A study on the extent of exchange coupling between (Ba0.5Sr0.5Fe12O19)1−x(CoFe2O4)x magnetic nanocomposites synthesized by solgel combustion method

Author’s Accepted Manuscript A study on the extent of exchange coupling between (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x magnetic nanocomposites synthesized by solgel combustion method V. Harikrishnan, R. Ezhil Vizhi www.elsevier.com/locate/jmmm

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S0304-8853(16)30233-5 http://dx.doi.org/10.1016/j.jmmm.2016.03.037 MAGMA61263

To appear in: Journal of Magnetism and Magnetic Materials Received date: 30 November 2015 Revised date: 7 March 2016 Accepted date: 8 March 2016 Cite this article as: V. Harikrishnan and R. Ezhil Vizhi, A study on the extent of exchange coupling between (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x magnetic nanocomposites synthesized by solgel combustion method, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.03.037 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.

A Study on the extent of exchange coupling between (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x magnetic nanocomposites synthesized by solgel combustion method V. Harikrishnan and R. Ezhil Vizhi* Materials Research Laboratory Department of Physics School of Advanced Sciences, VIT University, Vellore-632014, India *

[email protected], [email protected]. Tel: +91 416 2202358 , Fax: +91 416 2343092

Abstract One step citrate gel combustion method followed by high temperature annealing was employed for preparing (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x (x = 0.1, 0.2, 0.3) composite ferrite powders. The powders were subjected to annealing at 800oC in order to decisively study the phase evolution of the combined hard and soft ferrites. Thermogravitry (TGA) / differential scanning calorimetry (DSC) analysis exhibited three stages of decomposition in the precursor gels combined with an exothermic peak at 210oC. X-ray diffraction (XRD) analysis confirmed that the diffraction peaks were perfectly indexed to the hexagonal magnetoplumbite structure of Ba0.5Sr0.5Fe12O19 and the cubic spinel structure of CoFe2O4. Fourier transform infrared spectroscopy (FT-IR) analysis for the samples showed a Co-O stretching vibration accompanied with Co-O-Co or Fe-O-Fe bands at 1220 cm-1. The morphology of the samples were examined by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The crystallinity of a selected sample was evaluated by using the high resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) pattern. It confirmed the presence of planes comprising the hard and soft phases in the synthesised nanocomposites. The magnetic parameters like saturation magnetization MS, remanent magnetization MR, squareness ratio SR, coercivity HC and magnetic moment µB were evaluated using hysteresis by employing vibrating sample magnetometer (VSM). Maximum HC of 4.7 kOe and MS of 60.4 emu/g were obtained for (Ba0.5Sr0.5Fe12O19)0.9(CoFe2O4)0.1. Switching field distribution curves were analysed by using the demagnetization curve. The exchange coupling between the hard and soft phases were analysed by the dM/dH plots and it indicated the exchange coupling first increased with the increase in the concentration of spinels and then decreased. The possible comparison of exchange coupling between the hard and soft phases has been discussed in detail. Keywords: Composite materials, magnetoplumbite, sol gel auto combustion, exchange coupling

1. Introduction Magnetic nanocomposites have been receiving continuous attention due to their applications in permanent magnets, magnetic recording devices, sensors and ferro fluids [1,2]. Kneller and

Hawig had proposed the concept of nanocomposite magnets which was formed by the combination of exchange coupling effect between the hard and soft magnetic phases in the early 1990s [3]. They proposed that these composite magnets would possess high coercivity of hard magnetic phase and high saturation magnetization of the soft magnetic phase thus aiding excellent performance in permanent magnet industry. Recently, a number of studies on the microstructure for hard and soft exchange – coupled systems have been reported. Radmanesh et al., reported the magnetic composites consisting of SrFe12O19 and Ni0.7Zn0.3Fe2O4 by combining different weight fractions of both the synthesised powders [4]. Subhenjit Hazra et al., reported a novel one pot synthesis of composite powders consisting of Mn0.2Ni0.4Zn0.4Fe2O4 and BaFe12O19 [5]. Miao Liu et al., reported the synthesis of Ba0.5Sr0.5Fe12O19/Y3Fe5O12 using one step citrate gel combustion method [6]. Haibo Yang et al., reported the synthesis of BaFe12O19/CoFe2O4 using one step citrate gel combustion method and showed the fact that there was an enhancement in (BH)max[7]. They also substantiated the fact that composite powders synthesised by one step citrate gel combustion method exhibited good exchange coupling between the hard and soft phases. For composite particles and traditional nanofibres, contact area between the aggregated particles is limited so that cannot be sufficiently exchange coupled [8]. M type hexaferrites consisting of BaFe12O19 and SrFe12O19 has gained lot of importance in the commercial industries owing to their useful hard magnetic properties such as large saturation magnetization MS,

