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JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 43 (2010) 145003 (5pp)

doi:10.1088/0022-3727/43/14/145003

Current-induced nonlinear conduction of
two-electron doped manganites
Ca1−xCexMnO3
Y Yamato1 , M Matsukawa1 , Y Murano1 , S Kobayashi1 and
R Suryanarayanan2
1

Department of Materials Science and Engineering, Iwate University, Morioka 020-8551, Japan
Laboratoire de Physico-Chimie de L’Etat Solide, CNRS, UMR8182 Universite Paris-Sud,
91405 Orsay, France

2

Received 7 January 2010, in final form 25 February 2010
Published 23 March 2010
Online at stacks.iop.org/JPhysD/43/145003
Abstract
We have investigated the current-induced nonlinear conduction in a charge-ordered phase of
two-electron doped manganites Ca1−x Cex MnO3 (x = 0.1 and 0.167). The substitution of Ce4+
ion for Ca2+ site of the parent matrix causes two-electron doping with the chemical formula
3+
4+
4+
Ca2+
1−x Cex Mn1−2x Mn2x O3 . Seebeck coefficient data of Ca0.9 Ce0.1 MnO3 are, in both its
temperature dependence and its magnitude, very similar to those of Ca0.8 Sm0.2 MnO3 , leading
to further evidence for two-electron doping. The V I characteristics measured using a longpulsed current show a negative differential resistance and its associated giant electroresistanceeffect. It is demonstrated that a temperature rise across the samples due to Joule heating is not
responsible for a huge decrease in resistance observed here.
(Some figures in this article are in colour only in the electronic version)of an applied electric current/voltage on the resistance of hole
doped manganites causing a huge negative ER effect so far[3–13]. However, they have paid little attention to electron
doped manganite systems with a high concentration of Mn4+
ions [14, 15].
In this paper, we demonstrate current-induced nonlinear
conduction in a charge-ordered phase of two-electron doped
manganites Ca1−x Cex MnO3 (x = 0.1 and 0.167). The
substitution of Ce4+ ion for Ca2+ site produces a twoelectron doped manganite system with the chemical formula
3+
4+
4+
Ca2+
For a single electron doped
1−x Cex Mn1−2x Mn2x O3 .
2+3+system, we have Ca1−y Ry3+ Mn4+
1−y Mny O3 (R = trivalentrare-earth ions). It is well known that for Ca1−y Ry MnO3
(R = Sm, Pr, Nd, Eu, etc) the CMR effect appears in a very
narrow range of carrier contents [16]. Zeng et al showed that
Ca1−x Cex MnO3 is a two-electron doped CMR system througha scaling (x = 2y) of magnetic properties with respect to asingle electron doped system [17]. Furthermore, the x-ray
absorption spectroscopy result reveals that the formal valenceof Ce ion is 4+. In particular, at the x = 0.167 composition
with the concentration of Mn3+ = 1/3, the CO state is morestabilized.

1. Introduction
Perovskite manganites show a large variety of fascinating
properties such as the colossal magnetoresistance (CMR)
effect and the charge–orbital ordered (CO) insulating phase [1].
The most interesting one is the existence of a phaseseparated state, the coexistence of antiferromagnetic (AFM)
CO insulating and ferromagnetic metal (FMM) regions [2].
The phenomenon of microscopic phase separation into twocompeting ordered phases on various length scales has a close
relation to quenched disorder existing in doped manganites.
Recently, it has been reported that the resistance of CO
manganites exhibits a significant change when a strong electricfield or current is applied [3, 4]. The electric current induced
resistance change results in switching from the charge-orderedinsulating to ferromagnetic metallic state. This phenomenon
is called the colossal electroresistance (CER) effect, which
is analogous to the term of the CMR effect. The effect ofelectric fields on the CO state of the manganites is an issueof considerable interest not only on the basis of physicalphenomena but also from the viewpoint of technologicalapplications. There have been extensive studies on the effect0022-3727/10/145003+05$30.00

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J. Phys. D: Appl. Phys. 43 (2010) 145003

Y Yamato et al

2. Experiment

(a)

