Magnetic finite size effects, coercive field and irreversibility in sintered (1-x)BaTiO3–xCoFe2O4 nano-composites

Journal Pre-proof Magnetic finite size effects, coercive field and irreversibility in sintered (1-x)BaTiO3–xCoFe2O4 nano-composites G. Hassnain Jaffari, Junaid Ur Rehman, Layiq Zia, Azizur Rahman, S. Ismat Shah PII:

S0254-0584(20)30136-X

DOI:

https://doi.org/10.1016/j.matchemphys.2020.122757

Reference:

MAC 122757

To appear in:

Materials Chemistry and Physics

Received Date: 21 August 2019 Revised Date:

29 December 2019

Accepted Date: 31 January 2020

Please cite this article as: G.H. Jaffari, J.U. Rehman, L. Zia, A. Rahman, S.I. Shah, Magnetic finite size effects, coercive field and irreversibility in sintered (1-x)BaTiO3–xCoFe2O4 nano-composites, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/j.matchemphys.2020.122757. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Magnetic finite size effects, coercive field and irreversibility in sintered (1x)BaTiO3–xCoFe2O4 nano-composites

G. Hassnain Jaffari*†, Junaid Ur Rehman†, Layiq Zia†, Azizur Rahman∥ and S. Ismat Shah‡§

† ∥

Department of Physics, Quaid-i-Azam University Islamabad, Pakistan. Department of Physics, University of Science and Technology of China, Hefei 230026, China.

‡

Department of Material Science and Engineering, University of Delaware, Newark, DE, 19716, USA. §

Department of Physics and Astronomy, University of Delaware, Newark, DE, 19716, USA.

Abstract We present detailed magnetic properties of (1-x)BaTiO3–xCoFe2O4 (x = 1, 0.5, 0.4, 0.3, 0.2, 0.1, 0.75, 0.05, 0.025) nanocomposites, synthesized through multiple-step chemical synthesis technique. Stepwise synthesis and structural optimization of composites include oxalate route synthesis of BaTiO3 with specifically chosen tetragonality followed by mixing in citric acid solution, and a final addition of the precursor for CoFe2O4. Heat treatment temperature were appropriately chosen to obtain composites with distinct spinel and tetragonal perovskite phases. Peak broadening has been observed in composites being a consequence of presence of strain. Main goal of the present work is to investigate magnetic response as a function of temperature for various compositions and to compare them with the magnetic properties of CoFe2O4 nanoparticles. BaTiO3 acts as a barrier for the growth of the CoFe2O4 due to the phase separation between these constituent phases in the composites. The nanoscopic features are consistently observed in the magnetic properties of the composites. These features include increase in coercivity, stronger temperature dependence of coercivity and magnetic irreversibility. CoFe2O4 and BaTiO3 composites exhibit strong temperature dependence with a more than an order of magnitude increase in coercivity at low temperatures. 1

Keywords: Multiferroic; Magnetism; Coercivity; Kneller`s law; Anisotropy barrier

1. Introduction Multiferroics are known to exhibit both ferromagnetic and ferroelectric responses simultaneously which makes these materials interesting [1-4]. Coexistence of magnetic and electric phases triggers presence of new coupling mechanisms between the two phases which is known as magneto-electric coupling [2, 5, 6]. (1-x)BaTiO3–xCoFe2O4 is one of the well-known composite of interest in this class of materials. This composite can have different possible connectivity schemes, e.g. (0-3), (1-3), (2-2) connectivity [7, 8]. BaTiO3 (BTO) and CoFe2O4 (CFO) are chosen as constituents in this composite because they exhibit good piezoelectric[9, 10] and magnetostrictive [11] properties, respectively. CFO composited with BTO is expected to exhibit finite size effects due to the hinderance in growth provided by BTO phase which is due to the presence of intrinsic phase separation between the composite’s constituents. Therefore, nanoparticle like magnetic response such as super-paramagnetism, is expected in CFO-BTO composites. In general, CFO synthesized in the nanometric size range, is reported to exhibit interesting features such as superparamagnetism[12], surface spin glass phase[13], non-equilibrium cationic distribution[14], high field magnetic irreversibility[15], and high magnetic coercivity[16] etc. In addition, ferromagnetic coercivity is shown to depend on particle size[17] and it is also reported that it exhibits decreasing trend with temperature following Kneller’s law[18]. CFO and BTO nanoparticles were synthesized through different routes. In case of BTO nanoparticles, it has been shown that tetragonality decreases with decrease in the particle size [19] suggesting that the surface strain plays an important role in decreasing tetragonality. Small particles exhibit different phase transformation phenomena as compared with the bulk counterparts. Extra pressure due to the surface tension exerts extra pressure which leads to change in overall thermodynamics of the system when size of the the particle is small [20]. CFO is a well-known spinel ferrite which exhibits high anisotropy constant [21, 22]. In most of the cases synthesis routes are the same for various spinel ferrite. There are several routes to prepare spinel ferrite nanoparticles [13, 23-26]. In order to achieve the best multiferroic properties, one 2

