Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International
journal homepage: www.elsevier.com/locate/ceramint
Cation distributions and magnetism of Al-substituted CoFe2O4 - NiFe2O4
solid solutions synthesized by sol-gel auto-combustion method
R. Vijaya Kumara, A.V. Anupamaa, R. Kumara, H.K. Choudharya, V.B. Khopkara, G. Aravindb,
⁎
B. Sahooa,
a
b
Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
Methodist College of Engineering and Technology, Hyderabad 500001, India
A R T I C LE I N FO
A B S T R A C T
Keywords:
Citrate-gel auto-combustion method
Al-doped Co-Ni ferrites
Mössbauer spectroscopy
Rietveld refinement
Magnetic properties
Aluminium substituted cobalt-nickel ferrite nanoparticles were synthesized by citrate gel auto-combustion
method followed by annealing at 1000 °C for 1 h in air. Scanning electron micrographs of all the samples show
crystalline particles of irregular morphology with a small variation in particle sizes (~ 110–160 nm). From the
analysis of the X-ray diffraction results we observed that the unit cell parameter decreases linearly with increase
in aluminium concentration due to the smaller ionic radius of the Al3+ ions substituting the other cations such as
Co2+, Ni2+ and Fe3+ ions in the compounds. The room temperature Mössbauer spectra of the samples show
Zeeman split sextet patterns corresponding to the tetrahedral (Th) and octahedral (Oh) interstitial iron (Fe3+)
cations. The observed magnetic hyperfine field (Bhf) decreases with increase in Al-concentration due to the
distribution of diamagnetic Al3+ in the environment of 57Fe probe atoms. The saturation magnetization measured by Vibrating Sample Magnetometer (VSM) shows a similar trend like that of Bhf. The distributions of the
cations obtained from the Rietveld refinement and Mössbauer spectroscopy results indicate an increase in
Fe3+(Th)/Fe3+(Oh) occupancy-ratio on increasing Al3+ concentration, and Ni2+ cations prefer the octahedral
site, whereas Co2+ and Al3+ ions redistribute themselves in tetrahedral and octahedral sites, in the ratio 2:3.
1. Introduction
A variety of foreign cations can be incorporated into the tetrahedral
(Th) and octahedral (Oh) sites in the spinel structure of the ferrites. The
complex ordering of the incorporated cations at these sites drastically
modifies the properties of spinel compounds [1–5], providing the impetus for the materials to be tailored for different applications. The
spinel family of ferrites are also attractive because these oxides can
have interesting electrical properties, i.e., these compounds can be halfmetallic, ferrimagnetic insulators or transparent conductors [6,7]. The
high resistivity and high permeability of ferrites are of great importance
in academic research and industrial applications [8]. Tuning the saturation magnetization with a reasonable value of coercivity in nanocrystalline ferrites is the key to their applications. An important challenge is to control the crystallite size and to enhance the electrical
resistivity of ferrites for various applications such as in microwave
devices [9–11].
Cobalt ferrites, which belong to spinel ferrite family, have moderately high saturation magnetization with high coercivity, mechanical
hardness and chemical stability, and hence, are potential candidates for
⁎
high-density magnetic recording media [12]. The magnetic hardness of
cobalt ferrite can be brought down by substituting Co2+ by Al3+, which
also aids in achieving high electrical resistivity [9]. These electrical and
magnetic properties are ideal for applications such as; for power
transformer core materials in the field of electronics and telecommunication [9,13–15]. Such a change in physical properties due to
the substitution of host Co2+ by cations of different valencies (valence
states such as + 1 or + 3) are presumably due to the alteration in the
electric and magnetic environments (superexchange interactions) of the
interstitial sites (tetrahedral and octahedral). Hence, these changes are
helpful in tuning the magnetic and electrical properties of spinel ferrites
[14–16].
Despite a decrease in saturation magnetization, Al3+ substituted Niferrites possess high electrical resistivity, which is due to the decrease in
dielectric and eddy current losses. This makes Al-substituted Ni-ferrites
suitable for microwave devices which can operate at L, S and C band
frequencies with low insertion losses [10,17–20]. Furthermore, the
partial substitution of Ni2+ by Al3+ reduces the magnetic coercivity
and thus renders magnetic softness to the ferrite, which is an important
requisite for high frequency applications [17]. The substitution of Ni2+
Corresponding author.
E-mail address: bsahoo@iisc.ac.in (B. Sahoo).
https://doi.org/10.1016/j.ceramint.2018.08.065
Received 16 June 2018; Received in revised form 20 July 2018; Accepted 7 August 2018
0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Kumar, R.V., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.08.065
Ceramics International xxx (xxxx) xxx–xxx
R.V. Kumar et al.
by Al3+ also inhibits the grain growth in ferrites, while improving its
mechanical strength [21].
