Fig. 2 shows the spin polarized total DOS per formula unit of Ca2NiWO6 along with the PDOS of Ca-p, Ni-d, W-d and O-p. In the up spin channel the DOS shows an insulating behaviour with a gap size of 1.97 eV between the uppermost valence band and lowermost conduction band. It is observed in Figs. 1 and 2 that the valence band of Ca2MWO6 consists predominantly of O 2p states hybridized with Co(Ni) d states with a small admixture of W d states. It is observed in Fig. 1 that the crystalline field produced by the oxygen octahedra evidences a splitting between t2g and eg states of Co and W in Ca2CoWO6. Exchange splittings between t2g up and t2g down, and between eg up and eg down are also observed for Co ion, whereas for W no exchange splitting is observed due to the complete overlapping of spin up and spin down states. This indicates the non-magnetic behaviour of W, and suggests the W6 + (5d0) valence state in this compound. The exchange splitting between eg up and eg down states for Co is 2.79 eV, and the exchange splitting between t2g up and t2g down states is 1.53 eV.
Ca2NiWO6
5
10
Fig. 1. Total and partial densities of states of Ca-p, Co-d, W-d and O-p for Ca2CoWO6.
0.00482 0.06291 0.06454 0.06428
1.55868 0.03429 0.06221 0.06110 0.06739
5.99963
3.99683
The d-states of Ni and W in Ca2NiWO6 also split up into t2g and eg states due to the crystal field. It is observed in Fig. 2 that for Ni ion there is an exchange splitting between t2g up and t2g down, and between eg up and eg down states but the spin-up and spin-down states of W are completely overlapped showing the non-magnetic behaviour of W6 + (5d0) valence state in this compound. The exchange splitting between eg up and eg down states for Ni is 1.83 eV, whereas the exchange splitting between t2g up and t2g down states is 1 eV. From the above analysis it is found that the occupations for Co are t52ge2g and for Ni are t62ge2g , which further indicates the Co2 + and Ni2 + valence states in these materials. In Table 1 we have summarized the total and species decomposed spin moments of these compounds. It is observed that the magnetic moments of Co and Ni are somewhat smaller than the corresponding experimental values [18]. This difference is due to the additional magnetic moments distributed at the oxygen sites and the
Author's personal copy ARTICLE IN PRESS A. Dutta, T.P. Sinha / Journal of Magnetism and Magnetic Materials 322 (2010) L25–L27
interstitial region. The magnetic moment data also show strong evidence for Co2 + and Ni2 + in these oxides. We have observed that the sum of the magnetic moment of the constituent atoms is not equal to the total magnetic moment of the materials. The reason for this is that when we use full potential linearized augmented plane wave method to calculate the electronic and magnetic structure of the compounds, we use the muffin-tin spheres where some of the area are excluded, so the sum of the magnetic moment of the constituent atoms is always less than the total magnetic moment of the compounds.
4. Conclusion We have analyzed the electronic structure of Ca2MWO6 (M =Co, Ni) by full potential linearized augmented plane wave method. The ground state of Ca2CoWO6 is half-metallic while Ca2NiWO6 shows the insulating ground state. Co and Ni have great effect on the magnetic property of these compounds, because of the hybridization between d-states of metal and p-states of oxygen. The valence states of Co, Ni and W are found to be 2+ , 2+ and 6 +, respectively, in these materials.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgement This work is financially supported by the Department of Science and Technology of India under Grant no. SR/S2/CMP-01/ 2008. Alo Dutta acknowledges the financial support provided by the CSIR, New Delhi, in the form of SRF (Extended).
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[18] [19] [20]
[21]
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