Journal of Magnetism and Magnetic Materials 328 (2013) 86–90

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Journal of Magnetism and Magnetic Materials

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Role of grain size on the magnetic properties of La0.7Sr0.3MnO3

P.A. Yadav a, A.V. Deshmukh b, K.P. Adhi a, B.B. Kale c, N. Basavaih d, S.I. Patil a,n
a
Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune 411007, India
b
Department of Physics, Fergusson College, Pune 411004, India
c
Centre for Materials for Electronics Technology, Pune 411008, India
d
Indian Institute of Geomagnetism, New Mumbai 410218, India

a r t i c l e i n f o a b s t r a c t

Article history: Nanophasic La0.7Sr0.3MnO3 samples were synthesized using the citrate-gel method. The samples were
Received 10 December 2011 annealed at different temperatures ranging from 600 to 1200 1C. Grain size was observed to increase
Received in revised form with the increase in annealing temperature. Furthermore, the magnetization data of these samples
20 September 2012
show well defined hysteresis. Saturation magnetization was observed to increase with increase in
Available online 2 October 2012
particle size. This gives evidence of formation of a magnetically dead layer at the surface. The thickness
Keywords: of the dead layer has also been calculated. The coercivity of nanoparticles follows the same trend as
Nanostructure predicted theoretically and particles below 22 nm are found to be single domain. The ferromagnetic to
Chemical synthesis paramagnetic transition temperature also increases with increase in particle size.
XRD
& 2012 Elsevier B.V. All rights reserved.
FE-SEM

1. Introduction highest ferromagnetic to paramagnetic transition temperature

(Tc). In particular, the x ¼0.3 composition of La1  xSrxMnO3 has
Mixed valence manganites with perovskite structure have been extensively studied in the form of single crystal, bulk
been studied extensively for many years, due to their interesting polycrystalline and thin film form due to the wide variety of
physical, electrical and magnetic properties. Research on manga- properties exhibited by it, in addition to the highest reported
nites has led to the formulation of important physical concepts value of Tc (  360 K).
such as double exchange and Jahn–Teller distortion and also Currently, the effect of particle size on magnetic properties of
revealed new phenomenon of colossal magnetoresistance manganites is an interesting subject for experimental as well as
(CMR). In perovskite manganites, of the form R1  xAxMnO3 (where theoretical exploration. A clear understanding of magnetic prop-
R is a trivalent rare earth ion and A is a divalent alkali earth ion), erties of manganite nanoparticles is still lacking and more work is
the spin, lattice, charge and orbital degrees of freedom are needed on this topic for better understanding. Previous studies on
coupled to one another. The interaction energies are of the same the hole doped manganites in the nano-form have tried to
order of magnitude; therefore, their properties are extremely investigate the changes in the electrical properties [3,4] and
sensitive to small changes in the material parameters thereby metal–insulator transition temperature as a function of the
leading to a very rich phase diagram [1]. The technological A-site ionic mismatch [4,5]. In addition, the changes in magnetic
applications in the field of magnetic sensors and spin dependent properties like CMR [6,7], ferromagnetic to paramagnetic transi-
electron transport devices make these compounds very interest- tion temperature Tc [4–8] and hysteresis behavior [9,10] have also
ing for research purpose. The parent compound LaMnO3 is an been investigated when the particle size is in nanometer range.
antiferromagnetic insulator whereas the hole doped manganites The effect of particle size on coercivity is also an interesting
La1  xAxMnO3 (AQCa, Sr, Ba) show a variety of physical proper- subject to study. Roy et al. [11] have found that the coercivity of
ties resulting in a rich phase diagram depending on the dopant the La0.8Sr0.2MnO3  d nanoparticles increases as the particle size
and its concentration. Among the various manganite compounds, (D) varies from 53 nm to 21 nm and in the single domain region
the La1  xSrxMnO3 (LSMO) system has received a lot of attention the coercivity exhibits a d  1.125 behavior, where d is the thickness
because over the entire composition range the system exhibits of the dead layer. In the study of variation of saturation magne-
rich magnetic and electrical phase diagram [2] and also shows the tization with particle size, Sanchez et al. [8] observed that the
saturation magnetization enhanced with increasing particle size
for La0.67Ca0.33MnO3, whereas Zhang et al. [4] observed decrease
n
Corresponding author. Tel.: þ91 20 25692678; fax: þ91 20 25691684.
in the saturation magnetization as the particle size increased for
E-mail addresses: patil@physics.unipune.ac.in, the fine particles of La1  xSrxMnO3. Apart from these reports,
patil@physics.unipune.ernet.in (S.I. Patil). Zhang et al. [12] also observed that the saturation magnetization

