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 2 Ceramics International xxx (xxxx) xxx–xxx R.V. Kumar et al. 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 3 Ceramics International xxx (xxxx) xxx–xxx R.V. Kumar et al. 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 4 Ceramics International xxx (xxxx) xxx–xxx R.V. Kumar et al. 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 5 Ceramics International xxx (xxxx) xxx–xxx R.V. Kumar et al. 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 6 Ceramics International xxx (xxxx) xxx–xxx R.V. Kumar et al. 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. 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