5

10
Resistivity (Ωcm)

Polycrystalline Ca1−x Cex MnO3 (x = 0.1 and 0.167) weresynthesized by a standard solid-state reaction method. The
starting materials CaO, CeO2 and Mn3 O4 with high purities
were mixed, and the formed powders were annealed at 1000–
1200 ◦ C for 24 h with several grindings. The products were
finally pressed into pellets, and they were sintered in air at
1450 ◦ C for 24 h. Powder x-ray diffraction patterns showed
the samples are almost single phase with an orthorhombic
structure (P nma space group). The lattice parameters ofthe x = 0.1 sample are a = 5.3261 Å, b = 7.4863 Å andc = 5.2961 Å, which is fairly in agreement with a previous
work [18]. The cell parameters and the unit cell volume
are elongated upon increasing the Ce doping although theion radius of Ce4+ (1.14 Å) is smaller than the value of Ca2+
(1.34 Å). The Mn4+ ions (0.53 Å) are replaced by Mn3+ ions(0.645 Å) with one extra electron, which contributes to theenhanced cell volume. The temperature variation of electricalresistivity was measured with a standard four-probe method.
Electrical contacts were prepared with silver paste on a barshaped sample (typical dimension ∼8 × 2 × 2 mm3 ). Seebeckcoefficient was determined from both a temperature gradientand its associated thermal voltage across the sample when athermal current is supplied along its longitudinal direction.
The V I characteristics measurement was performed with a
two-probe method using a Keithley KE2400 source meter,
which enables us to monitor voltage on current contact. The
sample was mounted on the cold head of a closed type helium
cryocooler surrounded by a vacuum cryostat. Measurement ofmagnetization was carried out by a superconducting quantum
interference device magnetometer. A pulsed current with atime duration of 10 ms was applied from 1 up to 50 mA in stepsof I = 1 mA to reduce the Joule heating effect. The appliedcurrent was switched off for about 10 ms between successive
current steps. The temperature gradient generated across the
samples was monitored using a differential-type thermocoupleof Au+0.07%Fe alloy versus chromel when the applied current
was supplied. One junction of the thermocouple lead was set
on the top surface of the sample with a GE7031 varnish and theother one was attached at its bottom on the cold end. Underadiabatic conditions, heat flux generated flows from the hot
surface at the top of the sample towards its cold end when theapplied current is parallel to both surfaces. For comparison,
the V I data of the x = 0.1 and 0.167 samples dipped in liquid
nitrogen were taken using a stationary current.

1000
10
0.1

x=0.1
x=0.167

0.001
0

50

100 150 200
Temperature (K)

250

0.02

0.01
(b) ZFC 10 mT

0.015
M (emu/g)

300

0.0095

x=0.1

0.009

0.01

0.0085
0.008

0.005

x=0.167
0

0

50

100

150 200
T (K)250

0.00750.007
300

Figure 1. (a) Temperature variation of resistivity ρ ofpolycrystalline Ca1−x Cex MnO3 (x = 0.1 and 0.167) measured at asmall current of 0.01 mA. Arrows denote the charge-orderedtransition temperatures. (b) Temperature variation of zero fieldcooled (ZFC) magnetization of Ca1−x Cex MnO3 (x = 0.1 and 0.167)
measured under an applied field of 10 mT.

240 K. The magnetization data of the x = 0.1 sample, as
depicted in figure 1(b), reveal a pronounced peak around 170 K
indicating the paramagnetic to AFM transition accompanied bya transition into a charge-ordered insulating state. A similar
behaviour on the ZFC run is obtained at x = 0.167. Here, we
define the charge-ordered insulator transition temperature as
TCO . Next, we examine the Seebeck coefficient S of x = 0.1(figure 2), in order to check an electron-doped system of oursamples. The value of S shows a monotonic increase down
to TCO = 170 K upon cooling the x = 0.1 sample and thenrapidly starts to drop below TCO . After the value of S reaches
a broad minimum around ∼100 K, at lower temperatures itagain begins to exhibit a stable increase crossing the horizontal
axis. The Seebeck coefficient is a sensitive probe to examine
electronic states of dirty materials since it is less affected bygrain boundaries of polycrystalline samples than the resistivity
measurement. We note that the S data of Ca0.9 Ce0.1 MnO3 are,in both its temperature dependence and its magnitude, very
similar to those of Ca0.8 Sm0.2 MnO3 , indicating an additional
evidence for two-electron doping [19].
Now, let us show the voltage versus current (V I )characteristics of Ca1−x Cex MnO3 (x = 0.1) at selected
temperatures (figure 3(a)). Upon increasing the appliedcurrent, at smaller currents the I V data of the x = 0.1 sample