of the requirements is the formation of composite with individual phases well connected with each other. Furthermore, atomic inter-diffusion and extra phases should be minimized. Several techniques were employed to prepare these composites such as hydrothermal method [27], solgel chemistry [28, 29],mechanical milling method [30], etc. In the present work, (1-x)BTO–xCFO nano-composites are synthesized through multistep synthesis process. Where x represents CFO weight percentage in the composite, Optimization was carried out via the control of the tetragonality of the piezoelectric constituent (i.e. BTO). Step wise structural quantitative analysis are presented for both single phase and composite materials. Main goal of the present work is to study the magnetic response of the composite and to compare the results of the magnetic characterization with that for the pure CFO nanoparticles. BTO acts as a grain growth barrier affecting the growth kinetics of the CFO in the composite which is due to the phase separation between constituents in the composite. If the grain growth is severely restricted, nanoscopic features are expected in magnetic properties. These features include increase in coercivity, stronger temperature dependence of coercivity and noticeable magnetic irreversibility.

2. Experimental Section Individual BTO and CFO nanoparticles and their composites were prepared by using wet chemical method. Synthesis steps and heat treatment procedure were optimized to obtain composites instead of solid solutions. Structural analysis were performed using Empyrean PANalytical X-ray Diffractometer equipped with Cu-Kα radiation and wavelength of λ = 0.154 nm. Magnetic measurements in the temperature range of 50-300 K, were carried out using a commercial vibrating sample magnetometer (VSM), in the field of up to 30 kOe. High field and low temperature magnetic measurements, in the temperature range of 5-340 K, were carried out using a quantum design PPMS, up to the field as high as 60 kOe.

2.1.

Synthesis

2.1.1. Synthesis of CoFe2O4 nanoparticles For the synthesis of cobalt ferrite nanoparticles, various methods have been reported in literature. To control the size of particle at the nanometric scale, several chemical and mechanical routes can be used. These methods include sol-gel [31], citrate precursor [32], auto 3

combustion [33], electrodeposition [34]and hydrothermal[35]. Some methods require special equipment, high vacuum and/or high temperature. In present study, we synthesized the CFO nanoparticles through sol-gel method. The benefit of this method is that the particle size and morphology can be easily controlled. Stoichiometric amounts of Iron nitrate (Fe (NO3)3.9H2O) and Cobalt nitrate (Co (NO3)2.6H2O) were dissolved in 100 ml of distilled water and stirred for 20 minutes at room temperature, after this mixture was heated to 90°C with constant stirring at this temperature for 1 h . This resulting solution (“solution 1”) was then added to citric acid solution (“solution 2”) maintaining the molar ratio equal to 1:1, as shown schematically in figure 1. This mixture was then stirred for 2 h at room temperature for better homogenization. In order to adjust the pH to 7, ammonia was added drop wise to the prepared solution. Obtained solution was constantly stirred until black gel is formed. The resultant black gel was finally heated in an oven at 80°C for 24 h which resulted in a dry mass. This mass was grounded using mortar pestle to obtain fine black powder. Calcination of the obtained powder was carried out at 900°C for 2 h, to obtain singlephase Cobalt ferrite nanoparticles [36]. We note that after calcination the mass of the powder was reduced by about 20% of its original mass. This reduced mass is due to the removal of extra compound like water and other organic precursors and residues. 2.1.2. Synthesis of pure BaTiO3 nanoparticles The wet chemical techniques, known as oxalate route[37], was used for the preparation of BTO nanoparticles. Post-synthesis heat treatment was carried out in order to improve crystallinity and control tetragonality. This method gives a good chemical homogeneity of Ba and Ti ions through mixing at the molecular level. Barium chloride (BaCl2.2H2O) was dissolved in distilled water and oxalic acid was dissolved in propanol-1 with continuous stirring on magnetic stirrer for 0.5 h. Subsequently, titanium-Isopropoxide (3.6 ml) was added to oxalic acid solution and stirred for 1 h at 60°C. In this same solution, barium chloride solution was added drop at 60°C as the mixture was constantly stirred. As a result of continuous stirring, the mixed solution was transformed into a white gel, as shown schematically in figure 1. In this reaction Barium chloride was used as a precipitating agent. The obtained white gel was then dried at 70°C in the oven for 24 h followed by grinding and calcination at various temperatures, i.e. from 600°C to 1200°C, for 2 h each. After calcination, BTO nanoparticles with different degree of 4

