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

Magnetoresitive materials Conductive phase : external applied magnetic field magnetic order Insulating phase : high temperatures paramagnetic phase MR % = x 100  (H) -  (H=0)  (H=0) Magnetoresistance (MR) : magnetization  resistivity Sr 2 FeMoO 6 300K Sr 2 FeMoO 6 4.2K

Interest in Manganites and Double-Perovskites High Ferromagnetic Curie temperature (T C ), (up to 450 K in double-perovskites) Attractive both in terms of basic investigations and technological applications Half-Metallic Ferromagnetic (HMFM) ground state electrical current is 100% spin polarized T C 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

Technological applications 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 Magnetic layer Non-Magnetic layer Low resistanceHigh resistance

I Manganites Theory

Doped manganites O 2- Mn 3+ A = La 3+, Y 3+ … B = Sr 2+, Ca 2+ … Perovskite cellChemical formula A 1-x B x MnO 3 A = trivalent alkaline ion B = divalent rare earth Mn mixed valence : Mn 4+

The Ca-doped series : La 1-x Ca x MnO 3 LaMnO 3 CaMnO 3 Mn 3+ Mn 4+ Jahn-Teller active ionNon Jahn-Teller active x Ca Regular octahedron Axially elongated octahedron La 3+ and Ca 2+ are subsitutional La 1-x Ca x MnO 3 solid solution can be obtained with 0 ≤ x ≤ 1 :

 (M  · cm) M/M s T (K) Local structure and magneto-transport properties Magnetic transition (T C ) Metal-to-Insulator transition (T MI ) Structural transition (T S ) (T C  T MI  T S ) Local structure : Mn-O bond lengths Mn-O-Mn bond angles ferromagnetic conductive reduced distortion paramagnetic insulating enhanced distortion T C  260 K

Radial distribution of atoms around the absorber First shell Fourier Transform R(Å) |FT| (a.u.) Extendend X-ray Absorption Spectroscopy (EXAFS) Selective and local probe suitable to investigate the local structure around the absorber atom XANES valence state and geometry around absorber EXAFS coordination numbers (N) bond distances (R) local lattice distortions (  2 ) XANESEXAFS Absorption K-edge

Experimental I Manganites

Na-doped manganite thin films Sample thickness (Å)T C (K) c (Å)MR (%) insulating at any T La 0.87 Na 0.13 MnO 3 PLD grown on STO substrate x Na = 0.13  Max. MR P.Ghigna, University of Pavia Substrate affects the structure of thin films : out-of-plane a substrate STO MR film tensile stress compressive stress c a film-plane c Substrates : STO = SrTiO 3 (cubic 100) lattice mismatch = 0.5% NGO = NdGaO 3 (cubic 110) lattice mismatch = -0.54% Lattice mismatch a sub -a film a film substrate NGO MR film 100x Aim of this work is to study the evolution of the local structure as a function of the thickness

Strong signal from the STO substrate prevented fluorescence acquisitions TEY has limited penetration depth => lower signal from the substate Total Electron Yield (TEY) detector Challenging measurements : 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 e-e- polarized electrode He 2 X-rays to amplifier (10 10 ) TEY current ground sample insulating holders

Bragg condition: n  = 2sin  Total Electron Yield (TEY) detector Incident beam Scattered beam  d X-rays Sample ++ -- oscillation period < 1s  theo  rad  exp < 1° Still sample Oscillating sample

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

Lattice Mismatch ? (0.5 %) EXAFS : discussion Insulating behavior Strong static Jahn-Teller distortion of the MnO 6 octahedra Origin of the structural change : Mn-O bonds elongation 2% 50 Å film XANES pre-edge features An increased A 1 -A 2 pre-edge peak splitting is the signature of a enhanced Jahn-Teller distortion (Elfimov et al.) A1A1 A2A2

