Chemical Bonding Center | Research Theme IV: Ferromagnetic insulators and magnetic ferroelectrics (multiferroics)

The two B-sites in double perovskites A2BB'O₆ can be effectively exploited to combine magnetic and dielectric behavior. A number of new quintinary perovskites with general composition AA'MM'O₆ (A = Li⁺, Na⁺, K⁺; A' = La³⁺, Y³⁺; M = Mg²⁺, Sc³⁺; M' = W⁶⁺, Nb⁵⁺, Sb⁵⁺) have been synthesized; these are particularly promising for combining properties by incorporating di®erent ions.

References

[1] D. D. Awschalom, M. E. Flatté, and N. Samarth. Spintronics. Sci. Am. (Int. Ed.), 286:66, 2002.

[2] B. T. Matthias, R. M. Bozorth, and J. H. Van Vleck. Ferromagnetic interaction in EuO. Phys. Rev. Lett., 7:160-161, 1961.

[3] J. D. Garrett, J. E. Greedan, and D. A. MacLean. Crystal growth and magnetic anisotropy of YTiO₃. Mater. Res. Bull., 16:145-148, 1981.

[4] K. Kohn, K. Inoue, O. Horie, and S.-I. Akimoto. Crystal chemistry of MSeO₃ and MTeO₃ (M = Mg, Mn, Co, Ni, Cu, and Zn). J. Solid State Chem., 18:27-37, 1976.

[5] H. Chiba, T. Atou, and Y. Syono. Magnetic and electrical properties of Bi_1-xSr_xMnO₃: hole-doping effect on ferromagnetic perovskite BiMnO₃. J. Solid State Chem., 132:139-143, 1997.

[6] T. Dietl, H. Ohno, F. Matsukura, J. Cibèrt, and D. Ferrand. Zener model description of ferromagnetism in zinc-blende magnetic semiconductors. Science, 287:1019, 2001.

[7] A. S. Risbud, N. A. Spaldin, Z. Q. Chen, S. Stemmer, and R. Seshadri. Magnetism in polycrystalline cobalt-substituted zinc oxide. Phys. Rev. B, 68:205202, 2003.

[8] G. Lawes, A. S. Risbud, A. P. Ramirez, and R. Seshadri. Magnetic properties of Co and Mn substituted ZnO in bulk samples, 2004.

[9] N. A. Spaldin. Search for ferromagnetism in transition-metal-doped piezoelectric ZnO. Phys. Rev. B, 69, 2004.

[10] M. A. Hayward, E. J. Cusses, J. B. Claridge, M. Bieringer, M. J. Rosseinsky, C. J. Kiely, S. J. Blundell, I. M. Marshall, and F. L. Pratt. The hydride anion in an extended transition metal oxide array: LaSrCoO₃H_0.7. Science, 295:1882-1884, 2002.

[11] M. A. Hayward, M. A. Green, M. J. Rosseinsky, and J. Sloan. Sodium hydride as a powerful reducing agent for topotactic oxide deintercalation: Synthesis and characterization of the nickel (I) ioxide LaNiO₂. J. Am. Chem. Soc., 38:8843-8854, 1999.

[12] N. A. Hill. Why are there so few magnetic ferroelectrics? J. Phys. Chem. B, 104(29):6694-6709, 2000.

[13] N. A. Hill and K. M. Rabe. First principles investigation of ferromagnetism and ferroelectricity in BiMnO₃. Phys. Rev. B, 59:8759-69, 1999.

[14] R. Seshadri and N. A. Hill. Visualizing the role of Bi 6s ``lone pairs'' in the off-center distortion in ferromagnetic BiMnO₃. Chem. Mater., 13:2892-2899, 2001.

[15] A. M. dos Santos, S. Parashar, A. R. Raju, Y. S. Zhao, A. K. Cheetham, and C. N. R. Rao. Evidence for the likely occurrence of magnetoferroelectricity in the simple perovskite, BiMnO₃. Solid State Commun., 122:49-52, 2002.

[16] A. M. dos Santos, A.K. Cheetham, T. Atou, Y. Syono, Y. Yamaguchi, K. Ohoyama, H. Chiba, and C. N. R. Rao. Orbital ordering as the determinant for ferromagnetism in biferroic BiMnO₃. Phys. Rev. B, 66:064435, 2002.

[17] E. C. Subbarao. Ferroelectric and antiferroelectric materials. Ferroelectrics, 5:267-280, 1973.


