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Current-voltage characteristics of manganite heterojunctions:

Current-voltage characteristics of manganite heterojunctions: Unusual junction properties under magnetic field. T. Susaki, N. Nakagawa, and H. Y. Hwang. University of Tokyo June 17, 2005. Magnetic-field dependent manganite junctions. FM-I-FM TMR junction (1996 ~).

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Current-voltage characteristics of manganite heterojunctions:

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  1. Current-voltage characteristics of manganite heterojunctions: Unusual junction properties under magnetic field T. Susaki, N. Nakagawa, and H. Y. Hwang University of Tokyo June 17, 2005

  2. Magnetic-field dependent manganite junctions FM-I-FM TMR junction (1996 ~) Single-interface junction (2004 ~) Low resistance High resistance J. R. Sun et al. Appl. Phys. Lett. 84, 1528 (2004) J. Z. Sun et al. Appl. Phys. Lett. 69, 3266 (1996) • Both positive and negative junction magnetoresistance • What is the origin ?

  3. Questions • What is a key feature of these H-dependent single-interface junctions ? • How relevant (or irrelevant) is the semiconductor band picture for manganite-based junctions ? • This question may be reduced to • 1. difference between correlated and uncorrelated metal • 2. difference between correlated and uncorrelated • semiconductor • in the junction form • If there is a significant difference, what is the fundamental origin of such difference ?

  4. I-V measurement to probe interface under H • What techniques for the interface electronic structure under the magnetic field ? • PES: surface-sensitive, magnetic-field incompatible • EELS: variable magnetic-field incompatible • XAS: signals averaged along surface-normal direction • Kerr effect: • in-plane transport measurement (of superlattice): • Junction I-V measurement: • OK for both interface and magnetic-field study, but • Interpretation is not straightforward compared with electron spectroscopy techniques

  5. Sample preparation La1-xSrxMnO3 Oxygen deficiency shift the properties to LaMnO3 side Y. Tokura et al., JAP 79, 5288 (1996) • Tg: 700~750 oC • Nb:SrTiO3(100) (Nb 0.01 wt %) • PO2:1x10-3 torr (for La0.7Sr0.3MnO3-d) • & • 250 mtorr (for La0.7Sr0.3MnO3) • KrF excimer laser: • ~3 J/cm2, 4 Hz Junction structure

  6. I-V Characteristics under magnetic field La0.7Sr0.3MnO3-d La0.7Sr0.3MnO3 10K 10K N. Nakagawa et al., Appl. Phys. Lett. 86, 082504 (2005) Large negative junction magnetoresistance only in oxygen-deficient junction

  7. Temperature Dependent I-V under no magnetic field • In going from 400 K to 100 K, the slope in semi-log plot becomes sharper (Sawa et al., APL 86, 112508 (2005)) • cf. • The slope changes little below 100 K

  8. Temperature dependent I-V under 8T • Between 150 K and 75 K the slope does not show the temperature dependence and finally it decreases as the temperature is lowered below 50 K.

  9. I-V Characteristics of Au/heavily doped GaAs Thermally-assisted tunneling: Slope-1 = E0 =E00coth(E00/kT)) cooling Thermionic emission: Slope-1 = kT F. A. Padovani and R. Stratton, Solid-State Electron. 9, 695 (1966) Slower temperature dependence of the slope have been analyzed with “thermally-assisted tunneling” model

  10. Tunneling with and without thermal assistance • Large and temperature-independent impurity concentration N (degenerate semiconductor) – thin depletion layer • WKB(-like) calculation of tunneling probability • Direct tunneling at low T • Thermally-assisted tunneling at higher T • Fermi function incorporated • Image force correction neglected • Contribution of the free electrons to the space charge density neglected Thermally-assisted tunneling Em EB f x E Direct tunneling 0 x1 l x Barrier shape: F = Nq2 (x - l)2/2e

  11. Ideality factor and slope temperature According to thermionic-emission model, n: ideality factor nT: “slope temperature”

  12. Experiment and thermally-assisted tunneling model La0.7Sr0.3MnO3-d junction La0.7Sr0.3MnO3 junction 0T 0T 8T Calculated (blue) curves: Slope temperature (E00 estimated from the slope observed at 10 K)

  13. Discussion • As La0.7Sr0.3MnO3-d becomes more metallic, the junction must become closer to metal-semiconductor Schottky junction • This explains the fact that a slight deviation from thermally-assisted tunneling model in La0.7Sr0.3MnO3-d junction is absent in La0.7Sr0.3MnO3 junction but • I-V characteristics of La0.7Sr0.3MnO3-d junction under 8 T significantly deviate from thermally-assisted tunneling model !

  14. Conclusion • By applying magnetic field an unusual junction behavior emerges (Note: both magnetic field and recovery of oxygen stoichiometry increase the metallic character in La0.7Sr0.3MnO3-d) • Further question – the origin of such unusual T-dependence under the magnetic field: • Unoccupied electronic states of manganite ? • Interface ?

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