Synthesis of nanocrystalline YFeO3 and its magnetic properties

Article abstract of DOI:10.1016/j.jmmm.2009.05.061

Single phase nanocrystalline YFeO₃ has been synthesized by a simple solution method. The average particle diameter is 42.2 nm. The particles exhibit ferromagnetic behaviour in the temperature range 10-300 K with a coercivity of 23 kOe. The magn

Full text of DOI:10.1016/j.jmmm.2009.05.061

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Journal of Magnetism and Magnetic Materials 321 (2009) 3274–3277
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Synthesis of nanocrystalline YFeO3 and its magnetic properties
Ã
Ramaprasad Maiti a, Soumen Basu b, Dipankar Chakravorty c,
a Department of Electronics, Vidyasagar University, Midnapore, West Bengal 721102, India
b National Institute of Technology, Durgapur, West Bengal 713209, India
c DST Unit on Nano Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Single phase nanocrystalline YFeO3 has been synthesized by a simple solution method. The average
particle diameter is 42.2 nm. The particles exhibit ferromagnetic behaviour in the temperature range
10–300 K with a coercivity of 23 kOe. The magnetization versus temperature over the temperature range
2–300 K obeys Bloch equation with a Bloch constant value 9.98 ꢀ 10ꢁ6 Kꢁ3/2. Ferromagnetic hysteresis
loops have been observed up to a temperature of 300 K. At 10 K a field-cooled sample shows an
exchange bias field.
Received 18 April 2009
Received in revised form
19 May 2009
Available online 17 June 2009
Keywords:
Yttrium orthoferrite
Ferromagnetic behaviour
Exchange bias
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
continued for 2 h when Y2O3 was completely dissolved. Fe(NO3)3
(1.43 g) was added to the solution under stirring for 12 h.
Rare earth orthoferrites have attracted considerable attention in
recent years because of their interesting magnetic properties [1–5].
These exhibit a weak ferromagnetic behaviour. Yttrium orthoferrite
which has been investigated by several groups has a distorted
perovskite structure. A super-exchange magnetic interaction occurs
between two iron ions separated by an oxygen ion. The alignment
of iron moments is not perfectly antiparallel but there is a slight
canting. This results in a small magnetization giving rise to weak
ferromagnetism. Preparation of single phase yttrium orthoferrite
by conventional solid state reaction of precursor oxides has been
reported to be difficult because of the formation of secondary
phases like Fe3O4 and Y3Fe5O12 (yttrium–iron garnet) [1]. A novel
Y–Fe alkoxide has been developed to make YFeO3 by a sol–gel
process [4]. We have used a simple solution method to synthesize
YFeO3 in the nanocrystalline state. No secondary phase has been
observed. The magnetic properties have been delineated. The
details are reported in this paper.
A polyvinyl alcohol (Loba-Chemie, purity 98%) (PVA) solution was
prepared by taking 0.5 g PVA in a beaker containing 40ml distilled
water. The mixture was stirred continuously and the temperature
raised gradually to 347 K. The stirring and heating was continued
until the PVA was completely dissolved in water. The stirring
time was 2 h. The two solutions prepared as above (viz, Y(NO3)3+
Fe(NO3)3 and PVA, respectively) were mixed with a volume ratio
5:2. The final solution was allowed to form a clear gel at room
temperature. The gel was dried. The temperature was raised at a
rate of 5 K/min till it reached 693K. The solution was thereafter
furnace cooled to room temperature. These heating and cooling
cycles were repeated three times. By this process PVA was removed
completely. The resultant gel was subjected to heat treatment at
1073 K for 1 h to crystallize the YFeO3 phase.
2
The crystalline phase in the sample was identified by taking
X-ray diffractograms in a Rich Seifert diffractometer using CuK
a
radiation. The microstructure was investigated by a JEM 2010
transmission electron microscope operated at 200 KV. Magnetic
property measurements in the sample were carried out in a
Quantum Design MPMS system having a SQUID magnetometer in
the temperature range 2–300 K. The magnetization values were
measured under zero field-cooled (ZFC) and field-cooled (FC)
conditions at an applied magnetic field of 500 Oe
2. Experimental
The precursor solution was prepared using Y2O3 (Sigma-
Aldrich, purity 99.99%) and Fe(NO3)3.(Alfa-Aesar, purity 99.9%).
Y2O3 (0.4 gm) was taken in 16 ml distilled water. The mixture was
continuously stirred and the temperature raised slowly to 343 K
adding a few drops of concentrated HNO3. The stirring was
3. Results and discussion
Ã
Fig. 1 is the X-ray diffractogram obtained from the specimen
synthesized. All the lines have been identified with those of pure
Corresponding author.
E-mail address: mlsdc@iacs.res.in (D. Chakravorty).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.05.061

