Materials Science and Engineering B 266 (2021) 115080
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Materials Science & Engineering B
journal homepage: www.elsevier.com/locate/mseb
Chemically enabling CoFe2O4 for magnetostrictive strain sensing
applications at lower magnetic fields: Effect of Zn substitution
P.N. Anantharamaiah a, *, H.M. Shashanka a, R. Kumar b, J.A. Chelvane c, B. Sahoo b, *
a
Department of Chemistry, Faculty of Mathematical and Physical Sciences, M. S. Ramaiah University of Applied Sciences, Bangalore 560058, India
Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
c
Advanced Magnetic Group, Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500058, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cobalt ferrite
Zn-substitution
Cation distribution
Magnetostriction
Phase pure cobalt-ferrite (CoFe2O4) and Zn-substituted CoFe2O4 (Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4) nanopowders were synthesized by a glycine-nitrate auto-combustion route without any post-calcination process. The
as-synthesized nano-ferrite powders were first pelletized, sintered and studied. Our results show that the crystallographic site preference of Zn, cation distribution, change in the oxidation state of Co-cation (+2 to + 3), and
reduction in magnetic A-O-B superexchange interactions of the AB2O4 type spinel structure have a direct
consequence on the excellent magnetostriction behavior of the samples. Our results demonstrate that although
the observed λmax values of the Zn-substituted samples are lower than the unsubstituted sample, importantly, the
magnitude of the maximum strain sensitivity ([dλ/dH]max) of the Zn-substituted samples (~3.6 × 10-9 m/A) is
nearly 300% higher than the parent compound (~1.18 × 10-9 m/A), even at remarkably low magnetic fields.
This facilitates the direct use of our samples for highly sensitive strain sensor applications.
1. Introduction
Cobalt ferrite (CoFe2O4) is one of the versatile ceramic-based magnetic materials as it finds applications in a range of potential areas such
as catalysis, data storage, gas sensors, battery technology, optoelectronics, biomedical, electromagnetic interference (EMI) shielding, etc.
[1–6]. In addition to these applications, it is also projected as a suitable
smart magnetic material for sensor applications, due to its cost-effective
and soft magnetostrictive nature [7–8]. Owing to the wide range of
potential applications in electronic devices, sensors, actuators and
transducers, achieving higher magnetization and magnetostriction
strain sensitivity at lower applied magnetic fields, in polycrystalline
spinel ferrites, is of immense interest for the researchers. Furthermore,
metal oxide-based magnetostrictive smart materials have received
remarkable attention from the research community due to their superior
practical advantages over the alloy-based magnetostrictive smart materials such as Terfenol-D, Galfenol, and their derivatives [9–11].
Among the spinel ferrites, cobalt ferrite is well-known for its high
magnetostriction parameters (magnetostriction strain and strain sensitivity), due to spin–orbit coupling governed by the presence of Co2+ in
the octahedral coordination sites (B-sites) of the spinel structure [12].
The magnetostriction value of the single crystal cobalt ferrite is reported
as ~ 600 ppm along the [1 0 0] crystallographic direction, at an applied
field of 4 kOe [13]. However, for the sintered polycrystalline cobalt
ferrite, the magnitude of the maximum magnetostriction strain (λmax)
varies from 100 ppm to 400 ppm depending upon the synthesis methods
and processing conditions adapted during the fabrication of the samples
[7,14–16]. Particularly for the strain/torque sensor applications, materials with high magnetostriction strain (λ) and strain sensitivity (dλ/dH)
are indispensable [17]. Therefore, numerous attempts have been carried
out to attain higher magnetostriction parameters at low magnetic fields
by substituting various magnetic and non-magnetic metal ions either for
Co or Fe in CoFe2O4 [18–25]. In most of the metal ions substituted cobalt
ferrite systems, a higher magnitude of strain sensitivity has been achieved at low magnetic fields but at the cost of magnetostriction strain. A
few reports suggest that it is possible to achieve higher λmax at low
magnetic fields in the metal substituted cobalt ferrites samples sintered
from the nanocrystalline powder against the parent counterpart,
wherein the amount of metal substitution is low [26–29].
