Electrical properties and enhanced room temperature magnetoresistance in La0.7Ca0.2Sr0.1MnO3/Pd composites prepared by chemical plating

Article abstract of DOI:10.1016/j.jmmm.2012.05.018

A series of bulk polycrystalline La_0.7Ca_0.2Sr _0.1MnO₃ (LCSMO)/Pd composites were prepared by chemical plating and structural, electrical, magnetic, and magnetoresistance (MR) properties were investigated. It is found that Pd additions are uniformly distributed on the grain boundaries of the LCSMO grains, which decrease the resistivity and the saturation magnetic moment of the matrix. An interesting phenomenon is observed that at a given field, when the plating time increases, the MR increases at low addition level (0>t (plating time)<40 min) and decreases at high addition level (t>40 min), indicating an optimal plating time of 40 min, at which the MR value is maximum. Our analysis suggests that the improvement of grain boundaries originating from Pd addition plays an important role in enhancing the MR.

Full text of DOI:10.1016/j.jmmm.2012.05.018

Journal of Magnetism and Magnetic Materials 324 (2012) 3286–3290
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Electrical properties and enhanced room temperature magnetoresistance in
La Ca Sr MnO /Pd composites prepared by chemical plating
0
.7
0.2 0.1
3
n
F.Y. Chen, Y.Y. Wu, Y.H. Xiong, L.J. Li, Z.L. Liu, C.S. Xiong
Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of bulk polycrystalline La0.7Ca0.2Sr0.1MnO3 (LCSMO)/Pd composites were prepared by chemical
plating and structural, electrical, magnetic, and magnetoresistance (MR) properties were investigated.
It is found that Pd additions are uniformly distributed on the grain boundaries of the LCSMO grains,
which decrease the resistivity and the saturation magnetic moment of the matrix. An interesting
phenomenon is observed that at a given field, when the plating time increases, the MR increases at low
addition level (04t (plating time)o40 min) and decreases at high addition level (t440 min),
indicating an optimal plating time of 40 min, at which the MR value is maximum. Our analysis
suggests that the improvement of grain boundaries originating from Pd addition plays an important
role in enhancing the MR.
Received 13 December 2011
Received in revised form
1
5 May 2012
Available online 1 June 2012
Keywords:
Composite material
Chemical plating
Grain boundary
Electrical transport
Low-field magnetoresistance
&
2012 Elsevier B.V. All rights reserved.
1
. Introduction
However the additional material is preferred to be distributed on
the grain boundary of the matrix instead of coating, while
chemical plating allows for uniformly coatings on the surfaces
of any profiles of metallic, dielectric and semi-conducting materi-
als. Xiong et al. [20] reported the studies on the composites
Perovskite manganites with the form of R
x
A
3
1ꢀxMnO (where R
is a rare-earth ion, A is an alkaline-earth divalent ion) are widely
investigated for potential applications in next generation of
digital recording and sensing devices in recent years since the
discovery of colossal magnetoresistance effect (CMR) in these
materials [1,2]. Nowadays, many attempts have been made to
obtain a large value of the low field magnetoresistance (LFMR),
such as mixing CMR materials with secondary phases including
insulators [3–8], magnetic materials [9–14], and metals [15–19].
However, the mixture of the CMR material and the insulating
grains commonly results in a large increase of resistivity and
downshift of metal–insulator transition, which limits the practical
applications for the circuit-matching requirement. Numerous
efforts have also been down to obtain CMR at a low magnetic
field and about room temperature. Yuan et al. have reported a
high value of MR at 289 K and 1 T applied magnetic field of the
of La0.7Ca0.3MnO
successfully enhanced the low field magnetoresistance. In this
paper, we coat Pd onto La0.7Ca0.2Sr0.1MnO by chemical plating to
enhance the room temperature magnetoresistance.
3
coated by Cu with chemical plating, which
3
2. Experimental procedure
The pure LCSMO was prepared by the conventional solid-state
reaction method, which is similar to that of Ref. [20]. Briefly, the
chemical plating process contains two steps, sensitization–
activation and plating. In the first step, SnCl
sensitization and PdCl for activation. Then PdCl
was added dropwise to the mixture of NH Cl (stabilizing agent)
and NH O (pH value regulator) to prepare the plating solu-
ꢁ H
tion, in which equal amount of NaH PO (reduction agent) was
2
was used for
2
2
(main salt)
4
La0.67(Ca0.65Ba0.35
)
0.33MnO
3
–Pd composite (prepared by the sol–
gel method) [16,17]. Panwar et al. have also reported the studies
on the composites of Pr2/3Ba1/3MnO with PdO addition [18].
3
2
2
2
3
added before experiment. After mixing the plating solution with
the LCSMO powders, the chemical plating process was carried out
in the magnetic stirring apparatus at a constant temperature of
However high MR values could only be achieved at high applied
magnetic fields or at low temperatures, which was still unsuitable
for the practical application.
The conventional way to introduce the additional material
to the matrix is via mixing, grinding and subsequent sintering.
50 1C. With several tests, the reasonable chemical plating formula
is shown in Table 1.
After plating, the LCSMO/Pd powders were ground into quad-
rate thin slices, sintered at 1100 1C in the air for 2 h, and then
slowly cooled down for characterization. The structures of the
n
Corresponding author. Tel.: þ86 27 87556914; fax: þ86 27 87556914.
E-mail address: csxiong@mail.hust.edu.cn (C.S. Xiong).
specimen were examined by X-ray diffraction (XRD) using Cu Ka
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jmmm.2012.05.018

