Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Low field magnetoresistance and conduction noise in layered

manganite La1.4Ca1.6Mn2O7

Neeraj Kharea,*, Ajai K. Guptaa,b, G.L. Bhallab

aNational Physical Laboratory, Superconducting Device Group, Dr. K.S. Krishnan Road, New Delhi-110012, IndiabDepartment of Physics and Astrophysics, University of Delhi, New Delhi-110007, India

Received 12 August 2004; accepted 2 September 2004 by C.N.R. Rao

Available online 15 September 2004

Abstract

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La1.4Ca1.6Mn2O7

(DLCMO) are reported and compared with the infinite layered manganite La0.7Ca0.3MnO3 (LCMO). The double layered

manganite was prepared using standard solid state reaction method and had a metal–insulator transition temperature (TM–I) of

155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed

voltage noise spectral density (SV) shows 1/fa type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic

region (T!168 K), SV/V2 shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of

DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range

3D-ferromagnetic ordering at two different temperatures TC2 and TC1. In temperature range between TC1 and TC2, the

magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the

framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].

q 2004 Elsevier Ltd. All rights reserved.

PACS: 75.47.Gk; 75.47.Lx; 72.70.Cm; 75.30.Kz

Keywords: A. Layered manganite; B. Colossal magnetoresistance; C. Anisotropy; D. Conduction noise

1. Introduction

Mixed valence manganese oxides R1KxAxMnO3 [RZLa, Nd, Pr; AZCa, Ba, Sr, Pb] have drawn considerable

attention in recent years because of the colossal magnetore-

sistance (CMR) effect [1–3]. The CMR phenomena in these

systems are generally understood in terms of the double-

exchange mechanism [4] combining with the local Jahn–

Teller distortions of MnC3 ions [5]. Bulk samples and

polycrystalline films of R1KxAxMnO3 show large magne-

toresistance effect for the low field and even at temperatures

0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2004.09.002

* Corresponding author. Tel.: C91-11-25742610-2; fax: C91-

11-25726952.

E-mail address: nkhare@mail.nplindia.ernet.in (N. Khare).

much lower than TC [6,7]. The electrical transport across the

grain boundaries in these materials is proposed to be due to

spin polarized tunneling which produces low field magne-

toresistance [6,8]. More recently double layered manganites

R2K2xA1C2xMn2O7 have gained importance because of

their inherent anisotropy and its consequences for studying

low dimensional physics [9–13]. The double layered

manganite consists of the ferromagnetic metallic MnO2

bilayers separated by nonmagnetic (La,Ca)2O2 insulating

layers and are found to exhibit very large magnetoresistance

effect and two ferromagnetic transitions [10–14]. The

setting up of 2D-ferromagnetic ordering at temperatures

much above the 3D-ferromagnetic ordering is a very

interesting feature of the double layered manganite [10–

12,14–16]. Study of conduction noise in this material can

provide more details about it’s magnetotransport behaviour.

Solid State Communications 132 (2004) 799–803

www.elsevier.com/locate/ssc

N. Khare et al. / Solid State Communications 132 (2004) 799–803800

The excess 1/fa noise has an intrinsic material dependence

via the nature of the defect fluctuations or other physical

processes contributing to resistance fluctuations [17].

Conduction noise in infinite layered doped manganites has

been reported earlier [18–20]. However, no study of the

conduction noise of double layered manganite has been

reported yet. There has also not been detailed study of low

field MR behaviour of double layered manganite in the

intermediate temperature range between the 2D- and 3D-

ferromagnetic ordering. This paper reports the temperature

and frequency dependence of conduction noise and low field

magnetoresistance of double layered La1.4Ca1.6Mn2O7

(DLCMO) manganite. In the low temperature region, two

peaks in the normalised conduction noise spectrum are

observed which has been attributed to the excess noise

generated due to setting of 2D and 3D-ferromagnetic

ordering in the DLCMO at two different temperatures. The

observed magnetoresistance behaviour of La1.4Ca1.6Mn2O7

in the intermediate temperature range indicates different

origin of low field MR in La1.4Ca1.6Mn2O7 as compared to

the polycrystalline infinite layered manganite La0.7Ca0.3-MnO3 and it has been explained in terms of two phase model

(ferromagnetic domains and paramagnetic region).

