Combining Ferroelectricity, Magnetism, and

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1 Forschungszentrum Jülich Combining Ferroelectricity, Magnetism, and Superconductivity ty in Tunnel Junctions H. Kohlstedt 1, N. A. Pertsev 2,A. Petraru 1, U. Poppe 1, and R. Waser 1 1 CNI Center of Nanoelectronic Systems for Information Technology (IFF-IEM) 2 A.F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, , St. Petersburg, Russia Arusha, Tansania August 2007

2 Layer Sequence of a Tunnel Junction Tunnel Barrier (1 nm 3 nm) Dielectric Top electrode (50 nm) Substrate B Bottom Electrode (50 nm)

3 Outline Electron Tunneling and Junctions (Overview) Ferroelectric Tunnel Junctions Size Effects and Boundary Conditions Tunnel Junction: An Interfacial Device A novel zoo of Tunnel Junctions

4 Electron Tunneling and Junctions

5 Quantum Mechanical Electron Tunneling E φ e k x real k k x x real Ψ A imaginary Ψ C x Ψ B T Transmission coefficient t 2 = C exp 2m φ( x) dx h 0 Frenkel, Phys. Rev. 36 (1930).; A. Sommerfeld and H. Bethe, Handbuch der Physik, Springer 1933, XXIV, p.450 R. Holm and W. Meissner, Z. Phys. 74, 715 (1932).

6 Tunnel Junctions: A short Survey Me I Me Su I Su Mag I Mag Semic I Me E g I I ΔR/R I V V H V Sommerfeld/Frenkel/Holm Giaever/Josephson Jullieré/Moodera/Parkin/ (Sun/Fert for oxides) Esaki

7 Superconducting -, Magnetic-, and Ferroelectric Tunnel Junctions Dielectric barrier Density of states effects Superconductor Superconductor Magnet Magnet 2π e A I( V ) = T( E) n ( E ev ) n2 h ( E )[ f ( E ev ) f ( E) ]de 1 Ferroelectric tunnel junction: Metal Metal Cooperative phenomenon located in the barrier! Ferroelectric Barrier

8 Ferroelectric Tunnel Junctions

9 Ferroelectric Tunnel Junction Kohlstedt, Pertsev, Waser, Ferroelectric Thin Films X, Vol. 688 (Material Research Society) 2002, p European Patent: A1, 1994 R. M. Wolf and P. W. M. Blom, Philips Electronics, Eindhoven (NL). IBM Technical Disclosure Bulletin 13, 2161 (1971). Patent Experiment

10 Ferroelectric Tunnel Junction High-Resolution TEM, C. Jia, Jülich P SrRuO 3 e unit cells PZT A V 4 nm SrRuO 3 T Tunneling matrix element: x 2 1 = C exp 2m φ ( x ) dx h x 0

11 Quantum Mechanical Electron Tunneling Polarization State of PZT Current-Voltage Curve?

12 Possible Effects: Tunnel Current vs. Polarization H. Kohlstedt et al., Phys. Rev. B 72, (2005).

13 Strain: Barrier Effects a. Variation of barrier thickness t = t + o d33 V -V C t -t 0 V C Voltage b. Shift of the conduction and valence band edges * t m T C E E dz C z h 2 0 = exp 4π k k + 0 z k E C = z * 2 2m k = z 2 = E 0 C h + κ 3 S ( 33 E 0 C E z ) κ 3 Deformation Potential: Brooks 1955, Herring 1956, Kane 1970 c. Change of the electron effective mass * 2 = h m, k = 0 2 * a Δ E * m C m = m + 0 S 33 S a: lattice parameter, Tight binding approx. * * * m d33v = m S33 t0

14 Interfacial Effect Symmetric barrier structure φ 1 φ 2 t t` I φ 1 φ 2 φ 1 φ 2 V Electrode Ferroelectric Electrode Symmetric I-V Asymmetric barrier structure φ = ( φ 1 + φ 2 ) / 2 a variable fixed I φ = ( φ 1 + φ2) / 2 b φ 2 φ 1 φ 2 V Asymmetric I-V

15 Origin of Giant Electroresistance (from E. Tsymbal, U Lincoln, Nebraska) Metal FE Metal Metal FE Metal + + P P + Electrostatic Potential E E Tunneling E F E F Potential Different potential (and barrier width) for transport electrons M.Ye. Zhuravlev R. F. Sabirianov S. S. Jaswal and E.Y. Tsymbal, PRL 94, (2005)

