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Large enhancement of room temperature magnetoresistance in
Ag-added La0.67(Ca0.65Ba0.35)0.33MnO3
Xiao-Bo Yuana, Yi-Hua Liua,*, Bao-Xin Huanga,b, Cheng-Jian Wanga, Liang-Mo Meia
aDepartment of Physics and State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100,
People’s Republic of ChinabDepartment of Physics, Weifang College, Weifang 261043, People’s Republic of China
Received 13 October 2004; accepted 18 April 2005 by G. Luke
Available online 4 May 2005
Abstract
The electrical and magnetoresistant properties of La0.67(Ca0.65Ba0.35)0.33MnO3/Agx (abbreviated by LCBMO/Agx) have been
studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large
enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for xZ0.27 sample in 10 kOe
magnetic field is 41%, almost 20 times larger than that for xZ0 sample. This enhancement is related to that the Curie
temperature (Tc) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of
experimental results for xZ0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced
by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.
q 2005 Elsevier Ltd. All rights reserved.
PACS: 75.30.Vn; 75.50.Pp; 75.50.Dd; 72.60.Cg
Keywords: A. Perovskite manganites; A. Ag-added manganites; D. Intrinsic CMR; D. Room temperature magnetoresistance
1. Introduction
Since the discovery of colossal magnetoresistance
(CMR) in doped perovskite manganites R1KxAxMnO3 (R
is the rare-earth ion, A is alkaline-earth divalent ion),
extensive investigations have been focused on this subject in
the past few years [1–4]. However, in the most cases, the
intrinsic CMR is often triggered at high magnetic fields of
several Tesla and at comparative low temperature, which
severely limits its practical applications. Therefore, it is
desired to explore the CMR materials with high field
sensitivity at room temperature. Recently, the work about
low field magnetoresistance (LFMR) has been centered on
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2005.04.017
* Corresponding author. Tel.:C86 531 837 7035 8119; fax:C86
531 8565167.
E-mail address: [email protected] (Y.-H. Liu).
the granular system [5,6] and granular composites [7,8]. The
LFMR in these materials is closely related to the spin
dependent scattering and tunneling at the grain boundaries,
and remarkably enhanced LFMR has been observed in these
compounds. In addition, recent reports [9–12] revealed that,
adding Ag in polycrystalline film or bulk CMR compounds
can obtain large room temperature MR if Tc of the
compounds is near room temperature via the modifications
of the microstructural, electrical and magnetic inhomoge-
nities on the grains. Based on these investigations, we
combined Ag2O with granular compound La0.67(Ca0.65-Ba0.35)0.33MnO3 (LCBMO) synthesized by sol–gel method,
and systematically investigated the influence induced by Ag
addition on the electrical and magnetoresistant properties.
Tc of LCBMO is about 303 K, which is near room
temperature. It is noticeable that a large enhanced MR up
to 41% was obtained at room temperature, which would
promote the application studies.
Solid State Communications 135 (2005) 170–173
www.elsevier.com/locate/ssc
X.-B. Yuan et al. / Solid State Communications 135 (2005) 170–173 171
2. Experiments
LCBMO/Agx (xZ0, 0.23, 0.25, 0.27, 0.30, is the molar
ratio of Ag to LCBMO matrix) samples were prepared by
two steps. Firstly, the LCBMO powder was prepared by the
sol–gel process. The stoichiometric amounts of analytical
reagents of La2O3, Ca(NO3)2, Ba(NO3)2, Mn(NO3)2 were
dissolved in dilute HNO3 solution. Suitable amounts of
citric acid and glycol as chelating agents were added and a
homogeneous transparent solution was achieved. The
solution was evaporated by heating, and brown gel was
obtained. The gel was preheated in air at 600 8C for 2 h to
remove the remaining organic materials, then annealed at
900 8C for 5 h, and LCBMO powder can be obtained. The
average particle size of the LCBMO powder determined by
the scanning electron microscopy is about 150 nm. Sec-
ondly, appropriate amounts of LCBMO and Ag2O powders
were mixed according to the desired molar ratio. Then the
mixtures were ground and pressed into pellets. The pallets
were finally sintered at 1300 8C in air for 1 h, and then
slowly furnace-cooled to room temperature. The crystalline
structure of the samples was characterized by the X-ray
diffraction. Magnetic properties and MR were measured by
a vibrating sample magnetometer (VSM) in the temperature
range 77–400 K and in a field up to 10 kOe. The temperature
and magnetic field dependence of the resistivity were
measured with the standard DC four-probe method and the
applied field was parallel to the direction of current.