F.Y. Chen et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3286–3290
3287
radiation. Scanning Electron Microscopy (SEM) was used to study
the particle size and shape. The DC magnetization measurements
were taken using a vibrating sample magnetometer (VSM) under
an applied magnetic field of 6 kOe in the temperature from 130 to
are also shown in Fig. 2(b) as a comparison. It is demonstrated
that the metallic Pd segregated at the grain boundaries, consistent
with our conclusion of XRD analysis. Comparing Fig. 2(b) with
Fig. 2(c), it is clear that Pd introduced in by chemical plating was
distributed more uniformly than that by doping.
3
30 K. The temperature dependence of resistivity was measured
by a standard four probe technique in the temperature range from
1
30 to 330 K under either zero or 4 kOe magnetic field.
3.3. The magnetic characteristic of LCSMO/Pd composites
Fig. 3 presents the temperature dependence of specific satura-
3
. Results and discussions
tion magnetization
samples decreased monotonically and there were evident changes
from ferromagnetism to paramagnetism. Moreover, went down
as the plating time extended, while the magnetic transition
temperature (T ) was practically equal. It can be inferred that no
s
s
in 4000 Oe magnetic field.
s
s of all the
3.1. The analysis of the XRD patterns
s
s
The XRD patterns of the LCSMO/Pd samples made with
c
different plating times are shown in Fig. 1. Three components in
the LCSMO/Pd composites were observed, corresponding to
LCSMO, Pd and PdO. The XRD patterns of the samples with short
plating times were not sufficient enough to illustrate the exis-
tence of Pd phase due to phase coincidence of Pd and LCSMO.
With the elongated plating time, the third diffraction maximum
of Pd became visible. Meanwhile, the diffraction intensity of PdO
became stronger because Pd begins to be oxidized at around
chemical reaction took place between the LCSMO and Pd in the
viewpoint of magnetism. Combining with the analysis of XRD and
SEM, we can deduce that Pd is distributed on the grain boundaries
of LCSMO instead of replacement. As nonmagnetic Pd phases
affect magnetic LCSMO phase via dilution,
plating time extends.
s
s reduces when the
7
00 1C and PdO melts as the temperature reaches 800 1C. Simul-
3.4. The electronic transport properties of LCSMO/Pd composites
taneously, it can be seen in the XRD patterns that the structure of
the LCSMO composites was not destroyed, though Pd and PdO
were introduced. It indicated that Pd and PdO stayed on the grain
boundaries of LCSMO as independent phases rather than being
incorporated into the lattice of LCSMO crystal.
Fig. 4 shows the temperature dependence of zero-field elec-
trical resistivity(r), with the inset presenting the plating time
dependence of the metal–insulator transition temperature (T ) for
p
all the samples. Some interesting phenomena are observed. First
all the samples exhibited a transition from low-temperature
3
.2. The analysis of SEM micrographs
metallic behaviors at d
behaviors at d
/dTo0. Then as the plating time increased, the
electric resistivity reduced, the T shifted successively towards
r/dT40 to high-temperature insulating
r
These above mentioned conclusions were further confirmed
r
p
with the SEM images presented in Fig. 2. The LCSMO/Pd samples
with a Pd doping content of 10% prepared by solid state reaction
higher temperature and the transition peak became broader.
Generally, when high-impedance state insulative oxide is intro-
duced into manganese perovskite compound, the electrical resistiv-
Table 1
ity
MnO
1ꢀx)La2/3Ca1/3MnO
metal, the of composites decreases, such as L0.7
19], L0.7 0.3MnO/Ag [24], L0.67(C0.65 0.33MnO/Ag [25], L0.67
MnO/Fe [26] and L0.7 0.1MnO/Pd [27]. For the pure LCSMO,
r
3
of the composites goes up and T
1ꢀy/yZrO [21], (1ꢀx)La0.67Ca0.33MnO
/xSiO [23]. However if the second-phase is a
0.1MnO/Ag
p
falls, such as (La1ꢀxCa
x
The chemical plating formula.
)
3
/xZnO [22], and
2
(
3
2
PdCl
HCl(36%)
NaH PO
2
2 g/L
r
0.2
C S
10 mL/L
10 g/L
[
C
B )
0.35
C
0.33
2
2
ꢁ H
2
O
NH
NH
pH
4
Cl
27 g/L
0.2
C S
3
ꢁ H
2
O (28%)
16 mL/L
9.870.2
5072
the grain boundary is the barrier to electronic transport. This kind
of barrier caused by disordered structure of atom strengthens
the electron scattering, increasing the electric resistivity. However
T(1C)
Fig. 1. XRD patterns of the samples with different plating times. The inset shows a zoom in the region between 67.81 and 69.31 (t¼50 min).

