Tunneling characteristics and resistivity behavior of La
0.6
Pb
0.4
MnO
3
grain boundaries
P. Chowdhury,
1
S. K. Gupta,
1,
* N. Padma,
1
C. S. Viswanadham,
2
Santosh Kumar,
2
A. Singh,
1
and J. V. Yakhmi
1
1
Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Center, Mumbai 400085 India
2
Laser Processing and Advanced Welding Section, Bhabha Atomic Research Center, Mumbai 400085 India
Received 3 October 2005; revised manuscript received 15 February 2006; published 23 March 2006
The temperature dependences of resistivity, magnetoresistance MR, and current-voltage I-V characteris-
tics of grain boundaries GB in La
0.6
Pb
0.4
MnO
3
thin films prepared using bicrystal SrTiO
3
substrates have
been studied. Comparison of the resistances of the bridges, with and without GB, shows a contribution from
disordered region near the grain boundary. This has been taken into account for determination of the GB
characteristics. Analysis of the I-V characteristics shows that the transport through the grain boundary is
dominated by multistep inelastic tunneling as described by the Glazman and Matveev model. In addition a
small contribution of elastic tunneling in agreement with the Simmons model has been observed. Methodology
for direct determination of the contribution responsible for magnetoresistance has been developed and the
results suggest that only elastic tunneling contributes to magnetoresistance. Small value of elastic tunneling
contribution to total current is found to be responsible for small value of low field magnetoresistance.
DOI: 10.1103/PhysRevB.73.104437 PACS numbers: 75.47.Gk, 73.40.Gk, 75.47.Lx
I. INTRODUCTION
Several investigations on the transport characteristics of
magnetic tunnel junctions MTJ comprising of two ferro-
magnetic metal FM electrodes separated by a thin insulat-
ing barrier have been reported in the literature. These studies
are important due to interesting physics as well as potential
device applications. Charge transport through these junctions
depends on the relative spin orientation of FM electrodes. A
magnetic field applied to a MTJ leads to change in relative
spin orientation of the electrodes and thereby resistance of
the junction. Effect of magnetic field on ideal ferromagnetic-
insulator-ferromagnetic FIF junctions has been described
by Julliere model.
1
According to this model, the tunneling
magnetoresistance TMR, defined by R / R = R
ap
- R
p
/ R
p
is given by 2PP / 1- PP, where R
p
and R
ap
are resistances
with spins of two FM electrodes parallel and antiparallel to
each other, respectively, and P and P are conduction elec-
tron spin polarizations of two electrodes.
1
The rare earth manganites of the type La
x
A
1-x
MnO
3
A
=Ca, Sr, Ba and Pb have been extensively investigated due
to their diversified phase diagram.
2
Parent compound
LaMnO
3
is an antiferromagnetic insulating material with cu-
bic perovskite structure. On doping with divalent atoms, the
manganese ions become mixed valent with fraction x in the
tetravalent Mn
4+
and 1- x the trivalent Mn
3+
state. The
compounds become ferromagnetic insulator for x 0.15 and
ferromagnetic metal for 0.2 x 0.5. The ferromagnetic
transition is explained by double exchange interaction in-
volving simultaneous transfer of hole from Mn
4+
to O and O
to Mn
3+
.
2
The compound investigated in present study,
La
1-x
Pb
x
MnO
3
0.25 x 0.45, has slightly distorted rhom-
bohedral structure.
3,4
Single crystals of this material show a
paramagnetic insulator to ferromagnetic metal transition with
Curie temperature ranging from 315 to 350 K.
3
The manga-
nites have often been used for studying magnetic tunnel
junctions due to their half-metallic nature with almost com-
plete intrinsic spin polarization as the higher value of spin
polarization is expected to yield better TMR in terms of Jul-
liere model. The tunneling characteristics in these materials
have been generally investigated through two types of
samples. In some of the studies, polycrystalline materials,
with grain boundary GB tunnel junctions
5
have been used.
However, in polycrystaline materials it is difficult to separate
the GB and the intragrain characteristics. In other studies,
artificial tunnel junctions, such as, tri-layer,
6
step-edge,
7
bicrystal grain boundary,
8–15
and ramp
16
have been prepared
in epitaxial thin films. In these junctions, tunneling charac-
teristics may be accurately determined after taking into ac-
count the epitaxial film resistance. Artificial grain boundaries
prepared using bi-crystal substrates
10
and trilayers of
La
2/3
Sr
1/3
MnO
3
/ SrTiO
3
/La
2/3
Sr
1/3
MnO
3
6
have shown TMR
of 300 and 1800%, respectively, at 4.2 K. However, the
TMR is found to decrease on increase of temperature.
17
The
conduction mechanism through tunnel junctions has been
studied by measurement of the I-V characteristic, which are
generally nonlinear.
8,11,12,15
Two different mechanisms, elas-
tic and inelastic tunneling, have been used to understand the
I-V characteristics. Elastic tunneling refers to direct tunnel-
ing of charge carriers between two electrodes without scat-
tering or change in spin direction. Inelastic tunneling occurs
via impurities within the insulating barrier in single or mul-
tiple steps. This may involve flip in spin direction of the
carriers. In some of the measurements, inelastic tunneling
with loss of polarization at the interface was found to
dominate conduction mechanism.
18
In other studies, elastic
tunneling model
19
is reported to yield a reasonable fit to
I-V characteristics.
20
Gunnarson et al.
15
have shown that the
barrier height of the GB junctions depends on magnetization
of the sample. Therefore, it reduces with increase in tempera-
ture and is negligible near ferromagnetic transition tempera-
ture T
c
. Consequently, the excess resistance of the grain
boundary is finite for T T
c
only.
21
In a detailed study of
I-V characteristics, Paranjape et al.
11
have used multistep
tunneling model proposed by Glazman and Matveev GM
22
to analyze the results. The temperature and magnetic field
PHYSICAL REVIEW B 73, 104437 2006
1098-0121/2006/7310/1044377/$23.00 ©2006 The American Physical Society 104437-1