Electrical behavior of some silver-doped lanthanum-based CMR materials

Article abstract of DOI:10.1016/j.jmmm.2008.11.012

With a view to understand the structural, magnetic and electrical properties of La_1-xAg_xMnO₃ (x=0.05-0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found that both the metal-insul

Full text of DOI:10.1016/j.jmmm.2008.11.012

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Journal of Magnetism and Magnetic Materials 321 (2009) 1240–1245
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Journal of Magnetism and Magnetic Materials
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Electrical behavior of some silver-doped lanthanum-based CMR materials
Ã
Y. Kalyana Lakshmi, P. Venugopal Reddy
Department of Physics, Osmania University, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
ꢀx x 3
With a view to understand the structural, magnetic and electrical properties of La1 Ag MnO
Received 26 September 2008
Available online 21 November 2008
(x ¼ 0.05–0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found
that both the metal–insulator and ferro- to paramagnetic transition temperatures after increasing up to
the composition x ¼ 0.20, are found to remain constant thereafter. The electrical resistivity vs.
temperature plot of the sample x ¼ 0.10 is found to exhibit an insulating behavior below 36 K, while the
sample, x ¼ 0.20 exhibits two peaks, and the observed behavior is explained on the basis of the phase
PACS:
71.30.+h
74.25.Ha
7
7
5.47.Gk
5.47.Lx
separation model. The low-temperature (ToT
P
), electrical resistivity data were analyzed by a theoretical
2
4.5
model,
r
¼
0 2 4.5
r +r T +r T
, indicating the importance of grain/domain boundary effects, electro-
n–electron and two-magnon scattering processes. The low-temperature resistivity data (To50 K) were
fitted to an equation, which is based on the combined effect of weak localization, electron–electron and
electron–phonon scattering.
Keywords:
Metal–insulator
Phase separation
Two-magnon scattering
Weak localization
&
2008 Elsevier B.V. All rights reserved.
1
.
Introduction
(FMM) and antiferromagnetic charge and orbital ordered insulat-
ing phases, has recently been proposed to explain the CMR effect.
Most studies on manganites focused on the doping of a
divalent ion at the rare earth site. However, a few studies on
monovalent doped manganites are available [4–6]. In fact, the
High-storage-density magnetic devices require materials with
a large magnetoresistance (MR) near the room temperature. The
discovery of doped manganites, Ln1ꢀx MnO (Ln is a rare-earth
A
x
3
+
+
+
ion, while A is the divalent or monovalent dopant ion) meets this
technological demand to a large extent. For this reason, an intense
effort has been made to synthesize new doped manganites and to
understand the origin of the unique properties shown by these
intriguing materials [1,2]. In these mixed oxides, the colossal
magnetoresistance (CMR) phenomenon is accompanied by a wide
range of exotic behaviors such as magnetic-ordering, metal–insu-
lator transition, charge and orbital ordering. In addition, depend-
ing on temperature, pressure, doping material and its level, these
novel materials exhibit rich-phase diagrams, including insulating,
antiferromagnetic (AFM), paramagnetic and metallic ferromag-
netic phases [1,2].
The CMR effect can be qualitatively understood in the frame
work of the double-exchange (DE) interaction model proposed by
Zener [3]. However, subsequent studies clearly indicated that the
lattice distortion due to the Jahn–Teller (JT) effect is crucial in
explaining the magnetic and transport behavior of the mixed
valence manganites. Furthermore, the phase separation (PS)
model which assumes the coexistence of ferromagnetic metallic
substitution of monovalent ions (such as Na , K , Ag , etc.) for rare
earth ions can also lead to the change of the valence of Mn ions,
hence the DE interaction. Among these materials, manganite
doped with silver are of great interest [7–10] due to their high
sensitivity of magnetoresistance at room temperature via im-
proving the microstructure of the surfaces, electrical and
magnetic inhomogeneities on the grains. Moreover, as the high-
conducting metal Ag provides conduction paths among grains, the
resistivity of Ag-doped manganites can be much lower, making
them useful for potential applications. On the other hand, it is well
known that the preparation conditions can influence the micro-
structural, electromagnetic and transport properties of the
samples. In view of these facts, an effort has been made to
synthesize these materials by the polyvinyl alcohol (PVA) gel
method. In recent years, the PVA method was used to synthesize
various types of materials. For example, Das et al. [11] and Mandal
and Ram [12] have synthesized PZT powder using PVA solution by
the evaporation route. Generally PVA, is widely used as a binder
for green shaping of ceramic materials and decomposing
exothermally at low ignition temperatures (250 1C), leaving
behind hardly any residue [12]. This characteristic behavior
together with the water-soluble nature has motivated the authors
of the present investigation to prepare the samples by the PVA-gel
route. In fact, carbon atoms in the PVA solution not only control
Ã
Corresponding author. Tel.: +9140 27682242; fax: +9140 27090020.