0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jmmm.2012.09.056
 P.A. Yadav et al. / Journal of Magnetism and Magnetic Materials 328 (2013) 86–90 87

first decreases with the increase in particle size for lower doping
concentrations (x o0.25) in La1  xSrxMnO3 and for higher doping
concentrations (x 40.25) the saturation magnetization increases
with increase in particle size, since lattice distortions (like
structure relaxation) are mainly responsible for the changes in
the magnetism in this class of fine particles. On the other hand,
theoretical prediction by Battle and Labarta [13] suggests that
magnetization should increase with the reduction in particle size.
These controversies made us to believe that it was important to
carry out a systematic investigation of the annealing temperature-
dependent or grain size-dependent magnetism and coercivity in the
hole doped manganites. Hence in the present study, we examined
the influence of grain size on the structural and magnetic properties
of La0.7Sr0.3MnO3.

2. Experimental

Single phase nanocrystalline samples of nominal compositions

La0.7Sr0.3MnO3 were synthesized by the citrate-gel method. Suitable
proportions of Lanthanum acetate [C6H9LaO6], Strontium acetate Fig. 1. X-ray diffractograms of the LSMO samples annealed for 4 h at (a) 600 1C,
[Sr(OOCCH3)20  9H2O], Manganese acetate [C4H6MnO4  4H2O] and (b) 700 1C, (c) 800 1C, (d) 900 1C, (e) 1000 1C, (f) 1100 1C and (g) 1200 1C.
citric acid [C6H8O7] were used as starting materials. All these acetates
were mixed together and dissolved in distilled water. Further, these
acetates were converted into their respective nitrates by adding
concentrated HNO3 and then citric acid was added to it. The resulting
solution was dried on water bath at 96 1C for several hours. This
process was continued until a pale orange-colored gel was formed.
The gel when calcined at 400 1C resulted in highly porous black
powder. The resulting powder was separated into parts, ground using
an agate-mortar and a pestle and annealed for 4 h at temperatures
varying from 600 1C to 1200 1C in steps of 50 1C.
These samples were characterized by X-ray diffraction (Bruker
Axs D8 Advance Diffractometer) and the grain size was determined
independently from Scherrer’s formula and field-emission scanning
electron microscopy (FE-SEM-(S-4800 HITACHI)). The hysteresis
measurements of the samples were carried out using a Vibrating
Sample Magnetometer (VSM; Lakeshore Cryotronics, Model-7307). Fig. 2. Variation of particle size as a function of annealing temperature measured
by XRD and FE-SEM.
The magnetic transition was determined with help of a low field a.c.
susceptibility meter.

Table 1
Lattice constants a, b and c (Å) along with the unit cell volume are indicated for the
3. Results and discussion
LSMO samples annealed at various temperatures. LSMO-600 represents the LSMO
sample annealed at 600 1C.
The LSMO samples synthesized using the citrate-gel method
were investigated structurally using powder X-ray diffraction Sample a (Å) b (Å) c (Å) Unit cell volume
name (70.01) ( 70.01) (7 0.01) (Å3)
(XRD). Fig. 1a–g shows the y 2y diffraction patterns for the
LSMO samples annealed at various temperatures from 600 1C
LSMO-600 5.48 5.48 7.66 230.06
to 1200 1C in steps of 100 1C. Samples synthesized below 600 1C LSMO-700 5.49 5.47 7.63 229.148
(viz. 400 1C and 500 1C) were observed to be amorphous in nature LSMO-800 5.48 5.51 7.69 231.959
and hence were not considered for further studies. Fig. 1a shows LSMO-900 5.49 5.47 7.80 234.229
the XRD pattern of the LSMO sample annealed at 600 1C for 4 h. LSMO-1000 5.48 5.51 7.75 233.693
LSMO-1100 5.49 5.49 7.80 235.107
The Miller indices, corresponding to the perovskite structure LSMO-1200 5.48 5.51 7.80 235.288
(orthorhombic), are indicated in Fig. 1g. From the XRD patterns
it is clear that single phase LSMO with perovskite structure
(orthorhombic) has been formed at 600 1C (LSMO-600) after The increase in grain size also leads to an increase in the unit cell
annealing the powder for 4 h (Fig. 1a). Further heating helped in volume, which may be due to the relaxation of strain.
enhancing the grain growth of the samples, which is indicated by The surface morphology of the LSMO nanoparticles was
the reduction of the full width at half maximum (FWHM) of the examined using FE-SEM. Fig. 3a, b, c and d shows the representa-
diffraction peaks (Fig. 1b–g). It is found that the structure remains tive images of the LSMO samples annealed at 700 1C (LSMO-700),
unchanged with the increase in grain size due to increased 800 1C (LSMO-800), 1000 1C (LSMO-1000) and 1200 1C (LSMO-
annealing temperature. The grain sizes of the LSMO samples 1200) respectively. The figure indicates a mixed shape morphol-
annealed at various temperatures were determined using Scher- ogy comprising of faceted grains with edge curvatures. The
rer’s formula. The variation in the grain size as a function of variation in the average particle size measured using FE-SEM is
annealing temperature is plotted in Fig. 2. The structural para- also plotted in Fig. 2. The size determined by FE-SEM is found to
meters determined using the XRD patterns are listed in Table 1. be larger than that determined using Scherrer’s relation. This
88 P.A. Yadav et al. / Journal of Magnetism and Magnetic Materials 328 (2013) 86–90

Fig. 3. FE-SEM images of LSMO samples annealed at (a) 700 1C, (b) 800 1C, (c) 1000 1C and (d) 1200 1C.