Reduction of the out-of-plane parameter (lattice mismatch) apical JT component constrained in the film growth plane X-ray beam is polarized in the growth plane of the film We are sensitive only to in-plane bond distances ! Film growth plane EXAFS : discussion 50 Å film (insulating) R Mn-O  Å JT 2 x 2.07 Å 2 x 1.92 Å R exp.  Bulk powder sample insulating phase R Mn-O  1.98 Å JT 2 x 2.07 Å 4 x 1.92 Å

Large Jahn-Teller distortion with apical component oriented in the plane of the film Thinnest film (50 Å) : Fully strained structure (dead-layer) 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 Intermediate thickness film (125 Å) : Contributions from both fully strained (  40%) and fully relaxed structures Manganite thin films : conclusions

II Double-Perovskites Theory

Double-perovskite cell Two interpenetrating FCC sublattices Sr 2+ Fe 3+ Mo 5+ O 2- Sr 2 FeMoO 6 : crystalline structure

Sr 2 FeMoO 6 : mis-site disorder Sr 2 FeMoO 6 H(T) e- e-  Fe S=5/2 MoFeMo S=1/2 MoFeMoFe FMAFM FM Mis-site disorder : Non perfect ordering of Fe and Mo ions Mis-site disorder reduces MR

Subsituting Mo 5+ with W 6+ in Sr 2 FeMoO 6 we obtain the solid solution Sr 2 FeMo x W 1-x O 6 with 0  x  1 W-doping reduces the mis-site disorder rising T C x Mo Mo  W Sr 2 FeMoO 6 - Half metallic ferromagnet - High Curie temperature - Large negative MR between 5 and 300 K Sr 2 FeWO 6 - Insulating at all temperatures - Antiferromagnetic below 37 K A Metal to Insulator Transition (MIT) is expected at a certain value of x The W-doped series : Sr 2 FeMo x W 1-x O 6

Experimental II Double-Perovskites

Resistivity measurements indicate a critical concentration (x c ) in the interval 0.2 < x c < 0.3 MIT (x c  0.25) Insulators Conductors Aim of the work : Study of the evolution of the local structure as a function of the doping level Sr 2 FeMo x W 1-x O 6 samples Sr 2 FeMo x W 1-x O 6 Powder bulk samples (D.D.Sarma, Bangalore) : x = 0.0 Sr 2 FeWO 6 x = 0.05 x = 0.15 x = 0.3 x = 0.6 x = 0.8 x = 1.0 Sr 2 FeMoO 6

1 st R Sr-O N=12 shellpathdeg. 1 st R Mo-O N = 6 EXAFS results Mo Fe Fe K-edge Mo K-edge W L III -edge Sr K-edge Measured in transmission mode at 77K using Si 311 monocrhomator crystals O O O O O O Fe Mo O Fe O O O O O O Sr O O O O O

Abrupt change in the local structure crossing x c expansion of the FeO 6 octahedra  Fe 3+  Fe 2+ Contraption of MnO 6 octahedra and of the Sr-O bonds EXAFS : first shell results Fe-O Mo-O W-O Sr-O XRD Fe-O XRD Mo/W-O XRD Sr-O XRD data (Sanchez et al.) report a smooth evolution with x AFM insulating FM metallic FM metallic AFM insulating

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

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

XANES : Fe edge considerations XANES spectra of doped compounds can be fitted by a linear combination of the two end compounds ( Sr 2 FeMoO 6 - Sr 2 FeWO 6 ) with  as fitting parameter.  fit (x) =   exp ( Sr 2 FeMoO 6 ) + (1 -  )  exp ( Sr 2 FeWO 6 )  (E) Fit x = 0.6 Excess of the metallic Sr 2 FeMoO 6 - like structure in the FM phase The sistem does not change structure up to the critical concentration

Fe 3+  Fe 2+ FM Fe-Mo clusters are isolated by non magnetic Fe-W clusters 2. Percolative transition Mo 5+  W 6+ metallic Fe-Mo FM clusters connects each other permitting conduction Metal to Insulator Transition : two hypothesis (Kobayashi) Fe 2+ Mo 6+ W Valence transition HMFM region Insulating region Fe 3+ Mo 5+ W 5+ XANES  W does not change its valence state ! XANES  excess of metallic/Sr 2 FeMoO 6 -like structure in the FM phase Neither the valence transition nor the percolation scenario can describe the system !

