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Spin-polarized tunneling with magnetic oxide electrodes and barriers

Spin-polarized tunneling with magnetic oxide electrodes and barriers. Gervasi Herranz Unité Mixte de Physique CNRS / Thales, Route Départementale 128 91767 Palaiseau, France gervasi.herranz@thalesgroup.com Zaragoza, november 2005.

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Spin-polarized tunneling with magnetic oxide electrodes and barriers

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  1. Spin-polarized tunneling with magnetic oxide electrodes and barriers Gervasi Herranz Unité Mixte de Physique CNRS / Thales, Route Départementale 128 91767 Palaiseau, France gervasi.herranz@thalesgroup.com Zaragoza, november 2005

  2. M. Bibes 2, U. Lüders3,4, M. Gajek1,3, H. Béa1, R. Ranchal1,6, M. Bowen1, K. Bouzehouane1, S. Fusil5, E. Jacquet1, J.-L. Maurice1, A. Vaurès1, J.-F. Bobo4, M. Varela6, J. Fontcuberta3, J.-P. Contour1, A. Barthélémy1, J. M. De Teresa, A. Fert1… 1 Unité Mixte de Physique CNRS / Thales, Orsay (FRANCE) 2 Institut d’Electronique Fondamentale, Université Paris-Sud, Orsay (FRANCE) 3 Institut de Ciència de Materials de Barcelona, CSIC, Campus de la UAB, Bellaterra (SPAIN) 4 FRE CNRS-ONERA, Toulouse (FRANCE) 5 Université d’Evry (FRANCE) 6 Facultad de Física, Universidad Complutense de Madrid (SPAIN) 7 Facultat de Física Aplicada i Òptica, Universitat de Barcelona (SPAIN)

  3. Outline • Spintronics: Spin polarization, Spin-dependent tunneling • Half-metallic ferromagnets • Spin Polarization • Interface (electrical bondings) • Barrier (electronic structure) • Voltage bias • Temperature • Artificial high-spin polarization: Spin-filter devices • Spin-injection: diluted magnetic systems (DMS)

  4. Outline • Spintronics: Spin polarization, Spin-dependent tunneling • Half-metallic ferromagnets • Spin Polarization • Interface (electrical bondings) • Barrier (electronic structure) • Voltage bias • Temperature • Artificial high-spin polarization: Spin-filter devices • Spin-injection: diluted magnetic systems (DMS)

  5. Spin-dependent conduction: spin polarization (SP) Asymmetry in the DOS at the Fermi level Spin polarization (SP) Metal with magnetic order SP  0 N(EF)↑≠ N(EF)↓

  6. Determination of the spin polarization • Spin-resolved photoemission • Andreev reflection point contacts • Tunneling experiments: • FM/i/SC • FM/i/FM

  7. Physics of tunneling Transmission coefficient through a barrier: • Exponential dependence on barrier thickness(d) • Exponential dependence on h1/2 (barrier height) Conductance through a barrier: • Depends on the density of states (DOS) of the electrodes

  8. FM/I/SC tunnel junctions Ferromagnet/Insulator/Superconductor FM electrode: Asymmetry: N(EF)↑≠ N(EF)↓ Determination of SP in FM/Al2O3/Al junctions

  9. M R H H • FM/I/FM: Magnetic tunnel junctions (MTJs) Jullière’s model Jullière: TMR depends only on the electrode properties Tunel magnetoresistance (TMR)

  10. TMR and applications Moodera et al., PRL 74, 3273 (1995) CoFe / Al2O3 / Co, TMR  10% @ RT MRAMs

  11. 2SP 2SP 1 = TMR = TMR = 1 1 – – SP SP 1 • High-SP materials required: • Large TMR • Spin analyzers in spintronics devices SP2 = 1

  12. Materials for ferromagnetic electrodes: Half metals De Groot et al.; Phys. Rev. Lett. 50, 2024 (1983) EF  sf EF sf  SP=±100%  TMR For classification of different HM see: J.M.D.Coey and M.Venkatesan; J. Appl. Phys. 91, 8345 (2002)