Al-substituted NiFe2O4 shows distribution of Al in tetrahedral (Th)
and octahedral (Oh) sites, normally in the ratio of 2:3 [22]. Al-substituted CoFe2O4 shows an equal Al3+ distribution (1:1) at the Th and
Oh sites [23]. Such a distribution of Al at Th and Oh sites depending on
the host cations makes the Al-substituted spinel ferrite system interesting to further explore the Al-concentration dependent response of
the solid solution of Co-ferrite and Ni-ferrite. Moreover, it is fundamentally interesting to know the preferential occupation of the host
cations in the Co ferrite-Ni ferrite solid solution, i.e., Ni2+ and Co2+ on
the substitution of Al3+. In the present study, we explore such a distribution of cations (Al3+, Ni2+, Co2+ and hence Fe3+) at the Th and
Oh sites for different dopant (Al3+) concentrations.
The physical properties of (nano-)ferrites are highly sensitive to
various factors such as the method of preparation, synthesis conditions,
chemical composition, the type of substituents and the heat treatment
conditions, which further decide the grain size within the formed product and the distribution of cations at the available Th and Oh sites
[11,24–26]. There are different methods available such as sol-gel autocombustion [27], co-precipitation [28], hydrothermal [29], high-energy ball milling [30] and micro-emulsion [31] for the synthesis of
spinel ferrites. Among these, sol-gel auto combustion method is a
simple synthesis technique and advantageous in terms of factors such as
low product contamination, economical, homogeneous and stoichiometric product-formation due to high level of reactivity and formation
of nanocrystallites. Hence, we have followed sol-gel autocombustion
technique for the synthesis of Al doped Co0.2Ni0.8Fe2O4. Furthermore,
we have explored the cation distribution and magnetic properties resulting from Al3+ substitution in NiFe2O4 - CoFe2O4 solid solution.
corrections were considered. The chosen peak profile was ThomsonCox-Hastings (TCH). The occupancy factors of cations in each (tetrahedral (Th) and octahedral (Oh)) interstitial site of the cubic spinel
were refined. For each sample, the crystallite size of each phase was
obtained using Scherrer formula, considering a minimum of three
(deconvoluted) prominent Bragg peaks.
The morphology of the samples were imaged using an F-50 Field
Emission Scanning Electron Microscope (FE-SEM) instrument. For
characterization of the magnetic properties, e.g., saturation magnetization, coercivity and remnant magnetization of the synthesized powders at room temperature, a “Quantum Design MPMS SQUID magnetometer” was used. Using a constant acceleration Doppler velocity drive
with 57Co (Rh-matrix) source and a proportional counter Mössbauer
spectra were recorded in transmission geometry at room temperature.
For the quantitative evaluation, the Mössbauer spectra were least
square fit using ‘‘NORMOS” program [34,35].
4. Results and discussion
4.1. Morphology and structure of the samples
All the FESEM micrographs and particle size histograms are shown
in Fig. 1. The micrographs reveal that particles have well-defined edges,
however, are irregular (polygonal) in shape. All samples have an
average particle size ranging between ~ 110–160 nm, as shown in the
size distribution histogram (Fig. 1). The polygonal morphology with
soft edges for the particles of the x = 0.0 sample transform to welldefined plate-like morphology for the sample x = 0.8. The particle size
decreases from x = 0.0 to x = 0.6 and then increases again for x = 0.8.
The magnetostatic interaction between the particles seems to result in
aggregation of the particles.
The XRD patterns of all the studied AlxCo0.2Ni0.8Fe2-xO4 samples are
given in Fig. 2. There was no apparent impurity phase present at first
glance but a close inspection of the XRD peaks show asymmetrical peak
patterns. Each peak, in fact, is a double peak which account for two
spinel ferrite phases.
The double peak pattern is more prominent for the x = 0 sample. As
the Al-content (x) increases the peak becomes broader and the double
peak feature diminishes. The lattice parameters of these two phases
seems to be close to each other. XRD patterns were initially fit with a
single spinel (space group Fd 3̅ m) phase. However, the fitting was poor,
with χ2 = 4, Rp = 64 and Rwp = 29. The XRD patterns of each sample
were then fit with two spinel phases (Fig. 3), which led to better fitting
parameters (χ2 =1, Rp = 46 and Rwp = 14). Since CoFe2O4 has the
tendency to become inverse spinel and NiFe2O4 exhibits almost perfect
inverse spinel structure [36–38], the resultant preference of Co and Ni
towards octahedral site results in the splitting of the XRD lines for
undoped (x = 0.0) sample (Co0.2Ni0.8Fe2O4). Furthermore, the rest of
the samples (which contain Al in different proportions) also form two
separate phases having same spinel structure. The spinel phases differ
only slightly in their lattice parameters, hence, form overlapped peaks.
This aspect is demonstrated for x = 0.0 sample in Fig. 3, where a single
phase fit pattern shows poor peak fitting while addition of the second
phase clearly results in a better fit. Considering all the Ni2+ ions are at
the octahedral site, from the scale factor during Rietveld refinement,
one of the phases having higher peak intensity is unambiguously assigned to the Ni-rich (phase-1) ferrite while the phase corresponding to
lower peak intensity to the Co-rich ferrite (phase-2), in all the samples.
The Rietveld refined XRD patterns of all the samples are given in Fig. 2.