3. Results and discussion
Fig. 1 shows the XRD patterns of some samples. The
XRD analysis reveals that pure LCBMO has rhomobohedral
perovskite structure, while the crystal structure for Ag-
added samples are composed of two phases: a rhombohedral
perovskite phase and a cubic metal Ag phase (labeled by
Fig. 1. X-ray diffraction patterns for some typical samples.
asterisks). The intensity of Ag phase increases with x. It
suggests that Ag2O was decomposed into Ag during the
sintering process due to its low decomposition temperature
(around 300 8C). Therefore, a composite system consisting
of perovskite LCBMO and metal Ag was formed. The
magnetic measurements showed that Tc is not sensitive to
Ag addition, which ranges from 300 to 308 K. This indicates
that the Ag substituting effect in LCBMO is very limited.
The temperature dependence of resistivity (r) for the
composites has been measured in the temperature region
from 77 to 400 K with 4 kOe or without magnetic field. Fig.
2 gives the results measured in zero field. From this figure,
one can see two evident characteristics: (1) r decreases
dramatically with Ag addition, it reaches the minimum
when xZ0.27 and then increases. At 77 K, the r for xZ0.27
sample is only 1.2 mU cm, four orders of magnitude lower
than that for xZ0 sample, which is about 14.2 U cm. (2)
Adding Ag in LCBMO sharps the r peak and shifts the peak
temperature of r, Tp, to higher temperature. Tp for xZ0.27
sample is 316 K, almost 100 K higher than that of LCBMO
(216 K).
The lowering of r can be caused by three factors. Firstly,
metal Ag segregated at the grain surfaces or boundaries
improves the atomic disordered structure and magnetic state
on the grain surfaces and increases the connectivity between
the grains. As a result, the barriers formed by the atomic
disordered structure on the grain surfaces become thinner
even disappear. Therefore, r of Ag added LCBMO
decreases accordingly due to the less electron scattering
on the grain surfaces. The change of r peak can illuminate
the improvement of grain boundaries. For xZ0 sample, a
broad plateau on the r–T curve can be seen at low
temperature of about 216 K as shown in Fig. 3, which is
much lower than Tc (303 K). This is a typical feature of a
granular perovskite manganite [13]. The electron scattering
and tunneling at the grain boundaries result in the formation
of this low-T plateau. For xZ0.25 sample, double peaks on
Fig. 2. Resistivity r versus temperature T plots at zero field for
different Ag concentrations obtained in the warming process.
Fig. 3. Temperature dependence of resistivity for xZ0 and 0.25
sample.
Fig. 4. Field dependence of MR ratio (defined as Dr/r0) at 292 K for
all samples.
X.-B. Yuan et al. / Solid State Communications 135 (2005) 170–173172
the r–T curve are observed as seen in Fig. 3. Besides a
plateau low-T peak at about 224 K, there is a small sharp
peak appeared at 304 K, which is due to the decrease of r in
the low temperature region. This small peak appears at the
temperature around Tc, and is a metal–insulator transition
point. The appearance of this small peak is an intrinsic
behavior of the manganites. When xR0.27, the low-T peak
disappears and only a sharp peak exists at room temperature,
which indicates that the intrinsic properties of the compound
is dominant, and the influence of grain boundaries on the
electron transport disappears. Secondly, the existence of
high conductive metal Ag between the grains opens a new
conductive channel, which also decreases the resistivity of
the sample and is responsible for the enhancement of Tp[11]. The enhancement of Tp depends on the proportion of
the metallic conduction in the sample. In this composite
system, the more addition of Ag, the larger proportion of the
metallic conduction, and Tp is higher. Finally, the oxygen
released from Ag2O in the sintering process likely combines
to LCBMO [9], which would also lead to the decrease of r.
The increase of oxygen content not only improves the
magnetic inhomogenity of the samples and decreases the
microstructure deficiency, but also enhances the concen-
tration and hopping mobility of carriers through a little
change of Mn3C/Mn4C ratio. The increase of r for xR0.27
samples may accounts for the volatilization of extra Ag
during calcinations [12]. We suggest that the volatilization
of Ag would cause the emergence of lattice and structure
deficiencies, resulting in the additional electron scattering.
Shown in Fig. 4 are the curves of MRZDr/r0Z[rHKr0]/r0 versus applied field H obtained at 292 K for all
samples, where r0 and rH are the resistivities in zero and H
field. The MR ratio of LCBMO at 10 kOe is only 1.9%, but
it increases significantly with Ag addition, and reaches the
maximum of 41% for xZ0.27 sample. When x increases
further, the MR ratio decreases. This is also due to the
volatilization of extra Ag during calcination as the case in
r–T behavior. If we use the MR ratio definition as Dr/rHZ[rHKr0]/rH, the MR ratio at 10 kOe for xZ0.27 sample is
as large as 70%, which is larger than that observed in Co/Cu
multilayers, where the maximumMR ratio at 10 kOe is 65%
[14]. So large MR has never been obtained in the pure
perovskite manganites. This result is attractive for the CMR
application studies.