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13000
12000
11000
10000
9000
8000
7000
20
30
40
50
60
70
80
2θ
Fig. 1. X-ray diffractogram obtained from YFeO3 specimen.
Table 1
nature of the curves indicates that there is a superparamagnetic
behaviour exhibited by the particles. The latter have weak
ferromagnetic characteristics. Hence, this type of variation is
expected. The blocking temperature is estimated from the broad
maximum in the ZFC curve to be around 50 K. The data indicate a
distribution of particle sizes over a wide range. We have analyzed
the magnetization behaviour on the basis of Bloch equation [6,7],
given by
Refined structural parameters of YFeO3 (Rietveld analysis).
Compound
YFeO3
Crystal system
Space group
Orthorhombic
Pnma
˚
Lattice parameters (A)
a
b
c
5.584
7.597
5.274
MsðTÞ ¼ Msð0Þ½1 ꢁ BT3=2
ꢂ
ð1Þ
3
224.05
42.2
˚
Unit cell volume (A )
where Ms(T) and Ms(0) are the saturation magnetizations at
temperatures T and 0 K, respectively, and B is the Bloch constant.
It should be noted here that our samples do not show saturation
even at 50 KOe. We have therefore estimated Ms value at any
temperature by extrapolating the M vs. 1/H curve to Hꢁ1-0 [8].
Average grain size (nm)
R factors (%): Rw (%) ¼ 1.08, Rexp (%) ¼ 1.16, GoF ¼ (Rw/Rexp) ¼ 0.93.
Fig. 4 gives the plot of [Ms(0)ꢁMs(T)]/Ms(0) as a function of T3/2
.
The slope of the straight line fitted to the experimental data is
found to be B ¼ 9.98 ꢀ 10ꢁ6 Kꢁ3/2. This is much larger than those
YFeO3 as given in JCPDS file no. 39-1489. The (h k l) values have
been shown against the intensity peaks in the figure. Rietveld
analysis of the X-ray data was carried out and the results are
shown in Table 1. The average diameter of the orthoferrite grain
has been estimated to be 42.2 nm.
Fig. 2(a) is the transmission electron micrograph of a specimen.
There is a distribution of particle sizes and this particular area in
the micrograph gives large-sized particles. Fig. 2(b) is the electron
diffraction pattern obtained from Fig. 2(a). The interplanar
spacings were calculated from the positions of the diffraction
spots. These are summarized in Table 2. It is evident that all the
diffraction spots arise due to the presence of YFeO3 phase.
Fig. 3 shows the variation of magnetization as a function of
temperature under both zero field-cooled and field-cooled
conditions. The latter was cooled under a field of 500 Oe. The
observed in the case of ferromagnetic metal like a-Fe [4]. It may
be mentioned here that the value of B obtained by us is of the
same order as that reported by Mathur et al. for single phase
YFeO3 samples [4].
Figs. 5(a)–(c) show the magnetization versus magnetic-field
hysteresis loops for the YFeO3 specimen at temperatures 300, 200
and 100 K, respectively. The coercivity is found to be around
23 KOe. This is in agreement with values reported earlier [4].
Fig. 6 shows the hysteresis loop for YFeO3 sample at 10 K. The
data were collected after field cooling of the sample from 300 to
2 K. It can be seen that the loop is highly asymmetrical and an
exchange bias field of 18 KOe in the negative direction is obtained.
Such behaviour has not been reported before. It is believed that
this effect arises because of the presence of an antiferromagnetic
layer at the surfaces of the nanoparticles of YFeO3. These results

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FC
0.030
0.028
ZFC
0.026
0.024
0.022
0.020
0
50
100
150
200
250
300
T(K)
Fig. 3. Variation of magnetization as a function of temperature under zero field-
cooled (ZFC) and field-cooled (FC) condition for YFeO3 specimen.
0.06
B = 9.98x10-6(K-3/2
)
0.04
0.02
0.00
2.0x103
4.0x103
6.0x103
0.0
T3/2(K)
Fig. 4. Plot of [Ms(0)ꢁMs(T)]/Ms(0) as a function of T3/2
.
Fig. 2. (a) Transmission electron micrograph of YFeO3 specimen. (b) Electron
diffraction pattern obtained from (a).
Table 2
Comparison of interplanar spacings calculated from electron diffraction pattern of
YFeO3 with ASTM data.
substantiate the fact that the sample was field-cooled through the
ordering temperature of the antiferromagnetic layer referred to
above. It is not possible to determine the antiferromagnetic
ordering temperature because of the predominant effect of the
superparamagnetic blocking temperature. It has been shown
earlier that the exchange bias effect has its origin in the
exchange coupling between antiferromagnets and ferromagnets
[9,10].
In summary, single phase YFeO3 nanoparticles have been
synthesized by a simple solution method using Y(NO3)3 and
Fe(NO3)3 as precursors in a PVA medium. The magnetization
versus temperature variation over the range 2–300 K obeys Bloch
equation with a Bloch constant value higher than those of normal
ferromagnetic metals. Ferromagnetic hysteresis loops have been
observed at temperatures in the range 100–300 K. An exchange
bias field has been shown to occur at a temperature of 10 K for a
field-cooled sample.
Observed (nm)
ASTM
YFeO3 (nm)
0.2706
0.2644
0.2379
0.1920
0.1755
0.1706
0.1594
0.1574
0.1528
0.1444
0.1348
0.1180
0.1011
0.0996
0.2702
0.2641
0.2388
0.1920
0.1758
01713
01596
0.1572
0.1530
0.1444
0.1350
.01179
0.1011
0.1001

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1.5
1.0
a
1.2
0.6
0.5
0.0
0.0
-0.5
-1.0
-1.5
-0.6
-1.2
-60000 -40000 -20000
-60000 -40000 -20000
0
20000 40000 60000
0
20000 40000 60000
H (Oe)
H (Oe)
1.5
b
Fig. 6. Hysteresis loop for YFeO3 (field-cooled under 500 Oe) at 10 K.
1.0
0.5
Acknowledgements
0.0
The work has been supported by a project under the Nano
mission of DST, Government of India, New Delhi. D. Chakravorty
thanks Indian National Science Academy, New Delhi, for giving
him an Honorary Scientist position.
-0.5
-1.0
-1.5
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Fig. 5. Magnetization versus magnetic field hysteresis loops for YFeO3 specimen at
different temperatures: (a) 300 K, (b) 200 K and (c) 100 K.