  13. Materials for ferromagnetic electrodes: Half metals (1): Soulen et al.; Science 282, 85 (1998) (7) Anguelouch et al.; PRB64,180408 (2002) (2): Moon Ho Jo (8) Tanaka et al.; JAP(1999) (3): Bibes et al. APL83 (9) Kamper (1987) (4): Bowen et al, APL.82, 233 (2003) (10) Seneor et al.;APL74, 4017 (1999) (5); Park et al.; Nature (1998) (11) Gupta et al.; APL78, 1894 (2001) (6): Bertacco et al. (12) Dedkov et al.; PRB65, 064417 (2002)

  14. The sign of the spin polarization (SP) All SP > 0, except for: Co/STO J. M. De Teresa et al., Science 286, 507 (1999). FE3O4 Panchula, Ph.D. thesis, Stanford University, Stanford, 2003. SrRuO3/STO D. C. Worledge, PRL 85, 5182 (2000). La0.7Ce0.3MnO3 Mitra, PRL 90, 017202 (2003) Sr2FeMoO6Bibes et al. APL83 (2003) BUT contradictory with band calculations of TM ferromagnets

  15. Outline • Spintronics: Spin polarization, Spin-dependent tunneling • Half-metallic ferromagnets • Spin Polarization • Interface (electrical bondings) • Barrier (electronic structure) • Voltage bias • Temperature • Artificial high-spin polarization: Spin-filter devices • Spin-injection: diluted magnetic systems (DMS)

  16. Spin polarization depends on different factors Jullière’s model is insufficient: Spin Polarization is NOT ONLY a function of the electrode properties: • Interface bonding-types affect SP • The crystal symmetry of the barrier is important

  17. Effect of interface bondings on SP Calculations in a tight-binding approximation Tsymbal & Pettifor, J. Phys.: Condens. Matter 9 (1997) L411 The spin polarization of the tunnelling current depends strongly on the type of covalent bonding between the ferromagnet and the insulator. sss bonding  tunnelling current carried by the s electrons : SP > 0 sds bonding  tunnelling current carried by d-electrons : SP < 0 sss sss + sps, sds

  18. LSMO/STO/Co LSMO/Al2O3/Co 300 6 4.2 10 -6 Co -4 -30 6 280 4.1 10 LSMO -2 6 4 10 260 -20 0 6 3.9 10 Resistance (Ohms) 2 TMR (%) Resistance (Ohms) 240 TMR (%) 6 -10 4 3.8 10 6 6 3.7 10 220 0 8 (a) 6 3.6 10 10 200 6 10 3.5 10 -150 -100 -50 0 50 100 150 -100 -50 0 50 100 Magnetic Field (mT) Magnetic field (mT) LSMO/TiO2/Co SP depends on the inteface All MTJs with LSMO and Co electrodes Normal TMR SPCo > 0  de Teresa et al., Science 286, 509 (1999) s electrons are responsible for tunneling Inverse TMR SPCo < 0 d electrons with negative spin polarisation are responsible for tunneling

  19. 300 6 4.2 10 -6 -30 280 Co -4 6 4.1 10 LSMO 260 -2 -20 6 4 10 0 TMR (%) 240 6 Resistance (Ohms) -10 3.9 10 2 Resistance (Ohms) TMR (%) 6 4 3.8 10 220 0 (a) 6 6 3.7 10 200 8 10 6 3.6 10 10 -100 -50 0 50 100 6 3.5 10 Magnetic field (mT) -150 -100 -50 0 50 100 150 Magnetic Field (mT) Co Co SrTiO3 (STO) Al2O3 (ALO) LSMO LSMO 5 4.6 10 -4 5 4.4 10 0 TMR (%) 5 4 4.2 10 8 5 Resistance (Ohm) 4 10 12 5 3.8 10 16 5 3.6 10 20 -100 -50 0 50 100 Magnetic Field (mT) Co Al2O3(ALO) SrTiO3(STO) Role of interfaces on SP LSMO LSMO/STO 2.5nm/Co LSMO/Al2O330nm/Co LSMO/STO 1nm/Al2O3 1.5nm/Co Inverse TMR  SPCo <0 Normal TMR  SPCo >0 Hybridization at interface fixed the spin polarization ★ de Teresa et al., Science 286, 509 (1999)