3
288
F.Y. Chen et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3286–3290
Fig. 2. Scanning electron micrographs of LCSMO/Pd composites (a) the matrix; (b) the content of Pd is 10%; (c) the plating time of Pd is 20 min. The magnification times
are 10000.
Fig. 3. The temperature dependence of specific saturation magnetization
000 Oe magnetic field.
s
s
of
Fig. 5. The temperature dependence of the MR.
4
conductor, weakens the magnetic disorder of grain boundary to
get the barriers thinner. So the electron scattering on the grain
boundary is diminished and the possibility of electron tunneling is
raised. In this way the electric conductivity is infinitely enhanced.
Moreover, Pd provides a new channel for electron transport. Many
researches reveal that the oxygen content of LCSMO influences the
electric resistivity of the crystal grain [20,21]. In other words the
electric resistivity of the oxygen-rich samples is larger than that of
the oxygen-poor counterparts. The oxygen atoms, generated from
decomposition of PdO, raise the oxygen content of LCSMO, making
the electric resistivity of composites larger.
3.5. The magneto-resistor of composites
The temperature dependence of the MR for the composites
was shown in Fig. 5. Here MR is defined as (
r
0
–
r
H
)/
r
0
, where
r
0
and are the resistivity in zero and applied fields, respectively.
r
H
There were noticeable changes in the MR value from the LCSMO
matrix to the LCSMO/Pd composites prepared by chemical plat-
ing. Besides the MR value of the samples at the room temperature
rose first, followed by a decline of extend plating time.
With the plating time of 40 min, the MR value reached its
maximum of 11%, almost four times higher than that of the
matrix. The temperature of MR peak for both matrix and LCSMO/
Pd were identical and close to the room temperature, so it can be
classified as the intrinsic magneto-resistance, i.e. LCSMO crystal
grain magneto-resistance. At relatively low temperature, the
Fig. 4. The temperature dependence of zero-field
time dependence of the transition temperature T
r
. The inset shows the plating
p
for all the samples.
addition of Pd by chemical plating makes good contact with the
LCSMO grain boundary in the present case. Therefore the T of the
composites and crystallization of grain boundary are higher and the
nearby crystal grains are better bridged. Metallic Pd, as a good
p

F.Y. Chen et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3286–3290
3289
relation at 20 kOe field. It can be observed that the MR value
increased as the magnetic field did. At a given field H, the value of
MR increased at a low addition level (0oto40 min) and decreased
at a high addition level (t440 min) with increase of time. The MR
value reached the peak value of about 35% at 20 kOe, when plating
time was 40 min. These results are encouraging for the performance
of practical applications on magnetic memory.
4. Conclusions
The LCSMO/Pd composites were successfully prepared by
chemical plating. The nonmagnetic–magnetic structure was
investigated. Pd additions were uniformly distributed on the
grain boundaries of the LCSMO grains and brought no changes
to the internal structure of LCSMO. Pd addition decreased the
resistivity and the saturation magnetic moment of the matrix.
Interestingly, the Pd addition affected the MR value dramatically.
At a given field H, when time increased, the value of MR went up
at a low addition level (0oto40 min), followed by a decrease at
a high addition level (t440 min). The MR value reached the peak
value of about 35% at plating time of 40 min, magnetic field of
Fig. 6. The MR for some samples at a magnetic-field range from 0 to 20 kOe at
about room temperature. The inset shows the MR–time relation at 20 kOe field.
20 kOe and temperature of 305 K. These results suggest that the
materials in our studies are potentially useful for many devices.
sample is in the ferromagnetic state and the magnetic domains
are difficult to be influenced by external magnetic field. So the
externally-applied magnetic field slightly affects the resistivity,
thus, the MR value is relatively small. With the temperature close
Acknowledgments
The authors thank Analytical and Testing Center of Huazhong
University of Science and Technology, which supplied us the
facilities to fulfill the measurement. This work was supported by
the National Science Foundation of China (Grant no. 20971048).
c
to T , the consistency of magnetic domain tropism grows worse,
and the spin orientation of Mn ion disorders. The included angle
3
þ
4þ
of spin orientation between Mn
and Mn
ion rises and the
double-exchange effect weakens, so the resistivity of samples
increases. In this case the external magnetic field influences the
disorganized magnetic domain and the spin orientation of Mn ion,
significantly making their orientation identical. Accordingly, the
double-exchange effect is enhanced and the resistivity of samples
diminishes. The MR value increases obviously, which leads to that
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