Magnetic properties of nanoparticles of cobalt chromite

Article abstract of DOI:10.1016/j.jmmm.2011.01.040

Magnetic properties of cobalt chromite nanoparticles of size 812 nm synthesized through conventional coprecipitation route are reported. Magnetization versus temperature measurement plot reveals a transition from paramagnetic to superparamagnetic (SPM) phase in contrast with the transition from paramagnetic to long-range ferrimagnetic phase at Curie temperature, T _c, reported in bulk. The blocking temperature, T_b, of SPM phase is found to be 5060 K. On cooling in the presence of 10 kOe field these nanoparticles show an enhancement in coercivity and shifting of loop at 10 K, which is absent at 50 K. While the later observation supports the blocking temperature of the SPM phase, the former one is attributed to a disordered spin configuration at the surfaces and the distribution of nanoparticle sizes.

Full text of DOI:10.1016/j.jmmm.2011.01.040

Journal of Magnetism and Magnetic Materials 323 (2011) 1698–1702
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of nanoparticles of cobalt chromite
Chandana Rath a,n, P. Mohanty a, A. Banerjee b
a School of Materials Science & Technology, Institute of Technology, Banaras Hindu University, 221 005 Varanasi, India
b UGC-DAE Consortium for Scientific Research, Khandwa Road, 452017 Indore, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnetic properties of cobalt chromite nanoparticles of size 8–12 nm synthesized through conven-
tional coprecipitation route are reported. Magnetization versus temperature measurement plot reveals
a transition from paramagnetic to superparamagnetic (SPM) phase in contrast with the transition from
paramagnetic to long-range ferrimagnetic phase at Curie temperature, Tc, reported in bulk. The blocking
temperature, Tb, of SPM phase is found to be 50–60 K. On cooling in the presence of 10 kOe field these
nanoparticles show an enhancement in coercivity and shifting of loop at 10 K, which is absent at 50 K.
While the later observation supports the blocking temperature of the SPM phase, the former one is
attributed to a disordered spin configuration at the surfaces and the distribution of nanoparticle sizes.
& 2011 Elsevier B.V. All rights reserved.
Received 10 July 2010
Received in revised form
25 January 2011
Available online 3 February 2011
Keywords:
Nanoparticle
Chromite
Spinel
Superparamagnetism
Exchange bias
1. Introduction
induces the electric polarization and also a spontaneous magne-
tization for which it is said to be as multiferroic. Due to lack of
Chromite, cubic normal spinel compounds have recently
attracted much attention as multiferroic materials [1]. Cobalt
chromite (CoCr2O4), one of the spinel family, is ferrimagnetic in
nature in which magnetic Co2+ ions occupy A site and magnetic
Cr3+ ions occupy B site [2]. The strong interactions among
chromium ions in chromite control the magnetic order [3–5].
The antiferromagnetic alignment between A and B sites is
completely destroyed and system exhibits a screw ordering. This
is otherwise named as ferrimagnetic spiral wherein the spins lie
on the conical surfaces. The magnetic order is mostly studied in
bulk and single crystals. Menyuk et al. [6] studied the magnetic
ordering through neutron diffraction and magnetic measure-
ments of bulk samples and have shown that below Tc, magnetic
ordering consists of a ferrimagnetic component and a spiral
component. The ferrimagnetic component exhibits long range
order at all temperatures below Tc while the spiral component
exhibits a short range order. Tomiyasu et al. [7] revisited the
spiral ordering by neutron scattering and magnetic measure-
ments in CoCr2O4 single crystals and report a simultaneous
formation of long range order of ferrimagnetic component and a
short range order of the spiral component at lowest temperature
phase. In addition to the magnetic order investigated by Menyuk
et al. [6] in polycrystalline sample, a dielectric anomaly below
spiral magnetic order has been observed in polycrystalline sam-
ple [8] as well as in single crystals [1,9]. The spiral component
studies on nanoparticles of cobalt chromite, we have synthesized
these particles in nanometer range by conventional coprecipita-
tion route and studied the magnetic properties of these materials
by varying temperature and magnetic field. We observed a
transition from paramagnetic to superparamagnetic (SPM) phase
at Tc instead of a transition from paramagnetic to ferrimagnetic
phase. These nanoparticles exhibit an exchange bias phenomena
below Tb.
2. Experimental methods
2.1. Synthesis of cobalt chromite powder
Conventional coprecipitation technique was used to synthe-
size cobalt chromite powders. We have used cobalt nitrate
hexahydrate, Co(NO3)2 ꢀ 6H2O (molecular weight 291.03 g, 99%),
chromium nitrate nonahydrate, Cr (No3)3 ꢀ 9H2O (molecular
weight 400.15 g, 98%) and ammonia solution (30% by weight) to
synthesize cobalt chromite. Acetone of analytical grade was used
for drying the precipitate. Stock solutions of cobalt nitrate (0.5 M),
chromium nitrate (0.5 M) and ammonia solution (1.5 M) were
prepared in 500 ml volumetric flasks separately by dissolving
appropriate amount of Co(NO3)2 ꢀ 6H2O, Cr(NO3)3 ꢀ 9H2O and NH3
(about 30%) in double distilled water respectively. From the stock
solution, desired quantity of cobalt nitrate was taken in a 1000 ml
beaker. Desired quantity of chromium nitrate solution was added
slowly to the cobalt nitrate solution under continuous stirring.
The mixed solution was further stirred at room temperature
n Corresponding author.
E-mail address: chandanarath@yahoo.com (C. Rath).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2011.01.040

