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 ?