In these Rietveld refined XRD plots, the Miller indices given for the
spinel ferrite phase is valid for both the phases, which have overlapping
Bragg positions.
The cation distribution obtained from the Rietveld refinement results shows that Al is distributed in tetrahedral (Th) and octahedral
(Oh) sites by a ratio of 2:3. The obtained phase abundance and lattice
parameters after the refinement, and the crystallite sizes calculated
2. Materials and methods
A series on nanocrystalline ferrites with the composition
AlxCo0.2Ni0.8Fe2-xO4 (x = 0.0, 0.2, 0.4, 0.6 and 0.8) were synthesized
by citrate gel auto-combustion method using stoichiometric amount of
citric acid (C6H8O7·H2O, 99%, Merck), nickel nitrate (Ni(NO3)2·6H2O,
98.5%, Merck), aluminium nitrate (Al(NO3)3·9H2O, 98%, SigmaAldrich) and ferric nitrate (Fe(NO3)3·9H2O, 98.5%, Merck) as starting
materials. All chemicals were of analytical grade. The starting materials
were initially dissolved in deionized water. The pH of the solution was
adjusted to 7 by dropwise addition of ammonia with constant magnetic
stirring. The solution was stirred continuously for about four hours for
homogenization. The above solution was heated at 80 °C with continuous stirring, until the solution became a homogeneous viscous gel.
The magnetic beads were then taken out from the gel. After 30–45 min
the auto combustion of the viscous gel took place. After completion of
the combustion reaction a loose, brown colored flaky powder was obtained. This combustion product was then ground to obtain fine
powder. To improve the crystallinity of the products, the as-prepared
ferrite powders were annealed at 1000 °C for 1 h in air.
3. Characterizations
The structure of the AlxCo0.2Ni0.8Fe2-xO4 (x = 0.0–0.8) samples was
characterized by X-ray diffraction (XRD). The XRD data were recorded
using a PANalytical X′Pert Pro MPD diffractometer (Cu-Kα radiation, Ni
filter) in the 2θ range of 10–80° with a step size of 0.032°. The crystalline phases present in the samples were identified based on the Bragg
reflections with the help of crystallographic information files (CIF) from
Inorganic Crystal Structure Database (ICSD). For the extraction of structural details (such as the lattice parameter, phase abundance, the fullwidth at half maximum intensity of Bragg peaks, cation occupancy
etc.), Rietveld refinement [32] of XRD patterns was performed. The
Rietveld refinements of structural parameters were carried out using
the program FullProf [33]. During the refinement, instrumental error
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Fig. 1. The FESEM images and the histograms of the particle size distribution of AlxCo0.2Ni0.8Fe2-xO4 samples.
in ‘x’ is ascribed to the lower ionic size of Al3+ compared to that of the
host Fe3+ [39,40].
From the Rietveld refinement, the observed molar ratio between
phase-1 and phase-2 is 1:4. The cation distribution in the samples may
be written as CoAlxFe2-xO4 for phase-1 and NiAlxFe2-xO4 for phase-2,
respectively. The cation distribution within phase-1 is (Co0.4Al2×/
from the (311) Bragg peaks are listed in Table 1. Refinement showed
that the lattice parameter values for both phases decrease with increase
in Al-content in the sample following Vegard's law. A clear shift of the
(311) Bragg peak towards higher diffraction angle is demonstrated in
Fig. 4. A plot of lattice parameter (a) vs the Al concentration (x) for
both the phases is shown in Fig. 5(a). The decrease in ‘a’ with increase
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Fig. 2. The Rietveld refined XRD patterns for the AlxCo0.2Ni0.8Fe2-xO4 sample with x = 0.0–0.8 samples. The solid circles (red) are measured data points (Yobs). The
solid lines (black) are calculated patterns (Ycalc). The vertical lines (green) correspond to the Bragg positions. The difference between observed and calculated (YobsYcalc) plots (lines in blue) is also shown at the bottom of each pattern (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.).
progressive decrease in value with increase in Al-content, which is due
to the lower atomic weight of Al compared to the host Fe/Co/Ni atoms.
The maximum error in phase percentage estimation by Rietveld refinement of XRD data could be ~ 2% by weight. The refined lattice
parameter values are precise up to the third decimal position. The error
bar associated with crystallite size estimation could be about ± 2 nm.
5Fe0.6–2×/5) and [Co0.6Al3×/5Fe1.4–3×/5] at Th and Oh sites, respectively. Similarly, for phase-2, the Th and Oh site cation occupancies are
(Fe1–2×/5Al2×/5) and [NiAl3×/5Fe1–3×/5], respectively. The cation occupancies for each sample are listed phase-wise in Table 4.
For all the Al-containing samples, except for x = 0.0, the average
crystallite sizes were in the range of 60–70 nm and 20–30 nm for phase1 and 2, respectively. For the estimation of crystallite size (using
Scherrer formula), a minimum of three prominent peaks were considered. FWHM of each Bragg peak was deconvoluted corresponding to
phase 1 and 2, respectively. Interestingly, the crystallite sizes for phase1 and 2 of sample with x = 0.0 showed ~ 30 nm and 90 nm, respectively, which was in opposite trend as compared to other samples. Our
results indicate that the incorporation of Al in Co0.2Ni0.8Fe2O4 decides
the final distribution of cations in the lattice and eventually the grain
growth. Incorporation of Al seems to define the grain size as observed in
the SEM and XRD results in Table 1. Therefore, the samples are solid
solutions with different regions having Ni-rich or Co-rich ferrite structure.