C. Rath et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1698–1702
1699
for 2 h. Further, aqueous ammonia solution (1.5 M) was added
dropwise to the mixed solution till the pH of 8.9 was attained. The
hydroxide precipitate was filtered, washed several times with
distilled water till the filtrate attains a pH value of about 7. Finally
the hydroxide precipitate was washed with acetone and was
dried in an oven at 120 1C for 16 h to get the desired oxide.
The dried oxide was calcined at 600 1C for 4 h to get well
crystalline phase.
Diffractometer operating in the Bragg–Brentano geometry and
fitted with a graphite monochromator in the diffracted beam. Field
emission scanning electron microscopy (FESEM, ZEISS: SUPRA 40)
at 5 kV is used for structural characterization of CoCr2O4 nanopar-
ticles. We measured the temperature and field dependent dc
magnetization using Vibrating Sample Magnetometer insert of
Physical Properties Measurement System (PPMS-VSM) of Quantum
Design operating between 2 to 350 K. For frequency dependent ac
susceptibility measurements SQUID magnetometer (Quantum
Design) is used. We measured specific heat as a function of
temperature using the heat capacity insert of PPMS (Quantum
Design) for 8 mg pressed powder rectangular bar.
2.2. Characterization of cobalt chromite powder
The calcined powders were characterized by X-ray Diffraction
(XRD) using an 18 kW rotating anode (Cu K ) based Rigaku powder
a
3. Results and discussion
(Observed)
(Calculated)
(Difference)
(Bragg reflections)
Fig. 1 depicts the XRD pattern of CoCr2O4 calcined at 600 1C.
The pattern is fitted using Fullprof program with Fd3m space
group. The observed pattern, calculated data after Le-Bail analysis
and the difference pattern between observed and calculated one
are shown as dots, continuous line and as bottom line, respec-
tively. The tick marks above the difference plot show the posi-
tions of the Bragg peaks. The well defined peaks corresponding to
miller indices (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and
(4 4 0) are ascribed to cubic phase of CoCr2O4 (JCPDS file
no:780711). No peaks other than CoCr2O4 have been observed.
˚
The lattice parameter is found to be 8.310 A, which is in agree-
˚
ment with the bulk value (8.334 A). The mean crystallite diameter
along (3 1 1) calculated using the Scherrer formula after correct-
ing the instrumental broadening is found to be ꢁ8 nm. Field
emission scanning electron micrograph is shown in Fig. 2. The
particles are mostly agglomerated and majority of particles are in
8–12 nm range as observed from particle size distribution histo-
gram shown as inset of Fig. 2. Crystallite size is in good agreement
with the particle size measured from FESEM.
10
20
30
40
50
60
70
2θ (degree)
Fig. 1. X-ray diffraction spectrum of calcined CoCr2O4 nanoparticles synthesized
at pH 8.9 and fitted using Fullprof program with Fd3m space group. The observed
pattern, calculated data after Le-Bail analysis and the difference pattern between
observed and calculated one are shown as dots, continuous line and as bottom
line, respectively.
Fig. 3a shows magnetization as a function of temperature
under zero field cooling (ZFC) and field cooling (FC) condition
50
40
30
20
10
0
5
10
15
20
Particle Size (nm)
Fig. 2. Field Emission Scanning Electron Micrograph of CoCr2O4 nanoparticles calcined at 600 1C and inset shows the particle size distribution histogram.