crystallinity, depending on calcination temperature, were obtained. The results of the structural characterization were also compared with the literature [38]. For composites formation and analysis, BTO heated at 1000°C and 1200°C, was used for composite formation. 2.1.3. Synthesis of (1-x)BTO – xCFO nano-composites A three step procedure was followed to produce (1-x)BTO–xCFO nano-composites with distinct phases. These steps include: i) BTO nanoparticles were prepared through oxalate route and specifically chosen final powder was heated at 1000°C and 1200°C. ii) The BTO nanoparticles obtained in step i) were mixed in citric acid solution (i.e. “solution 2”), as shown in figure 1. iii) BTO nanoparticles carrying “solution 2” were added into precursor “solution 1”, to form the nano-composites with different values of x. Final product was heated in an oven for 24 h and calcined at 900°C for 2 h. In order to highlight finer details about the synthesis of the composites, a flow chart is shown in figure 1. “Solution 1” is obtained from stoichiometric amount of cobalt nitrate and iron nitrate that were dissolved in distilled water and stirred for 15 minutes. The temperature of solution was raised to 90°C with a dwell-time of 2 hours, to obtain “solution 1”. BTO nanoparticles annealed at 1000°C and 1200°C, were mixed with the citric acid solution to obtain solution 2. “Solution 1” and “solution 2” were mixed and stirred for 2 h. Ammonia (NH3) was added into final solution to adjust the pH to 7. Prepared solution was heated and stirred to make a gel which was heated in an oven at 70°C for 24 h. Finally, in order to obtain the BTO-CFO nanocomposite powder, heat treatment at 900°C for 2hrs was carried out. 30% of mass loss was observed which is due to the evaporation of organic compounds in the precursors. The final powder of (1-x)BTO–xCFO for (x = 0, 0.2, 0.3, 0.4, 0.5, 1) were pelletized using hydraulic press in disk forms with a diameter of 13 mm and thicknesses of 0.6 mm to 0.75 mm by applying 4.5 ton load. These pellets were then sintered at 900°C for 2 h and used for further characterizations. Details of the chemicals used during this study are summarized in table 1 of the supplementary information. 5

Figure 1. Schematic flow chart diagram of (1-x)BTO-xCFO nano-composites synthesis.

3. Results and Discussion 3.1.

Structural Characterization X-ray diffraction patterns of the BTO nanoparticles annealed at 600°C to 1200°C, are

shown in figure 2(a). Phase evolution as a function of annealing temperature was observed. It is clear that BTO nanoparticles require ex-situ annealing at around 1100°C to develop the tetragonal phase (exact temperature is determined from the rietveld refinement and discussed in the next section). Consequence of the heat treatment of BTO nanoparticles is the increase in crystallite size as well as particle size, which accompanies phase transformation into the tetragonal phase. The cubic to tetragonal structure transformation as a function of the annealing temperature is confirmed by the splitting of singlet cubic peak (110) at θ = 31.5o into a doublet

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(101) and (011) tetragonal peaks and the cubic (002) peak at θ = 45o into (002) and (200) tetragonal peaks, as shown in figure 2(b). Unlike cubic phase, lattice parameters c and a for the tetragonal phase are different, therefore the (110) and the (101) reflections are distinguishable. Same is the case for (002) and (200) reflections. XRD patterns of sample annealed at 1000°C and 1200°C match well with the reference cards JCPDS: 05-0626 and ICOD-00-022-1086, respectively. Additional shoulder at higher angle side of peaks indexed as (110) and (101), for sample annealed at 1100°C and 1200°C, has also been observed which arise due Kα2 component of incident X-rays.

Figure 2 (a) XRD patterns of BTO nanoparticles annealed at 600°C to 1200°C for 2hrs. (b) Evolution of (110) and (200) cubic reflections into tetragonal (110), (011) and (200), (002) reflections, respectively.