Somaiah et al. investigated the effect of Zn substitution for Fe on the
magnetostrictive properties of sintered cobalt ferrite samples (CoZnxFe2-xO4 (0 ≤ x ≤ 0.3)) synthesized by auto-combustion method and
reported a decrease in the λmax with increasing the amount of Zn content
in the spinel lattice [30]. Unlike the λmax, the maximum strain sensitivity
* Corresponding authors.
E-mail addresses: anantharamaiah.cy.mp@msruas.ac.in (P.N. Anantharamaiah), bsahoo@iisc.ac.in (B. Sahoo).
https://doi.org/10.1016/j.mseb.2021.115080
Received 12 October 2020; Received in revised form 5 January 2021; Accepted 22 January 2021
Available online 4 February 2021
0921-5107/© 2021 Elsevier B.V. All rights reserved.
P.N. Anantharamaiah et al.
Materials Science & Engineering B 266 (2021) 115080
Fig. 1. The schematic representation of ferrite samples fabrication and magnetostriction measurements.
is shown for the sample with × = 0.1. Bhame et al. studied the impact of
Zn substitution for Co on the magnetostrictive properties of cobalt ferrite
samples (Co1-xZnxFe2O4 (0 ≤ x ≤ 0.5)) synthesized by solid-state reaction route and shown a decrease in the λmax with increasing the Zn
content in the spinel lattice [31]. However, higher strain sensitivity is
shown for the samples with × = 0.2. Nlebedim et al. reported decrease
in the λmax as well as [dH/dH]max with ‘x’ in the Co1-xZnxFe2O4 samples
(x = 0.0, 0.02, 0.04, 0.06, 0.09, and 0.17) synthesized by the solid-state
ceramic route [32]. A thorough literature survey revealed that whether
the Zn is substituted either for Co or Fe in CoFe2O4, the λmax decreases
with increasing the Zn content and it is consistent with most of the reports. However, there was some disagreement in the values of strain
sensitivity, probably due to different synthesis methods and processing
conditions adapted by the authors to fabricate their samples.
The prime objective of the current study is to investigate the effect of
the same amount of Zn substitution for Fe in one case (CoFe1.8Zn0.2O4)
and for Co in another case (Co0.8Zn0.2Fe2O4) on the magnetic and
magnetostrictive parameters of the sintered polycrystalline CoFe2O4.
The materials were synthesized and processed under identical conditions, for better comparison and correlation. Zn-substituted samples
showed superior magnetic and magnetostrictive parameters at low
magnetic fields compared to the unsubstituted cobalt ferrite, signifying
that they can be employed as potential candidates for strain/torque
sensor application.
Fig. 2. X-ray powder diffraction patterns of (a) as-synthesized and (b) sintered CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4 samples. The patterns are compared
with the indexed simulated pattern of cobalt ferrite.
2
P.N. Anantharamaiah et al.
Materials Science & Engineering B 266 (2021) 115080
Fig. 3. SEM images of the sintered CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4 samples.