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0.04
0.03
0.02
0.01
0.00
0.025
ZFC (10 Oe)
FC
T
= 44.0K
max
ZFC (500 Oe)
FC
0.020
0.015
0.010
0.005
0.000
Field = 500 Oe
Field =10 Oe
T
s
T
= 54.7K
max
T
c
0
20 40 60 80 100 120
Temperature (K)
0
20 40 60 80 100 120
Temperature (K)
Fig. 3. (a) Temperature dependent zero field cooled (ZFC) and field cooled (FC) dc magnetization measured at H¼500 and 10 Oe. (b) Difference in magnetization observed
after field cooling and zero field cooling at 10 and 500 Oe plotted with respect to change in temperature.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
12
10
8
Field = 3 Oe
Field = 3 Oe
3 Hz
3 Hz
31 Hz
230 Hz
966 Hz
31 Hz
230 Hz
966 Hz
6
4
T
s
2
0
0
20 40 60 80 100 120
Temperature (K)
0
20
40
60
80 100 120
Temperature (K)
Fig. 4. (a) Real and (b) imaginary parts of ac susceptibility versus temperature measured at 3 Oe field and at different frequencies such as 3, 31, 230 and 966 Hz.
with field 500 and 10 Oe, respectively. ZFC does not fall upon FC
curve. The branching in FC and ZFC starts at 96 and 99 K,
respectively, in applied field of 500 and 10 Oe. Tc is derived by
extrapolating the linear part of magnetization to zero in high
temperature regime. It is found to be 97 K and 98.5 K which
matches well with the Tc of bulk [1] and single crystals [7] of
CoCr2O4. This suggests that Tc remains unaffected by reducing the
size to nanoscale range. However, in similar kind of spinel
structures like in ferrites, a large increase in Tc has been reported
by reducing the size to nanoscale range [10,11]. Below Tc,
magnetization shows a large increase in both ZFC and FC curves
and then decreases further with decrease in temperature. An
anomaly around a temperature ꢁ23 K in FC is observed in M
versus T curve obtained by applying field of 500 Oe. This tem-
perature is known as spiral ordering temperature, Ts. In bulk and
single crystals of CoCr2O4, Ts is observed at 31 and 24 K, respec-
tively [7,8]. Ts in our case lies in between Ts of bulk and single
crystals. Comparing the MZFC and MFC values with bulk and single
crystals, we observe no negative magnetization in MZFC as found
in bulk samples [8] and an order of magnitude higher MFC than
the single crystal and bulk [7,8]. The difference between MFC and
MZFC is plotted with temperature in Fig. 3b. The maximum
magnetization value shifts to lower temperature with increase
in applied field from 10 to 500 Oe, which is expected [12–13].
Below Ts, magnetization again increases. Though MZFC shows a
peak for both superparamagnets and spin glasses, the tempera-
ture dependence of FC susceptibility becomes saturated below the
peak temperature for spin glasses and continues to increase
below that temperature for superparamagnets [14]. The increas-
ing tendency of magnetization below Ts (Fig. 3a) shows therefore
an evidence of superparamagnets. The temperature dependence
of ac susceptibility measured at several frequencies ranging from
3 to 1000 Hz probes further the spin glass (SG) and SPM behavior.
The sample is first cooled from room temperature to 10 K in a