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The XRD patterns of (1-x)BTO-xCFO nanocomposites with x = 0, 0.2, 0.3, 0.4, 0.5 and 1, are shown in figure 3. In figure 3(a) and 3(b), BTO constituent phase in composite was presynthesized at 1000°C and 1200°C, respectively. Both CFO and BTO phases are observed for all values of x. The absence of any impurity and intermediate phases within the XRD detection limit, confirm the successful formation of the composite. It indicates that no significant chemical reaction occurred between the BTO-CFO phases. For comparison, the integrated intensity of the strongest peaks of BTO and CFO can be used as a reference for the estimation of the approximate amount of the constituent phases present in the composite. As the CFO concentration increases, intensity of CFO in the diffraction patterns, systematically increases. It can also be noted that all individual reflections of CFO and BTO phases were observed in BTOCFO composite[39]. The distinct diffraction peaks were indexed to the perovskite tetragonal structure of BTO with space group of P4mm and cubic spinel structure of CoFe2O4 with space group Fd-3m[40]. Hence presence of both titante (BTO) and ferrite (CFO) phases without any detectable impurity phase, has been observed through XRD analysis. For x = 0 and 1, XRD patterns shows the pure BTO and CFO phases. From figure 3, it is clear that the relative intensities of CFO phase increases, as its content increases in BTO-CFO composites. For instance, we see that the relative intensity of main peaks (220), (311), (222), (400), (422), (333), (440), (620), (533), and (444) referring to CFO phase increases gradually as the weight of x increases from x = 0.2 to x = 1. Alternatively, the relative intensities of main peaks (001), (101), (111), (002), (102), (112), (220) corresponds to BTO phase, decreases from x = 0.2 to 1[41]. For BTO nanoparticles annealed at 1200°C, splitting of the peaks have been observed which are due to the increase c/a ratio. In this case peaks with miller indices (001), (100), (101), (011), (111), (002), (200), (102), (210), (112), (211), (202), (220), (221), (103), (310), (311) were observed.

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Figure 3. XRD patterns of BTO-CFO nanocomposites sintered at 900°C for 2 h with (a) BTO pre-calcined at 1000°C and with (b) BTO pre-calcined at 1200 °C. ∗ and ♦ represent CFO and BTO XRD peaks, respectively.

As can see in figure 3, in relative terms the intensities of CFO phase peaks are not as pronounced as those for BTO. Hence, in order to further analyze structural results, more sophisticated analysis was carried out in order to highlight and confirm quantitative details related to actual composition, lattice parameters, structural symmetry, etc. Detailed Rietveld refinement was carried out on BTO nanoparticles and composites. Figure 4 shows refined patterns of BTO nanoparticles annealed at various temperatures ranging from 600°C to 1200°C having χ2 values in acceptable range, between 1.2 to 2.3. This analysis again confirms that crystal symmetry is P4mm and is consistent with the literature. A systematic increase in c/a ratio was observed with increasing annealing temperature. c/a ~ 1.0022, 1.0033, 1.0034, 1.0043, 1.0063, 1.007 and 1.0104 for annealing temperature of 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C and 1200°C, respectively, was calculate. Therefore, with the increase in annealing

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temperature, tetragonality increases. Tetragonality and various other refinement parameters are given in table 2 in the supplementary information.

Figure 4. Rietveld refined XRD patterns of BTO nanoparticles calcined at (a) 600°C, (b) 700°C (c) 800°C (d) 900°C (e) 1000°C (f) 1100°C (g) 1200°C.

In order to obtain composites, BTO nanoparticles calcined at 1000°C and 1200°C were used, as explained in the synthesis section. Quantitative analysis was also carried on composite compositions and is presented for the series, i.e. BTO nanoparticles calcined at 1200°C and composited with CFO, followed by the heat treatment at 900°C. Refinement results are shown in figure 5 and refinement parameters presented in table 3 in the supplementary information. It is confirmed from the refined results that BTO and CFO exhibited P4mm and Fd-3m symmetries, respectively. In addition, actual phase percentage of CFO is extracted to be 0, 14, 25, 34, 41, 100% for the x = 0, 0.2, 0.3, 0.4, 0.5 and 1, respectively. This shows that actual phase percentage of CFO is less than the nominal values. This shows that the heat treatment of the composites reduces the mass of the samples due to the evaporation of the organics along with the formation of the CFO phase. The values of tetragonality for the BTO phase in the composites remains almost same. This is due to the fact that final synthesis temperature for the composite (~900 °C) is less than the calcination temperature (~1200°C) of BTO nanoparticles. Finally, from the 10

refinement results unit cell were also generated, e.g. results for the composition with x = 0.5, are shown in figure 6. Shift of the Ti ion is obvious in case of BTO cell, which is main source of ferroelectric behavior in these samples. While in case of CFO, oxygen octahedra for various sites are presented in figure 6.

