Materials Science and Engineering B 266 (2021) 115080 Contents lists available at ScienceDirect 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 3 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. References: [1] J. Tong, W. Li, L. Bo, H. Wang, Y. Hu, Z. Zhang, A. Mahboob, Selective oxidation of styrene catalyzed by cerium-doped cobalt ferrite nanocrystals with greatly enhanced catalytic performance, J. Catal. 344 (2016) 174–481. [2] X. Gao, L. Liu, B. Birajdar, M. Ziese, W. Lee, M. Alexe, D. Hesse, High-Density Periodically Ordered Magnetic Cobalt Ferrite Nanodot Arrays by Template-Assisted Pulsed Laser Deposition, Adv. Funct. Mater. 19 (2009) 3450–3455. [3] S. Joshi, V.B. Kamble, M. Kumar, A.M. Umarji, G. Srivastava, Nickel substitution induced effects on gas sensing properties of cobalt ferrite nanoparticles, J. Alloys Compd. 654 (2016) 460–466. [4] Y.-Q. Chu, F.u. Zheng-Wen, Q.-Z. Qin, Cobalt ferrite thin films as anode material for lithium ion batteries, Electrochim. Acta 49 (2004) 4915–4921. [5] S.Y. Srinivasan, K.M. Paknikar, D. Bodas, V. Gajbhiye, Applications of cobalt ferrite nanoparticles in biomedical nanotechnology, Nanomedicine 30 (2018) 10. [6] S. Gupta, S.K. Sharma, D. Pradhan, N.H. Taia, Ultra-light 3D reduced graphene oxide aerogels decorated with cobalt ferrite and zinc oxide perform excellent electromagnetic interference shielding effectiveness, Compos. Part A Appl. Sci. Manuf. 123 (2019) 232–241. [7] K.K. Mohaideen, P.A. Joy, Enhancement in the Magnetostriction of Sintered Cobalt Ferrite by Making Self-Composites from Nanocrystalline and Bulk Powders, ACS Appl. Mater. Interfaces 4 (2012) 6421–6425. [8] R.S. Turtelli, M. Kriegisch, M. Atif, R. Grössinger, Co-ferrite – A material with interesting magnetic properties, IOP Conf. Series: Mater. Sci. & Engineering 60 (2014), 012020. [9] Y. Chen, J.E. Snyder, C.R. Schwichtenberg, K.W. Dennis, R.W. Macallum, D. C. Jiles, Metal-bonded Co-ferrite composites for magnetostrictive torque sensor applications, IEEE Trans. Magn. 35 (1999) 3652–3654. [10] G. Engdahl Hand book of Giant Magnetostrictive Materials. (Academic, San Diego) (2000). [11] M. R. J. Gibbs Modern Trends in Magnetostriction study and Application. (Kluwer Academic, Dordrecht, The Netherland)(2001). [12] J.C. Slonczewski, Anisotropy and magnetostriction in magnetic oxides, J. Appl. Phys. 32 (1961) 253S–263S. [13] R.M. Bozorth, E.F. Tilden, A.J. Williams, Anisotropy and magnetostriction of some ferrites, Phys. Rev. 99 (1955) 1788–1798. [14] Y.X. Zheng, Q.Q. Cao, C.L. Zhang, H.C. Xuan, L.Y. Wang, Study of uniaxial magnetism and enhanced magnetostriction in magnetic annealed polycrystalline CoFe2O4, J. Appl. Phys. 110 (2011), 043908. [15] M. Atif, R. Sato-Turtelli, M. Kriegisch, R. Grossinger, F. Kubel, T. Konegger, Large enhancement of magnetostriction due to compaction hydrostatic pressure and magnetic annealing in CoFe2O4, J. Appl. Phys. 111 (2012), 013918. 4. Conclusions We have demonstrated that irrespective of whether Zn is substituted 6 P.N. Anantharamaiah et al. Materials Science & Engineering B 266 (2021) 115080 [16] K.K. Mohaideen, P.A. Joy, Influence of initial particle size on the magnetostriction of sintered cobalt ferrite derived from nanocrystalline powders, J. Magn. Magn. Mater. 346 (2013) 96–102. [17] I.C. Nlebedim, D.C. Jiles, Suitability of cation substituted cobalt ferrite materials for magnetoelastic sensor applications, Smart Mater. Struct. 24 (2015) 025006–025013. [18] P.N. Anantharamaiah, P.A. Joy, Tuning of the magnetostrictive properties of cobalt ferrite by forced distribution of substituted divalent metal ions at different crystallographic sites, J. Appl. phys. 121 (2017), 093904. [19] J.A. Paulsen, A.P. Ring, C.C.H. Lo, J.E. Snyder, D.C. Jiles, Manganese-substituted cobalt ferrite magnetostrictive materials for magnetic stress sensor applications, J. Appl. Phys. 97 (2005), 044502. [20] G.S.N. Rao, O.F. Caltun, K.H. Rao, B.P. Rao, H.L. Wamocha, H.H. Hamdeh, Influence of silicon and cobalt substitutions on magnetostriction coefficient of cobalt ferrite, Hyperfine Interact. 