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Fe3O4 nanocomposites
Author’s Accepted Manuscript Exchange spring magnetic behavior BaFe12O19/Fe3O4 nanocomposites
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K.P. Remya, D. Prabhu, S. Amirthapandian, C. Viswanathan, N. Ponpandian www.elsevier.com/locate/jmmm
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S0304-8853(16)30024-5 http://dx.doi.org/10.1016/j.jmmm.2016.01.024 MAGMA61048
To appear in: Journal of Magnetism and Magnetic Materials Received date: 19 November 2015 Revised date: 6 January 2016 Accepted date: 9 January 2016 Cite this article as: K.P. Remya, D. Prabhu, S. Amirthapandian, C. Viswanathan and N. Ponpandian, Exchange spring magnetic behavior in BaFe12O19/Fe3O nanocomposites, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.01.024 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.
Exchange spring magnetic behavior in BaFe12O19/Fe3O4 nanocomposites K.P. Remyaa, D. Prabhub, S. Amirthapandianc, C. Viswanathana, N.Ponpandiana a
Department of Nanoscience and Technology, Bharathiar University, Coimbatore 641 046, India. b
c
Centre for Automotive Energy Materials, ARCI, Chennai 600 113, India.
Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
Abstract We report the investigation on exchange spring coupling behavior of BaFe12O19/Fe3O4 nanocomposite synthesized by simple mixing followed by heat treatment of individual ferrites. Morphologically tuned, well crystalline hard and soft ferrites were synthesized by simple chemical method and the phase composition, crystallinity, surface morphology and magnetic properties of the as prepared ferrites as well as the nanocomposites were studied by using XRD, FESEM and VSM respectively. Exchange coupling behavior is observed in the nanocomposite samples heated at 600 °C with simultaneous enhancements of (BH)max and remanence. Keywords: Exchange coupling, hard phase, soft phase, Magnetic studies
1. Introduction The interest in research and development in the field of nanostructured magnetic materials has been increasing both from the fundamental physics and the technological points of view as these materials exhibit many unique and interesting physical and chemical properties. Nanocomposite materials that constitute fine mixture of hard and soft magnetic phases show enhanced properties which make them suitable for various potential applications including permanent magnets [1-4]. In addition to the size, shape and distribution of the constituent grains, the exchange as well as the dipolar interaction also plays an important role in the magnetic property of a two phase magnetic nanocomposite. In order to realize the improved properties of these materials, the hard and soft magnetic phases are exchangecoupled so that the hard phase magnetization with higher coercivity (Hc) combines with the high magnetization (Ms) of the soft phase. The characteristic exchange spring behavior occurs when the applied magnetic fields are not large enough to reverse the hard magnetic phase while the soft magnetic phase undergoes reversible rotation [5]. Many research groups have investigated the fundamental mechanism of exchange interaction, magnetization and demagnetization behavior in nanostructured single-phase as well as composite materials to
understand their exchange-spring coupling characteristics [6,1], employing different techniques, such as melt-spinning [7,8], mechanical alloying [9,10], sputtering [11,12], selfassembly [13], etc., for their preparation. Recently intensive investigations are being done on ferrite composite powders owing to their simple methods of preparation and ease of tuning the magnetic properties with microstructural distribution of the hard and soft magnetic phases, including CoFe2O4/ZnFe2O4 [14], SrFe12O19/ZnFe2O4 [15], SrFe12O19/ Ni0.7Zn0.3Fe2O4 [16], BaCa2Fe16O27/Fe3O4 [17], BaFe12O19/Ni0.8Zn0.2Fe2O4 [18]. BaFe12O19/ CoFe2O4[19] and the reports showed excellent exchange coupling and enhanced remanence in some of these systems while others presented enhanced coercivity with decrease in magnetic saturation. Among different systems, those combining hexagonal ferrite with spinel/ inverse spinel ferrite have obtained noticeable attention by researchers. BaFe12O19, a well-known hard magnet is a hexagonal ferrite having high coercivity, high curie temperature, good mechanical hardness, superior chemical stability and corrosion resistance [20-22], making it suitable for different applications such as permanent magnets, magnetic recording media, magneto-optical materials and microwave filters, etc.,[23-26]. However, these technological applications require well defined micro structure, controlled homogeneity, particle size and magnetic characteristics. A wide range of synthetic methods starting from chemical coprecipitation to aerosol pyrolysis have been employed for the successful synthesis of these M-type barium ferrites focusing on the nanoscale size distribution of the material in order to improve their magnetic properties [27-30]. Superparamagnetic Fe3O4 nanoparticles have enticed much interest not only in magnetic recording media, magnetic fluids, and data storage but also in the fields of catalysis, hyperthermia and targeted drug delivery [31-36]. Other than conventional methods, chemical synthesis such as co-precipitation, sol-gel method, ultrasound irradiation, hydrothermal method, polyol method, etc. have been described to produce Fe3O4 nanoparticles [37-39]. Among transition metal oxides, magnetite exhibits the strongest magnetism in which the strong magnetic moment of iron atom comes from the four unpaired electrons in its 3d orbitals. Magnetite is ferrimagnetic at room temperature having a Curie temperature of 850 K [40, 41]. Synthetic methods and crystal morphology seems to have strong influence on the magnetic properties of these nanoparticles and coercivities ranging from 2.4 to 20 kA/m and magnetic saturation (Ms) of ~75 emu/g have been obtained by controlling their synthesis conditions [40]. In the present work we have prepared BaFe12O19/Fe3O4 nanocomposite ceramics by a novel and simple method to understand exchange spring behavior in ferrite composites.