Figure 5. Rietveld refined XRD patterns of (1-x)BTO-xCFO nano-composites with (a) x = 0, (b) x = 0.2, (c) x = 0.3, (d) x = 0.4, (e) x = 0.5, (a) x = 1.

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Figure 6. Unit cell of (a) BaTiO3 and (b) CoFe2O4 at x=0.5.

In order to highlight finer structural differences between the pure and composites, (101), (110), (200) and (002) reflections of BTO phase, are shown in the figure 7(a) and (b). In composites, full width at half maxima increases as compare to pure BTO sample. As explained above BTO particles were first prepared before chemical mixing with CFO and final heat treatment was carried out on pelletized samples i.e. process of sintering. For pure BTO before compaction and composite formation, (101) and (110) reflection with corresponding Kα2 contribution is clearly visible in XRD spectrum due to the narrow and well-defined peak with high tetragonality, as shown in figure 7a. However, in composites, increase in peak broadening was also observed along with decrease in c/a value. Taking c/a value into account, due to which difference between the (101) and (110) reflections decreases, the broadening is significantly large. Consistently same features are exhibited by (200) and (002) reflections, as shown in figure 7(b). This increase in peak broadness indicates presence of strain exerted by CFO phase due to the lattice mismatch compared to the BTO. This also indicates formation of composite with good mixing of the both constituent phases which is due to the sintering procedure adopted in the present studies.

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Figure 7. Zoomed in region of (101), (110), (200) and (002) reflections.

3.2 Magnetic response of pure CoFe2O4 composition Figure 8(a) shows magnetization versus field behavior of pure CFO nanoparticles, heat treated at 900 °C. Hysteresis loops presented in this figure were measured at 50 K, 200 K and 300 K. Saturation magnetization (Ms) values are 66.29, 67.1, and 61.2 emu/g, recorded at 50 K, 200 K and 300 K, respectively. Saturation magnetization in CFO nanoparticles is lower than bulk CFO ~ 80 emu/g. Lower value of Ms in CFO nanoparticles is due to their small particle size which triggers surface spin glass features and also affect degree of inversion[42]. Large surface area of CFO nanoparticles may also be the cause of low value of saturation magnetization. As surface area increases, surface energy also increase and surface tension become large in the nanoparticle of CFO[43]. High value of surface tension promotes concentration of anti-site defects which ultimately leads to lowering of the magnetization [44]. Therefore, change in degree of inversion in spinel structures, in case of nano-particles, leads to change in saturation magnetization [45]. Low field region of the loops is shown in the inset of figure 8(a). Coercivity obtained at 50 K, 200 K, and 300 K are 986, 336 and 100 Oe, respectively. Overall trend of coercive field and saturation magnetization are shown in figure 8(b). Decrease in saturation 13

magnetization and coercive field as a function of temperature confirms the presence of thermal fluctuations which is typical for the ferromagnetic materials[18]. Slight increase in Ms (recorded in 3 T field), gives hint of the presence of super-paramagnetic character which is due to the nanoscopic crystallite size [46, 47].

Figure 8. (a) M-H loops of CoFe2O4 nanoparticles, recorded at (a) 50 K, 200 K and 350K (b) Variation of Hc and Ms as a function of temperature.