The average X-ray density (ρX-ray) (considering both phases within
each sample) obtained from the Rietveld refinement (Fig. 5(b)) shows a
4.2. Magnetic properties
The magnetic hysteresis (M-H) loops of AlxCo0.2Ni0.8Fe2-xO4 samples measured at room temperature are shown in Fig. 6(a). The values
of saturation magnetization (Ms) and coercivity (Hc) were obtained
from the M-H loop as listed in Table 2. The Al-concentration (composition) dependent variation of Hc and Ms are shown in Fig. 6(b). It is
observed that the values of Ms decreases from 52.7 to 7.4 emu/g as the
Al-concentration (x) increases from 0.2 to 0.8. Such a decrease in the
values of Ms can be explained on the basis of the distribution of cations
at tetrahedral (Th) and octahedral (Oh) sites as per Néel's theory
[41,42]. It is known that the magnetic moments of Fe and Ni atoms at
the Th and Oh sites are positioned antiparallel to each other resulting in
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Fig. 4. (311) peak in the X-ray diffraction pattern of AlxCo0.2Ni0.8Fe2-xO4
samples.
Fig. 3. Rietveld refined (440) peak for the sample with x = 0.0 (as an example),
without (top) and with (bottom) consideration of the second spinel phase in the
sample. Note the better quality of the fit when the second spinel phase is
considered. High angle peak was chosen to clearly demonstrate the presence of
the two phases in the samples.
Table 1
The Rietveld refined structural parameters of the AlxCo0.2Ni0.8Fe2-xO4 samples.
‘a’ and ‘ρX-ray’ are cell parameter and X-ray density, respectively. The sizes (t) of
particles obtained from SEM images are compared to the crystallite size (L)
from XRD results. Crystallite size (L) was calculated from the most prominent
(311) Bragg peak by Scherrer method. The errors in the lattice parameter,
density and crystallite size are estimated to be ± 0.0005 Å, ± 0.02 g/cc
and ± 2.0 nm, respectively. The maximum error in the estimation of phases
present in each sample is ~ 2 wt%.
Sample
Phases
(wt%)
a (Å)
ρX-ray (g/cc)
L (nm)
t (nm)
X = 0.0
1
2
1
2
1
2
1
2
1
2
35.7
64.3
96.7
3.3
92.6
7.4
86.6
13.4
86.9
13.1
8.346
8.343
8.312
8.317
8.286
8.289
8.254
8.257
8.234
8.236
5.36
31.3
89.7
67.0
25.2
74.5
25.2
72.4
28.3
60.4
22.6
153 ± 19
X = 0.2
X = 0.4
X = 0.6
X = 0.8
5.27
5.20
5.12
5.02
Fig. 5. The variation of (a) lattice parameters (phase-wise) and (b) X-ray density (averaged for both phases within each sample), with Al concentration (x) in
AlxCo0.2Ni0.8Fe2-xO4 samples.
149 ± 24
139 ± 19
110 ± 15
Table 2
The room temperature magnetic parameters for AlxCo0.2Ni0.8Fe2-xO4 samples.
165 ± 34
Composition
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
the reduction of net magnetic moment. Since the net magnetization on
Oh sub-lattice is higher than that at Th sub-lattice, there is a non-zero
net magnetic moment. However, the substitution of magnetic (Fe3+) by
the non-magnetic (Al3+) results in decrease of net magnetic moment
and hence, reduces the saturation magnetization (and remanence) of
the ferrite [43,44]. The rapid decline in the values of Ms with progressive increase in Al-concentration is primarily due to the dilution of
the magnetic moment at the Th site, which also adversely affects the
moments at Oh site, leading to an abrupt decrease in the net moment
via a decreased Fe-Fe interaction (within the Oh sites) [40,41]. With
further increase in Al-concentration, the values of Hc was found to
Ms (emu/g)
Hc (Oe)
52.69
37
25.57
13.82
7.38
298.5
304.0
246
220
307
decrease (except for x = 0.8 sample). This maybe attributed to the
development of plate-like morphology as evidenced from SEM studies,
which agree with the reports in the literature [45,46]. In addition, the
decrease of Hc from 298.5 to 220 Oe (Table 3) with the increase in Alconcentration up to x = 0.6 can originate from decrease in the
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Table 3
The Mössbauer spectral parameters obtained from the least square fit to the respective Mössbauer spectra. S1 and S2 correspond to sextets due to Fe atoms at
tetrahedral and octahedral sites, respectively. The estimated errors in δ, Δ, Bhf, Γ and Area values are ± 0.005 mm/s, ± 0.01 mm/s, ± 1.0 T, ± 0.02 mm/s
and ± 2.0%, respectively.