Respective oxides with suitable microstructure were prepared using plain chemical methods, succeeded by mixing and proper heat treatment of the individual single phases. The simultaneous enhancements in remanence and in (BH)max compared with the parent hard ferrite (BaFe12O19) has been achieved.
2. Experimental 2.1 Synthesis of BaFe12O19/Fe3O4nanocomposite The precursors used for the present investigation includes, barium nitrate, ferric nitrate and ferric chloride, i.e., Ba(NO3)2, Fe(NO3)3.9H2O, FeCl3.6H2O (purity > 99%, sdFine), sodium hydroxide (NaOH) and urea were used as base medium and ethylene glycol and deionized water were the solvent medium. All the chemicals are analytically graded and used without further purification. The ferrites were synthesized separately by simple hydrothermal/solvothermal methods. The synthesis of the hard ferrite, barium ferrite nanoparticles (BaFe12O19) has been carried out by dissolving 1:12 ratio of Ba(NO3)2 and Fe(NO3)3.9H2O respectively, in distilled water followed by the addition of NaOH drop wise for the precipitation to occur. After stirring for 30 min, the resulting suspension was transferred to an autoclave having capacity of 70 ml and heated to 200 °C for 12 h. The obtained hydrothermal product was washed several times with water and ethanol and dried overnight at 70 °C. The synthesis of the soft ferrite, magnetite nanoparticles (Fe3O4) was done by solvothermal method. In a typical process 0.5mol FeCl3.6H2O was dissolved in ethylene glycol followed by the addition of required amount of NaOH and urea. The resultant suspension was vigorously stirred for 30 min and transferred to an autoclave which was heated at 200 °C for 12 h. The black product was washed many times with water and ethanol and dried overnight at 70 °C. The preparation of BaFe12O19/Fe3O4 nanocomposite has been done by mixing the soft and hard ferrite in different weight ratios such as 3:1, 6:1 and 9:1 respectively. The samples were divided into three batches which were heat treated at three different temperatures to realize the effect of temperature on the magnetic property of the composite. The samples were heat treated at 100, 300 and 600 °C for a time period of 2 hours.
2.2 Characterizations
The samples were characterized by means of X-ray diffraction using X-ray difractometer with high intensity Cu Kα radiation (Rigaku D/MAX -2400). The surface morphology of the samples was determined by using field-emission scanning electron microscope (FESEM, FEI quanta-250) equipped with energy dispersive X-ray spectroscopy (EDS). Magnetic hysteresis loops were measured by using vibrating sample magnetometer (VSM, EG&G Parc, Model 4500) at room temperature.
3. Results and Discussion 3.1 XRD Analysis The XRD patterns of pristine BaFe12O19 (BaM), Fe3O4
nanoparticles and the
nanocomposite of BaFe12O19/Fe3O4 with different ratios such as 9:1, 6:1 and 3:1 heat treated at 600 °C for 2 h are shown in Fig. 1. Figure 1(a) shows the crystalline structure of pure Fe3O4 nanoparticles. The observed patterns are indexed with the standard JCPDS # 89-2355. The diffraction peaks of (220), (311), (400), (422), (511) and (440) reflect the magnetite crystal with cubic spinel structure [42]. The average grain size of the prepared magnetite nanoparticles estimated from the full width at half maximum (FWHM) using Scherrer formula is found to be 59 nm.
