1 / 42

Perovskite-type transition metal oxide interfaces

Perovskite-type transition metal oxide interfaces. M. Matvejeff. 7.2.2011. Contents. Perovskites - Chemistry and properties Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs) Charge transfer at perovskite interfaces. A B O. Perovskites – Structure.

ward
Download Presentation

Perovskite-type transition metal oxide interfaces

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Perovskite-type transition metal oxide interfaces M. Matvejeff 7.2.2011

  2. Contents • Perovskites - Chemistry and properties • Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs) • Charge transfer at perovskite interfaces

  3. A B O Perovskites – Structure AO+BO2= ABO3 SrTiO3 (La,Sr)MnO3 (LSMO) 1 u.c. AO BO2 AO BO2 AO

  4. Perovskites – The Good Highly flexible cation stoichiometry Wide variety of functional properties through changes in cation stoichiometry (La1-xSrx)MnO3 (LSMO) Imada et al. Rev. Mod. Phys. 70

  5. A B O Perovskites – The Good Highly flexible cation stoichiometry Wide variety of functional properties through changes in cation stoichiometry Highly flexible oxygen content  Properties can be fine-tuned after synthesis AO1- + BO2 = ABO3- SrTiO3- (La,Sr)MnO3- (LSMO) 1 u.c. AO BO2 AO BO2 AO

  6. The flexibility of perovskite structure and the easy tunability of the functional properties are definite bonuses as long as bulk material is suitable for applications For example capacitors, catalytic converters and superconductors

  7. However, significant number of industrial applications rely on device structures consisting of several different functional material layers, in some cases only few atomic layers in thickness In these structures, such as field-effect transistors (FETs), the properties of the interface are often significantly more important to the correct function of the device than the properties of the bulk material

  8. A B O Perovskites – The Bad Highly 3-dimensional structure + Strong hybridization of 3d orbital of the transition metal B to neighboring oxygen 2p orbitals + Highly sensitive to small changes in transition metal oxidation state Properties at interfaces? 1 u.c. AO BO2 AO BO2 AO

  9. Contents • Perovskites - Chemistry and properties • Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs) • Charge transfer at perovskite interfaces

  10. CMR in manganites Manganites exhibit CMR i.e. strong change in resistivity under applied magnetic field The CMR effect can be used for example for magnetic sensor applications As the most properties of transition metal oxides, CMR is highly dependent on transition metal (Mn) oxidation state Colossal MR (CMR) in La2/3Ba1/3MnO3 R. von Helmholt APL 1993

  11. Electronic structure of manganites General formula AMnO3 A = divalent and/or trivalent cation (Ca, Sr, La, Nd...) To understand the origin of CMR phenomenon we need to first understand the electronic structure of manganites Itinerant electron Local electrons (La,Sr)MnO3 (LSMO) Mn3+ Mn4+ eg eg t2g t2g

  12. Chemical substitution means we’re directly playing with the average valence of Mn +2 +4-2 +3 +3-2 SrMnO3 LaMnO3 General formula AMnO3 A = divalent and/or trivalent cation (Ca, Sr, La, Nd...) +3 +2 3...4-2 La1-xSrxMnO3 x Mn4+ 1-x Mn3+ Itinerant electron Local electrons (La,Sr)MnO3 (LSMO) Mn3+ Mn4+ eg eg t2g t2g

  13. Mn3+/Mn4+-ratio (doping) has strong impact on magnetotransport properties In double-exchange (DE) model Itinerant eg electron is the charge carrier whereas the t2g electrons are localized +2 +4-2 +3 +3-2 SrMnO3 LaMnO3 +3 +2 3...4-2 La1-xSrxMnO3 x Mn4+ 1-x Mn3+ Itinerant electron Local electrons (La,Sr)MnO3 (LSMO) Mn3+ Mn4+ eg eg t2g t2g