  20. Role of interfaces on SP Co/Al2O3 interfaces: tunneling of s states through ALO No hybridization with Co-d orbitals (Al has no empty d-states to hybridize with Co-d orbitals) Co/STO & Co/TiO2 interfaces: tunneling of d states through STO DOS of Co Bias dependence of TMR Empty Ti d-shells from STO and TiO2 hybridize with Co-d orbitals

  21. SP depends on the complex electronic bands of the barrier D. Wortmann, J. Phys.:Condens. Matter 16, S5822 (2004). Butler et al., PRB 2001 Coherent spin tunneling in single crystalline MTJs Bloch states symmetry at the Fermi energy and their relationship to the symmetry of the slowly decaying evanescent states in the barrier layer affect tunnelling conductance.

  22. SP depends on the complex electronic bands of the barrier • Conventional MTJs with amorphous AlOx  TMR  70% @ RT • Fe/MgO(001)/Fe  TMR  180% @ RT • S. Parkin et al., Nat. Mater. 2004 • CoFe/MgO(001)/CoFe  TMR  220% @ RT • S. Yuasa et al., Nat. Mater. 2004 Strong enhancement of TMR due to coherent spin-polarized tunneling in highly oriented MgO (0 0 1) barriers Butler et al., PRB 2001 TMR oscillations (with thickness)  coherent tunneling

  23. Relevance of the electronic structure of the barrier LSMO/STO/Co B-site ions: Ti-d orbitals (octahedral sites) hybridize with Co-d orbitals Similar TMR(V) dependence LSMO/LAO/Co A-site ions (La): conduction band comes from La-4d in dodecahedral environment hybridizing with Co-d orbitals B-site (Al): no empty d-states can hybridize with Co-d V. Garcia et al., APL in press Electronic structure of the barrier is relevant to SP

  24. Relevance of the electronic structure of the barrier STO: Ti 3d states D1 (Tieg), D5(Ti2g) D2(Ti2g) • Calculations of the band structure of BO2/hcp0001-Co interfaces • Complex band structure of LAO LSMO D1(), D2() LAO: La 4d states D1 (dz2) D2(dx2-y2) V. Garcia et al., APL in press

  25. Outline • Spintronics: Spin polarization, Spin-dependent tunneling • Half-metallic ferromagnets • Spin Polarization • Interface (electrical bondings) • Barrier (electronic structure) • Voltage bias • Temperature • Artificial high-spin polarization: Spin-filter devices • Spin-injection: diluted magnetic systems (DMS)

  26. Growth of LSMO / STO / LSMO heterostructures by pulsed laser deposition TEM : excellent crystalline structure LSMO / STO / LSMO tunnel junctions M. Bowen et al., APL 82, 233 (2003) Junction processing : UV lithography and SIMS-monitored ion-beam etching Tunnel magnetoresistance (TMR) Junction size : 6 to >100 µm² 1800 % TMR at 4K PLSMO = 95% Confirmation of the HM state by transport measurements Assymetry related to the presence of a Co/CoO bilayer deposited onto the top LSMO electrode

  27. Sharp decrease due to electron-magnon scattering at LSMO/STO interfaces R(H) 1.0 I(V) Œ 0.8 0.6 Normalized TMR 0.4  0.2 0.0 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 V (V) DC LSMO / STO / LSMO tunnel junctions Bias dependence of the TMR Influence of the density of states (as in theoretical calculation of TMR(V) in HM / I / HM junctions) 4.2K A. Bratkovsky, PRB (1997) M. Bowen et al., PRL 95, 137203 (2005) Inflection point reflects the presence of a gap in the spin-down sub-band