Fig. 4. Field dependence of magnetization (M–H) curves at room temperature of

LSMO samples annealed at different temperatures: (a) 600 1C, (b) 700 1C,
(c) 800 1C, (d) 900 1C, (e) 1000 1C, (f) 1100 1C, and (g) 1200 1C. The inset shows
an expanded portion of the M–H loop near origin for LSMO-700. Fig. 5. Dependence of saturation magnetization on particle size.

deviation in size is due to the fact that one particle may consist of existence of a non-magnetic dead layer on the surface. Such a
more than one crystallite, as is the case for polycrystalline surface layer is known to exhibit spin glass behavior which can
samples. Similar type of behavior has also been reported pre- occur due to unavailability of exchange partners to the surface
viously [14]. In the present case, though there is a deviation in the atoms, surface strains and disorder [11,14, 15]. Thickness of this
particle size measured using two techniques, the variation of non-magnetic surface layer has been calculated by Lopez-Quintela
particle size as a function of annealing temperature follows a et al. [14] and by Sarkar et al. [16] independently, using the relation,


similar trend. Ms ðDÞ ¼ M s0 1 6t=D Þ proposed by Tang et al. [17], where Ms ðDÞ is
The room temperature magnetization as a function of applied saturation magnetization, D is the grain size, M s0 is spontaneous
magnetic field (up to 5000 G) was measured using VSM and is as magnetization for bulk sample and t is the thickness of the surface
shown in Fig. 4. All the samples show ferromagnetic nature, layer. The reported value of thickness of the dead layer is approxi-
with very low values of coercivity. The saturation magnetization mately 2–3 nm assuming spherical particles. In the present case the
(obtained from the hysteresis measurements, Fig. 4) is observed to calculated thickness of non-magnetic surface layer is also found to
increase with the increase in particle size as shown in Fig. 5. The lie between 2.7 and 3.0 nm.
decrease in the value of saturation magnetization with the reduction As the particle size is reduced, width of this surface layer
in particle size as observed can be explained by assuming the almost remains unchanged, but the surface contribution increases
 P.A. Yadav et al. / Journal of Magnetism and Magnetic Materials 328 (2013) 86–90 89

rotations. Further reducing the size, the coercivity becomes zero

because of the thermal effects which are strong enough
to spontaneously demagnetize the particles resulting in a super-
paramagnetic state [19].
Variation of the coercivity with particle size has been plotted
in Fig. 6. From Fig. 6, it may be concluded that the LSMO
nanoparticles with size above  22 nm are multidomain while
the nanoparticles with size below  22 nm are single domain.
However, using the citrate-gel process, we were unable to
synthesize samples with smaller particle size which would show
superparamagnetic behavior.
Fig. 7 shows the variation of the magnetic transition tempera-
ture obtained from the low field a.c. susceptibility measurements
for LSMO-600 to LSMO-1200. The figure indicates that ferromag-
netic to paramagnetic transition temperature (Tc) increases with
the increase in particle size. Similar behavior was observed by
Sarkar et al. [16]. They observed two regions of Tc variation with
the particle size and attributed them to the enhancement in
bandwidth W and finite size effects. When the size becomes
comparable to the magnetic correlation length x, which cannot
grow near the critical point due to the finite size of the sample,
Fig. 6. Particle size dependence of coercivity. the Tc reduced with the reduction of particle size. Hence we could
say that the observed variation may be due to the finite size
effect.

4. Conclusion

We have successfully synthesized single phase nanocrystalline

samples of La0.7Sr0.3MnO3 by the citrate-gel method. The size of
the particles increases with increase in annealing temperature.
The decrease in saturation magnetization with decrease in parti-
cle size suggests the formation of a magnetically dead layer on the
surface of the nanoparticles. The nanoparticles below 22 nm are
found to be single domain and above  22 nm are multidomain in
nature. The magnetic transition temperature also approaches the
bulk value with increase in the particle size.

Acknowledgment

One of the authors P.A. Yadav would like to thank Center for
Nano-Quantum Systems (CNQS), University of Pune, Pune, for
financial assistance.
Fig. 7. Variation of the magnetic transition temperature with particle size.

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