E-mail address: paduruvenugopalreddy@gmail.com (P. Venugopal Reddy).
0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2008.11.012

ARTICLE IN PRESS
Y. Kalyana Lakshmi, P. Venugopal Reddy / Journal of Magnetism and Magnetic Materials 321 (2009) 1240–1245
1241
the growth of metal atoms, but also reduce them to nano-level. In
view of this, a systematic investigation of electrical and magnetic
behavior of nano-crystalline La1ꢀxAg MnO prepared by the PVA-
x
3
gel route has been undertaken and the results of such a study are
presented here.
x-0.05
x-0.10
x-0.15
2.
Experimental details
The polycrystalline materials with the compositional formula,
x-0.20
La1ꢀxAg
method with a reactive polymer matrix of PVA and sucrose. In this
method, high-purity La (99.99%), AgNO (99.9%) and MnCO
99.9%) were taken in stoichiometric ratio as starting materials.
x
MnO
3
(x ¼ 0–0.3) were prepared by a novel chemical
x-0.25
x-0.30
2
O
3
3
3
(
Later, they were converted into nitrates and mixed thoroughly
maintaining pH at 1.0. To this, an aqueous polymer solution of PVA
and sucrose are added drop by drop and upon heating yields a
fluffy porous mass. On further heating the precursor mass in air, it
decomposes and burns to give nano-crystalline powders. The
powder after calcining at 600 1C was pressed into pellets and
finally sintered at 950 1C in air for 3 h.
20
30
40
50
60
70
80
2θ (Degree)
LA = 05
observed
The phase purity of the samples was checked by powder X-ray
difference
calculated
diffraction (XRD) using Philips (X pert) diffractometer with Cu K
radiation at room temperature. AC susceptibility ( ) measure-
ments were carried out over a temperature range 100–350 K, to
determine T values using a home-made sample holder designed
a
w
C
on the basis of mutual inductance principle. The valence states of
Mn ion and oxygen stoichiometry were determined using the
redox titration technique. Finally, the electrical resistivity and
magnetoresistance measurements were carried out using
a
superconducting magnetic system of OXFORD in applied fields
of 1, 3 and 5 T over a temperature range, 10–310 K using the four-
point probe method.
3. Results and discussion
3.1. Structural studies
20
30
40
50
60
70
80
2θ (Degree)
The XRD patterns of all the samples are shown in Fig. 1(a), and
it can be seen from the figure that the samples with xo0.2 have
Fig. 1. (a) X-ray diffraction patterns of the Ag-doped manganites and (b) Rietveld
single phase with Rhombohedral perovskite structure and R 3¯ c
profile fit for the X-ray diffraction pattern of LA-05 sample.
space group. The presence of second phase became apparent as Ag
concentration increases gradually beyond x40.2. After analyzing
the second phase it was identified as the unreacted metallic Ag,
3.2. Magnetic transitions
thereby indicating the limited solubility of Ag in LaMnO
3
and the
0
The AC susceptibility ( ) vs. temperature plots of Ag-doped
w
observation is in agreement with the reported ones [10]. Fig. 1(b)
shows the experimental and calculated XRD patterns of LA-05
sample. The structural cell parameters of all the samples refined
by the standard Rietveld technique are listed in Table 1. It can be
seen from the table that the lattice parameters are found
to increase initially up to the sample LA-15 and decreases
thereafter.
samples are shown in Fig. 2. It can be seen from the figure that all
the samples are found to exhibit a sharp ferro- to paramagnetic
0
transition and based on the inflection point of d
w
/dT vs. T curves,
T
C
values were determined (Table 2). It can be seen from the table
that T
C
values initially increase (x ¼ 0.20) and remain constant
with further increase in silver doping. The observed behavior may
be explained by considering the fact that doping of x Ag ions
results in creating 2x holes in the doping regime (xp0.20) with
The average crystallite sizes of the materials were estimated
using the peak broadening technique [13] and the Scherrer’s
3+
4+
Mn –Mn
Further, for the samples with xX0.20, T
may be attributed to the fact that the solubility of Ag saturates at
ratio favouring double-exchange ferromagnetism.
