Journal of Alloys and Compounds 479 (2009) 357–362
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Electrical properties and enhanced room temperature magnetoresistance in
(La
0.7
Ca
0.2
Sr
0.1
MnO
3
)
1−x
/Pd
x
composites
Caoshui Xiong
a,∗
, Haixin Hu
a
, Yonghong Xiong
a
, Zhenhua Zhang
b
, Houli Pi
a
,XiWu
a
,
Lijun Li
a
, Fenfen Wei
a
, Chongfeng Zheng
a
a
School of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
b
School of Physics and Electronic Science, Changsha University of Science and Technology, Changsha 410076, People’s Republic of China
article info
Article history:
Received 5 September 2008
Received in revised form 3 December 2008
Accepted 15 December 2008
Available online 25 December 2008
PACS:
75.47.Lx
72.80Tm
61.72.Mn
75.47.Gk
Keywords:
Composite materials
Solid state reaction
Grain boundary
Electrical transport
Low field magnetoresistance
abstract
The structural, electrical, magnetic, and magnetoresistance (MR) properties for perovskite
(1 − x)La
0.7
Ca
0.2
Sr
0.1
MnO
3
(LCSMO)/Pd
x
have been studied in this present work. The results of X-
ray diffraction, scanning electron microscopy and magnetic measurement indicate that no reaction
occurs between LCSMO and Pd grains, and that Pd segregates mostly at the grain boundaries of LCSMO.
Pd addition has a little influence on the Curie temperature (T
C
), but it decreases the resistivity ()dra-
matically and shifts the metal–insulator transition temperature (T
p
) towards a higher value substantially.
In the low-temperature ferromagnetic (FM) metallic region the data follow an empirical relation,
=
0
+
2.5
T
2.5
+
7.5
T
7.5
, reflecting that the conductive mechanism mainly arises from electron–electron,
electron–phonon, and electron–magnon scattering. But in the high-temperature paramagnetic (PM)
semiconductor region, the data for all samples follow the adiabatic small-polaron-hopping model.
Moreover, the –T curves for all samples can be fitted well to a phenomenological model derived from
spin-polarized tunneling at grain boundaries and a thermal activation. This result suggests a coexistence
of FM clusters and PM regions near T
C
and indicates the prominent intrinsic behaviors of the LCSMO
matrix. It is very interesting to note that the MR at room temperature is enhanced, which is encouraging
for potential applications.
© 2009 Published by Elsevier B.V.
1. Introduction
Perovskite manganites with the form R
x
A
1−x
MnO
3
(R is the
rare-earth ion, A is alkaline-earth divalent ion) have attracted con-
siderable attention in recent years because of the discovery of
colossal magnetoresistance effect (CMR) [1,2] in these materials
and their potential applications [3] in magnetoresistive transduc-
ers and sensors. Nowadays, researchers are focusing on how to
obtain a large value of the MR at a low field and room tempera-
ture in order to satisfy practical applications. Many attempts have
been made to enhance the low field magnetoresistance (LFMR)
effect of perovskite manganites by mixture of the CMR materi-
als with secondary phases including insulators [3–8], magnetic
materials [9–14], or metals [14–19]. However, the mixture of
the CMR material with the insulating grains commonly results
in large increase of resistivity and downshift of metal–insulator
transition, which limits the practical applications for the circuit-
∗
Corresponding author. Tel.: +86 27 87556914; fax: +86 27 87556914.
E-mail address: csxiong@hust.edu.cn (C. Xiong).
matching requirement. Recently, Yuan et al. have reported a
high value of MR at 289 K and 1 T applied magnetic field in
the La
0.67
(Ca
0.65
Ba
0.35
)
0.33
MnO
3
–Pd composite (prepared by the
sol–gel method) [16,17]. And Panwar et al. [18] have also reported
the studies on the composites of Pr
2/3
Ba
1/3
MnO
3
with PdO addi-
tion. However, the studies of the manganite/conductor composites
have been done in the materials showing a high value only at a
high applied magnetic field or at a low-temperature which lim-
its the practical applications. Here we report the studies on the
composites of La
0.7
Ca
0.2
Sr
0.1
MnO
3
/Pd.
In this present work, LCSMO and PdO are selected as the matrix
material and the additional one, respectively. There are two rea-
sons: (1) LCSMO is one of the well-studied CMR materials and its
T
p
and T
C
occur at about 319 K [19]. Usually, the intrinsic MR value
in perovskite composites can reach maximum near T
C
[20], and (2)
PdO can be decomposed into metal Pd during the sintering pro-
cess due to its low decomposition-temperature (∼750
◦
C) [16]. This
would reduce the resistivity of the composites. So one can expect
that the excellent metallic conductivity would enhance the MR of
these composites. Our experimental results have revealed that a
large room temperature MR up to 40% can be obtained.
0925-8388/$ – see front matter © 2009 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2008.12.087
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