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

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

Sr 2 FeMoO 6 : kinetic driven mechanism (D.D. Sarma 2001) 3d 5 4d 1 E cr y egeg egeg t 2g  t 2g  EexEex egeg egeg t 2g  E cry E ex t 2g  EFEF E ex > E cry E ex < E cry Fe-Mo hybrid levels in presence of hopping interactions AFM coupling between Mo delocalised and Fe localised electrons leads to FM coupling of the Fe sublattice Fe 3+ S=5/2 Mo 5+ S=1/2

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 (Fe 3+  Fe 2+ ) The greater ionic radius of Fe 2+ drives the observed transition of the local structure Metallic phase Insulating phase

Sr 2 FeMoO 6 : ground state 1 Ground state is Semimetallic : Up-spin : gap at the Fermi level (E F ) Down-spin : finite DOS at E F Up-spin states (  )  insulator Down-spin states (  ) : conductor Fully spin-polarized mobile charges ! O2p Mo/Fe t 2g O2p 3.9 eV EFEF Up Spin  Down Spin  Fe e g Mo t 2g 0.5 eV Fe e g

La 1-x Ca x MnO 3 x Ca T(K) paramagnetic insulating LaMnO 3 CaMnO 3 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 FM-MR conductive phase

Jahn-Teller polaron = charge carrier + Jahn-Teller lattice distortion Enhanced effective mass  reduced mobility Transport properties : antagonist mechanisms Electron-phonon coupling (Millis, 1994 …forty years later ! ) Strong on-site Hund coupling Transfer integral  cos(     ) Predicted T c is too high ! Double-Exchange (Zener, 1951)   : Mn 3+ - O 2- - Mn 4+ e g s=1/2 t 2g S = 3/2 Mn 4+ Mn 3+ O 2- t 2g S = 3/2     : Mn 4+ - O 2- - Mn 3+ e g s=1/2 t 2g S = 3/2 Mn 3+ Mn 4+ O 2- t 2g S = 3/2  

Summarising XANES and EXAFS : Abrupt change of the local structure crossing x c System does not change adding W until x c is reached W local structure does not change in the whole x range XANES : Excess of metallic/Sr 2 FeMoO 6 -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 Sr 2 FeMoO 6 and Sr 2 FeWO 6 changing in weight according to the nominal concentrations Neither the percolative nor the percolation scenario can describe the system !

O Mn

DE : delocalised Mo 4d 1 electron plays the role of the delocalised electron in manganites SE : the interaction is mediated by virtual electron hoppings into unoppupied Mo d states Double-Exchange (DE) vs Super-exchange (SE) Other mechanism ? 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 vs

Transport mechanism 1 Electronic levels 1.t 2g Localised electrons form a core with S = 3/2 2.e g conduction electron belongs to the Mn 3d – O 2p hybrid derived states 3.Strong on-site Hund strenght couples FM the localised and delocalised electrons 3d 4 Mn 4+ -site 3d 3 egeg t 2g d x 2 -y 2 dz2dz2 egeg Mn 3+ -site t 2g crystal field + Jahn-Teller

Peaks splitting originates from the crystal field which is influenced by the Jahn-Teller distortion egeg t 2g A2A2 A1A1  E(t 2g - e g )  E(A 1 - A 2 ) A1A1 A2A2 A1A1 A2A2 bulk 50 Å Energy (eV) absorption The large A 1 – A 2 energy splitting in the thinnest film is the signature of a large Jahn-Teller distortion  E(A 1 - A 2 )   E(t 2g - e g ) XANES