high

coercivity HC, high curie temperature TC, large uniaxial magnetic anisotropy and good chemical stability [9]. As far as soft magnetic phases are concerned, CoFe2O4 has gained a lot attention owing to their low eddy current losses [10]. Owing to these factors, a composite material with the combination of hexaferrites and spinel ferrites will pave way for the material in variety of applications like magnetic recording media, memory devices and permanent magnets.

Generally physical mixing method is employed to synthesize composite materials, where two different phases are synthesised individually and then mixed in certain fixed ratios. These are then subjected to grinding for several hours. Many reports have already come into existence for the physical mixing and wet chemical methods for the synthesis of nanocomposites with ferrite phases [11 - 14]. Among the wet chemical methods two step chemical coprecipitation, one step solgel combustion method are employed to synthesize the composite magnets. The present work relies on one step citrate gel combustion method to synthesize nanocomposites with the formula (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x (x = 0.1, 0.2 ,0.3). Switching field distribution curves were employed to analyse the exchange coupling interaction between the hard and soft phases of the magnetic nanocomposites. As a continuation of the present work, the synthesised magnetic nanocomposites will be subjected to spark plasma sintering for permanent magnet application in due course. 2. Experimental (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x nanocomposite powders were synthesized by a one-step citrate sol–gel method. x is the mass ratio of CoFe2O4 and was equal to 0.1, 0.2, 0.3 and 0.4.Sr(NO3)2, Ba(NO3)2, Fe(NO3)3.9H2O, Co(NO3)2.6H2O, ammonium solution and citric acid were acquired from Merck Co., Germany and were used without further treatment. Sr(NO3)2, Ba(NO3)2, Co(NO3)2.6H2O, Fe(NO3)3.9H2O were precisely weighed according to the stoichiometric ratio. Aqueous solutions of Sr(NO3)2, Ba(NO3)2, and Fe(NO3)2.9H2O were prepared in de-ionized water and simultaneously Co(NO3)2.6H2O was added to the homogenous solution according to the stoichiometric calculations. The molar ratio of metal nitrates to citric acid was kept as 1:2. Then the complete solution was transferred to the citric acid solution and stirred for 2h. The pH of the solution was maintained to 7-8 by adding ammonia. The homogenous solution was subjected to heating on a hotplate at 80oC for 2h. At the end greenish gel is formed which is further heated and this gel is transferred to powdered

sample by auto- combustion. The powders were subjected to heat treatment at 500oC for 5h. The resultant fine powders were subjected to annealing at 800oC for 2h. X-Ray diffraction (XRD) (BRUKER D8 advance Cu-Kα (1.5406 Ǻ) radiation) was used to analyse the phase evolution of the magnetic nanocomposites. Fourier transform infrared (FTIR) was recorded using FTIR-Shimazdu IR Affinity spectrophotometer. A field emission scanning electron microscope (FE-SEM) (QUANTA FEG) was analysed by for investigating the morphology of the samples. High resolution transmission electron microscope (HR-TEM) (JEOL JEM 2100) was employed to analyse the particle size for elected sample. The magnetic properties at room temperature were obtained using vibrating sample magnetometer (Lakeshore 7407) with maximum applied field 2 kOe. 3. Results and Discussion The thermal decomposition of the precursor gels were analysed by employing the thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Fig. 1 shows the thermogram of the precursor powder. A total weight loss of 83% was observed when the precursor gel was heated from 30oC to 900oC. Initially a weight loss of 46% was observed up to 210oC which may be attributed due to the continuous loss of moisture [13].A 25% weight loss was found between 210oC and 600oC which may be attributed to the oxidative decomposition of the precursor gel [14]. Then 12% weight loss was found between 620oC and 800oC which may be attributed due to the evolution of CO2 and NOx gasses [5]. There was no weight loss observed after 800oC, which established the fact that this temperature was suitable for the formation of phases of both the Ba0.5Sr0.5Fe12O19 and CoFe2O4. These results are substantiated by the XRD patterns of the sample annealed at 800oC. DSC (fig. 1 (inset)) of the precursor gel showed a distinct exothermic peak at 200oC.