2. Experimental methods and characterization techniques
The auto-combustion method was employed to synthesize nanocrystalline CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4 samples by
following the same procedure as described in ref. [16]. Briefly, metal
nitrates (Fe(NO3)3, Co(NO3)2 and Zn(NO3)2) were weighed according to
the stoichiometric compositions (see the electronic supporting information ESI file) to obtain CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4
into a 250 ml crystallizing dish and dissolved in an optimum volume of
distilled water. Later, a minimum volume of glycine (1 mol per mole of
metal ion) solution, prepared separately, was mixed with the metal nitrate solution and sonicated for a few minutes to obtain a homogeneous
reaction mixture. The reaction mixture was then placed on a pre-heated
hot plate at temperature 200 ◦ C. After evaporation of solvent molecules,
the brown-colored viscous gel was formed, dried and underwent selfcombustion leading to the formation of a fluffy mass of ferrite powders. The obtained ferrite powders were ground into fine powders using
agate mortar and pestle. Later, the resultant fine powders were lubricated with 2% PVA solution followed by moulding into the form of discshaped pellets by applying a pressure of 8 MPa. The compacted green
pellets were sintered at 1200 ◦ C for 2 h in the furnace atmosphere. Phase
formation and crystal structural parameters of the as-synthesized and
sintered samples were investigated using a powder X-ray diffractometer
(PANalytical X’pert PRO) using Cu Kα X-ray source and nickel filter.
Raman spectra of the sintered samples were recorded at ambient conditions using a micro-Raman spectrometer (Horiba JY Labraman HR
800) equipped with 533 nm excitation source. A vibrating sample
magnetometer (VSM) was used to probe the field-dependent and
temperature-dependent magnetizations of the sintered samples.
Magnetostriction measurements of the sintered and polished samples
were performed using 350 Ω resistive strain gauges having the gauge
factor of 1.96. The strain gauge was attached on one of the flat surfaces of
the sintered pellet using suitable adhesive followed by cured at 80 OC for
a few hours. The fabrication of the ferrite samples and magnetostriction
measurements carried out in the present study are illustrated in the form
of a schematic in Fig. 1.
Fig. 4. Raman spectra of sintered CoFe2O4, Co0.8Zn0.2Fe2O4
CoFe1.8Zn0.2O4 samples, recorded at room temperature.
and
(1200 ◦ C), all the sintered nano-ferrite samples exhibited a single phase
without any traceable secondary phases (see Fig. 2(b)).
Unlike the XRD patterns of the as-synthesized ferrite powders, the
diffraction peaks in the XRD patterns of the sintered samples are sharp
and intense due to the contribution of larger crystallites/grains. With
respect to the diffraction peaks of the unsubstituted cobalt ferrite, the
peaks of the CoFe1.8Zn0.2O4 sample are shifted towards lower diffraction
angles, whereas the peaks associated with the Co0.8Zn0.2Fe2O4 sample
are shifted towards higher diffraction angles. The changes in the
diffraction peaks positions of the Zn-substituted samples can be attributed to changes in the unit cell lattice parameter. The unit cell lattice
parameters, extracted by the least-square refinement of the powder
patterns using the PCW program [33], are found to be 8.390, 8.408 and
8.378 Å, respectively, for the CoFe2O4, Co0.8Zn0.2Fe2O4 and
CoFe1.8Zn0.2O4. A marginal increase in the lattice parameter for the
Co0.8Zn0.2Fe2O4 sample is primarily due to the replacement of smaller
cation Co2+ (0.58 Å for 4-fold coordination) by a slightly bigger cation
Zn2+ (0.60 Å for 4-fold coordination) in the tetrahedral site. Although
the ionic radius of Zn2+ is considerably bigger than that of the Fe3+
(0.49 Å for 4-fold coordination), the decrease in the lattice parameter of
the CoFe1.8Zn0.2O4 sample can be interpreted based on changes in the
oxidation of Co. In the CoFe1.8Zn0.2O4 samples, the divalent Zn2+ has
been substituted for trivalent Fe3+ and the ionic charges (net valencies)
of cations and anions of the sample are not completely compensated.
