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Local Structural Properties of Magnetoresistive Materials

Local Structural Properties of Magnetoresistive Materials. G.I.L.D.A. Fabrizio Bardelli. Outline : Magneto-Resistive materials I - Manganites II - Double-Perovskites. r (H) - r (H=0) r (H=0). MR % = x 100. Magnetoresitive materials. Sr 2 FeMoO 6 300K.

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Local Structural Properties of Magnetoresistive Materials

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  1. Local Structural Properties of Magnetoresistive Materials G.I.L.D.A Fabrizio Bardelli Outline : Magneto-Resistive materials I - Manganites II - Double-Perovskites

  2. r(H) - r(H=0) r(H=0) MR% = x 100 Magnetoresitive materials Sr2FeMoO6 300K • Magnetoresistance (MR) : magnetization  resistivity • Conductive phase : • external applied magnetic field • magnetic order • Insulating phase : • high temperatures • paramagnetic phase Sr2FeMoO6 4.2K

  3. Interest in Manganites and Double-Perovskites • High Ferromagnetic Curie temperature (TC), • (up to 450 K in double-perovskites) • Half-Metallic Ferromagnetic (HMFM) ground state • electrical current is 100% spin polarized • TC can be raised by changing doping species and • concentration, pressure, magnetic field… • Manganites : prototype of strong electron • correlated systems • Double-perovskites : new mechanism at the origin • of magnetotransport properties Attractive both in terms of basic investigations and technological applications

  4. Magnetic storage technology : • MR materials have been used • for years in reading-heads • of hard disks • Future spintronic devices : • Spin-driven electronic • devices : • spin-valves • spin-injectors • tunnel junctions Technological applications Magnetic layer Magnetic layer Non-Magnetic layer Non-Magnetic layer Magnetic layer Magnetic layer Low resistance High resistance

  5. I Manganites Theory

  6. Doped manganites Chemical formula A1-x Bx MnO3 A = trivalent alkaline ion B = divalent rare earth Mn mixed valence : Perovskite cell O2- A = La3+, Y3+… B = Sr2+, Ca2+… Mn3+ Mn4+

  7. The Ca-doped series : La1-xCaxMnO3 • La3+ and Ca2+ are subsitutional • La1-xCaxMnO3 solid solution can be obtained with 0 ≤x ≤1 : LaMnO3CaMnO3 Mn3+ Mn4+ Jahn-Teller active ion Non Jahn-Teller active xCa Axially elongated octahedron Regular octahedron

  8. ferromagnetic conductive reduced distortion paramagnetic insulating enhanced distortion Magnetic transition (TC) Metal-to-Insulator transition (TMI) Structural transition (TS) (TC ~TMI ~TS) Local structure and magneto-transport properties M/Ms r (MW· cm) T (K) TC~ 260 K • Local structure : • Mn-O bond lengths • Mn-O-Mn bond angles

  9. First shell Radial distribution of atoms around the absorber |FT| (a.u.) Fourier Transform R(Å) Selective and local probe suitable to investigate the local structure around the absorber atom Extendend X-ray Absorption Spectroscopy (EXAFS) XANES valence state and geometry around absorber EXAFS coordination numbers (N) bond distances (R) local lattice distortions (s2) Absorption K-edge XANES EXAFS

  10. I Manganites Experimental

  11. Substrate affects the structure of thin films : Lattice mismatch asub-afilm afilm Substrates : STO = SrTiO3 (cubic 100) lattice mismatch = 0.5% NGO = NdGaO3 (cubic 110) lattice mismatch = -0.54% out-of-plane c 100x MR film MR film substrate STO substrate NGO a tensile stress compressive stress c < a c > a film-plane Sample thickness (Å) TC (K) c (Å) MR (%) 750 260 3.87 45 250 260 3.87 78 125 248 3.85 78 50insulating at any T La0.87Na0.13MnO3 PLD grown on STO substrate xNa= 0.13  Max. MR P.Ghigna, University of Pavia Na-doped manganite thin films Aim of this work is to study the evolution of the local structure as a function of the thickness