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Theme IV: Ferromagnetic insulators and magnetic ferroelectrics (multiferroics) Theme Leader: Seshadri The recent research interest in the field of spin-based electronics (or spintronics)[1] has generated a demand for multifunctional materials that combine ferromagnetism with additional desirable properties, such as semiconducting transport or large magneto-optical response. This theme of the CDM focuses on the particularly challenging task of combining ferromagnetism with insulating behavior. In addition to having potential spintronic applications, strong insulation is also a prerequisite for polarity, since conductors cannot support electric polarization. The task is challenging because ferromagnetic insulators are scarce; among simple oxides, they include the f-electron system EuO, [2] (T_C = 79 K) and the Jahn-Teller (orbital-ordered) systems YTiO₃[3] (T_C = 29 K), SeCuO₃[4] (T_C = 29 K) and BiMnO₃[5] (T_C = 105 K). None of these compounds can be used at room temperature, since their T_Cs are near 100 K or below. The recent suggestion by Dietl and coworkers[6] that high-temperature (near the room temperature) ferromagnetism can be induced in wide band-gap semiconductors such as ZnO has so far not been realized[7,8,9]. Clearly new ways of inducing ferromagnetism in insulating compounds are needed. 2 K magnetization of the Dilute ferrimagnetic Semiconductor spinel [ZnGa₂O₄]_{1-x}[Fe₃O₄]_x with _x = 0.15. This compound maintains its optical band gap and yet shows long-range magnetic ordering at the room temperature. The inset shows a portion of the cation substructure of spinel, with octahedral (blue) and tetrahedral (black) cations. The first focus in this theme will be to develop new magnetic materials that order at temperatures near or above the room temperature, and yet are insulating. Seshadri has already made progress in the development of new wide band-gap ferrimagnets. The magnetic interactions in a ferrimagnet are antiferromagnetic; such interactions are robust in insulating oxides. As part of this task we will develop guidelines for controlling magnetic superexchange interactions through suitable modifications at the anion site. The fact that anions can be used to tune exchange interactions has been demonstrated by Rosseinsky and coworkers[10], who incorporated hydride anions in oxides and, as a result, significantly modified the magnetic exchange pathways. Making such hydrides requires the use rather sophisticated synthetic chemistry skills; reactions of oxides with metal hydride reductants at relatively low (below 573 K) temperatures[10,11]. Anion control of magnetism will complement anion control of polarity and thereby spawn a novel way of designing oxide materials. Concurrently with the search for new magnetic insulators, we will also explore the coexistence of magnetism and polarity. This is a difficult problem because the most widely occurring SOJT mechanism, d⁰-ness on the cation, is precluded in ferromagnetic ferroelectrics, because magnetism requires partially filled d or f states. This explains the scarcity of ferromagnetic ferroelectrics[12]. Instead, we will incorporate other mechanisms for polar behavior that are compatible with ferromagnetism. Existing mechanisms rely on electronic (such as the stereochemically active lone pair) or structural (such as intrinsically polar polyhedra) driving forces. In addition we will search computationally for as yet unidentified mechanisms for off-centering. Our earlier work in this area is highlighted by Spaldin's successful prediction of a ferromagnetic ferroelectric, BiMnO₃[13], in which the stereochemical activity of the Bi lone pair is exploited[14]. This material was subsequently synthesized and characterized[15,16] and shown to be ferromagnetic and ferroelectric as expected. However the samples were leaky and not of high enough quality for device applications. For further improvement of ferroelectric ferromagnets and discovery of new materials, we will pursue the incorporation of magnetic cations (d³ and d⁵), which are less prone to oxidation and reduction and therefore likely to form more insulating samples. Seshadri and Woodward will develop improved synthesis methods for stoichiometric samples, a prerequisite for strongly insulating behavior. Ferroelectric switching in the polar materials will be examined experimentally by Stemmer. Seshadri and Salvador will explore layered materials, such as the Aurivillius phases[17], using both traditional synthetic and thin film techniques. Aurivillius phases, such as Bi₄Ti₃O₁₂, have alternate Bi₂O₂ and perovskite blocks, and polar behavior arises from the lone-pair Bi₂O₂ block. This frees up the perovskite Ti-O block for substitution of magnetic ions. This approach constitutes an as-yet-unexplored avenue in the synthesis of multifunctional materials. Current reseach activities: Seshadri group undergraduate intern, Brent Mellot, has been exploring the formation of magnetic double per ovskites with the formula A₂BB'O₆ where B is a non-magnetic, "d⁰" cation (and therefore can contribute to ferroelectricity), while B' is a magnetic ion. To optimize insulting behavior, d³ Cr(III) was chosen for B' in the starting model compound; in a magnetic and octahedral environment, this is likely to give rise to a band insulator. Characterization of these phases, which suffer from antisite disorder where the B and B' ions sit in each others' sites, is diffcult. Seshadri has had a major breakthrough on this front, by devising a new way of treating X-ray diffraction data from these phases that permit such disorder to be quantified. Efforts are now in place to correlate the antiferromagnetic Néel ordering with antisite disorder. Following the theoretical predictions of Spaldin's group (described in Theme VI), that ferrimagnetic materials should form good multiferroics, Seshadri has extended his studies to ferrimagnetic insulators, and prepared the spinel compound CoCr₂O₄. They have determined that it is a novel example of a ferrimagnetic semiconductor, and neutron and magnetotransport studies of this material are underway. Seshadri has also started exploring an entirely new paradigm for developing ferromagnetic insulators (which was not suggested in the original proposal); that is by making porous magnetic materials. This is a previously completely unexplored area of functional materials. Graduate student Eric Toberer in the Seshadri group has made progress in novel preparative routes for porous magnetic materials. One publication has been submitted on this topic during the this reporting period, and is currently under review at Advanced Materials. Reseach findings: The two B-sites in double perovskites A2BB'O₆ can be effectively exploited to combine magnetic and dielectric behavior. A number of new quintinary perovskites with general composition AA'MM'O₆ (A = Li⁺, Na⁺, K⁺; A' = La³⁺, Y³⁺; M = Mg²⁺, Sc³⁺; M' = W⁶⁺, Nb⁵⁺, Sb⁵⁺) have been synthesized; these are particularly promising for combining properties by incorporating di®erent ions. Appropriate interpretation of X-ray diffraction data allows B-site disorder in such structures to be quantifid. Porous magnetic materials offer exciting new possibilities for designing magnetic insulators. The spinel compound CoCr,sub>2O₄ is a novel ferrimagnetic semiconductor. References [1] D. D. Awschalom, M. E. Flatté, and N. Samarth. Spintronics. Sci. Am. (Int. Ed.), 286:66, 2002. [2] B. T. Matthias, R. M. Bozorth, and J. H. Van Vleck. Ferromagnetic interaction in EuO. Phys. Rev. Lett., 7:160-161, 1961. [3] J. D. Garrett, J. E. Greedan, and D. A. MacLean. Crystal growth and magnetic anisotropy of YTiO₃. Mater. Res. Bull., 16:145-148, 1981. [4] K. Kohn, K. Inoue, O. Horie, and S.-I. Akimoto. Crystal chemistry of MSeO₃ and MTeO₃ (M = Mg, Mn, Co, Ni, Cu, and Zn). J. Solid State Chem., 18:27-37, 1976. [5] H. Chiba, T. Atou, and Y. Syono. Magnetic and electrical properties of Bi_1-xSr_xMnO₃: hole-doping effect on ferromagnetic perovskite BiMnO₃. J. Solid State Chem., 132:139-143, 1997. [6] T. Dietl, H. Ohno, F. Matsukura, J. Cibèrt, and D. Ferrand. Zener model description of ferromagnetism in zinc-blende magnetic semiconductors. Science, 287:1019, 2001. [7] A. S. Risbud, N. A. Spaldin, Z. Q. Chen, S. Stemmer, and R. Seshadri. Magnetism in polycrystalline cobalt-substituted zinc oxide. Phys. Rev. B, 68:205202, 2003. [8] G. Lawes, A. S. Risbud, A. P. Ramirez, and R. Seshadri. Magnetic properties of Co and Mn substituted ZnO in bulk samples, 2004. [9] N. A. Spaldin. Search for ferromagnetism in transition-metal-doped piezoelectric ZnO. Phys. Rev. B, 69, 2004. [10] M. A. Hayward, E. J. Cusses, J. B. Claridge, M. Bieringer, M. J. Rosseinsky, C. J. Kiely, S. J. Blundell, I. M. Marshall, and F. L. Pratt. The hydride anion in an extended transition metal oxide array: LaSrCoO₃H_0.7. Science, 295:1882-1884, 2002. [11] M. A. Hayward, M. A. Green, M. J. Rosseinsky, and J. Sloan. Sodium hydride as a powerful reducing agent for topotactic oxide deintercalation: Synthesis and characterization of the nickel (I) ioxide LaNiO₂. J. Am. Chem. Soc., 38:8843-8854, 1999. [12] N. A. Hill. Why are there so few magnetic ferroelectrics? J. Phys. Chem. B, 104(29):6694-6709, 2000. [13] N. A. Hill and K. M. Rabe. First principles investigation of ferromagnetism and ferroelectricity in BiMnO₃. Phys. Rev. B, 59:8759-69, 1999. [14] R. Seshadri and N. A. Hill. Visualizing the role of Bi 6s ``lone pairs'' in the off-center distortion in ferromagnetic BiMnO₃. Chem. Mater., 13:2892-2899, 2001. [15] A. M. dos Santos, S. Parashar, A. R. Raju, Y. S. Zhao, A. K. Cheetham, and C. N. R. Rao. Evidence for the likely occurrence of magnetoferroelectricity in the simple perovskite, BiMnO₃. Solid State Commun., 122:49-52, 2002. [16] A. M. dos Santos, A.K. Cheetham, T. Atou, Y. Syono, Y. Yamaguchi, K. Ohoyama, H. Chiba, and C. N. R. Rao. Orbital ordering as the determinant for ferromagnetism in biferroic BiMnO₃. Phys. Rev. B, 66:064435, 2002. [17] E. C. Subbarao. Ferroelectric and antiferroelectric materials. Ferroelectrics, 5:267-280, 1973.
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