Soft Mechanochemical Synthesis and
Characterization of Nanodimensional Spinel Ferrites
Zorica Lazarević*, Aleksandra Milutinović, Maja
Romčević, Nebojša Romčević
Institute of Physics
University of Belgrade
The Institute for Multidisciplinary Research
University of Belgrade
Dalibor Sekulić, Miloš Slankamenac
Faculty of Technical Sciences
University of Novi Sad
Novi Sad, Serbia
Abstract—NiFe2O4 and ZnFe2O4 ferrites have been prepared by
soft mechanochemical synthesis. The sintered samples were
analyzed by XRD and Raman spectroscopy. Investigation of the
magnetization as a function of magnetic field confirms an
expected change of the degree of inversion in the spinel structure
with the sintering. Impedance spectroscopy on the sintered
pellets of ferrites was performed in the wide frequency range
(100 Hz – 10 MHz) at different temperatures using an Impedance
/Gain-Phase Analyzer (HP-4194).
Keywords-ferrites; XRD; Raman spectroscopy; magnetic
measurements; impedance spectroscopy
It is well known that properties of ferrite materials strongly
depend on the preparation conditions. Consequently, different
preparation methods for ferrite are described in the literature
[1-3]. By choosing the method that reduces particle size,
magnetic properties (such as coercive field, Curie temperature,
saturation magnetization and absorption coefficients) may
change significantly in comparison with the bulk material.
The spinel ferrites with the general formula MFe2O4 (M =
Mn, Co, Ni, Cu, Mg or Zn) is well known due their interesting
magnetic and electrical properties on account of chemical and
thermal stabilities. NiFe2O4 and ZnFe2O4 are the most
important ferrite binary oxides with the spinel structure, and
are usually as a ferrimagnet with the strong dependence of
magnetic properties on the state of chemical order and the
cation site occupancy in the materials [4, 5]. The structural
formulas are generally written as (M2+
where round and square brackets denote sites of tetrahedral (A)
and octahedral [B] coordination, respectively, and where λ
represents the degree of inversion defined as the fraction of the
(A) sites occupied by Fe3+.
The mechanochemical synthesis can deliver the designed
phases and structures by a single-step of the high-energy
milling conducted in an enclosed activation chamber at room
temperature . Usually, the compete formation of spinel
ferrites was obtained only after milling followed by sintering,
i.e. by employing two processing steps. It is obvious that the
combined mechanochemical-thermal treatment yields a well-
ordered spinel phase in ferrites at lower annealing temperatures
and shorter durations than those required in conventional
ceramic methods . Of course, in such case the morphology
of crystallite, agglomerate and particle is changed significantly.
mechanochemically procedure is very suitable for the
activation or synthesis of inorganic precursors. This is reflected
primarily in the simplicity of the procedure and equipment
used . In many cases, when it comes to classical synthesis
reaction sintering process, requires high temperatures, which
can present an additional problem in industrial production.
Mechanochemical derived precursors exhibit significantly
higher reactivity and thus lower the calcination and sintering
method, and primarily soft
In this article, we investigated the sintered sample of
NiFe2O4 and ZnFe2O4 ferrites
mechanochemical treatment, via high-energy milling of binary
hydroxide precursors. In both cases, soft mechanochemical
reaction leading to formation of the spinel phase was followed
by X-ray diffraction and Raman spectroscopy. The magnetic
measurements at room temperature are done in magnetic fields
up to ±80 kOe. The electric property of the synthesized ferrites
has been analyzed by impedance spectroscopy.
prepared by soft
NiFe2O4 and ZnFe2O4 were prepared from stoichiometric
quantities of mixture of powders of Ni(OH)2-Fe(OH)3 and
Zn(OH)2-Fe(OH)3 by soft
Mechanochemical synthesis of both ferrites was performed in
air atmosphere in a planetary ball mill (Fritsch Pulverisette 5)
for 25 h and 18 h, respectively. The powder mixture pressed
into pallets using a cold isostatic press (8 mm in diameter and
~3 mm thick). The powder mixtures were sintered at 1100 °C
for 2h (Lenton-UK oven) without pre-calcinations step.