3. Results and discussion
First of all, we present in figure 1(a) the temperature variationof electrical resistivity ρ of polycrystalline Ca1−x Cex MnO3
(x = 0.1 and 0.167). The ρ value of the x = 0.1 sample
remains nearly constant at high temperatures and it thenexhibits a rapid increase below 170 K upon decreasing T ,suggesting the weakly metallic to charge-ordered insulatortransition. The resistivity of the x = 0.167 sample also
shows a semiconducting behaviour over a wide range oftemperatures and a further increase in ρ appears below
2

J. Phys. D: Appl. Phys. 43 (2010) 145003

40

Y Yamato et al5(a) x=0.1

20
4

0
-20

Voltage (V)S (µV/K)

50K
60K
70K
LN2
78K
90K

(a) x=0.1

-40
-60

3
2

-80
1

-100
-120

0

50

10

100

150
T (K)

200

250

300

0
0

(b) 50 mA

4

0

4

0

-20

5
ER (%)∆T (K)

20
30
I (mA)(b) x=0.1

0

7.5

2.5

-40

50K
60K
70K
78K
90K

-60

0
40

10

60

80

100

120

-80

T (K)
Figure 2. (a) Temperature variation of Seebeck coefficient S of
Ca0.9 Ce0.1 MnO3 . The arrow denotes the CO transition temperature.
(b) The temperature gradient T generated across the x = 0.1 and
0.167 samples at selected temperatures. T is monitored at
I = 50 mA. For comparison, the temperature rise of the x = 0.167
sample dipped in liquid N2 is given.

-100

0

10

20
30
I (mA)

Figure 3. (a) Voltage versus current (V I ) characteristics of
Ca1−x Cex MnO3 (x = 0.1) at selected temperatures under a vacuum
cryostat. For comparison, we add the I V curve recorded for the
sample dipped in liquid nitrogen bath. The arrows indicate thresholdcurrents. A negative differential resistance (dV /dI < 0) is observed
when the applied current exceeds the threshold value. (b) ER of thex = 0.1 sample as a function of applied current.

follow the Ohmic characteristics and then tend to saturate. As
T is lowered below 70 K, a negative differential resistance(NDR) feature occurs at higher currents above a thresholdvalue ranging from 35 mA at 70 K down to 16 mA at 50 K. The
current dependence of the corresponding resistance R = V /I
follows a strong nonlinearity and the magnitude of R rapidly
drops by more than one order of magnitude at higher currents.
Figure 3(b) represents the electroresistance ratio (ER) of thex = 0.1 sample as a function of applied current. Here, the ER
is defined as [R(I ) − R(Ilow )]/R(Ilow ) × 100, where Ilow is thelowest current value (1 mA). A giant electroresistance effectreaching ER-−90% at I = 50 mA for x = 0.1 is achieved
at 50 K. Our findings are in good agreement with a previous
work on the I V data of Ca1−x Cex MnO3 (x = 0.1) except forthe data at LN2 temperature [14].
To examine the influence of Joule heating on the nonlinear
I V characteristics, we check simultaneously a temperature riseof samples studied using differential thermocouples as shownin figure 2. For x = 0.1, we obtain T = ∼4 K (I = 50 mA)
at 50 K, which never reproduces a huge decrease in R by one
order of magnitude. In a similar way, we display in figure 4
the I V characteristics and the ER ratio in the charge-orderedstate of Ca1−x Cex MnO3 (x = 0.167) at selected temperatures.