formula, /SS ¼ K
l
/
b
cos
y
, where /SS is the average particle size
is the Cu K
is the corrected full-width half-maxima of XRD
peaks of the sample. SiO is used to correct the intrinsic width
C
is constant and this
˚
in A, K is the constant (shape factor 0.89),
l
a
wavelength and
b
3+
x40.20 forming a second phase. This in turn maintains Mn
/
2
4+
Mn ratio constant thereby allowing the Curie temperature at a
constant value.
associated with the equipment. The crystallite sizes are found to
be in the range 40–65 nm and are given in Table 1. In order to
evaluate Mn values and oxygen content in the samples, idometric
titrations [14] were carried out and the percentage of Mn are
also included in Table 1. No oxygen deficiency was found and
probably this could be due to filling of vacancies with silver
ions [7].
4
+
3.3. Electrical transitions
The variation of electrical resistivity with temperature of all
the samples is shown in Fig. 3 and the metal–insulator transition

ARTICLE IN PRESS
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Y. Kalyana Lakshmi, P. Venugopal Reddy / Journal of Magnetism and Magnetic Materials 321 (2009) 1240–1245
Table 1
Structural parameters of Ag-doped lanthanum manganites.
Sample code
Concentration of Ag
a ¼ b ( A˚ )
c ( A˚ )
%Mn
4
+
/SS (nm)
Mn–O–Mn (degrees)
Mn–O (A)
˚
LA-05
LA-10
LA-15
LA-20
LA-25
LA-30
x ¼ 0.05
x ¼ 0.10
x ¼ 0.15
x ¼ 0.20
x ¼ 0.25
x ¼ 0.30
5.5090
5.5124
5.5131
5.5117
5.5104
5.5045
13.3210
13.3217
13.3510
13.3580
13.3620
13.3520
18
21
25
30
33
35
65
45
65
65
35
40
173.68
173.69
173.70
173.70
173.70
173.70
1.9448
1.9432
1.9428
1.9428
1.9428
1.9428
0
0
0
0
0
0
.006
.005
.004
.003
.002
.001
model, the sample LA-10 can be considered as a mixed-phase
system with intrinsic magnetic inhomogeneity as a result of the
competition between AFM superexchange and FM double-
exchange interaction. The FM and CO clusters coexist at low
temperatures, and when the field is applied, these CO clusters will
collapse, resulting in further increase of FM regions which in turn
lowers the resistivity [23] thereby exhibiting large MR.
LA-05
LA-10
LA-15
LA-20
LA-25
LA-30
In contrast, the sample LA-15 is found to exhibit normal
colossal magnetoresistance behavior with clear electrical (T
P
) and
magnetic transitions (T ), thereby indicating that the CMR
C
phenomenon which started emerging slowly in the case of
LA-05 might have stabilized fully in the case of LA-15. Surpris-
ingly, the sample LA-20 exhibits two transitions—one at 306 K
(T
P1) and the other one at 285 K (TP2) (inset of Fig. 3). As the
transition at 306 K is in the vicinity of magnetic transition
temperature (T ), it is considered as normally observed meta-
l–insulator transition (T ), while the other one (TP2) observed at
85 K may be explained on the basis of the phase separation
C
P
100
150
200
T (K)
250
300
350
2
1
+
model. According to this model, for every Ag ion substitution at
La site, two Mn ions need to be oxidized to Mn ions, giving
rise to simultaneous occurrence of metallic Mn -rich ferromag-
3
+
3+
4+
0
Fig. 2. Variation of AC susceptibility (
0.05oxo0.30) manganites.
w
) with temperature for La1ꢀxAg
x
MnO
3
4
+
(
3+
netic domains in the neighborhood of Ag ions and Mn -rich
region in the neighborhood of La ions inducing phase or domain
separation [24,25]. Finally, on further doping beyond x ¼ 0.2, the
Ag segregate at the grain boundaries thereby improving the
conduction path between perovskite grains which in turn
decreases the resistivity drastically and shifting the second peak
gradually towards the high-temperature side and finally merges
with the first one [8].
P
(T ) temperatures (Table 2) are found to increase and remain
constant with increasing Ag doping. It can be seen from the figure
that the resistivity values of LA-05 are decreasing continuously
with increasing temperature without exhibiting any change of
slope within the measurement range of the present investigation.