XRD patterns of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x (x=0.1, 0.2, 0.3) nanocomposites shown in fig. 2 exhibited the diffraction peaks corresponding to both spinel Ba0.5Sr0.5Fe12O19 (JCPDS 51-1879) and CoFe2O4 (22-1086). In addition to these diffraction peaks, low intensities of αFe2O3 were found which inhibit the formation of Barium hexaferrites. A relative variation in intensities of (311) plane for CoFe2O4 are observed from the XRD patterns. The diffraction peaks of CoFe2O4 become strengthened gradually with increasing the concentration as reported by Haibo Yang et al., [6]. For x = 0.4, annealed at 800oC did not match with the JCPDS of both hard and soft phases. As a result of this, the results for x = 0.4 has not been concentrated in the present work. The average crystallite sizes of Ba0.5Sr0.5Fe12O19 and CoFe2O4 phases in (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x nanocomposites were calculated by X-ray peak broadening method using Scherrer’s equation and listed in table 1. For Ba0.5Sr0.5Fe12O19 phase, the diffraction peak at 2θ = 34.1o which corresponds to (114) plane and for CoFe2O4 phase, diffraction peak 2θ = 35.5o corresponding to (311) plane were used. The crystallite size of Ba0.5Sr0.5Fe12O19 phase decreased when compared with the parent structure. However the same trend was not observed in the case of CoFe2O4, the crystallite size of the pure CoFe2O4 was greater than that of the crystallite size of CoFe2O4 phases in the nanocomposite. An increase of crystallite size was observed for x = 0.3 for the CoFe2O4 phase which may be attributed due to the fact that there was a domination of the spinel phase over the M-type phase in the particular composition. Fig. 3 shows the FT-IR spectra of the (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x with x= 0.1,0.2,0.3. The phase formation of Ba0.5Sr0.5Fe12O19 and CoFe2O4 were already discussed in XRD patterns, FT-IR of nanocomposite powders were shown in fig. 3. The spectra reveal the absorption bands at 1367, 1220, 577, 538 and 421 cm-1. The absorption bands at 1367 cm-1 corresponds to the Co-O stretching vibration due to the incorporation of the good exchange coupling between Co2+ and Fe3+ ions [15]. The absorption bands around 1220 cm-1 can be

ascribed to the M-O-M such as Co-O-Co or Fe-O-Fe as reported by C. Sudakar et., [16]. FT-IR spectra were employed to study the two broad bands which are the characteristic bands of ferrites and usually occur due to the tetrahedral and octahedral complex. They are assigned due to vibration of ions present in the crystal lattice arising due to the stretching of tetrahedral and octahedral clusters (Fe3+-O2-) The bands around 577, 538, 421 cm-1 are due to the metal oxygen stretching vibrations of hexaferrites as reported in [7, 16-18]. The surface morphologies of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x with x = 0.1, 0.2, 0.3 annealed at 800oC were studied using FE-SEM are shown in fig. 4. The formation of irregular platelets (fig. 4a) reveals the fact that there is an incomplete formation of Ba0.5Sr0.5Fe12O19 platelets. The formation of circular grains is less seen in fig. 4(a) which may be due to the low concentration of CoFe2O4 phase. A distinct change in the grain morphology can be seen for (Ba0.5Sr0.5Fe12O19)0.8(CoFe2O4)0.2 and (Ba0.5Sr0.5Fe12O19)0.7(Co Fe2 O4)0.3 in fig. 4(b) and 4(c). There is clear evidence in the formation of circular grains as seen in fig. 4(b) and 4(c) representing CoFe2O4 phase in the nanocomposites. The transmission electron microscope (TEM) of (Ba0.5Sr0.5Fe12O19)0.7(CoFe2O4)0.3 are shown in fig. 5 (a). The high resolution transmission electron micrographs (HRTEM) and the selected area electron diffraction (SAED) are also shown in fig. 5 (b) and (c). The micrographs clearly indicate the formation of hexagonal plates resembling the hard phase and circular grains representing the soft phase with the particle size indicated in the fig. 5(a). The periodicity of the lattice fringes across the nanoparticle’s thickness were found in two different regions with 0.18 nm representing the hard phase (1 1 10) and 0.25 nm representing the soft phase (311). The SAED pattern indicates the crystalline nature of the sample. The magnetic properties of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x nanocomposites are compared with Ba0.5Sr0.5Fe12O19 and CoFe2O4. The room temperature magnetic hysteresis of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x (x = 0, 0.1, 0.2, 0.3, 1) are shown in fig. 6. The magnetic

properties are listed in table 2. A maximum HC of 5.1kOe was noticed for Ba0.5Sr0.5Fe12O19 and MS of 58.4 emu/g. The observed pattern is that the value of HC decreases with the increase in the content of CoFe2O4 as reported by Haibo Yang et al., [6]. However the value of MS increased to 60.4 emu/g for x = 0.1. This may be attributed due to the spring exchange coupling between hard and soft magnetic phases. The most important observation seen was that the hysteresis exhibited a single loop. The MS is slightly decreased for x = 0.2, this may be attributed due to the decrease in the values of magnetic moment of the whole system which is shown in table 2. The MS value of x = 0.2 is almost comparable with x = 0.3. The demagnetization curves are depicted in fig. 7 (a). Smooth demagnetization curves of the hysteresis loops establish the fact that there was good exchange coupling between the soft and hard magnetic phases. The switching field distribution curve (fig. 7 (b)) always gives a good account for the exchange coupling. There is appearance of peak (fig. 7 (c)) at higher fields for x = 0.1 & 0.2 which indicates the degree of exchange coupling interactions. For x = 0.3, there is splitting of peaks which shows less exchange coupling when compared with that of x = 0.1, 0.2. Moreover the low intensity of peaks further confirms the fact that there was higher degree of exchange coupling between the hard and soft magnetic materials. Exchange coupling interactions were explained in terms of the grain sizes of the soft and hard phases in magnetic nanocomposites. The random anisotropy model describes about the random anisotropy constant of soft grain as [19].

(1)

Where K1 is the first anisotropy constant of material, D is the diameter of grain, Lex is the range of the exchange-coupling interaction between the grains of hard and soft magnetic phases ie., the exchange length. According to Radmanesh et al., [20] this exchange length is defined as

(2)

where A is the exchange stiffness. From the above relation, it is clear that larger grain size reduces the exchange length. In the present case, the crystallite size is comparable for plane (114) but for plane (311), there is increase with the value of x. Hence there is probability of decrease in the exchange length between the hard and soft phases which in turn decreases the values of MS and are substantiated by the hysteresis. Conclusion One step citrate gel combustion method has been utilized to synthesize magnetic nanocomposites with the chemical formula (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x for x= 0, 0.1, 0.2, 0.3, 1. The TG/DSC analysis showed that the precursor gels contained three stages of decomposition attributed due to the loss of moisture, oxidative decomposition and evolution of CO2 and NOX gasses. XRD patterns confirmed the presence of Ba0.5Sr0.5Fe12O19 and CoFe2O4 phases. The crystallite sizes were evaluated for two different planes corresponding to the plane (311) for CoFe2O4 and plane (114) for Ba0.5Sr0.5Fe12O19. The FT-IR of the samples showed the formation of tetrahedral and octahedral M-O confined in the ferrite structure. FE-SEM micrographs of the samples showed the formation of circular and platelet shapes signifying the Ba0.5Sr0.5Fe12O19 and CoFe2O4 phases as the concentration of spinels increased. TEM micrographs of (Ba0.5Sr0.5Fe12O19)0.7(CoFe2O4)0.3 showed the clear formation of hexagonal plates and circular grains. The distance between the lattice fringes from the High resolution electron microscope micrographs were found to be 0.18 nm corresponding to the (1 1 10) plane of hard phase and 0.25 nm corresponding to the (311) plane of the soft phase. SAED pattern demonstrated the crystallinity of the sample. Room temperature hysteresis of the sample exhibited a highest MS of 60.4 emu/g and HC 4.7 kOe for x = 0.1. Switching field distribution curve showed smooth demagnetization curves. The dM/dH

curves showed the peaks near the maximum fields which confirmed the fact that there was good exchange coupling between the hard and soft phases. But however for x =0.3, there was split in the dM/dH peak. The exchange coupling for x= 0.3 was less when compared with that for x = 0.1, 0.2. This study signifies the fact that by introducing soft CoFe2O4 in the hard ferrite can lead to a good exchange coupling and there by tuning the magnetic properties of hard ferrites. Acknowledgements The authors are thankful to the management of VIT University, Vellore for their constant support, financial assistance to carry out this work and the characterization facilities provided through VIT-SIF and DST-FIST. Authors thank Dr. P. Saravanan, DMRL, Hyderabad for the discussions on magnetic characterizations. References [1]

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List of tables Table 1. Crystallite sizes for two different planes for Ba0.5Sr0.5Fe12O19 and CoFe2O4. Composition

Ba0.5Sr0.5Fe12O19 (Ba0.5Sr0.5Fe12O19)0.9(CoFe2O4)0.1 (Ba0.5Sr0.5Fe12O19)0.8(CoFe2O4)0.2 (Ba0.5Sr0.5Fe12O19)0.7(CoFe2O4)0.3 CoFe2O4

Plane (311) CoFe2O4 (nm) 22 24 57 40

Plane (114) Ba0.5Sr0.5Fe12O19 (nm) 44 39 40 36 -

Table 2. Magnetic parameters of samples from the hysteresis. Composition

Saturation Magnetization, MS (emu/g)

Coercivity HC, (kOe)

Squareness ratio, SR

Magnetic moment, µB

58.4 60.4

Remanent Magnetiza tion MR (emu/g) 31.4 34.7

Ba0.5Sr0.5Fe12O19 (Ba0.5Sr0.5Fe12O19)0.9 (CoFe2O4)0.1 (Ba0.5Sr0.5Fe12O19)0.8 (CoFe2O4)0.2 (Ba0.5Sr0.5Fe12O19)0.7 (CoFe2O4)0.3 CoFe2O4

5.1 4.7

0.53 0.57

11.355 10.824

54.9

29.3

3.6

0.53

9.004

56.9

31.9

3.3

0.56

8.467

80.2

29.9

0.7

0.53

3.366

Research Highlights  Magnetic nanocomposites comprising hard and soft phases were synthesised by citrate gel combustion method  TEM micrographs exhibited the presence of both hexagonal and circular grains for selected sample  Maximum MS of 60.4 emu/g and HC of 4.7 kOe were obtained for nanocomposites  Exchange coupling was analysed between the soft and hard phases

Figure

Fig 1. TGA of (Ba0.5Sr0.5Fe12O19)0.9(CoFe2O4)0.1 precursor gel and the inset shows the DSC of the precursor gel

Fig 2. X-ray diffraction patterns of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x (x = 0, 0.1, 0.2, 0.3, 1) annealed at 800oC

Fig 3. FT-IR of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x with x = 0.1, 0.2 ,0.3 annealed at 800oC

Fig 4. FE-SEM micrographs of (a) (Ba0.5Sr0.5Fe12O19)0.9(CoFe2O4)0.1 (b) (Ba0.5Sr0.5Fe12O19)0.8(CoFe2O4)0.2 (c) (Ba0.5Sr0.5Fe12O19)0.7(CoFe2O4)0.3

Fig 5.(a), (b), (c) represents the TEM, HRTEM and SAED pattern of (Ba0.5Sr0.5Fe12O19)0.7(CoFe2O4)0.3 respectively.

Fig 6. Hysteresis of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x with (x = 0, 0.1, 0.2 , 0.3, 1) samples

annealed at 800oC for two hours with maximum applied field of 2kOe.

Fig.7.(a) represents the demagnetization curves (Ba0.5Sr0.5Fe12O19)x(CoFe2O4)1-x (x= 0, 0.1, 0.2, 0.3, 1).

of

hysteresis

loops

for

(b) depicts the effect of CoFe2O4 concentrations on the switching field distribution (SFD) (dM/dH of demagnetizing curves) (c) represents the switching field distribution curve of (Ba0.5Sr0.5Fe12O19)1-x(CoFe2O4)x(x = 0.1, 0.2, 0.3)