BaTiO 3 : 5nm SrRuO 3 BaTiO 3 SrRuO 3 C (p pf) 16 14 12 10 8 6 E(kV/ (kv/cm) -1200-800 -400 0 400 800 1200 SrTiO 3 P (μc C/cm 2 ) 30 20 10 0-10 E (KV/cm) -1200-800 -400

16 BaTiO 3 : 5nm SrRuO 3 BaTiO 3 SrRuO 3 C (p pf) E(kV/ (kv/cm) SrTiO 3 P (μc C/cm 2 ) E (KV/cm) Hz 4 2 f=1000 Hz@ 300K -0,6-0,4-0,2 0,0 0,2 0,4 0,6 U(V) U (V) Similar to: Y.S Kim et al., APL 86, (2005).

17 I-V curve of a SRO/BTO/SRO Junction I(μA) nm BTO at 1 khz nm BTO ,0-0,5 0,0 0,5 1,0 U (V) Could not detect displacement current

18 Current Transport Measurements Current [ma] 0.5 Electric Field [kv/cm] nm PZT Voltage [V] K ty [ka/cm 2 ] Curre ent Densi Pt PbZr 0.52 Ti 0.48 O 3 SrRuO 3 SrTiO 3 J. Rodriguez Contreras et al., APL 83, 4959 (2003). No direct (elastic) tunnling Switching not caused by ferroelectricity! K. Szot et al., Nature Mat. 2006

19 Size Effects and Boundary Conditions

20 Ultra thin Ferroelectric Oxide Films nm Number of unit cells Junquera and Ghosez (BTO) Rappe (PTO) Tybell (PZT) Ghosez and Rabe (PTO) Pertsev (PTO) Streiffer (PTO) Streiffer (PTO) Lichtenste eiger (PTO) Gerra (BTO) Electron direct tunneling regime Kim (BTO) Nagarajan (PZT) nm Year

21 Strain enhanced Ferroelectricity N.A. Pertsev, et al., Phys. Rev. Lett. 80, 1988 (1998) K. J. Choi, et al., Science, (2004). Film Substrate: side view Enhancement of P possible S m = (b a 0 )/b b = Substrate lattice parameter a 0 = Equiv. cubic cell constant of free film, Prototypic cell out-of-plane c b in-plane a S m : Misfit strain

22 Electrical Boundary Conditions Metal _ E ds =0 E D P E D (only for perfect screening!!) _ Metal Φ t P. Würfel and I. P. Batra, Ferroelectrics 12, 55 (1976). J. Juncquera and Ph. Ghosez, Nature 422, 506 (2003).

23 Tunnel Junction: An Interfacial Device

24 Tunnel Junction: An interfacial device! Fermi s Golden Rule Initial State Travel Final State Tunneling electrons are extremely sensitive to barrier and interface properties!! examples?

25 Tunneling electrons - coupling to excitations (Inelastic) Electron Tunneling Spectroscopy Molecule and Phonon Electron-Phonon Magnons Spectroscopy Coupling α 2 (ω,k) P. Balk, JAP 1991 E. L. Wolf, PRB 1985 J. S. Moodera, PRL 1998 n-si/sio 2 /Al Nb/MgO/Ag Co/Al 2 O 3 /Ni 80 Fe 20

26 Magnetic Oxide Tunnel Junctions LSMO/SrTiO 3 K Y. Lu et al., PRB 54, R8357 (1996). 3 nm SrTiO 3 barrier 3

27 Interface Effect in Magnetic Tunnel Junctions Spinpolarization influenced by Barrier Material J. M. De Teresa et al., Science 286, 507 (1999).

28 Tunneling Magneto Resistance An Interface Effect! Fe MgO Fe Ch. Heiliger et al., Phys. Rev. B 73, J. S. Moodera, G. Mathon JMMM 200, 248 (1999). First layer adjacent to tunnel barrier is essential! Drastically change in TMR!!

29 Electrical Boundary Conditions: An endless story Metal _ E ds =0 E D P E D (only for perfect screening!!) _ Metal Φ P. Würfel and I. P. Batra, Ferroelectrics 12, 55 (1976). J. Juncquera and Ph. Ghosez, Nature 422, 506 (2003). t

30 Alternative Screening Mechanism Ionic Screening Fong, et al., Phys. Rev. B 71, (2005). Theoretically confirmed: G. Gerra et al., PRL (2006).