Fig.1. X-ray diffraction pattern of (a) Fe3O4 nanoparticles, (b) BaFe12O19, BaFe12O19/Fe3O4 nanocomposite mixed in ratio (c) 3:1, (d) 6:1 and (e) 9:1 respectively and heat treated at 600 °C for 2 h. The XRD pattern in Fig. 1b indicates a pure hexaferrite phase with all the major diffraction peaks matching the crystalline BaFe12O19 corresponds to JCPDS # 27-1029. All peaks belong to the phase of M type barium ferrite (magnetoplumbite) and no other impurity was observed. The average grain size estimated for the nanostructured hard ferrite is found to be 78 nm. Figure 1(c, d and e) shows the diffraction patterns for the nanocomposite with the BaM/Fe3O4 ratios of 9:1, 6:1, 3:1 respectively. All these diffraction patterns clearly show the characteristic diffraction peaks for BaM (*) and Fe3O4 (o) and no other impurity peak or change in peak position has been observed even after the samples are heat treated. The XRD patterns of nanocomposite also shows the peaks correspond to the soft ferrite phase are broader when compared to those of barium ferrite confirming the small size of soft magnetic phase in the nanocomposite. So it can be confirmed that the composite consists of two independent phases irrespective of chemical treatment. 3.2 Morphological analysis
Fig.2. FESEM micrographs of (a) pure Fe3O4 nanoparticles, (b) pure BaFe12O19 nanoflakes, (c) BaFe12O19/Fe3O4 nanocomposite mixed in the ratio 6:1 and (d) BaFe12O19/Fe3O4 nanocomposite mixed in the ratio 9:1, heat treated at 600 °C for 2 h, (e) energy dispersive Xray analysis of BaFe12O19/Fe3O4 nanocomposite. Microscopic studies of both pure and nanocomposites were done by using field emission scanning electron microscopy. The morphological images of pure Fe3O4 and BaM are shown in Fig. 2a and 2b respectively. It clearly shows the formation of uniformly distributed spherical nanoparticles of Fe3O4 with the average diameter of ~50 nm whereas the hydrothermally synthesized BaM nanoparticles show the formation of nanoflakes having randomly oriented hexagonal shapes. Figure 2(c and d) corresponds to the FESEM images of the nanocomposite of hard and soft magnetic ferrites mixed in different ratios of 6:1 and 9:1 and heat treated at 600 °C for 2h. The presence of both nanoflakes and nanospheres suggests the existence of both the magnetic phases in all the samples. As the ratio between BaM and Fe3O4 changes from 9:1 to 3:1, the spherical nanoparticles of Fe3O4 in the sample becomes abundant (image corresponding to 3:1 not shown here). The two phases in the nanocomposites are well distributed. The presence of Ba, Fe, O elements were further confirmed by the EDAX of both pure ferrites and the nanocomposite as shown in Fig.2e. 3.3 Magnetic Properties Figure 3 depicts the room temperature hysteresis loops for the as-prepared soft and hard magnetic nanoparticles and their nanocomposites heat treated at 100, 300 and 600 °C for 2 h respectively. Fe3O4 nanoparticles exhibit a superparamagnetic hysteresis loop with the saturation magnetization (Ms) of 75.3 emu/g and a very small coercivity of 100 Oe confirming the soft magnetic behavior of the nanoparticles. Hysteresis loop for the hard magnetic BaFe12O19 shows the intrinsic coercivity (Hci) of 3.135 kOe and the saturation magnetization of 33 emu/g. It has been reported that the theoretical value of saturation
magnetization for single crystalline barium ferrite is 72 emu/g [43]. The particle size effect of the hydrothermally synthesized BaFe12O19 nanoflakes and the effect of calcination temperature [44] can be accounted for the lower magnetization and coercivity compared to their bulk counterparts. The hysteresis loops of the nanocomposite BaFe12O19/Fe3O4 samples heat treated at 100 °C for 2 h (Set A) with different mass ratios of 3:1, 6:1, 9:1, show an increase in the saturation magnetization value with a decrease in the hard ferrite content in the nanocomposite. The samples do not exhibit a smooth hysteresis loop and they show the presence of two phases in the hysteresis loop instead of a single phase. They exhibit a dualmagnetic-phase phenomenon, and hence the formation of exchange spring nanocomposite is not successful. Figure 3b shows the magnetic hysteresis loop for the nanocomposite samples heat treated at 300 °C for 2 h (Set B). Though all the samples present a smooth hysteresis loop with an enhancement in saturation magnetization and changes in coercivity, the “beewaist” type hysteresis loop is observed for all the nanocomposites. It indicates the hard and soft magnetic phases are switching individually due to the in-complete exchange-coupling.