  14. What are magnetic tunnel junctions (MTJs)? Bulk CMR is not suitable for low field applications (magnetic field required is in order of several tesla) How to increase sensitivity? Significantly weaker field (~coercive field of the material) required in MTJs Tunneling current Tunneling current Magnetic tunnel junction (MTJ) FM FM Insulator (t = nm-Å) Insulator (t = nm-Å) FM FM

  15. Magnetization AP Magnetic field required is in order of tens to hundreds of Oe instead of several Tesla as for bulk CMR  low field sensors For maximum sensitivity RA-RAP has to be maximized  Degree of spin polarization is important! Junction resistance R P Applied field Applied field TMR RA(AP) Resistance in parallel (antiparallel) configuration P1,P2 Polarizations of electrodes 1 and 2 Tunneling current MTJ 1 FM Tunneling current Applied field Insulator 2 FM

  16. Half-metals – Because polarization does matter… R. von Helmholt APL 1993 P. M. Tedrow and R. Meservey PRB 1973

  17. Half-metallicity in bulk La0.7Sr0.3MnO3 Y. Lu, APL 1996 P ~ 95-100% in low T LSMO is a good candidate material for MTJs J.-H. Park Nature 1998

  18. 4.2K Good TMR only at low T TMR dissappears well below Tc Why? Tc ~ 350K LSMO STO LSMO T. Obata, APL 1999

  19. Dead layer La0.67Sr0.33MnO3 films grown on (110) NGO (NdGaO3) and (001) LAO (LaAlO3) substrates Clear thickness dependence in resistivity Dead (insulating) layer forms at the interface? How can we study this? J. Z. Sun APL 1999

  20. Dead layer (2-10 u.c. LSMO – 2 u.c. STO)10-20 superstructure LSMO = La1-xSrxMnO3, 0.2  x  0.4 By changing the thickness of conducting layers (LSMO) separated by the insulator (SrTiO3) we can probe the critical thickness for transition from ferromagnetic metal (FM) to antiferromagnetic insulator (AFI) STO (2 u.c.) LSMO (2-10 u.c.) STO (2 u.c.) LSMO (2-10 u.c.)

  21. Dead layer For all doping doping levels, decrease in Tc and magnetization with decreasing LSMO thickness Decrease is faster with higher x  Samples which are closer to metal to insulator-phase diagram line loose metallicity and magnetic order already in thicker films M. Izumi J. Phys. Soc. Jpn. 2002 Y. Tokura Rep. Prog. Phys. 2006

  22. Dead layer Same effect also observed in M-H measurements Also, for thinner films M-H does not saturate  This indicates competing FM and AFM interactions FM+AFM + ext. field! FM FM+AFM M. Izumi J. Phys. Soc. Jpn. 2002

  23. Dead layer So how does the dead layer actually form? H. Fujishiro J. Phys. Soc. Jpn 1998 From phase diagram we see transition from FM to AF state at x ~ 0.5 Is this related to the formation of dead layer at the interface? Y. Tokura Rep. Prog. Phys. 2006

  24. So what does actually happen at the interface layer? STO (2 u.c.) LSMO (2-10 u.c.) STO (2 u.c.) LSMO (2-10 u.c.)

  25. Dead layer M. Izumi J. Phys. Soc. Jpn. 2002 • Hole-doping at La1-xSrxMnO3-STO interface • x increases • The properties of the interface change Effect is stronger when x in the original phase is higher (already closer to critical limit of x ~ 0.5) Why does the hole-doping occur? La0.4Sr0.4MnO3 (x = 0.4) Bulk High Tc High magnetization FM La0.8Sr0.2MnO3 (x = 0.2) Bulk High Tc High magnetization FM STO (2 u.c.) STO (2 u.c.) Hole-doped LSMO (x 0.4) Faster decrease in properties Hole-doped LSMO (x 0.2) FM+AFM Lower Tc/magnetization xincreases (charge transfer) xincreases Y. Tokura Rep. Prog. Phys. 2006

  26. Contents • Perovskites - Chemistry and properties • Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs) • Charge transfer at perovskite interfaces