Sample code
Sub-spectrum designation
δ (mm/s)
Δ (mm/s)
Bhf (T)
Γ (mm/s)
Area (%)
x = 0.0
S1
S2
S1
S2
S1
S2
S1
S2
S1
S2
0.146
0.252
0.137
0.255
0.147
0.228
0.137
0.251
0.149
0.196
− 0.005
0.004
− 0.000
− 0.018
0.018
0.005
0.038
− 0.003
0.038
− 0.004
52.1
48.8
51.3
47.5
50.0
46.2
47.9
44.0
45.3
40.6
0.43
0.40
0.54
0.50
0.70
0.59
0.80
0.65
0.86
0.82
45
54
45
54
46
53
49
50
48
51
x = 0.2
x = 0.4
x = 0.6
x = 0.8
increased coercivity [6]. The remanence does not change significantly
with increase in concentration of Al in the samples [44].
Table 4
The extent of Al-occupation at the tetrahedral and octahedral sites, derived
from the XRD and Mössbauer results. In the formula, () and [] represent tetrahedral and octahedral sites, respectively. The molar ratio of phase 1 to phase
2 is 1:4.
Sample
Phase
Formula
x = 0.0
1
2
1
2
1
2
1
2
1
2
(Co0.08Fe0.92) [Co0.92Fe1.08]O4
(Ni0.08Fe0.92) [Ni0.92Fe1.08]O4
(Co0.4Al0.08Fe0.52) [Co0.6Al0.12Fe1.28]O4
(Al0.08Fe0.92) [NiAl0.12Fe0.88]O4
(Co0.4Al0.16Fe0.44) [Co0.6Al0.24Fe1.16]O4
(Al0.16Fe0.84) [NiAl0.24Fe0.76]O4
(Co0.4Al0.24Fe0.36) [Co0.6Al0.36Fe1.04]O4
(Al0.24Fe0.76) [NiAl0.36Fe0.64]O4
(Co0.4Al0.32Fe0.28) [Co0.6Al0.48Fe0.92]O4
(Al0.32Fe0.68) [NiAl0.48Fe0.52]O4
x = 0.2
x = 0.4
x = 0.6
x = 0.8
4.3. Mössbauer spectroscopy
The least-square fit Mössbauer spectra of all the studied samples are
shown in Fig. 7. The hyperfine parameters such as the isomer shift (δ),
nuclear quadrupole level shift (Δ), magnetic hyperfine field (Bhf) and
line-width (Γ) obtained from the least-squares fit of the Mössbauer
spectra are listed in Table 3. The spectrum for Co0.2Ni0.8Fe2O4 sample
(x = 0.0), shows two-sextets pattern and was fit accordingly, which
correspond to the contribution of Fe3+ at the tetrahedral (Th) and octahedral (Oh) sites. The sextet with higher value of Bhf is unambiguously assigned to the Th-site Fe atoms, while the sextet with
lower Bhf is assigned to the Oh-site Fe atoms [13,22,47]. The area under
the subspectrum corresponding to Th and Oh sites as observed from the
fittings are ~ 46% and 54%, respectively. As discussed in the XRD results and from the literature [44,45], the preferential occupancy of Ni at
the Oh-site [48], suggests that the Co atoms occupy the Th and Oh sites
in the ratio of 2:3 [49,50]. This can be understood considering the
discussion of the inverse spinel structure of Fe3O4 [26], wherein Th and
Oh sites are occupied by Fe ions in the ratio 1:2. Doping of Ni in this
structure, as all the Ni atoms go to the Oh-sites, the Fe atom occupation
ratio should become 1:1.2 (i.e., 45% and 55%). An occupation of Co in
the ratio 2:3 (i.e., 0.08 and 0.12 mol) at Th and Oh sites will lead to an
Fe occupation ratio of 0.92:1.08 (i.e., 46% and 54%), respectively. As
the Th and Oh site areas observed under the Mössbauer subspectra S1
and S2 for our samples are the same, the occupation of Co can be assumed to be in the ratio of 2:3. [23] The Rietveld refinement of XRD
data also confirms these results.
With increase in Al-content in the samples, the values of δ and Δ
were nearly unaffected but shows significant variation of Bhf, Γ and
cation occupancy (Area %) values. The Bhf corresponding to both Th
and Oh sites shows systematic decrease with increase in Al content, due
to magnetic dilution [43,45]. The broadening of Mössbauer lines (refer
to Γ values in Table 3) suggests that increase in Al content causes disorder in cation positioning among the available Th and Oh sites. This is
also evidenced as line-broadening of Bragg peaks of XRD patterns.