  12. TEY detector design goals: • Signal amplification • in gas phase • Low temperatures (down to 4.2 K) • Possibility to smear-out • eventual Bragg peaks • from the substrate ground to amplifier (1010) TEY current He2 e- X-rays polarized electrode insulating holders sample Challenging measurements : Total Electron Yield (TEY)detector Strong signal from the STO substrate prevented fluorescence acquisitions TEY has limited penetration depth => lower signal from the substate

  13. Still sample q d Oscillating sample Total Electron Yield (TEY)detector Incident beam Scattered beam Bragg condition: nl = 2sinq • oscillation period < 1s • Dqtheo»10-2rad • Dqexp < 1° +q Sample X-rays -q

  14. Mn-La Mn-Mn Mn-O Fourier transform First shell Average Mn-O bond lengths 50 Å RMnO(Å) 125 Å 250 Å 750 Å Increasing Mn-O distance with decreasing the film thickness EXAFS signal FIT EXAFS : results 750 Å 250 Å 125 Å 50 Å

  15. Mn-O bonds elongation 2% Strong static Jahn-Teller distortion of the MnO6 octahedra 50 Å film XANES pre-edge features A2 A1 An increased A1-A2 pre-edge peak splitting is the signature of a enhanced Jahn-Teller distortion (Elfimov et al.) Origin of the structural change : EXAFS : discussion Lattice Mismatch ? (0.5 %) Insulating behavior

  16. 2 x 2.07 Å 2 x 1.92 Å 2 x 2.07 Å 4 x 1.92 Å 50 Å film (insulating) RMn-O= 1.995 Å Bulk powder sample insulating phase RMn-O~ 1.98 Å JT JT Rexp.= 1.992 Film growth plane Reduction of the out-of-plane parameter (lattice mismatch) apical JT component constrained in the film growth plane EXAFS : discussion X-ray beam is polarized in the growth plane of the film We are sensitive only to in-plane bond distances !

  17. Thinnest film (50 Å) : • Fully strained structure • (dead-layer) Large Jahn-Teller distortion with apical component oriented in the plane of the film • Thicker films (250 and 750 Å): • Fully relaxed structure • (bulk values) As the structure relaxes there is no more a preferred orientation for the Jahn-Teller distortion Manganite thin films : conclusions • Intermediate thickness film (125 Å) : • Contributions from both fully strained ( ³ 40%) • and fully relaxed structures

  18. II Double-Perovskites Theory

  19. Sr2FeMoO6 : crystalline structure Double-perovskite cell Sr2+ Fe3+ Mo5+ O2- • Two interpenetrating FCC sublattices

  20. e-¯ Fe S=5/2 Mo Fe Mo S=1/2 Mo Fe Mo Fe Sr2FeMoO6 H(T) Mis-site disorder : Non perfect ordering of Fe and Mo ions Sr2FeMoO6 : mis-site disorder FM FM AFM Mis-site disorder reduces MR

  21. Sr2FeMoO6 - Half metallic ferromagnet - High Curie temperature - Large negative MR between 5 and 300 K Sr2FeWO6 - Insulating at all temperatures - Antiferromagnetic below 37 K xMo Mo ® W The W-doped series : Sr2FeMoxW1-xO6 • Subsituting Mo5+ with W6+ in Sr2FeMoO6 we obtain • the solid solution Sr2FeMoxW1-xO6 with 0 £x£1 • W-doping reduces the mis-site disorder rising TC A Metal to Insulator Transition (MIT) is expected at a certain value of x

  22. II Double-Perovskites Experimental

  23. Powder bulk samples (D.D.Sarma, Bangalore) : x = 0.0 Sr2FeWO6 x = 0.05 x = 0.15 x = 0.3 x = 0.6 x = 0.8 x = 1.0 Sr2FeMoO6 Sr2FeMoxW1-xO6 Resistivity measurements indicate a critical concentration (xc) in the interval 0.2 < xc < 0.3 Sr2FeMoxW1-xO6 samples Insulators MIT (xc~ 0.25) Conductors Aim of the work : Study of the evolution of the local structure as a function of the doping level