Another interesting observation is that although LA-05 is not
having the metal–insulator transition (T
exhibited T at 114 K thereby indicating that the region below
14 K may be considered as ferromagnetic insulating (FMI). The
P
), it has surprisingly
3.4. Conduction mechanism
C
1
observed FMI behavior cannot be explained within the frame
work of the double-exchange model because it predicts the
coexistence of both FM as well as the metallic behavior. Therefore,
the FM order at To114 K exhibited by the sample, LA-05 might
have originated from the superexchange mechanism and in fact a
similar conclusion was arrived at by Yang et al. [15].
The low-temperature resistivity data (ToT
P
) were fitted to
various empirical equations, to understand the relative strengths
of different scattering mechanism originating from different
contributions. The experimental data of the samples (xX0.15) of
the present investigation were fitted to the equation,
2
4.5
r
¼
r
0
+
r
2
T +
boundaries. On the other hand,
to electron–electron scattering, while
r
4.5
T
, where
0
r arises due to the grain or domain
2
In the case of LA-10 sample, the resistivity increases with
2
r T indicates the resistivity due
4
.5
decreasing temperature reaching a maximum value at 72 K (T
On further decrease of temperature, decreases after reaching a
P
).
r
4.5
T
is attributed to the
r
two-magnon scattering process. The two-magnon process is more
favorable in the half-metallic band structured materials such as
manganites. Fig. 4 shows the best fit for the samples, xX0.15. It
can be seen from the figure that all the samples are found to fit
well up to T450 K, and are found to deviate thereafter (inset of
Fig. 4). Therefore, the electrical conduction below To50 K is
explained separately, in the next section.
minimum value at 36 K (Tmin), and later on it increases once again
rapidly, which is a characteristic property of an insulator [2]. In
fact, a similar type of behavior was reported in the case of
nonstoichiometric and Bi-doped La manganites [16,17], and low
doped manganites [18–20]. Another interesting feature of this
material is that under the influence of high magnetic field (5 T),
T
min weekend considerably (Fig. 6(a)). Another noticeable feature
The best fit parameters
given in Table 2. It can be seen from the table that all the three
parameters, and 4.5 are found to decrease with increasing
dopant concentration and magnetic field and the observed
behavior may be explained as follows: when the magnetic field
r
0
,
r
2
and r4.5 of all the samples are
of the material is that its MR is not only very large but also
remains almost constant over a large temperature range. Our
results are in close agreement with the reports of Uehara et al.
r
0
,
r
2
r
[
21]. In view of this, the observed behavior may be explained by a
percolative phase separation model [2,22]. According to this
increases, the domain enlarges reducing the value of
0
r , while the

ARTICLE IN PRESS
Y. Kalyana Lakshmi, P. Venugopal Reddy / Journal of Magnetism and Magnetic Materials 321 (2009) 1240–1245
1243
Table 2
Experimental data of La1-xAg
x
MnO
3
(0.05oxo0.30) materials.
(K)
T (K)
ꢀ
2
ꢀ4.5
)
Sample code
T
P
(K)
T
C
D
MR% at
0 K
r
0
(O
cm)
r
2
(
O
cm K
)
r
4.5
(
O
cm K
1
T
P
LA-05
LA-10
LA-15
LA-20
LA-25
LA-30
ꢀ
114
127
280
300
308
308
114
55
13
80
82
46
37
36
37
ꢀ
91
33
31
28
36
ꢀ
ꢀ
ꢀ
ꢀ
6
72
6410.08
0.15042
1.0325
3.707 ꢁ10
4.478 ꢁ 10
ꢀ1.91 ꢁ10
ꢀ
ꢀ
6
ꢀ13
ꢀ13
ꢀ14
267
306
310
310
7.32 ꢁ10
1.36 ꢁ10
0.54 ꢁ10
0.62 ꢁ10
6
06
02
02
0.02935
0.01263
0.01544
ꢀ
6
6
ꢀ7.83 ꢁ10
ꢀ
ꢀ14
ꢀ7.61 ꢁ10
07
0.040
1
1
1
1
LA-20
0
0
.112
0
.035
.030
.025
.020
.015
05
.105
0.035
0
0
0
0
LA-20
LA-25
LA-30
LA=05
LA=10
LA =15
03
0.098
0T
1T
0
0
.030
.025
LA-20
240
270
T (K)
300
LA =20
LA =25
LA =30
01
0
20
40
60
1
0-1
T (K)
204060
0
50
100
150
T (K)
200
250
300
350
0
20
40
T (K)
80
100
Fig. 3. Resistivity vs. temperature plots for different Ag concentration. Inset shows
the LA-20 sample having double-peak behavior.
