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Multiferroicity from charge ordering ? a case study

Multiferroicity from charge ordering ? a case study. Manuel Angst. Peter Grünberg Institut PGI and Jülich Centre for Neutron Science JCNS, JARA-FIT, Forschungszentrum Jülich GmbH. 5 th Georgian -German School and Workshop in Basic Science, Tbilisi , Aug 6, 2012. Magnetism: Spins.

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Multiferroicity from charge ordering ? a case study

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  1. Multiferroicityfromchargeordering?a casestudy Manuel Angst Peter Grünberg Institut PGI and Jülich Centrefor Neutron Science JCNS, JARA-FIT, Forschungszentrum Jülich GmbH 5thGeorgian-German School and Workshop in Basic Science, Tbilisi, Aug 6, 2012

  2. Magnetism: Spins Ferroelectricity: Charge (Dipoles) M P S N + Multiferroics M P E H H E brokenspace-inversion brokentime-reversal Multiferroicity: Spins and Dipoles

  3. H E S N + Magnetism: Spins Ferroelectricity: Charge (Dipoles) Multiferroics : Cross-coupling M P M P E H H E Multiferroicity: Spins and Dipoles

  4. MRAM Readwith GMR Multiferroics for non-volatile memories Write : requires remagnetization – high currents (slow, high power consumption) Magnetization

  5. MRAM Write with aVoltage Multiferroics for non-volatile memories Write : requires remagnetization – high currents (slow, high power consumption) Readwith GMR MF-RAM : Multiferroic [M. Bibes and A. Barthélémy, Nat. Mater. 7, 425 (2008)]

  6. Magnetism: Spins Ferroelectricity: Charge (Dipoles) M P Very small overlapp ! M P E H S N + Multiferroics : only few materials Multiferroicity: Spins and Dipoles

  7. Ferroelectricity: Charge (Dipoles) P + Multiferroicity from charge order Any charge order breakinginversion-symmetry may be expected to be polar. • Can in principle lead to very large polarizations • Spins are for free ! • same electrons/sites involved in charge and spin order sizeable magnetoelectric coupling possible Examples ??? ? LuFe2O4 classicalmagnetite

  8. E || c Polarization (mC/cm2 ) P Temperature (K) LuFe2O4

  9. 100 keV x-ray diffraction at APS 6ID-D(normal Bragg-peaks are masked with lead)  Charge Order belowTCO~320 K 9 6 (t,t,3/2) 3 0 (1/3+d,1/3+d,3/2) -3 -6 -9 -12 -1 0 1 2/3 1/3 hh Room temperature 350 K • Sharp superstructurereflectionsrequire high degreeofstoichiometry & homogeneity • Still fairlylong-rangedcorrelationswithinthelayersaboveTCO

  10. Symmetry Analysis 2+ 2+ Fe Fe 3+ 3+ Fe Fe Polar bilayers Chargedbilayers (antiferroelectricallystacked) G2 G1 [M. Angst et al., PRL 101, 227601 (2008)]

  11. RefinedStructure Fesiteswithvalences O Lu TCO 210 K, Mo-Ka 1285 uniquereflections, 104 parameters R>4[F2] = 5.91% - spacegroupC2/m [de Groot et al., and MA, PRL 108, 187601 (2012)]

  12. RefinedStructure O Lu 2+ Fe Bond-Valence-Sum 3+ Fe G1 210 K, Mo-Ka 1285 uniquereflections, 104 parameters R>4[F2] = 5.91% - spacegroupC2/m (structuremodelwith polar bilayers (2) hasR>4[F2]~ 15%) [de Groot et al., and MA, PRL 108, 187601 (2012)]

  13. (1/3,1/3,0) TLT TN 3D magneticorderbelow 240 K DNS@FRMII Spin-flip Non-Spin-flip + in-Hneutrondiffraction, soft x-ray resonant diffraction, …

  14. 6ID-D@APS TN Ferrimagnetic charge ordered TCO Paramagnetic charge ordered Inhomogeneous“glassy” phase Short-range correlations TLT 6ID-D@ APS [de Groot et al., and MA, PRL 108, 037206 (2012)]

  15. Relation between Spin- & Charge Order X-rayMagneticCircularDichroism (XMCD) @ APS 4ID-C + Fe2+netspinalongH Smaller Fe3+netspinoppositetoH Similar XMCD results, but on notlong-rangespin- & chargeordered LuFe2O4: Ko et al., PRL 103, 207202 (2009);Kuepperet al., PRB 80, 220409(R) (2009)

  16. ? E || c Polarization (mC/cm2 ) P Temperature (K) LuFe2O4

  17. 0mA 50mA Real T is ~420 K At 250 K: current develops DL=0.00745 Å Change charge configurationwith an electric field ? At 150 K: 15 kV/cm (3000 V) noinfluence on CO6ID-B/D@APS 260K 2Θ Silverelectrodes:latticeparametersasthermometer Similarconclusionsbyin-situ neutrondiffraction:• Christansonet al., unpublished • Wen et al., PRB 81, 144121 (2010)

  18. [Niermann, Waschkowski, de Groot, MA, and Hemberger, PRL 109, 016405 (2012)] ? E || c Polarization (mC/cm2 ) P Temperature (K) LuFe2O4

  19. Structurerefinement: NO polar bilayers • XMCD combinedwithspinstructuresupportsthis • Correspondingly, dielectricspectroscopysuggestsabsenceof (anti)ferroelectricity Conclusion LuFe2O4is not ferroelectric A clearexampleof an oxidemultiferroic due tochargeorderhasyettobefound

  20. Complex ordering phenomena in multi-functional oxides Young-Investigators-Group fundedby Helmholtz association, partoftheinstituteofscatteringmethods JCNS-2 & PGI-4 (directorTh. Brückel) PankajThakuriaPhDstudent Thomas MüllerDiplomastudent Hailey WilliamsonMaster student Manuel Angst Group leader Joost de GrootPhDstudent Thesis subjectLuFe2O4 ShilpaAdigaPhDstudent Main externalcollaborations on theresultspresented Soft resonant x-raydiffraction Non-Resonant XRD Neutron diffraction Raphaël P. Hermann Karin SchmalzlWernerSchweika, Yixi Su, Walter Borghols Stew R. Bland, Peter D. Hatton Jong-Woo Kim , Phil Ryan, Zahir Islam, Douglas Robinson Racquel de Souza, Urs Staub Andy D. Christianson, Mark D. Lumsden, Karol Marty, Steve E. Nagler XMCD Dielectricspectroscopy Richard A. Rosenberg, David J. Keavney Daniel Niermann, Joachim Hemberger Zahra Yamani

  21. L Refinementof CO crystalstructure Supernova (210 K, Mo-Ka ) 9 6 3 3/2 0 3 domains of CO: AqA = (1/3,1/3,3/2)BqB = (-2/3,1/3,3/2)CqC = (1/3,-2/3,3/2) -3 -6 -9 -12 The domainscorrespondto3 monocliniccellsrotatedby120° -1 0 1 2/3 1/3 B A hh A C A (partof hex cell) C B B C 7.6 g B C C A B A • 0.25 x 0.17 x 0.03 mm

  22. Charge-frustration (V electrostatic interaction) Spin-frustration V V low valence ? V V ? V V high valence LuFe2O4 RFe2O4 ± nominal 2.5+ triangular Fe/O bilayers Lu Fe O Spins strongly Ising ||c c ~ 25 Å a ~ 3.4 Å rhombohedral()

  23. Floating Zone Growth of LuFe2O4 CO/CO2 mixture control O partial pressure~10-9 bar (& O-stoichiometry) ±

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