  24. Mo Fe O O O Fe O O O Sr O O O O O O O O Fe shell path deg. 1st RMo-ON = 6 O 1st RSr-O N=12 O O O EXAFS results Fe K-edge Mo Mo K-edge Mo Fe Mo W LIII-edge Sr K-edge Measured in transmission mode at 77K using Si 311 monocrhomator crystals

  25. XRD Sr-O XRD Fe-O XRD Mo/W-O • XRD data (Sanchez et al.) report a smooth evolution with x FM metallic EXAFS : first shell results AFM insulating FM metallic Fe-O AFM insulating Sr-O Mo-O W-O • Abrupt change in the local structure crossing xc • expansion of the FeO6 octahedra  Fe3+® Fe2+ • Contraption of MnO6 octahedra and of the Sr-O bonds

  26. XANES spectra : Fe and Mo K-edges insulating insulating x = 0.05 x = 0.0 x = 0.15 xc x = 0.05 xc x = 0.15 x = 0.3 x = 0.3 x = 0.6 x = 0.6 x = 0.8 x = 0.8 x = 1.0 x = 1.0 metallic metallic Energy (eV) Energy (eV) Huge and abrupt change of the charge distribution crossing xc Fe edge : change in the valence state (edge position) Mo edge : evidence of localization of the charge carrier in the insulating phase

  27. XANES spectra : W LIII- and Sr K-edges insulating insulating x = 0.0 x = 0.0 x = 0.05 x = 0.05 x = 0.15 x = 0.15 xc xc x = 0.3 x = 0.3 x = 0.6 x = 0.6 x = 0.8 x = 0.8 x = 1.0 metallic metallic W edge : No detectable changes, neither in the local structure nor in the valence state

  28. m(E) Fit x = 0.6 1.2 0.4 • Excess of the metallic Sr2FeMoO6 -like structure in the FM phase • The sistem does not change structure up to the critical concentration XANES : Fe edge considerations mfit(x) = a • mexp(Sr2FeMoO6) + (1 - a) • mexp(Sr2FeWO6) XANES spectra of doped compounds can be fitted by a linear combination of the two end compounds ( Sr2FeMoO6 - Sr2FeWO6 ) with a as fitting parameter.

  29. HMFM regionInsulating region 1. Valence transition Fe3+ Mo5+ W5+ Fe2+ Mo6+ W6+ 2. Percolative transition Mo5+®W6+ metallic Fe-Mo FM clusters connects each other permitting conduction Fe3+®Fe2+ FM Fe-Mo clusters are isolated by non magnetic Fe-W clusters Neither the valence transition nor the percolation scenario can describe the system ! Metal to Insulator Transition : two hypothesis (Kobayashi) XANES ®W does not change its valence state ! XANES ®excess of metallic/Sr2FeMoO6-like structure in the FM phase

  30. Double-perovskites : conclusions • EXAFS and XANES data depicts the microstructural • counterpart of the Metal to Insulator Transition • Contrary to XRD results we see an abrupt change of • the local structure crossing the critical concentration • XANES data show that neither the percolative • nor the valence transition are good models • to describe the system • More quantitative analisys is needed on the XANES • spectra

  31. Acknowledgements GILDA scientific group : Prof. S. Mobilio Dr. F. D'Acapito Dr. C. Maurizio M. Rovezzi Gilda technicians group: F. D'Anca F. Lamanna V. Sciarra V. Tullio Collaborators : C. Meneghini – University of "Roma Tre" P. Ghigna – University of Pavia D.D. Sarma – Bangalore Institute of Science

  32. Mo5+ S=1/2 Sr2FeMoO6 : kinetic driven mechanism (D.D. Sarma 2001) eg­ Fe3+ S=5/2 Ecry 4d1 Ecry eg¯ eg¯ Eex t2g­ t2g¯ Ecry 3d5 t2g¯ Eex EF t2g¯ eg­ Eex<Ecry Fe-Mo hybrid levels in presence of hopping interactions t2g­ Ecry Eex> Ecry AFM coupling between Mo delocalised and Fe localised electrons leads to FM coupling of the Fe sublattice

