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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: [email protected] (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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