The Fe-atoms redistribute themselves on increasing Al content in the
sample. By comparing the Fe-occupancy obtained from the least square
fit of Mössbauer spectra of the samples (in terms of area %, Table 3),
considering complete Ni occupation at the Oh site and distribution of
Co at Th and Oh sites in ratio of 2:3, it can be inferred that the rest of
the Th and Oh sites should be occupied by Al atoms. This estimation
accounts for Al atoms occupying the Th and Oh sites in the ratio 2:3 in
all samples. Such occupation of cations in the interstitial sites (Th and
Oh) is also confirmed by Rietveld refinement of XRD data. Considering
the cation occupancies for both phases in each sample confirmed by
Rietveld refinement, the obtained formulae of the compounds are given
in Table 4. Note that these two phases might be randomly present in all
Fig. 6. (a) Magnetic hysteresis loops and (b) Variation of Ms and Hc with Al
content (x) for AlxCo0.2Ni0.8Fe2-xO4 samples.
magneto-crystalline anisotropy. Similar results were reported on other
ferrite systems earlier [41,47]. For the sample with x = 0.8, we speculate that thin area of Al-O-Al structure surrounding the magnetically
ordered regions might have arrested the domain walls, resulting in
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saturation magnetization and coercivity in the sample decreases due to
magnetic dilution effects in the lattice. However, the coercivity increased for the sample with x = 0.8, which might be due to formation
of plate-like morphology of the particles. The cation distribution within
the samples obtained from the Rietveld refinement of XRD data is in
close agreement with Mössbauer spectroscopy and magnetometry results. According to earlier results, the Al-content in NiFe2O4 and
CoFe2O4 were 2:3 and 1:1 at the tetrahedral and octahedral sites, but
our results confirm that solid solution of Ni-ferrite and Co-ferrite accommodates Al in the ratio of 2:3 for all the studied samples.
Acknowledgments
R. Vijaya Kumar would like to thank University Grants CommissionNew Delhi, India for providing UGC-Postdoctoral Fellowship. The authors thank Prof. K. Narasimha Reddy, President of Telangana Academy
of Sciences-Hyderabad, Telangana for his constant support and encouragement.
References
[1] R. Pandit, K.K. Sharma, P. Kaur, R.K. Kotnala, J. Shah, R. Kumar, Effect of Al3+
substitution on structural, cation distribution, electrical and magnetic properties of
CoFe2O4, J. Phys. Chem. Solids 75 (2014) 558–569.
[2] V. Rathod, A.V. Anupama, V.M. Jali, V.A. Hiremath, B. Sahoo, Combustion synthesis, structure and magnetic properties of Li-Zn ferrite ceramic powders, Ceram. Int.
43 (2017) 14431–14440.
[3] V.D. Sudheesh, N. Thomas, N. Roona, H.K. Choudhary, B. Sahoo, N. Lakshmi,
V. Sebastian, Synthesis of nanocrystalline spinel ferrite (MFe2O4, M = Zn and Mg)
by solution combustion method: influence of fuel to oxidizer ratio, J. Alloy. Compd.
742 (2018) 577–586.
[4] V. Rathod, A.V. Anupama, R. Vijaya Kumar, V.M. Jali, B. Sahoo, Correlated vibrations of the tetrahedral and octahedral complexes and splitting of the absorption
bands in FTIR spectra of Li-Zn ferrites, Vib. Spectrosc. 92 (2017) 267–272.
[5] V.J. Angadi, A.V. Anupama, R. Kumar, H.K. Choudhary, S. Matteppanavar,
H.M. Somashekarappa, B. Rudraswamy, B. Sahoo, Composition dependent structural and morphological modifications in nanocrystalline Mn-Zn ferrites induced by
high energy gamma-irradiation, Mater. Chem. Phys. 199 (2017) 313–321.
[6] V.J. Angadi, A.V. Anupama, R. Kumar, S. Matteppanavar, B. Rudraswamy,
B. Sahoo, Observation of enhanced magnetic pinning in Sm3+ substituted nanocrystalline Mn-Zn ferrites prepared by propellant chemistry route, J. Alloy. Compd.
682 (2016) 263–274.
[7] V.A.M. Brabers, K.H.J. Buschow (Ed.), Handbook of Magnetic Materials (Ch. 3), 8
North-Holland Publishing Company, Amsterdam, the Netherlands, 1982.
[8] A.J. Moulson, J.M. Herbert, Electroceramics: Materials, Properties, Applications,
2nd Revised ed., Wiley-Blackwell, 2003.
[9] S. Ramachandran, A review on ferrite devices, J. lnst. Telecommun. Eng. 9 (1963)
600–623.
[10] G.F. Dipnne, A review of ferrites for microwave applications, in: Proceedings of the
IEEE, vol. 63, 1975, pp. 777–789.
[11] A. Hajalilou, S.A. Mazlan, A review on preparation techniques for synthesis of nanocrystalline soft magnetic ferrites and investigation on the effects of microstructure features on magnetic properties, Appl. Phys. A 122 (2016) 680.
[12] M.P. Sharrock, Particulate magnetic recording media: a review, IEEE Trans. Magn.
25 (1989) 4374–4389.
[13] S. Singhal, S.K. Barthwal, K. Chandra, XRD, magnetic and Mössbauer spectral studies of nano size aluminium substituted cobalt ferrites (CoAlxFe2-xO4), J. Magn.
Magn. Mater. 306 (2006) 233–240.
[14] A.S. Albuquerque, Jose´ D. Ardisson, W.A.A. Macedo, Nanosized powders of NiZn
ferrite: synthesis, structure, and magnetism, J. Appl. Phys. 87 (2000) 4352–4357.