Therefore, to keep the system electrically neutral, an equivalent amount
of high-spin Co2+ will be oxidized to low-spin Co3+ (0.545 Å for 6-fold
coordination) in the octahedral coordination site [27]. Due to changes in
3. Results and discussion
3.1. Powder X-ray diffraction (PXRD)
Fig. 2(a) shows the X-ray powder diffraction patterns of assynthesized cobalt ferrite and Zn-substituted cobalt ferrite samples,
wherein indexed simulated pattern of cobalt ferrite has been compared
with the observed patterns. Based on the comparison between observed
and simulated patterns, all the as-synthesized ferrite powders are found
to be phase-pure as there are no impurity peaks observed in the observed
patterns, other than those corresponding to the cubic spinel ferrite
phase. Furthermore, all the diffraction peaks in the observed patterns
are remarkably broad, owing to the nanocrystalline nature of the ferrite
particles [26]. The average crystallite sizes of the samples, estimated
using the Scherrer formula, are found to be in the range of 9–12 nm.
After sintering the nano ferrite powders at a higher temperature
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P.N. Anantharamaiah et al.
Materials Science & Engineering B 266 (2021) 115080
Fig. 5. (a) Magnetization versus magnetic field hysteresis loops measured at room temperature, and (b) magnetization versus temperature curves measured above
room temperature at a constant magnetic field 8 kA/m of the sintered samples.
the oxidation of Co from 2 + to 3 + and the smaller ionic radius of
Co3+compared to other cations Fe3+, Co2+ and Zn2+, the decrease in the
lattice parameter is observed for the CoFe1.8Zn0.2O4 sample.
substituted cobalt ferrite sample, the T2g(3) and A1g(2) bands are
clearly distinct for both Zn-substituted samples. It is interesting to
observe that the intensity of the A1g(2) band, due to CoO4 tetrahedron,
of the Co0.8Zn0.2Fe2O4 sample is considerably weaker than that of the
band observed for the CoFe1.8Zn0.2O4 sample, indicating Zn is fully
substituted for Co in the tetrahedral coordination site of the
Co0.8Zn0.2Fe2O4 sample. On the other hand, the intensity of the A1g(1)
band, due to FeO4 tetrahedron, of the CoFe1.8Zn0.2O4 sample is relatively weaker than the band obtained for the Co0.8Zn0.2Fe2O4 sample,
signifying all Zn ions are fully substituted for Fe in the tetrahedral coordination site of the CoFe1.8Zn0.2O4 sample. It is understood from this
study that Raman spectroscopy can be employed effectively to elucidate
the site preference of the substituent such as Zn.
3.2. Microstructural analysis
Scanning electron microscope (SEM) images of the sintered CoFe2O4,
Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4 samples were captured on the
fractured inner surfaces of the sintered pellets and are presented in
Fig. 3. For better comparison and correlation, all the images are in the
same scale and magnification. As can be seen from the SEM images, no
considerable changes in the microstructural features and moreover, all
the images appear almost the same. Unlike the nanocrystalline powders,
the grains of the sintered products are larger and are in micron-size, due
to significant grain growth induced by the sintering temperature.
3.4. Magnetic studies
Magnetic field-dependent and temperature-dependent magnetization curves of the sintered CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4
samples are presented in Fig. 5(a) and 5(b), respectively. As can be seen
from the inset of Fig. 5(a) that all the spinel ferrite samples exhibit welldefined hysteresis loops due to long-range ferrimagnetic behavior.
Compared to the unsubstituted CoFe2O4 sample, both Zn-substituted
cobalt ferrite samples are soft magnetic in nature owing to their high
magnetization and very low coercivity. Furthermore, the unsubstituted
sample magnetization is not fully saturated even at a maximum
measuring field of 1200 kA/m (1.5 T), but the magnetizations of the Znsubstituted samples attain almost saturation at the maximum field of
1200 kA/m. In the spinel ferrite systems, due to antiparallel coupling
between the magnetic moments of cations located on the octahedral
sites (B-sites) and the tetrahedral sites (A-sites) in AB2O4 spinel structure, the overall magnetization (M) of the system is given by M =
∑
∑
∑
∑
MB−sites − MA−sites , where
MB−sites and
MA−sites are the net
magnetizations of the B-sites and A-sites, respectively [40].