  33. Metallic phase Insulating phase Mo and W charge carriers belong to the Fe-Mo hybrid band Adding W changes neither the structure nor the charge distribution Below a critical concentration conduction band disappears due to the low level of Mo ions Charge carriers localize on Mo and Fe sites Charge localization induces a change of the Fe valence state (Fe3+® Fe2+) The greater ionic radius of Fe2+ drives the observed transition of the local structure

  34. Ground state is Semimetallic : • Up-spin : • gap at the • Fermi level (EF) • Down-spin : • finite DOS at EF Sr2FeMoO6 : ground state 1 O2p 0.5 eV Fe eg Fe eg Mo t2g Up Spin ­ EF Down Spin ¯ Mo/Fe t2g O2p 3.9 eV Up-spin states (­) : insulator Down-spin states (¯) : conductor Fully spin-polarized mobile charges !

  35. FM-MR conductive phase Temperaturevs doping phase diagram Doped manganites have complex phase diagram FM = FerroMagnetic AF = AntiFerromagnetic CAF = Canted AF FI = FM Insulator CO = Charge Ordered Maximum MR at x = 0.25 La1-xCaxMnO3 T(K) 350 300 250 200 150 100 50 0 paramagnetic insulating 0 0.2 0.4 0.6 0.8 1 xCa LaMnO3 CaMnO3

  36. eg s=1/2 q1 q2 eg s=1/2 q1 q2 t2g S = 3/2 t2g S = 3/2 Mn3+ O2- Mn4+ t2g S = 3/2 t2g S = 3/2 Y1 : Mn3+ - O2- - Mn4+ Mn4+ O2- Mn3+ Y2 : Mn4+ - O2- - Mn3+ Double-Exchange (Zener, 1951) Transport properties : antagonist mechanisms • Strong on-site • Hund coupling • Transfer integral • µ cos(q1- q2) Predicted Tc is too high ! Electron-phonon coupling(Millis, 1994 …forty years later !) Jahn-Teller polaron = charge carrier + Jahn-Teller lattice distortion Enhanced effective mass Þ reduced mobility

  37. XANES and EXAFS : • Abrupt change of the local structure crossing xc • System does not change adding W until xc is reached • W local structure does not change in the whole x range • XANES : • Excess of metallic/Sr2FeMoO6-like clusters in the FM phase • Evidences of charge localization on Mo and Fe sites in the AFM phase • W does not change valence ! • Valence state model predicts a change of the W valence state • Percolative model predicts Sr2FeMoO6andSr2FeWO6 changing in weight according to the nominal concentrations Summarising Neither the percolative nor the percolation scenario can describe the system !

  38. Mn O

  39. Double-Exchange (DE) vs Super-exchange (SE) SE : the interaction is mediated by virtual electron hoppings into unoppupied Mo d states vs DE : delocalised Mo 4d1 electron plays the role of the delocalised electron in manganites But : Localised up-spin band at Fe site is fully filled => Delocalised electron must be down-spin ! Therefore : Strong on-site Hund strenght, which couples FM localised and delocalised electrons in manganites, cannot be invoked in the case of double-perovkites Other mechanism ?

  40. Mn4+-site Mn3+-site dx2-y2 eg eg dz2 3d3 3d4 t2g t2g crystal field crystal field + Jahn-Teller Electronic levels Transport mechanism 1 • t2g Localised electrons form a core with S = 3/2 • eg conduction electron belongs to the Mn 3d – O 2p hybrid derived states • Strong on-site Hund strenght couples FM the localised and delocalised electrons

  41. 0.20 A2 A1 0.15 0.10 absorption 50 Å DE(A1 - A2) bulk 0.05 A2 A1 0.00 6536 6538 6540 6542 6544 Energy (eV) XANES eg DE(t2g - eg) t2g A1 A2 DE(A1 - A2) µDE(t2g - eg) Peaks splitting originates from the crystal field which is influenced by the Jahn-Teller distortion The large A1 – A2 energy splitting in the thinnest film is the signature of a large Jahn-Teller distortion

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