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: yhliu@sdu.edu.cn (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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