The magnitude of the magnetization in the metal ions substituted
spinel ferrites depends primarily on nature (magnetic or non-magnetic),
crystallographic site preference (A-site or B-site) and amount of the
substituents [28]. In the present study, the MS values of the samples,
obtained by linear extrapolating the M vs. 1/H curve to 1/H = 0, are
found to be 592, 663, 728 kA/m, respectively, for CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4 samples. The higher magnetization associated
with the Zn-substituted samples can be described based on the site
preference and the nature of the Zn2+ ion. The Zn2+ is a non-magnetic
transition metal ion with a strong preference for the tetrahedral site
3.3. Raman spectral studies
The Raman spectra of sintered CoFe2O4, Co0.8Zn0.2Fe2O4 and
CoFe1.8Zn0.2O4 samples recorded at room temperature are illustrated in
Fig. 4. Due to the inverse spinel structure (with a degree of inversion ~
0.6–0.7) of cobalt ferrite and its derivatives [27,34–39], more than five
active Raman bands have been observed for all the samples and are
designated as 2A1g, 3T2g and Eg bands [27]. More Raman bands associated with the inverse and mixed spinel structured compounds could be
attributed to the random distribution of divalent and trivalent cations in
the tetrahedral and octahedral crystallographic environments, as
documented in the literature [27–28]. The A1g(2) band situated at
wavenumber ~ 615 cm−1 is the prime characteristics band of the inverse
and mixed spinel compounds [27] and the origin of this band is pertaining to the divalent cation (Co2+) located in the tetrahedral site [27].
A1g(1) band at 685 cm−1 can be attributed to the symmetric breathing
mode of the FeO4 unit [27]. High-frequency T2g(3) band appears at ~
555 cm−1 arises due to the asymmetric bending motion of the oxygen coordinated to Co2+ in tetrahedral sites. T2g(2) band at ~ 470 cm−1 pertains to the motion of oxygen atoms bonded to Fe3+ in the octahedral
sites (FeO6). Eg mode is related to the symmetric bending motion of the
oxygen anions in the tetrahedral AO4 units [27]. A weak signal at ~ 210
cm−1, designated as T2g (1), is related to the translational motion of the
BO6 units against the A-sites cation [27].
In the Raman spectrum of cobalt ferrite, T2g(3) and A1g(2) bands are
not well-defined and are merged, probably due to the rough surface of
the sample on which the spectrum was recorded. In contrast to the un4
P.N. Anantharamaiah et al.
Materials Science & Engineering B 266 (2021) 115080
Table 1
Magnetic parameters of the sintered samples.
Table 2
Magnetostriction parameters of the sintered samples.
Sample
MS (kA/m)
HC (kA/m)
TC (OC)
CoFe2O4
Co0.8Zn0.2Fe2O4
CoFe1.8Zn0.2O4
592
663
728
21
3
3
522
378
348
Sample
CoFe2O4
Co0.8Zn0.2Fe2O4
CoFe1.8Zn0.2O4
λmax
(ppm)
Field @
λmaxobtained
−212
−112
−64
480
125
62
[dλ/
dH]max ×
10-9 m/A
−1.18
−3.85
−3.30
Field @ [dλ/
dH]maxobtained
160
25
27
Fig. 6. Magnetostriction strain curves of the sintered CoFe2O4, Co0.8Zn0.2Fe2O4
and CoFe1.8Zn0.2O4 samples, measured along the parallel direction to the
applied magnetic field.
Fig. 7. Magnetostriction strain values of the sintered CoFe2O4, Co0.8Zn0.2Fe2O4
and CoFe1.8Zn0.2O4 samples obtained at low applied magnetic fields.
(A-site) for almost all of the known ferrites [41–47]. In our present
system, the substitution of Zn2+ for either Co2+ or Fe3+ in cobalt ferrite
enhances the overall magnetization of the material due to the A-site
magnetic dilution [18].
