Currentvoltage characteristics of manganite heterojunctions:

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 700750 oC
• NbSrTiO3(100) (Nb 0.01 wt )
• PO21x10-3 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
x1
l
x
0
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 ?