31 Alternative Screening Mechanism metal ferroelectric Thomas-Fermi screening and Kretschmer-Binder effect C TF C KB Bond charge compensation by free carriers in the ferroelectric Extension of the ionic i polarization into the metal; Ionic distortion also in the metal Sketch taken from G. Gerra et al., PRL (2006). Fig.1

32 Magnetoelectric Interface Effect Fe/BaTiO 3 P P Interface between a ferromagnet and a ferroelectric Top interface Minority-spin charge density DOS Paraelectric BTO Bottom interface Ferroelectric BTO C.-G. Duan, S.S. Jaswal and E. Y. Tsymbal, PRL 97, (2006). E F

33 Interface without Ionic Screening BaTiO 3 + SrRuO 3 -

34 Interface with Ionic Screening Ferroelectric + Magnet or Superconductor Local atomic rearrangement at the interface Variation of DOS at the interfaces Tunneling current modified by interface properties - TMR vs. P? Josephson-Effects vs. P?

35 Magnetic and superconducting junctions with ferroelectric barrier La 0.33 Sr 0.67 MnO 3 BaTiO 3 P La x Sr 1-x CuO 3 BaTiO 3 La 0.33 Sr 0.67 MnO 3 La xsr 1-x CuO 3 The (ferroelectric) polarization might modify the spin polarization and superconducting order parameter ( ξ 0.1 nm) (at the interfaces) Tunneling magneto resistance as well as quasiparticle Tunneling magneto resistance as well as quasiparticle current and Josephson current should depend on P!

36 An optimistic Outlook: A novel zoo of tunnel junctions Paramagnet (Anti)-Ferromagnet P, M Superconductor Multiferroic (Insulator) (Tunnel Barrier) Ferroelectric Anti-ferroelectric Pyroelectric Piezoelectric Dielectric Magnetic Anti-ferromagnetic Josephson-Junction Junction with a ferroelectric barrier dc and ac Josepson Effect vs. P? Magnetic Tunnel Junction Tunnel Magneto Resistance vs. P? E. Y. Tsymbal and H. Kohlstedt, Science 2006

37 Multiferroic Tunnel Junctions M. Gajek et al., Tunnel junctions with multiferroic barriers Nature Mat La: BiMnO 3 Sheng Ju et al, PRB 75, bit Memory: 2 from Ferroelectricity 2 from Magnetism More about Multiferroics: N. A. Spaldin and M. Fiebig, Science (2005). R. Ramesh et al. Phil Mag. Lett. (2007). W. Eerenstein, ee N. D. Mathur, u, J. F. Scott Nature (2006).

38 Conclusion Quantum Mechanical Electron Tunneling and Multiferroic i Materials: Development of new tunnel junctions New Functionalities Will propel exciting theoretical approaches Tunneling Electrons will be applied as an analytical l tool Better understanding of multiferroic materials on the nm level Multiple size effects Challenge: Defect free/ideal ferroelectric tunnel barriers, I- V curves alone are not sufficient to extract the underlying switching mechanism There is a gap between theory and experiment!

Acknowledgement Sponsors: Center of Nanoelectronic Systems for Information Technology Volkswagen-Foundation: Nano-sized ferroelectric hybrids under contract t number I/77 737 Joint NSF-DFG Project:

39 Acknowledgement Sponsors: Center of Nanoelectronic Systems for Information Technology Volkswagen-Foundation: Nano-sized ferroelectric hybrids under contract t number I/ Joint NSF-DFG Project: University of Berkeley (Material Science Department) University of Aachen (RWTH) Research Center Juelich DFG: Displacive and Conductive Phenomena in Ferroelectric Thin Films: Scaling effects and switching properties Synchrotronstrahlungsexperimente zu Skalierungseffekten und ungewöhnlichen Phasen epitaktischer, perowskitscher Schichten

Hermann Kohlstedt. Technical Faculty Nanoelectronics

Hermann Kohlstedt. Technical Faculty Nanoelectronics Forschungszentrum Jülich Institut für Festkörperforschung Jülich, Germany Complex Oxide Tunnel Junctions Hermann Kohlstedt Christian-Albrechts-University Kiel Technical Faculty Nanoelectronics Germany