Effect of particle size on the magnetic and transport properties of La
0.875
Sr
0.125
MnO
3
Anulekha Dutta,* N. Gayathri,
²
and R. Ranganathan
‡
ECMP Division, Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Kolkata-700064, India
Received 27 January 2003; published 29 August 2003
The magnetic and transport properties of nanocrystalline La
0.875
Sr
0.125
MnO
3
have been studied. Raw
samples, prepared by the sol-gel method, were annealed at different temperatures to produce samples of
particle sizes 18 nm, 36 nm, and 50 nm. Unlike bulk and single-crystalline samples of the same composition,
magnetization data of the nanoparticle samples show no signature of structural transitions. The effects of
particle size on the magnetic and transport properties have been attributed to the domain status, changes in the
Mn-O-Mn bond angle, and Mn-O bond length in comparison with bulk samples. Field-cooled hysteresis
measurements suggest a cluster glasslike behavior in the nanoparticle samples.
DOI: 10.1103/PhysRevB.68.054432 PACS numbers: 75.47.Lx, 73.63.Bd
I. INTRODUCTION
Manganites having colossal magnetoresistive properties
have been the subject of various research activities because
of the close interplay between the magnetism and the trans-
port exhibited by these manganites.
1
A number of structural
and magnetotransport studies of the polycrystalline, single-
crystal, and thin films of the doped manganites show that the
ferromagnetic Curie temperature and the metal-
semiconductor transition temperature are strongly dependent
on the preparation method. Of the various manganese per-
ovskites, the La
1 -x
Sr
x
MnO
3
system has received a lot of
interest because it exhibits the highest T
c
among all the
doped manganite samples. Over the entire composition
range, the system exhibits rich magnetic and electrical
phases.
1
At low doping levels ( x 0.2) the system is a canted
antiferromagnetic insulator for x 0.1, a ferromagnetic insu-
lator for 0.1x 0.15, and a ferromagnetic metal for x
0.15. In the low doped regime of the La
1 -x
Sr
x
MnO
3
sys-
tem single crystals and bulk polycrystalline samples have
been extensively studied because of the series of structural,
magnetic, and electronic phase transitions they undergo as a
function of temperature which is also a paradigmatic ex-
ample of the interplay between carrier, lattice, spin, and or-
bital degrees of freedom.
2–4
The composition of
La
0.875
Sr
0.125
MnO
3
is in the region of ferromagnetic insula-
tor. Exhaustive studies using single-crystal and bulk poly-
crystalline samples have been performed on this composi-
tion. The La
0.875
Sr
0.125
MnO
3
single crystal shows a series of
structural transitions as a function of temperature.
3–13
The
first transition is from the high-temperature rhombohedral
phase to an orthorhombic O phase at 450 K. The second
transition occurs at around 270 K ( T
OO
'
) which is associated
with the O to O' the Jahn-Teller distorted orthorhombic
phase transition. This O' phase distorts to the orthorhombic
O phase at T 140 K ( T
O
'
O
). At T 180 K ( T
C
)
La
0.875
Sr
0.125
MnO
3
shows a paramagnetic to a ferromagnetic
metal FMM transition which is associated with a decrease
in the resistivity with temperature (d /dT 0). As the tem-
perature is lowered the FMM state gives way to a ferromag-
netic insulator FMI at T 140 K, which coincides with the
T
O
'
O
structural transition. The magnetization shows a sud-
den increase at this temperature. Detailed neutron-diffraction
and high-energy x-ray-diffraction studies have given evi-
dence of charge ordering at this temperature.
3,10–12
Though
there were contradictory reports suggesting that the low-
temperature phase be called an orbital ordered phase and not
a charge ordered phase,
8,9,13,14
now a consensus has been
reached that the phase is a charge ordered CO one. Dab-
rowski et al. have done a detailed magnetic, structural, and
electronic properties study of La
1 -x
Sr
x
MnO
3
around the
composition x =0.125.
15
They have observed for x =0.125
that T
C
190 K and there exists a small anomaly at T
135 K where magnetization decreases. The resistivity also
shows a decrease at T
C
and then again an increase below 140
K which coincides with the T
CO
observed in single crystals.
The transition is rather broad because of the polycrystalline
nature of the sample. An earlier report on a similar compo-
sition but with a varying oxygen content
16
shows an insulat-
ing behavior throughout the temperature range and also a
very broad ferromagnetic transition with no particular
anomalies in the magnetization.
In this study we examine the influence of particle size on
this particular composition of the Sr-doped system. It is well
known that reduction in particle size has a direct conse-
quence on the electronic and magnetic properties of a sys-
tem. Some salient features observed as we reduce the particle
size of magnetic systems are i a decrease and broadening of
the ferromagnetic transition temperature T
C
, ii a decrease
in the magnetization in comparison with single-crystal and
bulk polycrystalline samples, showing superparamagnetic
behavior at very low particle size, iii an increase in resis-
tivity, showing insulating properties at very low particle size,
and iv a higher value of magnetoresistance compared to the
bulk samples especially at low temperatures which is attrib-
uted to a spin-polarized intergrain tunneling mechanism.
17
These observations can be logically explained by assuming
the increase of an insulating grain-boundary contribution as
the particle size decreases.
18–20
Formation of grain bound-
aries causes broken bonds at the surface, which causes a
decrease in the magnetization value. The increase in resistiv-
ity is a consequence of the broken bonds as well. This is the
most general observation in the case of nanoparticles of the
manganite system.
18,21
There are a few exceptions to this
general rule. A recent report by Fu
22
on the La
0.8
Ca
0.2
MnO
3
PHYSICAL REVIEW B 68, 054432 2003
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