Temperature dependence of the parameters such as coercivity and saturation magnetization, extracted from the hysteresis loops, is presented in figure 8(b). Similar temperature dependent trend of these two parameters were previously reported and explained on the bases of simple model of thermal activation of magnetic moment of particles over the anisotropy barrier, known as Kneller`s law[18]. According to this law temperature dependence of the coercivity is given below:

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=

1−

(1)

Temperature dependence of Hc of CFO nanoparticles presented in this paper show a decreasing trend with temperature.Saturation magnetization is known to exhibit following temperature dependence: ( )=

(0) 1 −

(2)

which is known as Bloch law. Decreasing trend of saturation magnetization above the peak value was observed, as shown in figure 8(b). Coercivity and saturation magnetization decrease with the increase in temperature because at low temperature magnetic moments are frozen. As a result, when external field is applied, it is difficult to reorient them in the opposite direction leading to high coercivity and saturation magnetization. At higher temperature, due to thermal fluctuations, moments easily reorient themselves in opposite direction and results in a decreased value of coercivity and saturation magnetization. Similar decreasing trend was also reported in the literature [48, 49]. Moreover, as shown in figure 8, the Ms versus temperature curve peaks at a certain temperature. This peak indicates presence of blocking temperature around 150K under an applied field of 3T. This again confirms the presence of nanometric grain size of the system. Additionally, broad nature of the curve shows the presence of particle size distribution and dipolar interaction between the crystals.

3.2 Magnetic response of composites The magnetic hysteresis loop of pure CFO and (1-x)BTO-xCFO nanocomposites were recorded at various temperatures ranging from 50K to 350K. Figure 9 shows hysteresis loops recorded at 50K, 200K and 300K. Figure 9 (a, b and c) and (d, e and f), are the loops of the composites formed with the addition of BTO and annealed at 1000°C and 1200°C with various degrees of tetragonality. Qualitative comparison of the loops of pure CFO and BTO-CFO nanocomposites shows that high corecivity (Hc) was observed in composites, as compare to the pure 15

CFO. Furthermore, saturation magnetization (Ms) scales with the CFO concentration in the composites.

Figure 9. Magnetic hysteresis for (1-x)BTO-xCFO with increasing CFO concentration, measured at (a) 50K (b) 200K (c) 300K for BTO calcined at 1000°C and (d) 50K (e) 200K (f) 300K for BTO calcined at 1200°C.

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For the comparison of compositional dependence of various magnetic parameters, i.e., Hc, Ms and Mr, these parameters were extracted from the hysteresis loops. Variation of coercivity as a function of CFO concentration x, is shown in figure 10. Figure 10 (a, b and c) and (d, e and f), show Hc as a function of x, for composites formed from BTO that was annealed at 1000°C and 1200°C and had various degrees of tetragonality. In both cases saturation magnetization increases systematically with an increase in CFO content, which is expected, because CFO is the magnetic constituent in the composite. However, value of the coercive field is significantly larger in the case of the composites as compared to pure CFO nanoparticles. Value of coercivity (Hc) for pure CFO nanoparticles is 0.986, 0.336 and 0.100 kOe, recorded at 50, 200 and 300K, respectively. While the values of coercivity (Hc) for composite with BTO pre-annealed at 1000°C lies around 10.5, 2.2 and 0.5kOe, recorded at 50, 200 and 300 K, respectively. The values of coercivity (Hc) for composite with BTO pre-annealed at 1200°C lies around 10, 2 and 0.32kOe, recorded at 50, 200 and 300 K, respectively. CFO-BTO composites exhibit at least an order of magnitude higher values of coercivity.

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Figure 10. Coercivity and saturation magnetization versus compositions ( for x= 0.2, 0.3 0.4, 0.5 and 1) at three various temperatures (a) 50 K (b) 200 K (c) 300 K for BTO calcined at 1000°C and for (d) 50 K (e) 200 K (f) 300 K for BTO calcined at 1200°C. Temperature dependence of magnetic parameters i.e. Hc and Ms, has been presented in figure 11. Ms and Hc versus temperature, extracted from the loops of the composites formed from BTO pre-annealed at 1000 °C and 1200 °C, are shown in figure 11 (a, b) and (c, d), respectively. In both cases of composites, Ms (T) and Hc (T) curves exhibited similar trend. strong temperature 18

dependence of Hc was observed in composites which is the manifestation of finite size of the magnetic nanoparticles, i.e. their size is smaller as compared to the pure CFO sample. In pure sample, on heat treatment, size of the particles grows significantly compared to the CFO-BTO composite particles. Magnetic data shows that CFO nanoparticles had smaller size due to the barrier provided by BTO that restricts grain growth.

Figure 11. Coercivity and saturation magnetization as a function of temperature when BTO nanoparticles were pre-calcined at (a, b) 1000°C and (c, d) 1200°C.