Fig. 5. Low-temperature electrical resistivity fitting for Ag-doped samples using
1
/2
2
5
the equation
r
(T) ¼
r
0
ꢀr
1
T
2 5
+r T +r T . Inset shows fitting data under 5 T
magnetic field for LA-20 sample.
0
0
0
0
0
0
0
.14
.12
.10
.08
.06
.04
.02
LA-20
LA-25
LA-30
3
.5. Low-temperature behavior
Electrical resistivity vs. temperature plots of all the samples are
shown in Fig. 3. It can be seen from the figure that the first two
samples exhibit insulating behavior, while the remaining ones
show a slight upturn below 50 K (Fig. 5). The resistivity upturn in
polycrystalline manganites is usually attributed to a different
phenomenon such as Kondo effect [27], intergrain spin-polarized
tunneling (ISPT) [28], enhanced electron–electron (e–e) interac-
tion [29,30], etc. The Kondo effect is generally exhibited by a
20
40
T (K)
60
80
nonmagnetic system with
a small percentage of magnetic
impurity. As the samples under consideration have strong
ferromagnetic metallic nature, possibility of Kondo effect is ruled
out. According to the intergranular spin-polarized tunneling, the
resistivity minimum flattens gradually with increasing field and
vanishes at a critical field. In the present investigation as the
resistivity minimum is still intact even in
a field of 5 T,
0
50
100
150
200
250
300
the application of the ISPT model does not arise. Therefore all
the three models failed to explain the observed behavior.
However, the data is fitted using an equation which results
from the combined effect of weak localization, electron–electron
and electron–phonon scattering mechanism as
T (K)
Fig. 4. Plots of resistivity (
gives the best fitting using the equation
deviation of the equation at low temperatures (To50 K).
P
r) vs. temperature below T for xX0.20. The solid line
2
4.5
r
¼
r
0
+r
2
T +
r
4.5
T
. Inset shows the
1
=2
2
5
r
ðTÞ ¼ f1=ða þ bT Þg þ r2T þ r5
T
(1)
reduction in
2
r and r4.5 could be attributed to decrease in electron
spin fluctuations in the presence of magnetic field. Intrinsically,
the scattering effects are suppressed from various contributions
because of the parallel configuration of spins present in the
domain [26].
where the term in parentheses arises due to the weak localization
effect [29], ‘a’ is a temperature-independent residual conductivity
2
and ‘b’ is the diffusion constant. The other two terms, namely
and
2
r T
5
5
r T , arise due to the electron–electron and electron–phonon

ARTICLE IN PRESS
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Y. Kalyana Lakshmi, P. Venugopal Reddy / Journal of Magnetism and Magnetic Materials 321 (2009) 1240–1245
Table 3
1
/2
Low-temperature resistivity fitting (o50 K) using equation
r(T) ¼
0
r ꢀ
1
r T
+
2
5
2 5
r T +r T .
0
T
90
5T
LA-10
2
2
1
1
5000
0000
5000
0000
5000
0
ꢀ1//2
ꢀ2
ꢀ5
cm K )
Sample code
r
0
(
O
cm)
r
1
(
O
cm K
)
r
2
(
O
cm K
)
r
5
(
O
5T
85
LA-05
LA-10
LA-15
LA-20
LA-25
LA-30
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
80
75
70
ꢀ
6
ꢀ12
ꢀ12
ꢀ12
0.17819
0.03714
0.0147
0.004120
0.000093
0.00027
0.00022
6.528 ꢁ 10
8.429 ꢁ10
8.399 ꢁ10
2.833 ꢁ10
ꢀ
6
1.061 ꢁ10
4.732 ꢁ10
3.210 ꢁ10ꢀ
ꢀ
6
6
ꢀ12
0.01732
1.031 ꢁ10
65
0
25
50
T (K)
75
100
scattering, respectively [31]. However, we can expand Eq. (1)
binomially as
1
=2
2
5
r
ðTÞ ¼ r0
ꢀ
r1
T
þ
r
r2T þ r5
T
(2)
2
where
The
r
0
¼ 1/a and
1
¼ b/a are constants.