Fig.3. Magnetic hysteresis loops of nanocomposite samples prepared by mixing the hard and soft magnetic phases in three different ratios of 9:1, 6:1 and 3:1 heat treated at (a) 100, (b) 300 and (c) 600 °C for 2 h. The hysteresis loops for the nanocomposite heat treated at 600 °C for 2 h (Set C) are shown in Fig. 3c. It shows that similar to the pure-phase hysteresis loops of BaM and Fe3O4 at room temperature the nanocomposites also show a single loop for all the different ratios of hard and soft magnetic phases. This single-shaped hysteresis loop indicates the magnetization of both magnetic phases reverses conjointly. All the nanocomposites present the typical smooth single-phase hysteresis loop indicating the exchange coupling between the hard (BaM) and soft (Fe3O4) magnetic phases. A superposition of the two individual peaks might occur in the absence of exchange coupling. The nanocomposites heat treated at 600 °C for 2 h show the smooth hysteresis loop which confirms that the crystallographically two different magnetic phases in the nanocomposites are well coupled [45, 46, 47, 48]. The saturation magnetization of the nanocomposite increases with increase in Fe3O4 content. It can be seen from Table 1 that the magnetization increases from 37 to 56 emu/g and it is close to that of pure Fe3O4 when the ratio of the hard and soft magnetic phase reaches 3:1. The remanence of the BaM/Fe3O4 nanocomposite is larger than those of pure BaFe12O19 and Fe3O4 nanostructures which also can be inferred from the Table 1. This enhancement in remanence for the nanocomposite occurs due to the excellent exchange coupling between the hard and soft magnetic phases. Similar observations have already been reported in thin films as well as composite ceramics [49, 50]. The ratio of the remanent magnetization with the saturation magnetization provides the magnetization squareness (S=Mr/Ms) which is found to be 0.34 for the nanocomposite sample mixed in the ratio 9:1 which also decreases with decrease in the hard magnetic content. Sample BaFe12O19 Fe3O4 BaM/Fe3O4(9:1) BaM/Fe3O4 (6:1) BaM/Fe3O4 (3:1)
Ms (emu/g) 32.7 75.2 36.7 41.1 55.6
Mr(emu /g) 11.60 6.16 12.30 13.30 14.60
Hc (kOe) 3.1 0.1 3.17 2.50 0.67
S= Mr/Ms 0.35 0.08 0.34 0.32 0.26
(BH)max (MGOe)
4.70 1.03 5.04 3.03 2.20
Table 1. Magnetic parameters of the prepared nanocomposites heat treated at 600 °C for 2h.