The large enhancement of room temperature MR in the
present system is due to two factors. On the one hand, all
samples have Tc near room temperature. Usually, the
intrinsic CMR in perovskite compounds reaches its
maximum near Tc. On the other hand, the Ag addition
improves the crystal structure and magnetic homogeneity of
the grains and grain boundaries. These improvements are
favorable to increase the intrinsic properties of the
compounds, especially to the decrease in r dramatically.
The r decrease is also an important factor that induces MR
ratio to increase. The smaller is the r of the sample, the
larger is the relative change of Dr/r0. In these composites,
27% mole ratio Ag addition results in an optimum
improvement on the crystal structure and magnetic
homogeneity of the LCBMO grains, and grain boundaries.
This causes xZ0.27 sample having a minimum resistivity,
which is favorable to the increase in MR ratio.
In order to illuminate the intrinsic property of the MR
behavior observed in this Ag added system, we model the
MR versusH curves by using Brillouin function BJ proposed
by Wagner, et al. [15]. According to this model, CMR is
related to the spin-dependent hopping of charge carriers
between the spin clusters. The CMR usually reaches its
maximum around Tc because the disordering of spin clusters
is maximum in the vicinity of Tc. When the temperature is
over Tc, the size and population of spin clusters will decrease
because of the thermal fluctuation. The spin-dependent
hopping process is reduced thereby. The field-induced
resistivity decrease, Dr(H)Zr0KrH, is proportional to the
Brillouin function BJ in the ferromagnetic state and B2J in the
Fig. 5. MR ratio versus H curves for xZ0.27 sample. Line a is
obtained at 292 K and line b is obtained at 317 K. The solid lines are
the experimental results and the dotted lines are the theoretical fit to
Eqs. (1) and (2).
X.-B. Yuan et al. / Solid State Communications 135 (2005) 170–173 173
paramagnetic state [15],
DrF ZAðTÞBJ
gmBJðTÞH
KBT
� �(1)
DrP ZAðTÞB2J
gmBJðTÞH
KBT
� �(2)
Where r0 and rH are the resistivities in zero and H field,
respectively, gZ2 the gyromagnetic ratio, mB the Bohr
magneton, J(T) the average spin moment of the spin clusters
at hopping sites, KB the Boltzmann constant, and A(T) the
temperature dependent CMR amplitude which is related to
the population of spin clusters in the compounds. We use
Eqs. (1) and (2) to fit the experimental results for xZ0.27
sample. Fig. 5 gives the dependence of Dr/r0Z[r0KrH]/r0on the magnetic field H obtained by the experimental (the
solid line) and fitting (the dotted line) results, where line a is
obtained at 292 K and line b is obtained at 317 K. Line a fits
well with Eq. (1) because the sample is in the ferromagnetic
state at 292 K (TcZ300 K), while line b fits Eq. (2) due to
the samples is in the paramagnetic state. The fitting
parameters are JZ780Z and A0ZA(T)/r0ZK56 for line
a, and JZ700Z and A0ZK48 for line b. The good fits in
Fig. 5 indicate that the MR behavior in the Ag added
LCBMO is induced by the spin-dependent hopping of the
electrons between the spin-clusters, which is an intrinsic
property of the CMR materials. Due to Ag addition, the
crystal structure of the LCBMO grains is improved and
probably becomes perfect for xZ0.27 sample. In this case, a
large intrinsic CMR at room temperature is resulted.
4. Conclusion
We have prepared the granular composite system
La0.67(Ca0.65Ba0.35)0.33MnO3/Agx by two-step chemical
route, and studied systematically the effects of Ag addition
on the electric and magnetoresistant properties of LCBMO.
Ag addition induces the decrease in r dramatically. When
xZ0.27, the resistivity at 77 K is only 1.2 mU cm, almost
four orders of magnitude lower than that of pure LCBMO.
Ag addition also induces a large enhancement of the room
temperature MR ratio. The MR ratio at 292 K for xZ0.27
sample in HZ10 kOe is 41%, it is 20 times larger than that
for xZ0 sample. This enhancement is related to two factors.
One is Tc near room temperature. The MR ratio always
reaches its maximum near Tc. So this large MR is an
intrinsic effect of the LCBMO compounds. Another is the
significant reduction in r of the sample. The good fits of
experimental results for xZ0.27 sample to Brillouin
function indicate that the MR behavior in the Ag added
LCBMO is induced by the spin-dependent hopping of the
electrons between the spin clusters, which is an intrinsic
property of the CMR materials.
Acknowledgements
This work is supported by the State Key Project of
Fundamental Research.
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