r
(T) data for xX0.15 samples are fitted to Eq (2) (Fig. 5)
0
50
100
150
T (K)
200
250
300
350
and the best fit parameters are given in Table 3. It can be seen
from the figure that Eq. (2) fits well both in presence and absence
of magnetic field (inset of Fig. 5). In fact, Neeraj et al. [32] have
also used this model to explain the low-temperature resistivity
data of PBMO/Ag composites. It can be seen from Table 3 that the
fitting parameters decrease with increasing doping concentration
as well as magnetic field. This indicates that the weak localization,
electron–electron and electron–phonon scattering decreases with
Ag doping, wherein disorder decreases and the material become
ordered.
55
9
0
5T
5
4
4
3
0
5
0
5
80
LA-05
LA-10
7
6
0
0
50
40
3.6. Magnetoresistance
0
30 60 90 120 150 180
T (K)
The percentage of magnetoresistance of all the samples has
been calculated using the well-known formula
30
LA-15
LA-20
LA-25
LA-30
r
ꢀ r
ðHÞ
ð0Þ
ð0Þ
r
MR% ¼
ꢁ 100
(3)
2
2
5
0
where
r
(0) is the resistivity at zero magnetic field and
r
(H) is the
resistivity in a applied field. The
r vs. T for LA-10 sample in the
0
100
200
300
presence of 5 T field is shown in Fig. 6(a). The calculated values at
low and MI transitions are given in the Table 2. Fig. 6(b) shows the
MR% as a function of temperature for Ag-doped samples. It can be
seen from the inset of Fig. 6(b) that, the percentage of MR is
maximum at low temperatures for LA-05 samples. Surprisingly,
large MR is exhibited by the sample, LA-10 over a wide
T (K)
Fig. 6. (a) Resistivity vs. temperature for LA-10 sample in presence of 5 T field
inset shows the MR% vs. T at 5 T). (b) MR% vs. temperature plots of the Ag-doped
samples for xX0.15. Inset shows the MR variation for x ¼ 0.05 and 0.10 samples at
T field.
(
5
P
temperature region below T (i.e. entire ferromagnetic metallic
large magnetoresistance of 80% at 3 T field over
a wide
region). In contrast to the first three samples of the present
investigation, MR of the samples with xX0.15 is found to increase
almost linearly with decreasing temperature, which is a typical
characteristic of a granular system. It reflects the spin-dependent
electron tunneling or scattering process taking place at the grain
boundaries [33], which is an extrinsic property of the granular
manganites. This behavior is found to decrease with increasing Ag
doping concentration, thereby indicating that the MR property of
the samples slowly changes from extrinsic to intrinsic nature.
temperature range in the low-temperature region for x ¼ 0.10
sample is achieved. From a practical point of view, the large MR
over a wide temperature range is beneficial for the application of
magneto-electronics devices fabricated by the CMR materials.
Acknowledgements
The first author is grateful to the University Grants Commis-
sion (UGC) for providing RFSMS fellowship. Authors are thankful
to Dr. Rajeev Rawat, UGC-DAE Consortium for scientific Research,
Indore (CSR), India, for providing MR facilities.
4.
Conclusions
In conclusion, the effect of silver doping on electrical and
magnetic properties is discussed. It has been found that both the
metal–insulator and ferro- to paramagnetic transitions after
increasing up to x ¼ 0.20, are found to saturate thereafter. The
electrical resistivity vs. temperature plot of the sample x ¼ 0.10 is
found to exhibit insulating behavior below 36 K, while the sample
with x ¼ 0.20 is found to exhibit two peaks, and the observed
behavior is explained on the basis of phase separation. Further,
References
[1] M. D Coey, Adv. Phys. 167 (1999) 48.
[2] E. Dogotto, T. Hotta, A. Moreo, Phys. Rep. 344 (2001) 1.
[
[
3] C. Zener, Phys. Rev. 82 (1951) 403.
4] C. Shivakumara, B. Manjunath Bellakki, S. Annigere Prakash, Y. Nagsampagi
Vasanthachary, J. Am. Ceram. Soc. 90 (2007) 3852.
[5] L.W. Lei, Z.Y. Fu, J.Y. Zhang, Mater. Lett. 60 (2007) 970.