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One of the general results is the observation of significantly high value of coercivity for all the composites at 50K, as shown in figure 10. Similar results have also been observed in the case of the loops measured at 200K and 300K. In order to highlight finer detail, coercivity versus CFO concentration x recorded at the fixed temperatures, is presented. At 50K, it is clear that the magnitude of the coercivity is significantly higher for composites compared to pure CFO sample. This difference decreases, as the temperature increases to from 50K to 200K to 300K. This shows that first CFO is present in the nanosize range compared to the x = 1 where grain size is high enough that the system exhibits bulk like features. For x<1, indicates presence of single domain like character where, due to coherent rotation of the magnetic moment, high coercivity is obtained.

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After establishing the fact that CFO exist as phase separated nanoparticles within the BTO-CFO composite, additional samples with even much smaller concentration of CFO particles was also been prepared. This includes composites with concentrations x = 0.025, 0.05 and 0.75. For all these cases, it is noticeable that there is a significant increase in the value of coercive field for the loops recorded at 5 K compared to loops measured at 340K, as shown in figure 12. For x = 0.025, 0.05, 0.75 and 1; Hc = 8.1, 10.5, 8.6 and 11.4kOe, respectively, recorded at 5 K. Decrease in the value of Hc for the lower values of x compared to bulk counterpart, again confirms superparamagnetic features exhibited by small particles which is a well-known phenomenon. In addition to that, for x = 0.025, 0.05, 0.75 and 1; Hc = 0.99, 0.92, 1.2 and 0.705kOe, respectively, recorded at 340K. Non-zero and relatively high value of coercive field for temperature above room temperatures (specifically 340K), confirm high anisotropy constant of CFO[50-52]. Second interesting feature in the magnetic behavior of the composites is

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lack of saturation, even up to very high field (~60kOe). The bulk counterpart has loops that are

relatively well saturated. This confirms that in the case of composites, small size of the CFO constituent have additional contribution emanating from the surface anisotropy [13, 53, 54]. Decrease in the value of saturation magnetization confirms low concentration of the magnetic component in the composites with x = 0.025, 0.05 and 0.75. Third noticeable feature in the loops is, at least an order of magnitude decrease in the value of the Hc recorded at 5K compared to the value measured 340K was observed. This shows strong temperature dependence of the anisotropy constant in CFO [12, 14].

Figure 12. M-H loops of (1-x)BTO-xCFO with increasing CFO recorded at two different temperatures, i.e. 50K and 340K, for (a) x = 0.025 (b) x = 0.05 (c) x = 0.075 and (d) x = 1. This is the case when BTO for composite synthesis was calcined at 1200°C. 22

It has been identified that high value of anisotropy plays vital role in the magnetic response of the CFO-BTO composites. Figure 13 shows magnetization versus temperature (M(T)) curves for x = 0.025, 0.05 and 0.75 along with pure sample, with x = 1. M(T) curves were recorded after cooling the sample from 340 K to 5K in zero field (ZFC) and under a cooling field (FC). Magnitude of the cooling field was 100Oe. It is well-known that the difference in these two branches (i.e. magnetic irreversibility) arises due to the presence of the anisotropy barrier associated with the spin glass phase [55-57]. This phase arises due the large surface of the nanoparticles which also yields anisotropic effect. As clear from the figures (13 a, b, c and d) that magnetic irreversibility is larger in the case of composites compared to the CFO bulk counterpart. This again confirms the presence of surface anisotropy due to the small size of CFO in composite samples. In addition, in case of the composite samples, irreversibility lies in the entire temperature range and the energy barrier is high compared to the energy provided by the field. Therefore, blocking/unblocking peak in the ZFC curve, lies well above the room temperature, which again is a manifestation of high anisotropy value [13, 14, 54, 58-61]. The detailed magnetic properties of pelletized nano-composites are presented. Main emphasis is given to the magnetic properties of this system. In the present study, peak broadening is the manifestation of strain exerted on BTO by CFO which confirms formation of composite with good mixing. Interesting features such as increase in coercivity and stronger temperature dependence of coercivity and magnetic irreversibility, have been exhibited by the composites. Order of magnitude increase in coercivity at low temperatures for composites has been observed. Hence, clear nanoscopic features are demonstrated to be present which also makes it a promising candidate which exhibits magnetoelectric coupling.