Although the amount of Zn substitution is the same in both the cases,
it is interesting to note that MS of the CoFe1.8Zn0.2O4 sample is considerably higher than that obtained for the Co0.8Zn0.2Fe2O4 sample. This is
because in the Co0.8Zn0.2Fe2O4 sample, non-magnetic cation Zn2+ replaces the low-magnetic moment cation Co2+(3μB) from the A-site,
whereas in the CoFe1.8Zn0.2O4 sample, the same Zn2+ replaces the highmagnetic moment cation Fe3+ (5μB) from the A-site. As a result, the
average magnetization of the B-sites will be higher than that of the Asites (due to more A-site magnetic dilution) in the CoFe1.8Zn0.2O4 sample as compared to the Co0.8Zn0.2Fe2O4 sample, which has less A-site
magnetic dilution, and hence the higher magnetization for the
CoFe1.8Zn0.2O4 sample. The HC is found to be nearly the same (3 kA/m)
for both Zn-substituted samples, and it is considerably lower than the
value obtained for the unsubstituted CoFe2O4 sample (21 kA/m), due to
a decrease in the magnetocrystalline anisotropy as well as the A-O-B
superexchange interaction.
The Curie temperature (TC) is a measure of the strength of the A-O-B
superexchange interactions in the spinel ferrites [34], and in the present
study TC of all the samples was extracted by taking the first derivative of
magnetization versus temperature (M vs T) curves recorded in the
presence of constant magnetic field strength of 8 kA/m. The obtained TC
of the samples is listed in Table 1. The TC of the unsubstituted sample
found in the present study is consistent with the values reported for the
sintered polycrystalline cobalt ferrites [17–18]. As expected, TC of both
Zn-substituted samples is significantly lower than the unsubstituted
sample due to a considerable decrease in the A-O-B superexchange interactions governed by the presence of non-magnetic Zn2+ in the A-site.
Between the Zn-substituted samples, the strength of the A-O-B exchange
interaction is maximum for the Co0.8Zn0.2Fe2O4 sample compared to the
CoFe1.8Zn0.2O4 sample. Due to less A-site magnetic dilution in the
Co0.8Zn0.2Fe2O4 sample as the Zn2+ substituted for Co2+ in the tetrahedral site, more thermal energy is required to overcome the exchange
interaction and hence the higher TC for the Co0.8Zn0.2Fe2O4 sample over
the CoFe1.8Zn0.2O4 sample.
3.5. Magnetostriction studies
In Fig. 6, room temperature magnetostriction strain curves of the
sintered CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4 samples are
plotted as a function of the magnetic field. These strain curves were
measured along the parallel direction (strain gauge axis) to the applied
magnetic field. The values of the maximum magnetostriction strain
(λmax) and the fields at which those values obtained are summarized in
Table 2. The obtained λmax of the unsubstituted cobalt ferrite is comparable with the values often reported for the sintered polycrystalline
cobalt ferrites [17–19]. After introducing Zn into the cobalt ferrite lattice, the magnitude of the λmax decreases along with the fields required
for the same, and the present results are consistent with most of the
reports on the magnetostrictive properties of metal ions doped cobalt
ferrites [19,25,30]. At low magnetic fields, both Zn-substituted samples
show higher magnetostriction strain as compared to the unsubstituted
cobalt ferrite sample. For instance, at an applied magnetic field of 50
kA/m, the magnetostriction values are −1, −85 and −63 ppm, respectively, for the CoFe2O4, Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4 samples. A
plot of the magnetostriction values obtained at low applied magnetic fields
for all three samples is presented in Fig. 7. Obtaining higher magnetostriction values at low magnetic fields in the Zn-substituted samples is
primarily attributed to a strong reduction in the A-O-B superexchange
5
P.N. Anantharamaiah et al.
