ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1816–1817
Half-metallic transition metal oxides Z. Szoteka,*, W.M. Temmermana, A. Svaneb, L. Petitb, G.M. Stocksc, H. Winterd a Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark c Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA d INFP, Forschungszentrum Karlsruhe GmbH, Postfach 3640, D-76021 Karlsruhe, Germany b
Abstract We present an application of the self-interaction corrected local spin density approximation to study the electronic structure of half-metallic double perovskites Ba2 FeMoO6 ; Ca2 FeMoO6 ; Sr2 FeMoO6 ; Ca2 FeReO6 ; and charge order in Fe3 O4 : r 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Gg; 75.47.Pq Keywords: Half metals
Interest in ordered double perovskites has started with the discovery of the room-temperature collosal magnetoresistance (CMR) in Sr2 FeMoO6 [1]. The high transition temperature Tc and low field magnetoresistance indicate half-metallic behaviour in this compound. In calculations it is seen through a well defined gap in the majority spin channel, and metallic behaviour in the minority spin channel, with strongly hybridized bands of Fe 3d ðt2g Þ; Mo 4d ðt2g Þ; and O 2p character at the Fermi level, making it a system of interest for spinelectronics, utilizing both the spin and charge of electrons. Based upon its high magnetoresistive properties, also magnetite ðFe3 O4 Þ is of interest for technological applications, as e.g. computer memory, magnetic recording, etc. Magnetite is thought to be half-metallic, with a highest known Tc of 860 K; but at about TV ¼ 122 K it undergoes a transition to an insulating state, associated with some kind of charge order, setting in on the octahedral sites, and lattice distortion from cubic to monoclinic [2,3]. In this paper we apply the self-interaction corrected (SIC) local spin density (LSD) [4] to study Ba2 FeMoO6 ; Ca2 FeMoO6 ; Sr2 FeMoO6 ; and Ca2 FeReO6 compounds, concentrating on Fe valence and the electronic and *Corresponding author. Tel.: +44-1925-603227. E-mail address:
[email protected] (Z. Szotek).
magnetic properties of these compounds [5]. In magnetite we study charge order postulated by Verwey [2] who argued that below TV the Fe3þ and Fe2þ cations order on the octahedral sites in alternate (0 0 1) planes, respectively, and he interpreted this transition as an electron localization-delocalization transition. The SICLSD method, treating both localized and delocalized electrons on equal footing and allowing to realize various valence configurations in an ab-initio manner, is a suitable approach for studying a possible charge order in this system [4,6]. The SIC-LSD calculations find all the double perovskites studied to have half-metallic groundstate, with 100% spin-polarized conduction electrons [5]. In Table 1, the half-metallic character is reflected in the total spin magnetic moments that are all integer. The moments induced by hybridization on the Mo sites are of similar magnitude, about 0:4 mB ; independently of the alkaline earth element, while that of Re is of the order of 1:1 mB : These induced spin magnetic moments are antiparallel aligned with the Fe spin moment. It is interesting to see how little the magnetic properties of these compounds are affected by the kind of the alkaline earth atom, and the substantial change in their volumes. Also densities of states are very similar, showing a gap for majority spin states and strongly hybridized states for the other spin at the Fermi level. Concerning the Fe
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.818
ARTICLE IN PRESS Z. Szotek et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1816–1817
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Table 1 Total and species decomposed spin magnetic moments (in mB ) as calculated self-consistently within SIC-LSD band structure method. In the last row the respective volumes per formula unit are quoted (in (atomic unit)3 ). Here AE stands for alkaline earth, namely Sr, Ca or Ba Moment
Ca2 FeReO6
Ca2 FeMoO6
Sr2 FeMoO6
Ba2 FeMoO6
Mtotal MFe MMo MRe MAE MO1 MO2 MO3 Volume
3.00 3.87 — 1.12 0.02 0.02 0.01 0.11 773.8
4.00 3.76 0.40 — 0.01 0.10 0.11 0.11 777.5
4.00 3.71 0.43 — 0.02 0.11 0.11 — 830.2
4.00 3.81 0.41 — 0.02 0.09 — — 884.0
valence, for all the compounds the trivalent configuration has been most favourable, followed by the tetravalent and divalent (in this case all the compounds are found to be insulating) ones. For the magnetite we have studied three different arrangements of divalent and trivalent Fe cations on the octahedral sites of the cubic (high temperature phase) and monoclinic (low temperature phase) structures, while keeping the tetrahedral sites always trivalent, trying to establish whether the Verwey charge order is the lowest energy solution. Apart from the Verwey charge order, we have studied the case where, like the tetrahedral sites, all the octahedral sites are occupied by Fe3þ ; and finally the scenario with all the octahedral sites occupied by the Fe2þ ions. Our total energy calculations, both for the cubic and monoclinic structures, have shown that not the Verwey phase, but the scenario with all interstitial sites occupied by Fe3þ ; is the ground state, followed by the Verwey phase, and then the all Fe2þ scenario. Thus, we conclude that, if at all, the charge order must be much more complex than postulated by Verwey. In the Verwey phase, we have calculated magnetite to be insulating with a gap of B0:35 eV in cubic phase (B0:1 eV in monoclinic phase), while the Fe3þ scenario, depending on structure, has given rise to half-metallic or metallic (though with high spin polarization) solution. The total spin magnetic
moment of 4 mB per formula unit is mostly due to Fe2þ cations since the tetrahedral and octahedral sublattices are antiferromagnetically aligned [6]. This work was partially sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC05-00OR22725. The computational work was partly performed at the Center for Computational Sciences (CCS) at ORNL and at NERSC at LBNL.
References [1] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [2] E.J.W. Verwey, P.W. Haayman, Physica (Utrecht) 8 (1941) 979. [3] F. Walz, J. Phys.: Condens. Matter 14 (2002) R285. [4] W.M. Temmerman, et al., in: J.F. Dobson, G. Vignale, M.P. Das (Eds.), Electronic Density Functional Theory: Recent Progress and New Directions, Plenum Press, New York, London, 1998. [5] Z. Szotek, W.M. Temmerman, A. Svane, L. Petit, H. Winter, Phys. Rev. B 68 (2003) 104411. [6] Z. Szotek, W.M. Temmerman, A. Svane, L. Petit, G.M. Stocks, H. Winter, cond-mat/0302229.