Heating rate was 10 °C min-1, with nature cooling in air
atmosphere. The formation of phase and crystal structure of
NiFe2O4 and ZnFe2O4 were approved via the X-ray diffraction
measurements (XRD). Model Philips PW 1050 diffractometer
equipped with a PW 1730 generator (40 kV x 20 mA) was used
with Ni filtered CoKα radiation of 1.78897 Å at the room
temperature. Measurements were done in 2θ range of 10-80°
with scanning step width of 0.05° and 10 s scanning time per
Raman measurements of sintered samples were performed
using Jobin-Ivon T64000 monochromator in spectral range
The magnetization measurements were done at room
temperature using VSM 200 cryogenic magnetometer in
magnetic field from 0 kOe to ± 80 kOe.
Impedance measurements were carried out in the frequency
range 100 Hz to 10 MHz on a HP–4194A impedance/gain-
phase analyzer using a HP–16048C test fixture at the
temperature of 323-398 K.
RESULTS AND DISCUSSION
is a nondestructive material Raman spectroscopy
characterization technique and is sensitive to structural
disorder. It provides an important tool to probe the structural
properties of mechanochemicaly synthesized materials.
Figure 1. Raman spectra of the NiFe2O4 and ZnFe2O4.
There are five first order Raman active modes (A1g + Eg +
3F2g) in the normal spinel structure, and all these modes were
observed at ambient conditions, as shown in Fig. 1. The A1g
mode is due to symmetric stretching of oxygen atoms along Fe-
O (or M-O) tetrahedral bonds, Eg and F2g(3) are due to
symmetric and asymmetric bending of oxygen with respect to
Fe (M), respectively and F2g(2) is due to asymmetric stretching
of Fe (M) - O bond, F2g(1) is due to translational movement of
the whole tetrahedron.
Raman spectrum of sintered ZnFe2O4 ferrite exhibits a
normal spinel structure with symmetric peaks. In the case of
the NiFe2O4 all five Raman peaks seem asymmetric (or
dissociated). Deconvolution of spectrum demonstrates that
each peak can be presented like a doublet, what is a
characteristic of the inverse spinel structure [8, 9].
The magnetization curves of the nickel and zinc ferrites
measured at room temperature are shown in Fig. 2. We have
characterized the magnetic behavior of mechano-synthesized
NiFe2O4 and ZnFe2O4 focusing on the study of the size-
dependent magnetization. These measurements have revealed
that the magnetic behavior of soft mechanochemical
synthesized NiFe2O4 is different from that of the NiFe2O4
powder prepared using the conventional ceramic method. As
can be seen (Fig. 2),
mechanochemical synthesized sample does not saturate even
at the maximum field attainable (Hext = 80 kOe). This is in
contrast to the magnetic behavior of the bulk NiFe2O4, whose
magnetization reaches a saturation value easily. An absence of
saturation can be attributed to the efect of spin canting. Even
in sintered mechano-synthesized
surface/volume ratio is rather great and the dangling bonds
(uncompensated electrons) at the surface of the particles can
contribute to magnetization.
the magnetization of the
nikel ferrite, the
Figure 2. The magnetic hysteresis curves of NiFe2O4 and ZnFe2O4. The
insets show low magnetization behavior.
The measurement of the coercivity warrants the
determination of the magnetization response with better
accuracy and resolution particularly at small applied fields.
Hence, a separate set of hysteresis curves was showed for each
sample with an applied field of −1500 to 1500 Oe at room
temperature (the inset M–H curves in Fig. 2). The hysteresis
curve at low applied fields shows the values of the coercive
field, Hc ≈ 80 Oe and the remanent magnetization, Mr ≈ 8
emu/g and Mr ≈ 0.25 emu/g for sample of NiFe2O4 and
ZnFe2O4, respectively. The extrapolated magnetization M0
measured on the NiFe2O4 spinel ferrite obtained by a soft
mechanochemical synthesis is higher than that for spinel
ferrites produced by other methods .