Materials Science & Engineering B 266 (2021) 115080
either for Co or for Fe in cobalt ferrite, the magnetic and magnetostriction parameters of the substituted samples are much better (higher)
than the parent compound due to the non-magnetic nature and tetrahedral site preference of Zn. To emphasize these behaviours, we have
chosen the same amount of Zn substitution in both the samples
(Co0.8Zn0.2Fe2O4 and CoFe1.8Zn0.2O4). Moreover, a slightly lower
magnetostriction parameter for the CoFe1.8Zn0.2O4 sample compared to
the Co0.8Zn0.2Fe2O4 sample is attributed mainly due to the changes in
the oxidation state of Co from + 2 to + 3 in the octahedral site. The
experimental results for our samples indicate that Zn-substituted samples are highly suitable candidates for sensor applications. This is further
supported by the concomitant advantage that the samples exhibit high
magnetic and magnetostriction parameters even at much lower applied
magnetic fields than the pure CoFe2O4 sample.
5. Data Availability Statement
The raw/processed data required to reproduce these findings cannot
be shared at this time as the data also forms part of an ongoing study.
Declaration of Competing Interest
Fig. 8. Strain sensitivity curves of the sintered CoFe2O4, Co0.8Zn0.2Fe2O4 and
CoFe1.8Zn0.2O4 samples.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
interactions, as revealed by the Curie temperature measurements.
Although the amount of Zn substitution is the same in both the Znsubstituted cobalt ferrite samples, the λmax of the Co0.8Zn0.2Fe2O4 sample is considerably higher than of the CoFe1.8Zn0.2O4 sample. This can
reasonably be interpreted based on changes in the oxidation state of Co
in the octahedral site (B-site). In CoFe2O4, the magnetostriction arises
primarily due to the presence of Co2+ in the octahedral coordination
environment and therefore, any changes in the oxidation state of Co
would impact strongly on its magnetostriction value. In the Co0.8Zn0.2Fe2O4 sample, some fractions of Co will be in the + 3 oxidation state to
keep the system electrically neutral as divalent Zn has been substituted
for trivalent Fe. Due to the presence of low-spin non-magnetic Co3+ ions
in the octahedral site, the magnitude of the magnetostriction is
considerably lower for the CoFe1.8Zn0.2O4 sample as compared to that of
the Co0.8Zn0.2Fe2O4 sample.
Strain sensitivity (dλ/dH) is one of the important parameters of
magnetostrictive smart materials. The materials with a high magnitude
of dλ/dH at low magnetic fields are essential to developing highperformance sensor (torque) devices. In Fig. 8, the strain sensitivity
curves of all the samples are plotted as a function of the applied magnetic field. The maximum strain sensitivity ([dλ/dH]max) of ~ 1.18 × 109
m/A is obtained for cobalt ferrite sample at the applied magnetic field
of 160 kA/m and the value is comparable with the reported values
[23,28]. However, the [dλ/dH]max values of Co0.8Zn0.2Fe2O4 and
CoFe1.8Zn0.2O4 samples are found to be 3.85x 10-9 and 3.30 × 10-9m/A,
respectively, at the applied magnetic fields of 25 and 27 kA/m. The [dλ/
dH]max of Zn-substituted cobalt ferrite samples is nearly 300% higher
than the parent compound, even at much lower magnetic fields. The
prime reason for having a higher magnitude of strain sensitivity in the
Zn-substituted cobalt ferrite is due to their lower magnetocrystalline
anisotropy, reduced A-O-B super-exchange interaction and higher
magnetization at lower magnetic fields as compared to the parent
unsubstituted cobalt ferrite. It can be realized that the magnetic
anisotropy of pure metallic Co (and pure Co-ferrite) is higher than that of
any of the other elements used in our study [48–49]; hence, along with
many other applications [50–56], Co-based materials are very useful for
magnetostrictive devices [57].
Acknowledgment
Dr. P. N. Anantharamaiah is indebted to the Science and Engineering
Research Board (SERB), Department of Science and Technology (DST),
Govt. of India, for the financial support through the sanction order
number, CRG/2018/002925.
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