Fig. 1. Temperature dependence of resistivity of bulk pellets of

La1.4Ca1.6Mn2O7 (DLCMO) and La0.7Ca0.3MnO3 (LCMO).

2. Experiment

Bulk sample of doubled layered La1.4Ca1.6Mn2O7

(DLCMO) manganite was prepared by standard solid state

reaction method. Stoichiometric amounts of La2O3, CaCO3

and MnO2 were taken, mixed and ground using mortar and

pestle. Calcination was done for 48 h at 950 8C. The

resultant mixture was reground and pellets were prepared.

The pellets were calcined again at 950 8C for 24 h. Sintering

was done at 1200 8C for 48 h and then at 1300 8C for 15 h

followed by slow cooling to room temperature. X-ray

studies reveals that the resultant sample of double layered

La1.4Ca1.6Mn2O7 was of single phase and its structure was

tetragonal perovskite with lattice parameters as aZ3.858 A,

cZ19.291 A. The values of lattice parameters are similar to

that reported by others [10] for the same composition of

double layered manganite. For the preparation of infinite

layered La0.7Ca0.3MnO3 (LCMO), stoichiometric mixture

of La2O3, CaCO3 and MnO2 were calcined at 950 8C for

24 h. Pellets were prepared and again calcined at 950 8C for

24 h. Sintering was done at 1300 8C for 24 h followed by

slow cooling to room temperature.

AC susceptibility was used to determine the ferromag-

netic transition temperature of the sample. The temperature

dependence of electrical resistivity of double layered

La1.4Ca1.6Mn2O7 and infinite layered La0.7Ca0.3MnO3

were measured by four-probe technique in the temperature

range from 77 to 300 K. For magnetoresistance studies, an

electromagnet was used for applying the magnetic field and

magnetoresistance (MR) has been calculated using the

relation,

MRZ ½fRð0ÞKRðHÞg=Rð0Þ�!100%

where R(H) and R(0) are the resistance of the sample in the

presence and in the absence of the magnetic field

respectively.

Conduction noise of the bulk pellets was also measured

using four-probe technique. A 2 mA current was passed

through the sample using a battery operated low noise

current source. The voltage signal was dc filtered, amplified

by a low noise amplifier and measured by a dynamic signal

analyser for observing the frequency spectrum. Frequency

spectrum was recorded at different temperatures. All the

measuring instruments were interfaced with a computer for

automatic data collection.

3. Results and discussion

Fig. 1 shows the temperature dependence of resistivity of

double layered La1.4Ca1.6Mn2O7 (DLCMO) and infinite

layered La0.7Ca0.3MnO3 (LCMO) samples. The resistivity

of the DLCMO sample was much larger than that of infinite

layered LCMO sample at all temperature range. This

observed large resistivity of the double layered sample

could be ascribed to intrinsic anisotropic property of the

double layered compound [11,12]. The metal–insulator

transition temperature (TM–I) of the double layered sample

was 155 K, which was much lower than the metal–insulator

transition temperature of infinite layered compound (TM–

Iz200 K).

Fig. 2 shows the temperature dependence of real part of

ac-susceptibility (c 0) of DLCMO in the temperature range

from 77 to 200 K. The c 0 shows a gradual increase as

temperature is lowered from 168 K. It shows to attain a

saturation at TZ110 K. Such a wide ferromagnetic

transition width is a typical feature of double layered

R2K2xA1C2xMn2O7 and arises due to appearance of short

range magnetic order at temperatures much higher than the

Fig. 2. Temperature dependence of ac magnetic susceptibility (c 0)

of La1.4Ca1.6Mn2O7 (DLCMO). The inset shows temperature

dependence of 1/c 0.