C. Rath et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1698–1702
1701
zero magnetic field. Then a probing ac magnetic field of 3.0 Oe is
applied to measure the susceptibility as the temperature is slowly
raised. Fig. 4a and b shows the real and imaginary parts of
susceptibility. We note that w0 decreases with increase in fre-
quency, showing that w0max is independent of frequency, which
eliminates the possibility of spin glass behavior. Such frequency
independent behavior has also been observed in other nanopar-
observe any hysteresis (Fig. 5b). Magnetization readily increases
at low field and then linearly increases up to the maximum
applied field (60 kOe). At 50 K, we observed hysteresis loop with
non-saturation of magnetization up to maximum applied field of
60 kOe (Fig. 5c). While long range ferrimagnetic phase below Tc
has been seen in bulk phase [8], the absence of hysteresis loop at
60 and 80 K eliminates such long range ferrimagnetic order in
these nanoparticles below Tc, which persists up to 60 K. The
appearance of loop at 50 K and its disappearance at 60 K suggest
that the nanoparticles are SPM in nature and the blocking
temperature is between 50 and 60 K. The definition of SPM
materials meets at least two requirements. First, above Tb, the
system must not show any hysteresis, which we observed at 60 K.
Second, the magnetization curve must be temperature dependent
to the extent that curves taken at different temperatures must
(approximately) superimpose when being plotted against H/
T [16]. The plot of magnetization versus H/T measured at 60 and
80 K shown in Fig. 6 confirms the SPM phase below Tc. In contrast
ticle system [15]. w00
does not change with temperature and
max
frequency. w00 shows a peak at low temperature (ꢁ23 K) corre-
sponding to Ts. Tc at 97 K and Ts at 23 K, associated with the
transition from paramagnetic to long range ferrimagnetic and
from long range ferrimagnetic to long/short range spiral magnetic
structures, respectively, are in broad agreement with the specific
heat versus temperature measurement (figure not shown here).
The variations in magnetization with applied magnetic field at
several temperatures after ZFC are shown in Fig. 5. Above Tc, the
M versus H taken at 100 K shown in Fig. 5a confirms the
paramagnetic phase. Below Tc, i.e. at 60 and 80 K, we do not
4
3
@100K
80K
60K
3
2
1
0
2
1
0
0
20
40
60
0
20
40
60
Applied Field (kOe)
Applied Field (kOe)
4
2
4
3
10K
10K (@1Tesla)
2
1
0
0
-1
-2
-3
-4
-2
-4
50K (@1Tesla)
50K
-60 -40 -20
0
20
40
60
-60 -40 -20
0
20
40
60
Applied Field (kOe)
Applied Field (kOe)
Fig. 5. Magnetization versus applied magnetic field measured at (a) 100 K, (b) 80 and 60 K, (c) 50 K and (d) 10 K of CoCr2O4 nanoparticles calcined at 600 1C. In (c) and
(d) measurement of magnetization with varying external field after cooling under 10 kOe magnetic field at 50 and 10 K are shown respectively.

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C. Rath et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1698–1702
by the external field. Depending on the strength of the AFM
1.2
0.9
0.6
0.3
0.0
anisotropy, one may find a shift in hysteresis loop or enhance-
ment in Hc without any loop shift during field cooling procedure
([18] and references there in). On the other hand, both effects may
also be observed simultaneously due to for example structural
defects or grain size distribution ([18] and references there in).
The origin of Hc enhancement with shifting of loop observed at
10 K could be ascribed to the distribution of particles, which is
observed from FESEM and disordered spin configuration existing
at the surface evidenced from the non-saturation of magnetiza-
tion and one order magnitude higher Hc. The absence of loop shift
or absence of Hc change at 50 K further confirms the blocking
temperature of SPM phase [18–20].
60K
80K
4. Conclusion
Nanoparticles of cobalt chromite synthesized through conven-
tional coprecipitation technique undergo paramagnetic to super-
paramagnetic phase transition at Tc in contrast with paramagnetic
to ferrimagnetic phase transition in bulk. Further decrease in
temperature transforms superparamagnetic to long range ferri-
magnetic phase keeping Tc and Ts almost the same as in the bulk
phase. The blocking temperature of superparamagnetic phase lies
in between 50 and 60 K. Exchange bias phenomenon has been
observed in these nanoparticles of CoCr2O4. The absence of loop
shift or absence of enhancement in Hc at 50 K supports the
blocking temperature of SPM phase.
Acknowledgments
0
20
40
60
80
100
UGC-DAE Consortium for Scientific Research (CSR), Indore, is
acknowledged for the support. DST, Government of India, is
acknowledged for funding the 14T-PPMS-VSM for magnetic
measurement at CSR, Indore.
H/T (Oe/K)
Fig. 6. Magnetization M as a function of H/T at T¼60 and 80 K of CoCr2O4
nanoparticles calcined at 600 1C.
to paramagnetic to ferrimagnetic phase transition at Tc in bulk
CoCr2O4, we observed paramagnetic to a SPM phase followed by a
long range ferrimagnetic phase. The intermediate SPM phase has
not been observed even in nanoparticles of cobalt chromite
prepared by sonochemical method [17].
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terms of an alignment of the AFM spins at FM–AFM interface
parallel to FM spins occurring during the field cooling procedure.
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