  28. t 2g P A/V) e e e - - 1 AP g m d eV > d DC G(V) ( T=4K ) 0 2 P A/V t 2g AP m ( e 2 g e e - - 2 d I/dV d eV < DC d 2 0 d 0.0 0.1 0.2 0.3 0.4 0.5 V (V) DC LSMO / STO / LSMO tunnel junctions Bias dependence of the conductance Spin-resolved spectra taken at 100 K for a STO/LSMO interface. M. Bowen et al., PRL 95, 137203 (2005) From the TMR(V) and G(V) data, we deduce Eg = 380 meV (in agreement with spin-polarized inverse photo- emission) R. Bertacco et al, JMMM (2002) Spectroscopic nature of spin-dependent tunneling from a half-metal

  29. LSMO / STO / LSMO tunnel junctions Temperature dependence of the spin-polarization TMR = 2P2/(1 - P2) STO, TiO2, LAO Interfaces continuity of the oxygen octahedra Bulk M Air Surface LSMO Surface Park et al, PRL 1999 LSMO discontinuity of the oxygen octahedra The interfacial SP decays more slowly than the surface SP. The temperature dependence is similar for all interfaces. V. Garcia et al., PRB 69, 052403 (2004)

  30. Outline • Spintronics: Spin polarization, Spin-dependent tunneling • Half-metallic ferromagnets • Spin Polarization • Interface (electrical bondings) • Barrier (electronic structure) • Voltage bias • Temperature • Artificial high-spin polarization: Spin-filter devices • Spin-injection: diluted magnetic systems (DMS)

  31. J=0  large J 2 Non magnetic metal Ferro insulator HM or Ferromagn metal Parallel: large current  2 smallJ 2 J small J=0  2 J large Antiparallel: small current Materials for the barrier : Spin filters ☆ Hao et al., Phys. Rev. B42, 8235 (1990)

  32. Spin filter devices Antiparallel configuration: Non-magnetic electrode Parallel configuration: Magnetic insulating barrier Magnetic electrode

  33. Materials for the barrier : Spin filters Leclair; APL80, 625 (2002) Al/EuS/Gd Tc=16K =0.36eV

  34. c c Mn Mn d d +3 +3 4 4 z z y y x x O O - - 2 2 Bi Bi +3 +3 b b a a La1-xBixMnO3 – based spin filters Distorted perovskite structure (Monoclinic symmetry) Ferromagnetic insulator with Eg = 0.5 eV and Eg = 2.6 eV Ferromagnetic order TC=105K, Ms=3.6 µB

  35. I Au LSMO La1-xBixMnO3 – based spin filters BiMnO3, LaBiMnO3 BiMnO3 M. Gajek PRB 72 020406 (2005) TcBulk=105K La0.1Bi0.9MnO3 187%

  36. B A Fe Fe Ni 3+ 3+ 2+ 3d 3d 3d 5 5 7 T =850K C 5µ 5µ 2µ B B B Ferrimagnet with a F - coupling saturation moment of 2 µ /f.u. AF - coupling B NiFe2O4 – based spin filters B sites A sites A sites : tetrahedral B sites : octahedral Cubic spinel a=b=c=8.33 Å Mismatched by ~6% with STO and LSMO Inverse spinel structure Insulator with Eg=1.8 eV and Eg= 0.4 eV D. Szotek et al, JPCM 2005

  37. NiFe2O4 – based spin filters Insulating NFO grown in Ar/O2: magnetic barrier in spin-filter devices Au/NFO/La2/3Sr1/3MnO3 on STO • maximum TMR  50% • filter efficiency  22% Temperature dependence

  38. NiFe2O4 – based spin filters Parallel state Antiparallel state Insulator with Eg=1.8 eV and Eg= 0.4 eV Low barrier for majority spin carriers in FM High barrier for majority spin carriers in FM Au/NFO/LSMO TMR > 0 ! BUT Negative TMR should be expected • Possible explanations: • Positive spin-filtering efficicency •  Eg > Eg (opposite of what is found with calculations) • Modified electronic structure at interface ? • Role of symmetry filtering (like with MgO) ?

