Current-voltage characteristics of manganite heterojunctions: Unusual junction properties under...
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Current-voltage characteristics of manganite heterojunctions:
Unusual junction properties under magnetic field
T. Susaki, N. Nakagawa, and H. Y. Hwang
University of TokyoJune 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 mag
netoresistance • 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)
10-4
10-3
10-2
10-1
100
Re
sist
ivity
(
cm)
4003002001000
Temperature (K)
0T 4T 8T
La0.7Sr0.3MnO3-film
La0.7Sr0.3MnO3 film
Nb doped SrTiO3 crystal
La0.7Sr0.3MnO3- film
Gold electrode
aluminumelectrode
VF
IF
H
Junction structureJunction structure
LaLa1-x1-xSrSrxxMnOMnO33
• Tg: 700~750 oC• Nb:SrTiO3(100) (Nb 0.01 wt %)• PO2:1x10-3 torr (for La0.7Sr0.3MnO3-)
& 250 mtorr (for La0.7Sr0.3MnO3)• KrF excimer laser: ~3 J/cm2, 4 Hz
Oxygen deficiency shift the properties to LaMnO3 side
I-V Characteristics under magnetic field
4
3
2
1
0Cur
rent
Den
sity
(10
-2A
/cm
2)
1.00.90.80.70.60.5
Bias Voltage (V)
0.5
T0
T
1T
2T
3T
4T
5T
6T
8T
7T
LaLa0.70.7SrSr0.30.3MnOMnO3-3-
10K 10K
LaLa0.70.7SrSr0.30.3MnOMnO33
Large negative junction magnetoresistanceLarge negative junction magnetoresistance o only in oxygen-deficient junctionnly in oxygen-deficient junction
4
3
2
1
0Cur
rent
Den
sity
(10
-2A
/cm
2)
0.70.60.50.40.30.2
Bias Voltage (V)
8T 0T
N. Nakagawa N. Nakagawa et alet al., Appl. Phys. Lett. ., Appl. Phys. Lett. 8686, 082504 (2005), 082504 (2005)
-20
-15
-10
-5
lnJ
(A/c
m2)
0.60.40.20.0
Bias Voltage (V)
10
K5
0K
10
0K
15
0K
20
0K
22
5K
30
0K
35
0K
40
0K
12
5K
17
5K
25
0K
27
5K
32
5K
37
5K
La0.7Sr0.3MnO3-20
-15
-10
-5ln
J (A
/cm
2)
1.00.80.60.40.20.0-0.2
Bias Voltage (V)
50
K
10
0K
15
0K
20
0K
25
0K
30
0K
35
0K
75
K
10
K
12
5K
17
5K
25
K
32
5K
37
5K
40
0K
La0.7Sr0.3MnO3-
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
])/)(exp[( nkTqVTJsJ
-20
-15
-10
-5ln
J (A
/cm
2)
1.00.80.60.40.20.0-0.2
Bias Voltage (V)
50
K
10
0K
15
0K
20
0K
25
0K
30
0K
75
K
10
K
12
5K
17
5K
25
K
La0.7Sr0.3MnO3-
8T-14
-12
-10
-8
lnJ
(A/c
m2)
0.700.650.600.550.500.45
Bias Voltage (V)
50
K
10
0K
15
0K
75
K
10
K
12
5K
25
K
La0.7Sr0.3MnO3-
8T
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 = E0 =E00coth(E00/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:
= Nq2 (x - l)2/2
EBEm
x1 l x
E
0
Thermally-assisted tunneling
Directtunneling
1
2
3
4
567
10
2
Idea
lity
fact
or
4003002001000
Real temperature (K)
La0.7Sr0.3MnO3- 0T La0.7Sr0.3MnO3- 8T La0.7Sr0.3MnO3 0T
400
300
200
100
0
Slo
pe te
mpe
ratu
re (
K)
4003002001000
Real temperature (K)
La0.7Sr0.3MnO3- 0T La0.7Sr0.3MnO3- 8T La0.7Sr0.3MnO3 0T
Ideality factor and slope temperature
kT
ETATJs
nkTqVTJsJ
Bexp)(
])/)(exp[(
2*
According to thermionic-emission model, n: ideality factornT: “slope temperature”
La0.7Sr0.3MnO3- junctionLa0.7Sr0.3MnO3 junction
0T 0T 8T
400
300
200
100
0
Slo
pe te
mpe
ratu
re (
K)
4003002001000
Real temperature (K)
400
300
200
100
0
Slo
pe te
mpe
ratu
re (
K)
4003002001000
Real temperature (K)
400
300
200
100
0
Slo
pe te
mpe
ratu
re (
K)
4003002001000
Real temperature (K)
Experiment and thermally-assisted tunneling model
Calculated (blue) curves: Slope temperature
)/coth(/ 00000 kTEEE (E00 estimated from the slope observed at 10 K)
Discussion
• As La0.7Sr0.3MnO3- becomes more metallic, the junction must become closer to metal-semiconductor Schottky junction
• I-V characteristics of La0.7Sr0.3MnO3- 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 La0.7Sr0.3MnO3- junction is absent in La0.7Sr0.3MnO3 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 La0.7Sr0.3MnO3-)
Further question – the origin of such unusual T-dependence under the magnetic field:• Unoccupied electronic states of manganite ?• Interface ?