184 (2008) 179–184. [21] P.N. Anantharamaiah, P.A. Joy, High magnetostriction parameters of sintered and magnetic field annealed Ga-substituted CoFe2O4, Mater. Lett. 192 (2017) 169–172. [22] S.D. Bhame, P.A. Joy, Enhanced magnetostrictive properties of Mn substituted cobalt ferrite Co1.2Fe1.8O4, J. Appl. Phys. 99 (2006), 073901. [23] N. Ranvah, I.C. Nlebedim, Y. Melikhov, J.E. Snyder, D.C. Jiles, A.J. Moses, P. I. Williams, F. Anayi, Sang-Hoon Song, Temperature Dependence of Magnetostriction of Co1+xGexFe2-2xO4for Magnetostrictive Sensor and Actuator Applications, IEEE T.Magn. 44 (2008) 3013–3016. [24] M.V. Reddy, A. Lisfi, S. Pokharel, D. Das, Colossal piezomagnetic response in magnetically pressed Zr4+ substituted cobalt ferrites, Sci. Rep. 7 (2017) 7935. [25] B.C. Sekhar, G.S.N. Rao, O.F. Caltun, B.D. Lakshmi, B.P. Rao, P.S.V. Subba Rao, Magnetic and magnetostrictive properties of Cu substituted Co-ferrites, J. Magn. Magn. Mater. 398 (2016) 59–63. [26] P.N. Anantharamaiah, P.A. Joy, Magnetic and magnetostrictive properties of aluminium substituted cobalt ferrite synthesized by citrate-gel method, J. Mater. Sci. 50 (2015) 6510–6517. [27] P.N. Anantharamaiah, P.A. Joy, Enhancing the strain sensitivity of CoFe2O4 at low magnetic fields without affecting the magnetostriction coefficient by substitution of small amounts of Mg for Fe, Phys. Chem. Chem. Phys. 18 (2016) 10516–10527. [28] P.N. Anantharamaiah, P.A. Joy, Effect of size and site preference of trivalent nonmagnetic metal ions (Al3+, Ga3+, In3+) substituted for Fe3+ on the magnetostrictive properties of sintered CoFe2O4, J. Phys. D: Appl. Phys. 50 (2017), 435005. [29] P.N. Anantharamaiah, P.A. Joy, Large enhancement in the magnetostriction parameters of the composite of CoFe2O4 and CoFe1.9Ga0.1O4, Mater. Lett. 236 (2019) 303–306. [30] N. Somaiah, T.V. Jayaraman, P.A. Joy, D. Das, Magnetic and magnetoelastic properties of Zn-doped cobalt-ferrites—CoFe2-xZnxO4 (x=0, 0.1, 0.2, and 0.3), J. Magn. Magn. Mater. 324 (2012) 2286–2291. [31] S.D. Bhame, P.A. Joy, Enhanced strain sensitivity in magnetostrictive spinel ferrite Co1-xZnxFe2O4, J. Magn. Magn. Mater. 447 (2018) 150–154. [32] I.C. Nlebedim, M. Vinitha, P.J. Praveen, D. Das, D.C. Jiles, Temperature dependence of the structural, magnetic, and magnetostrictive properties of zincsubstituted cobalt ferrite, J. Appl. Phys. 113 (2013), 193904. [33] W. Kraus, G. Nolze, POWDER CELL - a program for the representation and manipulation of crystal structures and calculation of the resulting X-ray powder patterns, J. Appl. Cryst. 29 (1996) 301–303. [34] L. Kumar, P. Kumar, V. Kuncser, S. Greculeasa, B. Sahoo, M. Kar, Strain induced magnetism and superexchange interaction in Cr substituted nanocrystalline cobalt ferrite, Mater. Chem. Phys. 211 (2018) 54–64. [35] N. Thomas, V.D. Sudheesh, H.K. Choudhary, B. Sahoo, S.S. Nair, N. Lakshmi, V. Sebastian, Magnetic Properties of MFeCrO4 (M = Co/Ni) Prepared by Solution Combustion Method, J. Supercond. Nov. Magn. 32 (2019) 2973–2979. [36] R. Vijaya Kumar, A.V. Anupama, R. Kumar, H.K. Choudhary, V.B. Khopkar, G. Aravind, B. Sahoo, Cation distributions and magnetism of Al-substituted CoFe2O4 - NiFe2O4 solid solutions synthesized by sol-gel auto-combustion method, Ceram. Int. 44 (2018) 20708–20715. [37] S.V. Bhandare, R. Kumar, A.V. Anupama, H.K. Choudhary, V.M. Jali, B. Sahoo, Mechanistic insights into the sol-gel synthesis of complex (quaternary) Co–Mn–Znspinel ferrites: An annealing dependent study, Ceram. Int. 46 (2020) 17400–17415. [38] S.V. Bhandare, R. Kumar, A.V. Anupama, M. Mishra, R.V. Kumar, V.M. Jali, B. Sahoo, Effect of Mg-substitution in Co–Ni-Ferrites: Cation distribution and magnetic properties, Mater. Chem. Phys. 251 (2020), 123081. [39] D. Carta, M.F. Casula, A. Falqui, D. Loche, G. Mountjoy, C. Sangregorio, A. Corrias, A Structural and Magnetic Investigation of the Inversion Degree in Ferrite Nanocrystals MFe2O4 (M = Mn Co, Ni), J. Phys. Chem. C 113 (2009) 8606–8615. [40] J. Smith, H.P.J. Wijin, Ferrites, Philips Technical Library, Eindhoven, The Netherland, 1965. [41] 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. Alloys Compd. 