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Figure 13. FC/ZFC Magnetization versus temperature response of (1-x)BTO-xCFO composites with (a) x = 0.05, (b) x = 0.025, (c) x = 0.075 and (a) x = 1, measured in an applied field of 100Oe. 4. Summary and Conclusion The detailed magnetic properties of (1-x)BaTiO3–xCoFe2O4 nano-composites are presented. Comprehensive multiple step chemical synthesis and structural optimization of composites was also presented. Distinct spinel and tetragonal perovskite phases were achieved through post-synthesis thermal processing. BTO particles were first prepared before chemical mixing with CFO and final heat treatment was carried out on pelletized samples (i.e. sintering). For pure BTO before compaction and composite formation, XRD peaks were sharp, while in case of compacted composite, peak broadening has been observed. Peak broadness is the manifestation of strain exerted on BTO by CFO confirming formation of composite with good mixing of the both constituent phases. Magnetic response as a function of temperature for 24

various compositions were recorded and compared with the CoFe2O4 nanoparticles. It has been noted that the grain growth of the CoFe2O4 grain was prevented due to the phase separation between these constituent phases in the composites, where BaTiO3 acts as a barrier. In magnetic response, features such as increase in coercivity and stronger temperature dependence of coercivity and magnetic irreversibility, were exhibited by the composites. CoFe2O4 in the composite exhibited an order of magnitude increase in coercivity at low temperatures. Hence, clear nanoscopic features are demonstrated to be present in magnetic properties of the composites. Acknowledgment: GHJ acknowledges support of the Pakistan Higher Education Commission (HEC) for funding this research work (NRPU project number 8342).

Figure Captions Figure 1. Schematic flow chart diagram of (1-x)BTO-xCFO nano-composites synthesis. Figure 2 (a) XRD patterns of BTO nanoparticles annealed at 600°C to 1200°C for 2hrs. (b) Evolution of (110) and (200) cubic reflections into tetragonal (110), (011) and (200), (002) reflections. Figure 3. XRD patterns of BTO-CFO nano-composites sintered at 900°C for 2h with (a) BTO pre-calcined at 1000°C and with (b) BTO pre-calcined at 1200°C. ∗ and ♦ represent CFO and BTO XRD peaks. Figure 4. Rietveld refined XRD patterns of BTO nanoparticles calcined at (a) 600°C, (b) 700°C (c) 800°C (d) 900°C (e) 1000°C (f) 1100°C (g) 1200°C. Figure 5. Rietveld refined XRD patterns of (1-x)BTO-xCFO nano-composites with (a) x = 0, (b) x = 0.2, (c) x = 0.3, (d) x = 0.4, (e) x = 0.5, (a) x = 1. Figure 6. (a) Unit cell of BaTiO3 and (b) CoFe2O4 at x=0.5. Figure 7. Zoomed in region of (101), (110), (200) and (002) reflections. Figure 8. (a) M-H loops of CoFe2O4 nanoparticles, recorded at (a) 50K, 200K and 350K. (b) Variation of Hc and Ms as a function of temperature.

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Figure 9. Magnetic hysteresis for (1-x)BTO-xCFO with increasing CFO concentration, measured at (a) 50K (b) 200K (c) 300 K for BTO calcined at 1000°C and (d) 50K (e) 200K (f) 300K for BTO calcined at 1200°C. Figure 10. Coercivity and saturation magnetization versus compositions ( for x= 0.2, 0.3 0.4, 0.5 and 1) at three various temperatures i.e. for (a) 50K (b) 200K (c) 300K for BTO calcined at 1000°C and for (d) 50 K (e) 200 K (f) 300 K for BTO calcined at 1200°C. Figure 11. Coercivity and saturation magnetization as a function of temperature when BTO nanoparticles were pre-calcined at (a, b) 1000°C and (c, d) 1200°C. Figure 12. M-H loops of (1-x)BTO-xCFO with increasing CFO recorded at two different temperatures, i.e. 50 K and 340K, for (a) x = 0.025 (b) x = 0.05 (c) x = 0.075 and (d) x = 1. This is the case when BTO was calcined at 1200°C. Figure 13. FC/ZFC Magnetization versus temperature response of (1-x)BTO-xCFO composites with (a) x = 0.05, (b) x = 0.025, (c) x = 0.075 and (a) x = 1, measured in applied field of 100Oe.

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• • • • •

Multiple-step chemical synthesis of (1-x)BaTiO3–xCoFe2O4 nano-composites. Tetragonality variation through pre-mixing processing. Sintering led to observation of strain. Nanoscopic features observed in the magnetic properties e.g. Increase in Hc. Temperature dependence of coercivity and magnetic irreversibility.

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: