Influence of Co-substitution on the structural and magnetic properties of nanocrystalline Ba0.5Sr0.5Fe12O19

Author’s Accepted Manuscript Influence of Co-substitution on structural and magnetic properties of nanocrystalline Ba0.5Sr0.5Fe12O19 R. Ezhil Vizhi, V. Harikrishnan, P. Saravanan, D. Rajan Babu www.elsevier.com/locate/jcrysgro

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S0022-0248(16)30007-0 http://dx.doi.org/10.1016/j.jcrysgro.2016.01.026 CRYS23173

To appear in: Journal of Crystal Growth Received date: 23 September 2015 Revised date: 20 January 2016 Accepted date: 24 January 2016 Cite this article as: R. Ezhil Vizhi, V. Harikrishnan, P. Saravanan and D. Rajan Babu, Influence of Co-substitution on structural and magnetic properties of nanocrystalline Ba0.5Sr0.5Fe12O19, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.01.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Influence of Co - substitution on structural and magnetic properties of nanocrystalline Ba0.5Sr0.5Fe12O19 Ezhil Vizhi. R a* Harikrishnan. V a, Saravanan. P b and Rajan Babu. D a a- Materials Research Laboratory School of Advanced Sciences, VIT University, Vellore-632014 b- Defence Metallurgical Research Laboratory, Hyderabad, Telangana-500058 Abstract One-step citrate gel combustion method was employed to synthesize cobalt substituted barium strontium hexaferrite with a chemical formula of Ba0.5Sr0.5Fe12-xCoxO19 (x=0, 0.5, 0.7, 0.9) where all the samples were annealed at 800oC for 2 h. A Combination of thermo gravimetric analysis and differential scanning calorimetry was employed to understand the thermochemical behaviour of Ba0.5Sr0.5Fe12O19. X-ray diffraction was used to evaluate the hexagonal phase evolution for the samples, and showed the formation of secondary phase: αFe2O3 for Ba0.5Sr0.5Fe12O19. Raman spectroscopy confirmed the presence of different sublattices of Fe3+ present in the hexaferrite structure. Fourier transform infrared spectroscopy demonstrated the usual stretching vibrations of tetrahedral and octahedral M-O bands. The morphologies of the samples were analysed by field emission scanning electron microscopy and transmission electron microscopy along with energy dispersive x-ray analysis. Selected area electron diffraction showed the crystalline nature of the samples. Lattice fringes for the samples were also visible during the analysis. The magnetic parameters like saturation magnetization MS, coercivity HC and remanent magnetization MR where obtained from the hysteresis. Maximum MS of 70.5 emu/g was obtained for Ba0.5Sr0.5Fe11.5Co0.5O19. Finally the possible formation mechanism during the citrate gel combustion synthesis of Ba0.5Sr0.5Fe12O19 is being discussed in detail. Keywords: A1. X-ray diffraction, A1. Nanostructures, A1. Stirring, B1. Nanomaterials, B1. Oxides, B2. Magnetic materials.

1. Introduction M-type hexaferrite possesses the crystal structure of magnetoplumbite, a mineral with approximate composition of PbFe7.5Mn3.5Al0.5Ti0.5O19. These hexaferrites are available at low cost and possess large intrinsic coercivity HC, moderate saturation magnetization MS and high uniaxial magneto crystalline anisotropy constant. For these reasons, they are widely used in the production of electromechanical devices, electronic toys, microwave and high frequency devices [1]. M-type hexaferrites are denoted by MFe12O19, where M corresponds to barium or strontium. In order to manipulate the magnetic properties of Ba0.5Sr0.5Fe12O19, we need to closely understand its structure. The magnetism displayed by MFe12O19 can be considered due to the Fe3+ ion sublattices present in the structure. They are distributed in three octahedral (12 k, 2a, 4f2), one tetrahedral (4f1) and one bipyramidal sites (2b). From these sites, 12k, 2a, 2b are represented as the high spin states and 4f1 and 4f2 are considered as the low spin states [2]. The formation mechanism of SrFe12O19 can be largely divided into two intermediate processes. There is formation of SrFeO3-x and also Fe2O3 at first, then SrFeO3-X grows on the surface of Fe2O3. There is more probability of the formation of rodlike shapes rather than hexagonal platelets in the structure. [3]. Over the past several decades, considerable efforts have been made to process these hexaferrites through various synthesis strategies. Kanagesan et al., have reported the synthesis of Ba0.5Sr0.5Fe12O19 through sol-gel using D-fructose as a fuel and were able to obtain a MS of 46.48 emu/g and HC of 1.9 kOe [4]. Zhang Junli et al., have also reported the synthesis of Ba0.5Sr0.5Fe12O19 by chemical coprecipitation method and obtained MS of 51 emu/g and HC of 5.5kOe [5]. The reported values of MS have not exceeded 65 emu/g.

In order to obtain single phase M-type

hexaferrites, several techniques have been employed such as chemical coprecipitation method [6], solution combustion [7], solid state reaction method [8], citrate precursor method [9],

hydrothermal method [10] and sol gel [11]. The magnetic properties of hexaferrites are strongly correlated with the particle size, microstructure and morphology [1]. Along these lines, we herein exploit citrate gel combustion method to synthesise nanoparticles of Ba0.5Sr0.5Fe12-xCoxO19(x=0, 0.5, 0.7, 0.9). This method can be considered as one of the efficient methods to produce particles in the nanometer range with excellent homogeneity [12]. We also discuss the whole formation mechanism of the Barium strontium ferrite during the synthesis process. A higher combination with MS of 70.5 emu/g and HC of 4.2 kOe was achieved for Ba0.5Sr0.5Fe11.5Co0.5O19 annealed at 800oC for 2 h. 2. Experimental Sr(NO3)2, Ba(NO3)2, Fe(NO3)3.9H2O, Co(NO3)2.6H2O, ammonium solution and citric acid were acquired from Merck Co. (99% purity), Germany and were used without further treatment. Sr(NO3)2 (0.5M), Ba(NO3)2 (0.5M), Co(NO3)2.6H2O (0.5, 0.7, 0.9M), Fe(NO3)3.9H2O (12, 11.5, 11.7, 11.9 M) were precisely weighed according to the stoichiometric ratios desired. At first aqueous solution of Sr(NO3)2 and Ba(NO3)2 were prepared in a separate beaker. To this, aqueous solution comprising Fe(NO3)3.9H2O and Co(NO3)2.6H2O were added to the homogenous solution according to the stoichiometric calculation. The resulting solution was transferred to the citric acid solution and stirred for 2 h; the molar ratio of metal nitrates to citric acid was kept as 1:2. The pH of the solution was maintained to 7-8 by adding ammonia solution. The homogenous solution was subjected to heating at 80oC for 2 h. A greenish gel was formed which was subjected to combustion at 200oC. The resultant fluffy mass was subjected to heat treatment at 500oC for 5 h. Finally the samples were annealed at 800oC for 2 h. To ascertain the steps involved in the formation of Ba0.5Sr0.5Fe12O19, thermogravimetric analysis (TGA) combined with differential scanning calorimetry (DSC) of the dried sol gel

product was carried out by raising its temperature at a rate of 20 oC per minute up to 1000oC in a computer controlled instrument set up (NETZSCH STA model 449 F3A). X-Ray diffraction patterns of the samples were recorded on a BRUKER D8 Advance powder diffractometer using Cu-Kα (1.5406 Ǻ) radiation. Raman spectra were recorded with a 514 nm excitation source using a confocal scanning spectrometer (Renishaw InVIA) for the selected samples. Fourier transform infrared (FTIR) spectra were recorded using an FTIRShimazdu IR affinity spectrophotometer. Identification of particle morphology was done with a field emission scanning electron microscope (ZEISS) and high resolution transmission electron microscope (JEOL JEM 2100) attached with energy dispersive x-ray analysis (EDAX). The magnetic properties were studied with a maximum applied field of 2 kOe at room temperature (LAKESHORE 7407 instrument). 3. Results and Discussion The TGA analysis was carried out for the sample Ba0.5Sr0.5Fe12O19 shown in Fig. 1(a). A total weight loss of 80% was observed when the precursor gel was heated from 30o to 1000oC. Initially a weight loss of 8% was observed until 200oC which was attributed due to the continuous loss of moisture. The next significant weight loss, observed in the temperature range of 200oC to 400oC, was due to the oxidative decomposition of the precursors. The third major weight loss was observed between 400oC and 800oC due to the evolution of CO2 and nitrates. This was accompanied by an exothermic peak at 435oC in DSC, as shown in Fig. 1(b). The rate of weight loss was approximately constant and small after 450oC which confirmed the fact that full decomposition of the carbonaceous mass occurred below 450oC [13]. The XRD patterns of Ba0.5Sr0.5Fe12-xCoxO19 (x=0, 0.5, 0.7, 0.9) annealed at 800oC are shown in Fig. 2. All the diffraction peaks are perfectly indexed to M-type hexagonal structure

according to the standard JCPDS card no. 00-051-1879 with a space group of P63/mmc. A secondary peak of α-Fe2O3 was observed for x=0 which can be attributed to the fact that during the preparation process of Ba0.5Sr0.5Fe12O19, a mass of Ba and Sr ions will be lost due to thermal decomposition, resulting in surplus Fe3+ ions. The lattice parameters a and c for each XRD pattern were calculated (see Table 1) by the following relation [14]. (

)

(1)

where (h k l) are the Miller indices, d is the interplanar spacing. The values of a and c obtained for the unsubstituted sample, 5.886 Ǻ and 23.15 Ǻ respectively, are nearly equal to the earlier reported values for Ba0.5Sr0.5Fe12O19 [5]. By substituting Co2+, the lattice parameter a shows very little variation while the value of ‘c’ slightly decreases with the increasing concentration of Co. This variation is due to the smaller ionic radii of Co2+ which substitute the slightly larger Fe3+ ions in the hexagonal lattice. It supports the fact that Co2+ ions simply replaced the Fe3+ ions without distortion in the hexagonal lattice. The crystallite sizes of the samples were evaluated (see Table 1) using the well-known Scherrer equation D=0.9λ/βCosθ, where λ is the wavelength of the X-Ray radiation, θ being the diffraction angle, β being the full width at half maximum of an observed peak. Another important factor in deciding the validation of M-Type structure is the c/a ratio. It is generally observed that the c/a ratio is smaller than 3.98 for magnetoplumbites and above this value for β-alumina (Is that where [16] should be positioned?). It is to be noted that the c/a parameters obtained in the present study are indicative of the magnetoplumbite structure [16]. Raman spectra were taken basically to analyze the presence of the different sublattices of Fe3+ in the M-Type hexaferrites. As discussed in the earlier section, M-Type hexaferrites consist of five different sublattices, 2b representing the bipyramidal layer, 4f2, 2a and 12k

representing the octahedral lattice sites. All the sublattices were identified from the Raman spectra taken for x = 0, 0.5, 0.7 and are shown in Fig. 3. A band near 682 – 687 cm-1 represented the 2b sublattice with bipyramidal structure. The bands representing the mixed lattices of 2a and 12k (octahedral) were only visible for x = 0, 0.5. 4f2 sublattice was represented by the bands ranging from 616 – 618 cm-1. The whole spinel blocks were represented by the bands ranging from 177- 179 cm-1 [17]. The FT-IR spectra of the samples for the frequency ranging from 4000 cm -1 to 400 cm-1 are shown in Fig. 4 (a). The peaks from 1000 cm-1 to 400 cm-1 are also shown in fig 4 (b) for viewing the M-O stretching bands clearly. Ferrites bands occur due to tetrahedral and octahedral complexes. The bands are assigned to the vibration of ions present in the crystal lattices, caused by the tetrahedral and octahedral Fe3+–O2− distances. Ferrites should possess two important bands, ν1 assigned to the tetrahedral with high wave number and ν2 assigned to the octahedral clusters [18]. Very weak satellite peaks can be noticed at 426 and 407 cm−1, these peaks are assigned to the vibrations of metal ion– oxygen complexes in the tetrahedral (ν1) and octahedral (ν2) sites respectively. For x = 0.9, Co-O anti-symmetric stretching can be observed which is attributed to the interaction of Co2+ and Fe3+ ions resulting in the formation of a secondary phase of CoFe2O4 [15]. The field emission scanning electron microscope (FE-SEM) micrographs of the samples annealed at 800oC are shown in Fig. 5. The micrographs resembled hexagonal platelet nanocrystals with random orientations. The particles are ultrafine and are closely packed. As the annealing temperature is 800oC, there is always a tendency of the particles to join together and constitute larger particles with hexagonal shape and this was evidently realized in the samples. The transmission electron microscope (TEM) micrographs of Ba0.5Sr0.5Fe12O19 and Ba0.5Sr0.5Fe11.1Co0.9O19 are shown in Figs. 6 (a) and (b), as well as selected area electron diffraction (SAED) patterns. The micrographs clearly indicate the formation of hexagonal

platelets and hexagonal rod-like morphologies. The particle sizes are also indicated in the TEM micrographs, which show that there is a decrease in the particle size as Co 2+ is substituted in the Ba0.5Sr0.5Fe12O19. The periodicity of the lattice fringes across the nanoparticle’s thickness of 0.26 nm indicates that the atomic rows are (114) planes. The SAED pattern shows the crystalline nature of the samples with higher particle size for Ba0.5Sr0.5Fe12O19 and lower particle size for Ba0.5Sr0.5Fe11.9Co0.9O19 which is in good agreement with the crystallite size calculations from the XRD patterns. Energy dispersive XRay analysis (EDAX) technique has been adopted for determination of composition for Ba0.5Sr0.5Fe12O19 and Ba0.5Sr0.5Fe11.1Co0.9O19 .The EDAX spectrum of Ba0.5Sr0.5Fe12O19 and Ba0.5Sr0.5Fe11.1Co0.9O19 is shown in Fig. 7 and is in good agreement with the molarity of the elements used during the synthesis of the samples. The M-H loops of Ba0.5Sr0.5Fe12-xCoxO19(x = 0, 0.5, 0.7, 0.9) were measured up to an applied field of 2 T and are shown in fig 8. From these loops the values of saturation magnetization (MS), remanent magnetization (MR), coercivity (HC) and squareness ratio for all the samples were obtained and these values are provided in table 1. It has to be noted that the relative preference of FeCo substitution in the different sites of Fe3+ ions in hexaferrites leads to the variation of magnetic parameters. There are five different sites for Fe3+ in hexaferrites: mainly 2a, 2b, 12k which are considered as spin up states and 4f1, 4f2 which are considered as spin down states. In addition it can be observed that the MS value is maximum for x= 0.5 which can be explained by the site preference of Co2+ ions at low concentrations in the 4f1 and 4f2 sublattice of Fe3+ ions. This in turn decreases the negative magnetic moment of Fe3+ resulting in the increase in value of MS. For x = 0.7, 0.9, decrease in the value of MS can be seen which can be attributed due to the site preference of substitution of Co2+ in the 2a and 12k sublattice of Fe3+ ions at higher concentrations [15]. The continuous decrease in the

values of HC with x can be explained on the basis of the decrease in the particle size which was evident from the TEM analysis. Formation mechanism of barium strontium ferrites and the role of Co substitution The Fe2O3 structures are classified into three types α, γ and ε. α-Fe2O3 possesses orthorhombic hexahedron structure and inhibits the growth of MFe12O19. There are two types of reaction mechanisms suggested for the formation of MFe12O19. The first reaction mechanism as explained by Junli Zhang et al. for the synthesis of BaFe 12O19 is that Fe2O3 surrounds the intermediate BaFe2O4 during the calcination process and annealing at higher temperature, which results in the formation of BaFe12O19 [16]. However another formation mechanism of SrFe12O19 is explained by D.Y Chen et al., that there is always an interaction between the two intermediate spherical phases of SrFeO3-δ and Fe2O3 in order to form either rod-like shapes or hexagonal shapes [3]. In the present study, an attempt has been made to focus on the formation mechanism of Ba0.5Sr0.5Fe12-xCoxO19 (x = 0, 0.5, 0.7, 0.9) based on the other formation mechanisms discussed earlier. An insight has also been made into the substitution of Co2+ for the Fe3+ ions of Ba0.5Sr0.5Fe12O19. Based on the FE-SEM and HRTEM micrographs it is clear that rods and platelets with random orientations have been formed in the present case. The Fe2O3 grains accumulate on the edges of Ba0.5Sr0.5Feo3-δ and tend to form a rod as depicted in Fig. 9. For the unsubstituted sample, hexagonal platelets are in greater number than rods as seen from the TEM micrographs. For x = 0.9, conversion from hexagonal platelets to rods is seen which may be attributed to the variation of the lattice parameter ‘c’ evident from the XRD analysis. Conclusion Co2+ ion substituted Ba0.5Sr0.5Fe12O19 nanoparticles have been synthesised successfully by citrate gel combustion method. The TGA analysis showed three steps of decomposition in

the gel which were attributed to the loss of moisture, oxidative decomposition and also the evolution of CO2 and nitrates. This was accompanied by a sharp endothermic peak at 435 oC. The X-Ray diffraction reveals the formation of M-type hexagonal structure with space group of p63/mmc for x = 0.5, 0.7, 0.9 and for x = 0, a secondary phase of α-Fe2O3 is seen. The FTIR spectra showed a Co-O stretching for x = 0.9 which was due to the formation of secondary phase of CoFe2O4 evident from the XRD patterns. The morphology of the samples were analysed by high resolution FE-SEM and TEM micrographs. The micrographs showed hexagonal platelets nanocrystals for unsubstituted Ba0.5Sr0.5Fe12O19 and some of the grains were of rod shape for the substituted hexaferrites. Interplanar distances from the lattice fringes showed the presence of (114) plane in the nanocrystals. The grain size of the nanocrystals decreased with increase in concentration of Co2+ ions. This was evident from the slight decrease of the crystallite size from the XRD analysis and particle size measurements from the TEM micrographs. The average particle sizes for Ba0.5Sr0.5Fe12O19 and Ba0.5Sr0.5Fe11.1Co0.9O19 were 42 nm and 25 nm respectively obtained from the TEM analysis. The magnetic properties at room temperature revealed the fact that there are certain preference sites for Co2+ ions at lower and higher concentrations. For x= 0.5, Co2+ ions were substituted in the 4f1 and 4f2 sublattices of Fe3+ ions. However for x= 0.7, there was partial substitution of Co2+ in the 2a and 4f1 sublattices. At higher concentration of Co2+, there was a complete substitution of Co2+ ion in the 2a, 2b sublattices of Fe3+ ions. Maximum values of MS 70 emu/g and HC of 4.2 kOe were obtained for x = 0.5. Based on the formation of hexagonal platelets and rods evident from the FE-SEM and TEM micrographs, a formation mechanism was explained. This explained? the fact that increasing substitution of Co2+ for Fe3+ ions in Ba0.5Sr0.5Fe12O19 led to decreased particle size. XRD analysis hinted at the decrease in crystallite size; this was substantiated by the TEM analysis. The present study

paves the way for obtaining high values of MS and HC by substituting Co2+ ions in Ba0.5Sr0.5Fe12O19 in optimal amount. Acknowledgements The authors are thankful to the management, VIT University for their constant support and encouragement. One of the authors R. Ezhil Vizhi thanks the organizers of ACCGE 20 and the management of VIT University for providing financial assistance to attend the conference. Thanks to DST SERB for the travel grant provided. References [1]

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Figure 4 (a) FTIR of all the Co substituted barium strontium ferrites from 4000 – 400 cm-1 & (b) from 1000-400 cm-1. Figure 5 FE-SEM micrographs of (a) Ba0.5Sr0.5Fe12O19 (b) Ba0.5Sr0.5Fe11.5Co0.5O19 (c) Ba0.5Sr0.5Fe11.7Co0.7O19 (d) Ba0.5Sr0.5Fe11.1Co0.9O19. Figure 6 (a) TEM micrograph of Ba0.5Sr0.5Fe12O19 and the HR-TEM image showing the lattice fringes, inset shows the SAED pattern (b) TEM micrograph of Ba0.5Sr0.5Fe11.1Co0.9O19 and the HRTEM image showing the lattice fringes, inset showing the SAED.

Figure 7 EDAX patterns of (a) Ba0.5Sr0.5Fe12O19 and (b) Ba0.5Sr0.5Fe11.1Co0.9O19. Figure 8 Room temperature magnetic hysteresis loops of samples annealed at 800oC for 2 h. Figure 9 Formation mechanism of Ba0.5Sr0.5Fe12O19 starting from the combustion process. Table captions Table 1 Crystallite size, Lattice constants, c/a ratio, volume and Magnetic parameters of Ba0.5Sr0.5Fe12-xO19 (x = 0, 0.5, 0.7, 0.9) annealed at 800oC.

Composition

c/a ratio

Volume (Ǻ3)

MS emu/g

MR emu/g

HC kOe

R= (MR/MS)

23.15

3.933

694

64.6

35.2

4.9

0.54

5.884

23.10

3.925

692

70.5

38.1

4.2

0.54

38.1

5.879

23.14

3.936

692

68.9

36.6

3.6

0.53

35.2

5.883

23.06

3.919

693

62.9

31.1

2.9

0.49

XRD Crystallite size (nm)

Lattice constant a(Ǻ)

c(Ǻ)

Ba0.5Sr0.5Fe12O19

43.9

5.886

Ba0.5Sr0.5Fe11.5Co0.5O19

40.2

Ba0.5Sr0.5Fe11.3Co0.7O19 Ba0.5Sr0.5Fe11.1Co0.9O19

Table 1 Crystallite size, Lattice constants, c/a ratio, volume and Magnetic parameters of

Ba0.5Sr0.5Fe12-xCoxO19 (x = 0, 0.5, 0.7, 0.9)

(a)

(b)

Fig. 1 (a) TGA curves of barium strontium ferrite (b) DSC of barium strontium ferrites

Fig. 2 XRD patterns of Ba0.5Sr0.5Fe12-xCoxO19 (x = 0, 0.5, 0.7, 0.9)

Fig. 3 Raman spectra of samples annealed at 800oC for 2 h

(a)

(b)

Fig. 4 (a) FTIR of all the Co substituted barium strontium ferrite from 4000 – 400 cm-1 & (b) from 1000-400 cm-1.

100 nm

100 nm

100 nm

100 nm

Fig. 5 FE-SEM micrographs of (a) Ba0.5Sr0.5Fe12O19 (b) Ba0.5Sr0.5Fe11.5Co0.5O19 (c) Ba0.5Sr0.5Fe11.7Co0.7O19 (d) Ba0.5Sr0.5Fe11.1Co0.9O19.

(a)

42 nm

(b)

Fig. 6 (a) TEM micrograph of Ba0.5Sr0.5Fe12O19 and the HR-TEM image showing the lattice fringes, inset shows the SAED pattern. (b) TEM micrograph of Ba0.5Sr0.5Fe11.1Co0.9O19 and the HR-TEM image showing the lattice fringes, inset showing the SAED pattern.

Fig. 7 EDAX pattern of (a) Ba0.5Sr0.5Fe12O19 and (b) Ba0.5Sr0.5Fe11.1Co0.9O19

Fig. 8 Room temperature magnetic hysteresis loops of samples annealed at 800oC for 2 h.

Fig. 9 Formation mechanism of Ba0.5Sr0.5Fe12-xCoxO19 (x = 0, 0.5, 0.7, 0.9) starting from the combustion process Highlights    

Co2+ substituted Ba0.5Sr0.5Fe12O19 with randomly oriented rods and platelets were synthesised Particle sizes of 25 nm were achieved for Ba0.5Sr0.5Fe11.1Co0.9O19 analysed from TEM Maximum MS of 70 emu/g and HC of 4.2 kOe at room temperature for Ba0.5Sr0.5Fe11.5Co0.5O19 was obtained Formation mechanism of Ba0.5Sr0.5Fe12-xCoxO19 (x = 0, 0.5, 0.7, 0.9)during annealing process has been newly proposed