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 ~) J. R. Sun et al. Appl. Phys. Lett. 84, 1528 (2004) Single-interface junction (2004 ~) Low resistance High resistance J. Z. Sun et al. Appl. Phys. Lett. 69, 3266 (1996) Both positive and negative junction magnetoresistance What is the origin ?

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 ?

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

Sample preparation Y. Tokura et al., JAP 79, 5288 (1996) Junction structure La 1-x Sr x MnO 3 Tg: 700~750 o C Nb:SrTiO 3 (100) (Nb 0.01 wt %) PO 2 :1x10 -3 torr (for La 0.7 Sr 0.3 MnO 3-  ) & 250 mtorr (for La 0.7 Sr 0.3 MnO 3 ) KrF excimer laser: ~3 J/cm 2, 4 Hz Oxygen deficiency shift the properties to LaMnO 3 side

I-V Characteristics under magnetic field La 0.7 Sr 0.3 MnO 3-  10K La 0.7 Sr 0.3 MnO 3 Large negative junction magnetoresistance only in oxygen-deficient junction N. Nakagawa et al., Appl. Phys. Lett. 86, (2005)

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, (2005)) cf. The slope changes little below 100 K

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.

I-V Characteristics of Au/heavily doped GaAs Slower temperature dependence of the slope have been analyzed with “thermally-assisted tunneling” model Thermionic emission: Slope -1 = kT Thermally-assisted tunneling: Slope -1 = E 0 =E 00 coth(E 00 /kT)) cooling F. A. Padovani and R. Stratton, Solid-State Electron. 9, 695 (1966)

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 Barrier shape:  = Nq 2 (x - l) 2 /2  EBEB EmEm x1x1 lx  E 0  Thermally-assisted tunneling Direct tunneling

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

La 0.7 Sr 0.3 MnO 3-  junction La 0.7 Sr 0.3 MnO 3 junction 0T 8T Experiment and thermally-assisted tunneling model Calculated (blue) curves: Slope temperature (E 00 estimated from the slope observed at 10 K)

Discussion As La 0.7 Sr 0.3 MnO 3-  becomes more metallic, the junction must become closer to metal-semiconductor Schottky junction I-V characteristics of La 0.7 Sr 0.3 MnO 3-  junction under 8 T significantly deviate from thermally-assisted tunneling model ! This explains the fact that a slight deviation from thermally-assisted tunneling model in La 0.7 Sr 0.3 MnO 3-  junction is absent in La 0.7 Sr 0.3 MnO 3 junction but

Conclusion By applying magnetic field an unusual junction behavior emerges (Note: both magnetic field and recovery of oxygen stoichiometry increase the metallic character in La 0.7 Sr 0.3 MnO 3-  ) Further question – the origin of such unusual T- dependence under the magnetic field: Unoccupied electronic states of manganite ? Interface ?