  27. Let’s study the following a quantum well structure… In theory the Ti valence changes sharply at the interface between SrTiO3 (STO) and LaTiO3 (LTO) • Sr2+ and O2- • Ti4+ • (2 + x + 3*(-2) = 0) SrTiO3 • La3+ and O2- • Ti3+ • (3 + x + 3*(-2) = 0) LaTiO3 SrTiO3

  28. La Sr Ti O Ti4+ SrTiO3 Ti3+ Ti4+ LaTiO3 SrTiO3

  29. La Sr Ti O However in practice it has been found out that Ti3+ oxidation state is not limited to the LTO layers… Ti4+ SrTiO3 Ti3+ fraction Ti3/4+ Ti3+ LaTiO3 Ti3/4+ Ti4+ SrTiO3 Ohtomo A. et al., Nature, 2002 … i.e. charge transfer (transfer of electrons) occurs from LTO into STO layers forming mixed valence interface layer

  30. Now, our ideal TMR device the LSMO/STO/LSMO tunnel junction LSMO TC~ 350 K in the bulk phase FM LSMO Insul. FM STO LSMO

  31. Y. Tokura Rep. Prog. Phys. 2006 In practice, charge transfer over the interface Strong impact on carrier density (valence of Mn) at the interface Instead of FM, LSMO at interface either P or AF Formation of dead layer and TC 100 K instead of 350 K! FM LSMO P/AF Insul. STO P/AF FM LSMO

  32. A B O Perovskite - recap Alternating AO and BO2 layers Formula: ABO3 3D structure is the problem! So what about structures which aren’t (fully) 3D? 1 u.c. AO BO2 AO BO2 AO

  33. Ruddlesden-Popper structure Closely related to perovskite structure Alternating AO and BO2 layers Formula: An+1BnO3n+1 (i.e. one extra AO-layer compared to perovskites, ABO3)

  34. Perovskite: 3D structure vs Ruddlesden-Popper (RP): 2D High anisotropy (ab-plane vsc-axis) n = 2 RP (A3B2O7) 1 formula unit 1 u.c. AO AO AO AO AO BO2 BO2 BO2 BO2 BO2 Perovskite (ABO3) BO2 AO AO AO AO AO BO2 AO BO2 AO BO2 AO BO2 c-axis AO

  35. Charge carriers T. Kimura & Y. Tokura, Annu. Rev. Mater. Sci., 2000 La1.4Sr1.6Mn2O7 1 formula unit Charge carriers AO AO AO AO AO BO2 BO2 BO2 BO2 BO2 BO2 AO AO AO AO A= La, Sr B = Mn

  36. Strong interaction Modulation of interface properties Perovskite 1 Perovskite 1 Perovskite Perovskite 2 RP Weak interaction Clean interface, little or no modulation Perovskite

  37. So does it actually work? In perovskite-type interface between (La,Sr)MnO3/(La,Sr)FeO3electrons are transferred from Mn eg states to Fe eg states We can study the interface electronic structure in XPS… Kumigashira et al. APL 2004

  38. … to determine the occupation of eg and t2g states As LSFO layer thickness is increased, the charge transfer increases and eg electron occupation decreases (Mn valance increases) LSFO (t = 1-7 layers) LSMO t2g eg Itinerant electron Local electrons Mn3+ Mn4+ eg eg t2g t2g

  39. LSFO Strong interaction Large change in LSMO valence LSMO LSFO Weak interaction Clean interface Small change in LSMO valence? LSMO

  40. Perovskite t2g eg RP-type interface (LSMO layer thickness = 3 u.c.)

  41. Conclusions • Perovskite phases exhibit interesting functional properties in bulk form • Applications, however, are often based on device structures built from functional layers at times only few atomic layers in thickness • Interface effects arising from the 3-dimensional nature of the perovskite structure dominate the behavior of the devices Interface effects can be, at times, partially compensated for, but this leads to expensive production processes where device properties are difficult to predict and/or control • Best solutions would be based on integrating, property-wise, 2-dimensional materials into device structures to create not only structurally but also electronically sharp interface structures

More Related