The magnetic properties of the NiFe2O4 with an inverse
spinel structure can be explained in terms of the cations
distribution and magnetization originates from Fe3+ ions at both
tetrahedral and octahedral sites and Ni2+ ions in octahedral
sites. Hysteresis loops in Fig. 2 are typical for soft magnetic
materials and the “S” shape of the curves together with the
negligible coercivity (Hc ≈ 80 Oe) indicate the presence of
small magnetic particles exhibiting
behaviors. In superparamagnetic materials, responsiveness to
an applied magnetic field without retaining any magnetism
after removal of the magnetic field is observed. This behavior
is an important property for magnetic targeting carriers. In fact,
the difference between
superparamagnetism fabricates in the particle size. Literature
data imply that when the diameter of particles is less than 50
nm, the particles show the character of superparamagnetism.
Figure 2 shows the tiny magnetic hysteresis loop for
ZnFe2O4 sample. The curve is “S” shape with low coercivity
(80 Oe) and extremely small remanence magnetization. This
sample showes superparamagnetic behavior superponed to
paramagnetic behavior. In fact, the magnetic behavior of zinc
ferrite is very sensitive to the crystallinity and particle and
grain size . Greather particle size (and smaller tension
inside the particle) implies a structure that is closer to normal
spinel structure: means Fe only in octahedral positions, with
very low exchange interaction. The bulk zinc ferrite is
paramagnetic at ambient temperature.
In the present investigation, the impedance spectroscopy is
used as well-developed tool to separate out the grain/bulk and
grain boundary contribution to the total conductivity .
Depending on the electrical properties of the ferrite, the AC
response can be modeled with two semi-circles in the
impedance plane, the first in a low frequency domain
represents the resistance of grain boundary. The second one
obtained in a high frequency domain corresponds to the
resistance of grain or bulk properties [13, 14].
Cole-Cole plots of impedance data for sintered NiFe2O4 and
ZnFe2O4 ferrites as a function of frequency at different
temperatures are shown in Fig. 3. Two semicircles are
observed for both samples between the frequency range 100 Hz
to 10 MHz. It is noticeable that the impedance spectrum
includes both grain and grain boundary effects. The diameters
of the semicircles exhibit decreasing trends with the increase in
temperature. It indicates that the conductivity increases with
increase in temperature supporting the typical negative
temperature coefficient of resistance behavior of the NiFe2O4
and ZnFe2O4 usually shown by semiconductors. On impedance
spectra is observed that NiFe2O4 ferrite has a higher impedance
value of ZnFe2O4 ferrite at all temperatures, so it follows that
the conductivity of the ZnFe2O4 is higher than the NiFe2O4. The
impedance values of both ferrite samples are decreased by two
orders of magnitude, which are due to thermal activation
mechanism. The rise of temperature brings with an enhanced
conductivity, and hence, decreasing the impedance values.
Successful modeling of the impedance response is achieved
using an equivalent circuit consisting of two serially connected
parallel R-CPE elements taking into account grain and grain
boundary effects, see Fig. 4.
Figure 3. Cole–Cole plots for the sample of the NiFe2O4 and ZnFe2O4
ferrites at different temperatures.
Figure 4. Proposed equivalent circuit model for analysis of the impedance
The ferrites have been prepared by soft mechanochemical
synthesis starting from the mixture of the powders. Single
phase nanosized of NiFe2O4 and ZnFe2O4 ferrites were
synthesized for 25 h and 18 h by ball milling and sintered at
1100 °C for 2 h. X-ray diffraction of the prepared samples
show single phase cubic spinel structure. In the Raman spectra
is observed all of five first-order Raman active modes in the
form characteristic for NiFe2O4 inverse and ZnFe2O4 normal
spinel structures. Both of the ferrites were superparamagnetic
at room temperature. The analysis of the complex impedance
data shows that the capacitive and reactive properties of the
sintered ferrites are mainly attributed due to the processes
which are associated with the grain and grain boundary. The
grain and grain boundary resistances of sintered sample
exhibit decreasing trends with the increase in temperature.
This research was financially supported by the Serbian
Ministry of Education and Science through Projects No. III
45003 and III 45015.
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