Perovskite-type transition metal oxide interfaces 7.2.2011 M. Matvejeff

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

Perovskites – Structure A B O AO BO2 AO + BO2 = ABO3 SrTiO3 (La,Sr)MnO3 (LSMO)

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

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 AO 1 u.c. BO2 AO BO2 AO AO1- + BO2 = ABO3- SrTiO3- (La,Sr)MnO3- (LSMO)

For example capacitors, catalytic converters and superconductors 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

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

Properties at interfaces? Perovskites – The Bad A B O 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? AO 1 u.c. BO2 AO BO2 AO

Perovskites - Chemistry and properties Contents Perovskites - Chemistry and properties Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs) Charge transfer at perovskite interfaces 10

CMR in manganites Colossal MR (CMR) in La2/3Ba1/3MnO3 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

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 (La,Sr)MnO3 (LSMO) Local electrons Mn3+ t2g eg Mn4+

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

LaMnO3 SrMnO3 La1-xSrxMnO3 In double-exchange (DE) model x Mn4+ 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 LaMnO3 +3 +3 -2 SrMnO3 +2 +4 -2 La1-xSrxMnO3 +3 +2 3...4 -2 x Mn4+ 1-x Mn3+ Itinerant electron (La,Sr)MnO3 (LSMO) Local electrons Mn3+ t2g eg Mn4+ t2g eg

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 Magnetic tunnel junction (MTJ) FM FM Tunneling current Tunneling current Insulator (t = nm-Å) Insulator (t = nm-Å) FM FM

RA(AP) Resistance in parallel (antiparallel) configuration P1,P2 Polarizations of electrodes 1 and 2 TMR R P AP Magnetization Applied field resistance Junction 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! Applied field MTJ 1 FM Tunneling current Tunneling current Applied field Insulator 2 FM

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

LSMO is a good candidate material for MTJs J.-H. Park Nature 1998 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

TMR dissappears well below Tc 4.2K Tc ~ 350K Good TMR only at low T TMR dissappears well below Tc Why? 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

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) LSMO (2-10 u.c.) STO (2 u.c.)

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 Y. Tokura Rep. Prog. Phys. 2006 M. Izumi J. Phys. Soc. Jpn. 2002

Dead layer Same effect also observed in M-H measurements M. Izumi J. Phys. Soc. Jpn. 2002 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 FM+AFM + ext. field!

Dead layer So how does the dead layer actually form? H. Fujishiro J. Phys. Soc. Jpn 1998 So how does the dead layer actually form? 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

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

Dead layer Hole-doping at La1-xSrxMnO3-STO interface x increases 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? x increases STO (2 u.c.) La0.4Sr0.4MnO3 (x = 0.4) Bulk High Tc High magnetization FM Hole-doped LSMO (x  0.4) Faster decrease in properties (charge transfer) x increases STO (2 u.c.) La0.8Sr0.2MnO3 (x = 0.2) Bulk High Tc High magnetization FM Hole-doped LSMO (x  0.2) FM+AFM Lower Tc/magnetization Y. Tokura Rep. Prog. Phys. 2006

Perovskites - Chemistry and properties 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) SrTiO3 LaTiO3 Sr2+ and O2- Ti4+ (2 + x + 3*(-2) = 0) La3+ and O2- Ti3+ (3 + x + 3*(-2) = 0)

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

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

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

In practice, charge transfer over the interface 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

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

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)

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

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

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

We can study the interface electronic structure in XPS… So does it actually work? In perovskite-type interface between (La,Sr)MnO3/(La,Sr)FeO3 electrons 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

t2g eg Mn3+ Mn4+ … 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) t2g eg LSMO LSFO (t = 1-7 layers) Itinerant electron Local electrons Mn3+ t2g eg Mn4+ 39

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

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 42