  39. Outline • Spintronics: Spin polarization, Spin-dependent tunneling • Half-metallic ferromagnets • Spin Polarization • Interface (electrical bondings) • Barrier (electronic structure) • Voltage bias • Temperature • Artificial high-spin polarization: Spin-filter devices • Spin-injection: diluted magnetic systems (DMS)

  40. Transition metals and the problem of spin injection in semiconductors Spin LED Fiederling et al., Nature, 402, 787 (1999) Spin FET Datta & Das, Appl. Phys. Lett., 56, 665 (1990)

  41. ferromagnetic ( ex: Fe, Co ) Non magnetic ( ex: Cu, GaAs ) Spin accumulation lsfFM lsfNM s s EF, EF, d d Ferromagnet/semiconductor interface Number of spin flips in F: NSFF NSFF>> NSFSC  Unpolarized current in SC Solution: to insert a thin Insulating barrier Or ferromagnetic semiconductors Schmidt; PRB62, R4790 (2000); Fert; PRB64, 184420 (2001); Jaffres; JAP91, 8111 (2001)

  42. Materials for spin injection in semiconductors: DMS DMS

  43. E E E E E G G F F F Materials for spin injection in semiconductors: DMS Paramagnetic DMS (II,Mn)-VI (Zn,Mn)-Se (Zn,Mn)-S (Cd,Mn)-Te No carrier for coupling Paramagnetic, AF or spinglass (III,Mn)-V (Ga,Mn)-As (In,Mn)-As (Ga,Mn)-Sb Ferromagnetic DMS

  44. Materials for spin injection in semiconductors: DMS Field effect transistor Ohno, Nature 408, 944 (2000) Electric control of magnetism: evidence of ferromagnetism induced by carriers

  45. Mn2+ β itinerant carriers β Materials for spin injection in semiconductors: DMS Dietl, Science 287, 1019 (2000) AF coupling between holes and ions Hole induced ferromagnetism

  46. DMMS: Co-doped (La,Sr)TiO3 DMS: semiconductor hosts doped with low concentrations of magnetic ions n  1018 – 1020 cm-3 Doping a metallic host: Co-(La,Sr)TiO3, n  1022 cm-3 Existence of ferromagnetism in La0.5Sr0.5TiO3− doped with Co, TC 500 K Y.G. Zhao et al., Appl. Phys. Lett. 83, 2199 (2003). High carrier density: effect on the magnetic interactions between magnetic ions

  47. DMMS: Co-doped (La,Sr)TiO3 Growth by PLD on STO (001) (T=700°C, PO2=10-7-10-4 mbar) target comp : La0.63Sr0.37Ti0.98Co0.02O3 Co, x = 1% Auger electron spectroscopy Homogeneous Ti and Co distribution No Co-rich clusters larger than 10 nm

  48. g [020] Ti Co La Black is zero; white is 50% in Ti and La images, 1% in Co image. DMMS: Co-doped (La,Sr)TiO3 Transmission electron microscopy  Good structural quality Absence of Co-clusters EELS Homogeneous element distribution; Co content ~0.01 No Co-rich phases larger than 10 nm detected

  49. DMMS: Co-doped (La,Sr)TiO3 Co-doped (La,Sr)TiO3 thin films: XAS & XMCD • Multiplet structure in Co-L2,3 XAS and XMCD spectra • Ionic state of Co (Co2+ in Oh symmetry) • No evidence of metallic Co Metallic Co Ionic Co M  2 mB/Co

  50. -6 p = 1.5*10 mbar O 2 2 -6 p = 5*10 mbar O 2 -6 p = 5*10 mbar (a.u.) O 2 reported T 300 K 1 C M/M 0 0 200 400 600 Temperature (K) DMMS: Co-doped (La,Sr)TiO3 Films are ferromagnetic @ RT

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