ARTICLE IN PRESS
Y. Kalyana Lakshmi, P. Venugopal Reddy / Journal of Magnetism and Magnetic Materials 321 (2009) 1240–1245
1245
[
[
6] Soma Das, T.K. Dey, Solid State Commun. 134 (2005) 837.
7] O.Yu. Gorbenko, O.V. Melnikov, A.R. Kaul, A.M. Balagurov, S.N. Bushmeleva, L.I.
Koroleva, R.V. Demin, Mater. Sci. Eng. B 116 (2005) 64.
[19] Y. Moritomo, A. Asamitsu, Y. Tokura, Phys. Rev. B 56 (1997) 12190.
[20] T. Tang, R.S. Huang, S.Y. Zhang, Solid State Commun. 147 (2008) 190.
[21] M. Uehara, S. Mori, C.H. Chen, S.-W. Cheong, Nature 399 (1999) 560.
[
[
8] S.L. Ye, W.H. Song, J.M. Dai, K.Y. Wang, S.G. Wang, C.L. Zhang, J.J. Du, Y.P. Sun,
J. Fang, J. Magn. Magn. Mater. 248 (2002) 26.
9] L. Pi, M. Hervieu, A. Maignan, C. Martin, B. Raveau, Solid State Commun. 126
[22] M. Mayr, A. Moreo, J.A. Verges, J. Arispe, A. Feiguin, E. Dogotto, Phys. Rev. Lett.
86 (2001) 135.
[23] T. Tang, C. Tien, B.Y. Hou, J. Alloys & Compds. 461 (2008) 42.
[24] G.H. Rao, J.R. Sun, J.K. Liang, Phys. Rev. B 55 (1997) 3742.
[25] S.L. Ye, W.H. Song, J.M. Dai, S.G. Wang, K.Y. Wang, C.L. Yuan, Y.P. Sun, Appl.
Phys. 88 (2000) 5915.
(2003) 229.
[
[
10] M. Battabyal, T.K. Dey, J. Phys. Chem. of Solids 65 (2004) 1895.
11] R.N. Das, A. Pathak, S.K. Saha, A. Sannigrahi, P. Pramanik, Mater. Res. Bull. 36
(
2001) 1539.
[26] G. Venkataiah, P. Venugopal Reddy, J. Magn. Magn. Mater. 285 (2005)
343.
[
[
12] T.K. Mandal, S. Ram, Mater. Lett. 57 (2003) 2432.
13] G. Venkataiah, Y. Kalyana Lakshmi, P. Venugopal Reddy, J. Phys. D: Appl. Phys.
[27] J. Kondo, Prog. Theor. Phys. 32 (1964) 37.
4
0 (2007) 721.
[28] M. Auslender, A.E. Karkin, E. Rosenberg, G. Gorodetsky, J. Appl. Phys. 89
(2001) 6639.
[
[
[
[
[
14] A.I. Vogel, A Text Book of Quantitative Inorganic Analysis including
Elementary Instrumental Analysis, fourth ed., Longman, London, 1978.
15] J. Yang, W.H. Song, R.L. Zhang, Y.Q. Ma, B.C. Zhao, Z.G. Sheng, G.H. Zheng, J.M.
Dai, Y.P. Sun, Solid State Commun. 131 (2004) 393.
16] V. Markovich, E. Rozenberg, G. Gorodetsky, M. Greenblatt, W.H. McCarroll,
Phys. Rev. B 63 (2001) 054423.
[29] P.A. Lee, T.V. Ramakrishnan, Rev. Mod. Phys. 57 (1985) 287.
[30] B.L. Altshuler, A.G. Aronov, in: A.L. Efros, M. Pollak (Eds.), Electron-Electron
Interactions in Disordered Systems, North-Holland, Amsterdam, 1985.
[31] J.M. Ziman, Electrons and Phonons, Oxford University Press, Oxford, 1967.
[32] P. Neeraj, D.K. Pandya, S.K. Agarwal, J. Phys.: Condens. Matter 19 (2007)
456224.
17] Y.D. Zhao, J. Park, R.-J. Jung, H.-J. Noh, S.-J. Oh, J. Magn. Magn. Mater. 280
(2004) 404.
[33] X.B. Yuan, Y.H. Liu, B.X. Huang, C.J. Wang, L.M. Mei, J. Phys D: Appl. Phys. 38
(2005) 1.
18] A. Dutta, N. Gayathri, R. Ranganathan, Phys. Rev. B 68 (2003) 054432.