At the lowest temperature of 70 K, a strong NDR appears evenat low currents applied over a threshold value of 5 mA and theapplication of the high currents up to 50 mA results in a veryhuge decrease in R by more than one order of magnitude. The
corresponding ER attains −98% at I = 50 mA. For x = 0.167,
we obtain T = ∼8 K (I = 50 mA) at 70 K but it does not
explain such a huge decrease in R. We expect from figure 1(a)that a temperature rise due to Joule heating suppresses the valueof ρ from 840  cm at 70 K down to 260  cm at 78 K, givinga gradual drop in R by a factor of about one-third. To examine
the difference in the thermal environment of the sample in a
vacuum cryostat and a liquid N2 bath, the I V data of the sample
immersed in liquid N2 are given in figure 4(a). In the liquidN2 bath, we have both a negative differential resistance and its
associated huge decrease in R, giving a CER effect achieving
ER = −85% at I = 50 mA. The corresponding temperaturerise is T = ∼3 K at I = 50 mA for x = 0.167 (for x = 0.1,
T is negligible). It is evident that the effect of Joule heating
is not responsible for the huge decrease in R.
3

J. Phys. D: Appl. Phys. 43 (2010) 145003

Y Yamato et al50(a) x=0.167
40
Voltage (V)our knowledge, we have no experimental evidence for theenhanced bandwidth effect due to the application of a strong
electric field on manganites.

70K
78K
LN2
90K
120K

4. Summary30

We have investigated the nonlinear V I characteristics ina charge-ordered phase of two-electron doped manganites
Ca1−x Cex MnO3 (x = 0.1 and 0.167). The substitution of
Ce4+ ion for Ca2+ site produces two-electron doping with the3+4+4+chemical formula Ca2+
1−x Cex Mn1−2x Mn2x O3 . It is pointed
out that the Seebeck coefficient data of Ca0.9 Ce0.1 MnO3 arevery similar to those of Ca0.8 Sm0.2 MnO3 , indicating additional
evidence for the two-electron doping. We demonstrate that atemperature rise across the samples is not responsible for thehuge decrease in R. The V I characteristics measured using
a long pulsed current show a negative differential resistanceand concomitant giant electroresistance effect. The larger ER
effect enhanced for the x = 0.167 sample is probably attributed
to its stable CO state.

20
10
0
0

10

20
30
I (mA)(b) x=0.167

0ER (%)

-20
-40

40

50

70K
LN2
78K
90K
120K

Acknowledgments
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Japan Society of the Promotionof Science. The authors thank A Tamura for his technicalsupport.

-60-80

References
-100
0

10

20
30
I (mA)

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50

Figure 4. (a) Voltage versus current (V I ) characteristics and (b) ER
ratio of Ca1−x Cex MnO3 (x = 0.167) at selected temperatures.

It has been reported that the CO state of the x = 0.167
sample is stable in comparison with that of x = 0.1 because
the concentration of magnetic frustrated bonds decreases withincreasing Mn3+ ions [20]. In addition, the fraction of CO
phase in the x = 0.1 sample is estimated to be at most 30%
with a majority (70%) of the C-type AFM phase at 12 K, while
for the x = 0.167 sample it reaches about 80%, suggestinga major fraction of the CO phase. We thus believe that theapplication of a stronger electric field is needed to transformthe stable CO phase into a low resistive state.
Here, we assume that CO clusters are pinned to the latticethrough the Jahn–Teller effect of Mn3+ ions as pointed out
in [21]. We then understand that they are depinned by theapplication of a strong electric field, resulting in the nonlinear
ER effect. Moreover, upon increasing T , a thermally activateddepinning becomes more dominant, leading to a smaller ER
effect [4]. This pinning model qualitatively describes thegiant ER effect enhanced at low T . As for the ER effect in
the ferromagnetic insulating phase of La0.82 Ca0.18 MnO3 , theauthors argue that the electric current enhances transfer integral
between eg orbitals of neighbouring Mn sites, i.e. increases
the eg bandwidth, which results in a resistance drop [6]. To
4

J. Phys. D: Appl. Phys. 43 (2010) 145003

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