742 (2018) 577–586. [42] M.K. Anupama, N. Srinatha, S. Matteppanavar, B. Angadi, B. Sahoo, B. Rudraswamy, Effect of Zn substitution on the structural and magnetic properties of nanocrystalline NiFe2O4 ferrites, Ceram. Int. 44 (2018) 4946–4954. [43] 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 γ-irradiation, Mater. Chem. Phys. 199 (2017) 313–321. [44] 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. [45] A.V. Anupama, V. Kumaran, B. Sahoo, Synthesis of highly magnetic Mn-Zn ferrite (Mn0.7Zn0.3Fe2O4) ceramic powder and its use in smart magnetorheological fluid, Rheol. Acta 58 (2019) 273–280. [46] A.V. Anupama, V. Kumaran, B. Sahoo, Application of Ni-Zn ferrite powders with polydisperse spherical particles in magnetorheological fluids, Powder Technol. 338 (2018) 190–196. [47] A.V. Anupama, V. Kumaran, B. Sahoo, Steady-shear magnetorheological response of fluids containing solution-combustion-synthesized Ni-Zn ferrite powder, Adv. Powder Technol. 29 (2018) 2188–2193. [48] R. Kumar, M. Manjunatha, A.V. Anupama, K.P. Ramesh, B. Sahoo, Synthesis, composition and spin-dynamics of FCC and HCP phases of pyrolysis derived Conanoparticles embedded in amorphous carbon matrix, Ceram. Int. 45 (2019) 19879–19887. [49] T.M. Ali, S.M. Ismail, S.F. Mansour, et al., Physical properties of Al-doped cobalt nanoferrite prepared by citrate–nitrate auto combustion method, J. Mater. Sci.: Mater. Electron. (2021), https://doi.org/10.1007/s10854-020-05059-y. [50] H.K. Choudhary, M. Manjunatha, R. Damle, K.P. Ramesh, B. Sahoo, Solvent dependent morphology and 59Co internal field NMR study of Co-aggregates synthesized by a wet chemical method, Phys. Chem. Chem. Phys. 20 (2018) 17739–17750. [51] H.K. Choudhary, R. Kumar, S.P. Pawar, U. Sundararaj, B. Sahoo, Enhancing absorption dominated microwave shielding in Co@C–PVDF nanocomposites through improved magnetization and graphitization of the Co@C-nanoparticles, Phys. Chem. Chem. Phys. 21 (2019) 15595–15608. [52] R. Kumar, H.K. Choudhary, A.V. Anupama, A. Menon, S.P. Pawar, S. Bose, B. Sahoo, Nitrogen doping as a fundamental way to enhance the EMI shielding behavior of cobalt particle-embedded carbonaceous nanostructures, New J. Chem. 43 (2019) 5568–5580. [53] R. Kumar, A. Kumar, N. Verma, R. Philip, B. Sahoo, Mechanistic insights into the optical limiting performance of carbonaceous nanomaterials embedded with core–shell type graphite encapsulated Co nanoparticles, Phys. Chem. Chem. Phys. 22 (2020) 27224–27240. [54] R. Kumar, A. Kumar, N. Verma, R. Philip, B. Sahoo, FeCoCr alloy-nanoparticle embedded bamboo-type carbon nanotubes for non-linear optical limiting application, J. Alloys Compd. 849 (2020), 156665. [55] S.M. Azab, A.M. Fekry, Electrochemical design of a new nanosensor based on cobalt nanoparticles, chitosan and MWCNT for the determination of daclatasvir: a hepatitis C antiviral drug, RSC Advances 7 (2017) 1118–1126. [56] A.M. Fekry, M. Shehata, S.M. Azab, A. Walcarius, Voltammetric Detection of Caffeine in Pharmacological and Beverages Samples Based on Simple Nano- Co (II, III) Oxide Modified Carbon Paste Electrode in Aqueous and Micellar Media, Sensors and Actuators: B. Chemical 302 (2020), 127172. [57] P.N. Anantharamaiah, B.P. Rao, H.M. Shashanka, J.A. Chelvane, V. Khopkar, B. Sahoo, Role of Mg2+ and In3+ substitution on magnetic, magnetostrictive and dielectric properties of NiFe2O4 ceramics derived from nanopowders, Phys. Chem. Chem. Phys. 23 (2021) 1694–1705. 7