[15] A. Ahlawat, V.G. Sathe, V.R. Reddy, A. Gupta, Mössbauer, Raman and X-ray diffraction studies of superparamagnetic NiFe2O4 nanoparticles prepared by sol–gel
auto-combustion method, J. Magn. Magn. Mater. 323 (2011) 2049–2054.
[16] P. Chandramohan, M.P. Srinivasan, S. Velmurugan, S.V. Narasimhan, Cation distribution and particle size effect on Raman spectrum of CoFe2O4, J. Solid State
Chem. 184 (2011) 89–96.
[17] D.R. Mane, U.N. Devatwal, K.M. Jadhav, Structural and magnetic properties of
aluminium and chromium co-substituted cobalt ferrite, Mater. Lett. 44 (2) (2000)
91–95.
[18] M. Mozaffari, J. Amighin, Preparation of Al-substituted Ni ferrite powders via
mechanochemical processing, J. Magn. Magn. Mater. 260 (2003) 244–249.
[19] S. Rashonlel, N.D. Sharma, S.P. Taneja, A. Gupta, Mössbauer, X-ray and magnetization studies of Ni0.5Zn0.5AlxFe2-xO4 system, Ind. J. Pure Appl. Phys. 43 (2005)
119–122.
[20] S.E. Shirsath, B.G. Toksha, K.M. Jadhav, Structural and magnetic properties of In3+
substituted NiFe2O4, Mater. Chem. Phys. 117 (2009) 163–168.
[21] J.A. Gomes, M.H. Sousa, F.A. Tourinho, J. Mestnik-Filho, R. Itri, J. Depeyro,
Rietveld structure refinement of the cation distribution in ferrite fine particles
studied by X-ray powder diffraction, J. Magn. Magn. Mater. 289 (2005) 184–187.
Fig. 7. Mössbauer spectra of AlxCo0.2Ni0.8Fe2-xO4 samples.
particles in the samples.
5. Conclusions
Aluminium substituted Co-Ni ferrite (AlxCo0.2Ni0.8Fe2-xO4,
x = 0.0–0.8) nanopowders were prepared successfully by the citrate
gel-auto combustion method. All the samples show double phase cubic
spinel structure due to competing preference between Ni2+ and Co2+ at
the octahedral sites. As the Al3+ content increases, the lattice parameter and crystallite size decreases due to lower ionic radius of Al3+
which also inhibits grain growth. The irregular spherical particles
transform to plate-like morphology for the high Al-containing (x = 0.8)
sample. As the Al3+ concentration in the ferrite increases, the
7
Ceramics International xxx (xxxx) xxx–xxx
R.V. Kumar et al.
(1987) 417–432.
[36] E.W. Gorter, Saturation magnetization and crystal chemistry of ferrimagnetic
oxides, Philips Res. Rep. 9 (1954) 403–443.
[37] Y.H. Hou, Y.J. Zhao, Z.W. Liu, H.Y. Yu, X.C. Zhong, W.Q. Qiu, D.C. Zeng, L.S. Wen,
Structural, electronic and magnetic properties of partially inverse spinel CoFe2O4: a
first-principles study, J. Phys. D: Appl. Phys. 43 (2010) 445003.
[38] V.G. Ivanov, M.V. Abrashev, M.N. Iliev, M.M. Gospodinov, J. Meen, M.I. Aroyo,
Short-range B-site ordering in the inverse spinel ferrite NiFe2O4, Phys. Rev. B 82
(2010) 1–8 (024104).
[39] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic
distances in halides and chalcogenides, Acta Cryst. A 32 (1976) 751–767.
[40] David van Horn, Electronic table of shannon ionic radii, 2001. 〈http://v.web.umkc.
edu/vanhornj/shannonradii.htm〉.
[41] A.A. Ati, Z. Othaman, A. Samavati, F.Y. Doust, Structural and magnetic properties
of Co–Al substituted Ni ferrites synthesized by co-precipitation method, J. Mol.
Struct. 1058 (2014) 136–141.
[42] S. Zare, A.A. Ati, S. Dabagh, R.M. Rosnan, Z. Othaman, Synthesis, structural and
magnetic behavior studies of Zn–Al substituted cobalt ferrite nanoparticles, J. Mol.
Struct. 1089 (2015) 25–31.
[43] I. Maghsoudi, H. Shokrollahi, M.J. Hadianfard, J. Amighian, Synthesis and characterization of NiAlxFe2-xO4 magnetic spinel ferrites produced by conventional
method, Powder Technol. 235 (2013) 110–114.
[44] J.C. Maurya, P.S. Janrao, A.A. Datar, N.S. Kanhe, S.V. Bhoraskar, V.L. Mathe,
Evidence of domain wall pinning in aluminum substituted cobalt ferrites, J. Magn.
Magn. Mater. 412 (2016) 164–171.
[45] H.M. Zaki, S.H. Al-Heniti, A. Hashhash, Effect of Al3+ ion addition on the magnetic
properties of cobalt ferrite at moderate and low temperatures, J. Magn. Magn.