Fig. 4. Temperature dependence of normalized conduction noise, SV(3 Hz)/V2 for La1.4Ca1.6Mn2O7 (DLCMO) and La0.7Ca0.3MnO3

(LCMO) samples.

N. Khare et al. / Solid State Communications 132 (2004) 799–803 801

3D-ferromagnetic transition temperature [11,12]. The inset

of the Fig. 2 shows the plot of 1/c 0 vs. T. This indicates that

onset of short range ferromagnetic ordering occurs at 168 K.

This short range ordering is converted into long range

ferromagnetic ordering as the temperature decreased to

114 K.

Fig. 3 shows variation of voltage noise spectral density,

SV (3 Hz) for DLCMO and LCMO samples with tempera-

ture. For LCMO, SV–T curve shows a peak at 200 K which is

the TM–I for the sample. The temperature dependence of SVof DLCMO also shows a peak at TM–Iz155 K similar to

that of LCMO. A small peak in SV–T curve of DLCMO was

also observed at 114 K which seems to corresponds to 3D-

ferromagnetic transition temperature of the DLCMO.

Fig. 4 shows the temperature dependence of normalized

noise (SV/V2) of DLCMO and LCMO samples. For LCMO,

in the paramagnetic region (TOTCZ245 K), the noise

shows a decrease with the decrease in temperature.

Fig. 3. Variation of voltage noise spectral density, SV (3 Hz) with

temperature for La1.4Ca1.6Mn2O7 (DLCMO) and La0.7Ca0.3MnO3

(LCMO) samples.

However, for T!TC, SV/V2 increases with the decrease in

temperature. This type of temperature dependence of

normalized noise of LCMO is a typical feature of the

infinite layered manganites and has been reported pre-

viously by other workers [18–20]. In the case of DLCMO, in

the paramagnetic region, the noise decreases with the

decrease in temperature similar to that of LCMO. However,

in the ferromagnetic region (T!168 K) the behaviour is

different. The SV/V2 increases as temperature decreases to

168 K and shows a peak at 164 K. Another peak was also

observed at 114 K. Appearance of peak in SV/V2 at 114 K

and 164 K shows that the setting up of short range 2D-

ferromagnetic ordering and long range 3D-ferromagnetic

ordering at z164 K and z114 K generates excess

fluctuations.

The frequency dependence of voltage noise (SV) was

found to follow 1/fa type of behaviour at all temperatures.

Fig. 5 shows the values of a at different temperatures for

DLCMO sample. The value of a was found to vary with

temperature and it shows dips at 165 and 112 K. This

Fig. 5. Variation of noise parameter a with temperature.

Fig. 7. Variation of magnetoresistance (MR) with the applied

magnetic field for La1.4Ca1.6Mn2O7 (DLCMO) and La0.7Ca0.3MnO3

(LCMO) samples at 123 K.

N. Khare et al. / Solid State Communications 132 (2004) 799–803802

indicates that there was enhancement in noise fluctuations at

165 K and 112 K which has changed the value of a.

Fig. 6 shows the temperature dependence of magnetore-

sistance (MR) for LCMO and DLCMO samples in the

presence of 1.5 kOe magnetic field. MR for both the samples

decreases with the increase in temperature. The MR for

infinite layered LCMO decreases almost linearly with

temperature and shows a slight hump at ferromagnetic

transition temperature (TCz245 K). For DLCMO variation

of MR with temperature for T!114 K was similar to that of

LCMO sample. However, for TO114 K, the variation of

MR for DLCMO was different from that of LCMO.

The double layered manganite has two ferromagnetic

transition temperatures [11,12]. For DLCMO at

TC2z168 K, two dimensional ferromagnetic ordering

occurs and at TC1z114 K, 3D-ferromagnetic ordering is

established. Compared to DLCMO, the infinite layered

manganite (LCMO) has only one 3D-ferromagnetic tran-

sition temperature at 245 K. The result shown in Fig. 5

indicates that when 3D ferromagnetic ordering are present

in DLCMO, the origin of low field MR is similar to that of

LCMO. However, for temperatures when 2D-ferromagnetic

ordering is dominating, the origin of low field MR seems to

be different.

