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Jeroen van den Brink

LaOFeAs -- multiferroic manganites. Jeroen van den Brink. Gianluca Giovannetti,Luuk Ament,Igor Pikovski,Sanjeev Kumar,Antoine Klauser,Carmine Ortix. George Sawatzky,Frank Kruger,Ilya Elfimov,Jan Zaanen. Krakaw 19/6/2008. t 2g. t 2g. e g. e g. S=2. S=1. S=0. LaOFeAs.

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Jeroen van den Brink

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  1. LaOFeAs -- multiferroic manganites Jeroen van den Brink Gianluca Giovannetti,Luuk Ament,Igor Pikovski,Sanjeev Kumar,Antoine Klauser,Carmine Ortix George Sawatzky,Frank Kruger,Ilya Elfimov,Jan Zaanen Krakaw 19/6/2008

  2. t2g t2g eg eg S=2 S=1 S=0 LaOFeAs Layered crystal structure tetrahedral surrounding Fe d6 exp. ordered moment: 0.4 B C-type magnetic order (bad) metal

  3. LaOFeAs: LDA+U, C-type magnetic order S=2 Fe 3d-band width ~4 eV gap ~0.5 eV Giovannetti, Kumar, JvdB, arXiv:0804.0866 Phys B. doi:10.1016 (2008)

  4. Stochiometric LaOFeAs S=2, Mott insulator high spin Fe found in XAS Bernd Buchner, Martin Knupfer arXiv:0806.2625 Experimental small moment and “metallicity” are due to off-stochiometry Giovannetti, Kumar, JvdB, Phys B. doi:10.1016 (2008)

  5. Fundamental interest: Why rare? How to get around? How to get strong coupling between magnetic and ferroelectric orderparameter? Multiferroics = Magnetic Ferroelectrics Why study them? Magnetic ferroelectrics are very rare! Van Aken et al., Nature 449, 702 (2007) Cheong and Mostovoy, Nature Mat. 6, 13 (2007) Eerenstein et al., Nature 442, 759 (2006) Pimenov et al., Nature Phys. 97, 100 (2006) Ikeda et al., Nature 436, 1136 (2005) Lottermoser et al., Nature 430, 541 (2004) Zheng et al., Science 303, 661 (2004) Kimura et al., Nature 426, 55 (2003) Hur et al., Nature 429, 392 (2003)

  6. Tferroelectric=Tmagnetic magnetism induces FE -- TbMnO3, DyMnO3, Ni3 V2 O8 chiral magnetic order (spin spiral) does the job Maxim Mostovoy, PRL 96, 067601 (2006) -- Pr1-xCaxMnO3, HoMn2O5 magnetism induces FE, but: no chiral symmetry breaking Observed multiferroic couplings Tmagnetic<<Tferroelectric orderparameters barely couple -- BiFeO3 and BiMnO3 ???

  7. Mn3+ Mn3+ Mn4+ Mn4+ HoMn2O5

  8. electronic and ionic + + + - - - displacements due to dislocated SDW / magneto-striction ferroelectric polarization HoMn2O5 conceptual picture Mn3+ Mn4+ Betouras, Giovannetti, JvdB, PRL 98, 257601 (2007)

  9. experimental polarization ~80 nC/cm2 ferroelectric domains? Ab initio bandstructure computations ~1200 nC/cm2 Incorporating electron correlation effects with LDA+U fixes the problem

  10. small polarization due to near cancellation of Pion and Pelec Gianluca Giovannetti and JvdB, PRL 100, 227603 (2008)

  11. Perovskite crystal structure of Pr1-xCaxMnO3 0.4 < x < 0.5 Pr3+/Ca2+ Oxygen2- Mn4+ / Mn3+

  12. eg t2g Near x=0.4 : Bond-centered charge/spin ordering Dimer A.Daoud-Aladine et al., PRL 89 97205 (2002)

  13. Near x=0.5 : Site-centered charge/spin ordering E.O. Wollan and W.C. Koeler, Phys. Rev. 100, 545 (1955)

  14. x=0.4 x=0.5 0.4 < x < 0.5 Bond centered spin/CO Site centered spin/CO intermediate Ferro-electric groundstate It is allowed by symmetry: can happen will happen observed to happen Jooss et al., PNAS 104, 13597 (2007) Ferroelectric?

  15. multiferroicity ....and it first happened in microscopic DDEX model JvdB, Khomskii, PRL 82, 1016 (1999) stabilization of dislocated SDW phase Efremov, JvdB, Khomskii, Nature Mat. (2004)

  16. Continous transition from Site centered CO to Bond centered CO “in between order” Breaking of inversion symmetry in the intermediate phase Ferro-electricity Magnetism Efremov, JvdB, Khomskii, Nature Mat. (2004) Computed phase diagram of Pr1-xCaxMnO3

  17. Giovannetti, Kumar, JvdB, Picozzi, preprint (2008)

  18. Conclusions we predict.... LaOFeAs to be Mott insulator with Fe high spin La1/2Ca1/2MnO3 to be strongly multiferroic

  19. suggests FE in quasi-1D organic charge transfer salts e.g.: S. Brazovskii, Physics of Organic Superconductors and Conductors Springer Series in Materials Sciences (2008). which is observed! Simple 1D Picture longitudinal charge displacements

  20. Perovskite HoMnO3 with GdFeO3 distortion Magnetic E-phase S. Picozzi et al., Phys. Rev. Lett. 99, 227201 (2007).

  21. all oxygens move down! transversal charge displacements Simple 1D Picture Superexchange strengthens bonds of antiparallel spins Double exchange strengthen bonds of parallel spins JvdB and Daniel Khomskii, J. Phys. C.M., in press (2008)

  22. HoMn2O5 ICM + PE CM+FE ICM+PE Kimura, Kamada, Noda, Kaneko, Metoki, Kohn cond-mat/0602226 (2006)

  23. 1. Phenomenological approach

  24. Magneto-electric coupling: Ginzburg-Landau Electric polarization couple these two orderparameters Magnetization Free energy must be invariant for: time reversal spatial inversion

  25. We are interested in ferroelectrics: uniform electric polarization so that which implies that: Magneto-electric coupling: Ginzburg-Landau To build an invariant we need SPIRAL

  26. becomes active if SDW dislocated magnetization is shifted with respect to the lattice (but inversion invariant) Our key observation: Betouras, Giovannetti, JvdB, PRL 98, 257601 (2007) Giovannetti and JvdB, PRL 100, 227603 (2008)

  27. with Ansatz for polarization gives finite p0 and p1 only when qm = q/2 For multiferroic coupling it is sufficient to have commensurate dislocated magnetic order Minimize Free Energy with

  28. YMn2O5 ICM ICM: incommensurate magnetic ordering CM: commensurate ICM CM indeed only commensurate magnetic phase is ferroelectric! Chapon, Radaelli, Blake, Park, Cheong Phys. Rev. Lett.96, 097601 (2006)

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