Mater. 401 (2016) 1027–1032.
[46] A.A. Ati, Z. Othaman, A. Samavati, Influence of cobalt on structural and magnetic
properties of nickel ferrite nanoparticles, J. Mol. Struct. 1052 (2013) 177–182.
[47] V. Kuncser, G. Schinteie, B. Sahoo, W. Keune, D. Bica, L. Vekas, G. Filoti, Magnetic
interactions in water based ferrofluids studied by Mössbauer spectroscopy, J. Phys.:
Condens. Matter 19 (2007) 1–16 (016205).
[48] M.V. Ushakov, B. Senthilkumar, R.K. Selvan, I. Felner, M.I. Oshtrakh, Mössbauer
spectroscopy of NiFe2O4 nanoparticles: the effect of Ni2+ in the Fe3+ local microenvironment in both tetrahedral and octahedral sites, Mater. Chem. Phys. 202
(2017) 159–168.
[49] G. Chandraa, R.C. Srivastava, V.R. Reddy, H.M. Agrawal, Effect of sintering temperature on magnetization and Mössbauer parameters of cobalt ferrite nanoparticles, J. Magn. Magn. Mater. 427 (2017) 225–229.
[50] B. Ndlovu, J.Z. Msomi, T. Moyo, Mössbauer and electrical studies of NixCo1-xFe2O4
nanoparticles, J. Alloy. Compd. 745 (2018) 187–195.
[22] S. Kamali, Spin structure, magnetism, and cation distributions of NiFe2-xAlxO4 solid
solutions, J. Magn. Magn. Mater. 433 (2017) 155–161.
[23] L. Kumar, M. Kar, Influence of Al3+ ion concentration on the crystal structure and
magnetic anisotropy of nanocrystalline spinel cobalt ferrite, J. Magn. Magn. Mater.
323 (2011) 2042–2048.
[24] S.V. Bhandare, R. Kumar, A.V. Anupama, H.K. Choudhary, V.M. Jali, B. Sahoo,
Annealing temperature dependent structural and magnetic properties of MnFe2O4
nanoparticles grown by sol-gel auto-combustion method, J. Magn. Magn. Mater.
433 (2017) 29–34.
[25] V.J. Angadi, A.V. Anupama, R. Kumar, H.M. Somashekarappa, K. Praveena,
B. Rudraswamy, B. Sahoo, Evidence of structural damage in Sm and Gd co-doped
Mn–Zn ferrite ceramics due to high-energy gamma irradiation, Ceram. Int. 42
(2016) 15933–15939.
[26] A.V. Anupama, W. Keune, B. Sahoo, Thermally induced phase transformation in
multi-phase iron oxide nanoparticles on vacuum annealing, J. Magn. Magn. Mater.
439 (2017) 156–166.
[27] H.K. Choudhary, R. Kumar, A.V. Anupama, B. Sahoo, Effect of annealing temperature on the structural and magnetic properties of Ba-Pb-hexaferrite powders
synthesized by sol-gel auto-combustion method, Ceram. Int. 44 (2018) 8877–8889.
[28] M. Houshiar, F. Zebhi, Z.J. Razi, A. Alidoust, Z. Askari, Synthesis of cobalt ferrite
(CoFe2O4) nanoparticles using combustion, coprecipitation, and precipitation
methods: a comparison study of size, structural, and magnetic properties, J. Magn.
Magn. Mater. 371 (2014) 43–48.
[29] R.S. Melo, F.C. Silva, K.R.M. Moura, A.S. de Menezes, F.S.M. Sinfrônio, Magnetic
ferrites synthesised using the microwave-hydrothermal method, J. Magn. Magn.
Mater. 381 (2015) 109–115.
[30] Y. Zhang, Y. Wu, Q. Qin, F. Wang, D. Chen, A study of the mechanism of microwave-assisted ball milling preparing ZnFe2O4, J. Magn. Magn. Mater. 409
(2016) 6–9.
[31] R. Ali, A. Mahmood, M.A. Khan, A.H. Chughtai, M. Shahid, I. Shakir, M.F. Warsi,
Impacts of Ni–Co substitution on the structural, magnetic and dielectric properties
of magnesium nano-ferrites fabricated by micro-emulsion method, J. Alloy. Compd.
584 (2014) 363–368.
[32] R.A. Young, The Rietveld Method, Oxford University Press, Oxford, 1993, pp. 1–39
(IUCr 5).
[33] J. Rodriguez-Carvajal, FULLPROF: a program for rietveld refinement and pattern
matching analysis, in: Proceedings of the Abstracts of the Satellite Meeting on
Powder Diffraction of the XV Congress of the IUCr, p. 127, Toulouse, France, 1990.
[34] R.A. Brand, Improving the validity of hyperfine field distributions from magnetic
alloys: part I: unpolarized source, Nucl. Instrum. Methods B 28 (1987) 398–416.
[35] R.A. Brand, Improving the validity of hyperfine field distributions from magnetic
alloys: part II: polarized source and spin texture, Nucl. Instrum. Methods B 28
8