In order to further investigate the magnetoresistive

behaviour, we have studied the variation of MR with field

for DLCMO and LCMO sample at 123 K. At this

temperature in DLCMO, 2D ferromagnetic ordering was

dominating, whereas in LCMO 3D-ferromagnetic ordering

was present. Fig. 7 shows variation of MR with magnetic

field (upto 3 kOe) at 123 K for DLCMO and LCMO

samples. The nature of variation of MR for the LCMO and

DLCMO samples are different. For LCMO, MR shows a

sharp increase for a field of 300 Oe and afterwards it shows a

slow increase with the magnetic field. For DLCMO, MR

rises slowly and keeps on increasing with the increase in the

Fig. 6. Variation of magnetoresistance (MR) of La1.4Ca1.6Mn2O7

(DLCMO) and La0.7Ca0.3MnO3 (LCMO) samples with temperature

for HdcZ1.5 kOe.

magnetic field. The difference in the MR behaviour of the

DLCMO and LCMO indicates that the origin of low field

MR at temperatures between the 3D- and 2D-ferromagnetic

ordering temperatures in double layered manganite is

different from the infinite layered manganite. For the infinite

layered polycrystalline manganite sharp increase in MR at

low magnetic field is due to spin polarized tunneling across

the grain boundary in the material [6,8] and slow increase in

MR at large field was attributed to field induced reduction of

magnetic disorder at the grain boundaries [21]. In the double

layered manganite for T!TC2, short range 2D-ferromag-

netic ordering starts growing and at TZTC1, 3D-long range

ordering is established. Thus, for the temperatures between

TC1 and TC2, the double layered manganite sample can be

thought of as a mixture of ferromagnetic-metallic (FM)

domains and paramagnetic-insulating (PMI) regions. In

such a two phase system, transport properties can be

determined by the percolation [22]. The resistance of the

double layered manganite for the temperatures between TC1and TC2 can be written as

RZ ð1K f ÞRPM C fRFM (1)

where RFM and RPM are resistances of ferromagnetic and

paramagnetic regions in the sample and f is the fraction of

the FM phase in the sample. For TOTC2 the value of f

would be zero and for T!TC1, the value of f would be one.

At temperatures between TC1 and TC2, the application of

magnetic field will increase the ferromagnetic-metallic

region. Thus the resistance of the sample will decrease. In

such a two phase system, the application of magnetic field is

not expected to lead a sharp change in the resistance and the

MR vs. magnetic field curve is expected to show a slow

increase as we have observed for the DLCMO sample.

N. Khare et al. / Solid State Communications 132 (2004) 799–803 803

4. Conclusions

We have synthesized and investigated the temperature

and frequency dependence of conduction noise and low field

magnetoresistance of double layered manganite La1.4Ca1.6-Mn2O7 (DLCMO) from 77 K to 300 K. The conduction

noise shows 1/fa type of behaviour at all the temperature

range from 77 K to 300 K. The temperature dependence of

normalised conduction noise of double layered DLCMO

was found to be different from that of infinite layered

LCMO. The observation of two peaks in SV/V2 vs. T curve

for DLCMO and also dips in a vs. T curve at 165 and 112 K

indicates that the appearance of short range 2D-ferromag-

netic ordering at higher temperature and long range 3D-

ferromagnetic ordering at lower temperature generate

excess noise. For the temperature range between 2- and

3D-ferromagnetic ordering, the low field MR occurred due

to increase in ferromagnetic-metallic regions in the two

phase mixture of ferromagnetic and paramagnetic regions.

Acknowledgements

The authors gratefully acknowledge the support and

encouragement received from Prof. Vikram Kumar, Direc-

tor, NPL, New Delhi, India and usefull discussions with Dr.

A.K. Gupta (NPL, New Delhi, India) and Prof. O.N.

Srivastava (BHU, Varanasi, India). One of us (AKG) is

thankful to CSIR, New Delhi, India for the award of Junior

Research Fellowship.

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