The variation of remanence and coercivity for the nanocomposite with increasing hard magnetic content is shown in Fig. 4a. When the concentration of hard magnetic BaM is 90 wt % and that of the soft magnetic phase is 10 %, the coercivity increases and reaches a value of 3.2 kOe but it decreases with the increasing concentration of Fe3O4. When there is no soft magnetic phase, the direct coupling among the hard grains influences the magnetization of the sample, which decreases with the incorporation of soft magnetic phase to hard magnetic phase. In a nanocomposite where hard and soft magnetic phases are exchange coupled, the magnetic moments of both the phases interact parallelly in the absence of a reverse field. When a reverse field is applied, there will be a rotation of the magnetic moment of the soft phase along the direction of field. As the field increases progressively, the domain wall of the soft phase reaches the interface between the hard and soft phases. Hence, at lower concentration of soft magnetic phase, the hard magnetic phase exerts more exchange interaction on the soft phase and as a result the coercive field increases. However, the increase in the concentration of soft magnetic phase further decreases the coercivity and reached a minimum value of 0.67 kOe. This may be due to the increased interaction among the soft grains rather than that between the soft and hard phases. Similar results have been already reported for the nanocomposite of BaFe12O19 with CoFe2O4 and Ni0.8Zn0.2Fe2O4 [50, 51]. Demagnetization curves of the nanocomposite heat treated at 600 °C for 2h along with the parent hard (BaFe12O19) and soft magnet (Fe3O4) are shown in Fig.4b. The maximum energy product of the BaFe12O19/Fe3O4 nanocomposite were calculated and the maximum value of (BH)max is found for the nanocomposite mixed in the ratio 9:1 attaining a value of 5.04 MGOe as given in Table. 1. This enhancement in (BH)max also concludes the presence of well exchange coupling between the hard magnetic (BaFe12O19) and soft magnetic (Fe3O4) phases. The exchange coupling of 600 °C heat treated nanocomposites were further evaluated by plotting their switching field distribution curves (dM/dH of the demagnetization curves) as shown in Fig. 4c. The switching field distribution shows one single peak for hard magnetic BaFe12O19, indicating that there is one-step completion of magnetic reversal in the hard magnetic phase. The dM/dH curves in Fig.4c, for the BaFe12O19/Fe3O4 nanocomposite in three different ratios exhibits a broad single peak suggesting the strong exchange coupling in the prepared samples. Generally, if the hard and soft magnetic phases are uncoupled or coupling between the phases is weak two distinct peaks can be observed in the switching field distribution [52]. The intensities of the distribution peaks decreases with the increasing hard magnetic content (3:1>6:1>9:1), which indicates that better exchange coupling occurs
for nanocomposite mixed
in 9:1 ratio of hard /soft magnetic phases. The magnetic
orientation at the interface between the hard and soft grains is maximum when there is 10% soft magnetic phase in the nanocomposite. When we increase the soft magnetic content, the interaction at the interface gradually decreases due to the decrease in exchange coupling.
Fig.4. (a) The variation in coercive field and magnetic remanence with increasing hard magnetic content, (b) demagnetization curve of the samples and (c) switching field distribution curves of parent hard ferrite and nanocomposite 4. Conclusion In summary, we have synthesized and observed the exchange spring action between hard BaFe12O19 and soft Fe3O4 nanostructures after suitable heat treatment adapting the microstructural distribution of the two phases. They are mixed together in three different ratios and heat treated at three different temperatures and their corresponding magnetic properties comfirms the exchange spring action. The nanocomposites heat treated at 600 °C show better performance than the parent materials, having well exchange coupling between the components. Though more optimizations have to be carried out for improving the exchange spring magnetism in the nanocomposite, this study shows the stable formation of
BaFe12O19/Fe3O4 nanocomposite under mild conditions without high temperature sintering and the possibility of increasing magnetic properties of permanent magnets. Acknowledgements The authors are thankful to UGC-DAE CSR, Indore and Kalpakkam Node for financial support through collaborative research project. The authors are also thankful to DST-FIST and DST-PURSE for the characterization of the samples.
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Highlights
Hard/Soft magnetic nanocomposite was prepared by simple mixing and heat treatment.
Simple chemical method (hydrothermal/solvothermal) was employed for the synthesis of pristine hard and soft magnetic nanostructures.
Microscopic studies show the presence of both phases.
Exchange-spring behavior was observed in BaFe12O19/Fe3O4 nanocomposite, further evaluated using switching field distribution curves.
in
K.P. Remya, D. Prabhu, S. Amirthapandian, C. Viswanathan, N. Ponpandian www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(16)30024-5 http://dx.doi.org/10.1016/j.jmmm.2016.01.024 MAGMA61048
To appear in: Journal of Magnetism and Magnetic Materials Received date: 19 November 2015 Revised date: 6 January 2016 Accepted date: 9 January 2016 Cite this article as: K.P. Remya, D. Prabhu, S. Amirthapandian, C. Viswanathan and N. Ponpandian, Exchange spring magnetic behavior in BaFe12O19/Fe3O nanocomposites, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.01.024 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.
Exchange spring magnetic behavior in BaFe12O19/Fe3O4 nanocomposites K.P. Remyaa, D. Prabhub, S. Amirthapandianc, C. Viswanathana, N.Ponpandiana a
Department of Nanoscience and Technology, Bharathiar University, Coimbatore 641 046, India. b
c
Centre for Automotive Energy Materials, ARCI, Chennai 600 113, India.
Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
Abstract We report the investigation on exchange spring coupling behavior of BaFe12O19/Fe3O4 nanocomposite synthesized by simple mixing followed by heat treatment of individual ferrites. Morphologically tuned, well crystalline hard and soft ferrites were synthesized by simple chemical method and the phase composition, crystallinity, surface morphology and magnetic properties of the as prepared ferrites as well as the nanocomposites were studied by using XRD, FESEM and VSM respectively. Exchange coupling behavior is observed in the nanocomposite samples heated at 600 °C with simultaneous enhancements of (BH)max and remanence. Keywords: Exchange coupling, hard phase, soft phase, Magnetic studies
1. Introduction The interest in research and development in the field of nanostructured magnetic materials has been increasing both from the fundamental physics and the technological points of view as these materials exhibit many unique and interesting physical and chemical properties. Nanocomposite materials that constitute fine mixture of hard and soft magnetic phases show enhanced properties which make them suitable for various potential applications including permanent magnets [1-4]. In addition to the size, shape and distribution of the constituent grains, the exchange as well as the dipolar interaction also plays an important role in the magnetic property of a two phase magnetic nanocomposite. In order to realize the improved properties of these materials, the hard and soft magnetic phases are exchangecoupled so that the hard phase magnetization with higher coercivity (Hc) combines with the high magnetization (Ms) of the soft phase. The characteristic exchange spring behavior occurs when the applied magnetic fields are not large enough to reverse the hard magnetic phase while the soft magnetic phase undergoes reversible rotation [5]. Many research groups have investigated the fundamental mechanism of exchange interaction, magnetization and demagnetization behavior in nanostructured single-phase as well as composite materials to
understand their exchange-spring coupling characteristics [6,1], employing different techniques, such as melt-spinning [7,8], mechanical alloying [9,10], sputtering [11,12], selfassembly [13], etc., for their preparation. Recently intensive investigations are being done on ferrite composite powders owing to their simple methods of preparation and ease of tuning the magnetic properties with microstructural distribution of the hard and soft magnetic phases, including CoFe2O4/ZnFe2O4 [14], SrFe12O19/ZnFe2O4 [15], SrFe12O19/ Ni0.7Zn0.3Fe2O4 [16], BaCa2Fe16O27/Fe3O4 [17], BaFe12O19/Ni0.8Zn0.2Fe2O4 [18]. BaFe12O19/ CoFe2O4[19] and the reports showed excellent exchange coupling and enhanced remanence in some of these systems while others presented enhanced coercivity with decrease in magnetic saturation. Among different systems, those combining hexagonal ferrite with spinel/ inverse spinel ferrite have obtained noticeable attention by researchers. BaFe12O19, a well-known hard magnet is a hexagonal ferrite having high coercivity, high curie temperature, good mechanical hardness, superior chemical stability and corrosion resistance [20-22], making it suitable for different applications such as permanent magnets, magnetic recording media, magneto-optical materials and microwave filters, etc.,[23-26]. However, these technological applications require well defined micro structure, controlled homogeneity, particle size and magnetic characteristics. A wide range of synthetic methods starting from chemical coprecipitation to aerosol pyrolysis have been employed for the successful synthesis of these M-type barium ferrites focusing on the nanoscale size distribution of the material in order to improve their magnetic properties [27-30]. Superparamagnetic Fe3O4 nanoparticles have enticed much interest not only in magnetic recording media, magnetic fluids, and data storage but also in the fields of catalysis, hyperthermia and targeted drug delivery [31-36]. Other than conventional methods, chemical synthesis such as co-precipitation, sol-gel method, ultrasound irradiation, hydrothermal method, polyol method, etc. have been described to produce Fe3O4 nanoparticles [37-39]. Among transition metal oxides, magnetite exhibits the strongest magnetism in which the strong magnetic moment of iron atom comes from the four unpaired electrons in its 3d orbitals. Magnetite is ferrimagnetic at room temperature having a Curie temperature of 850 K [40, 41]. Synthetic methods and crystal morphology seems to have strong influence on the magnetic properties of these nanoparticles and coercivities ranging from 2.4 to 20 kA/m and magnetic saturation (Ms) of ~75 emu/g have been obtained by controlling their synthesis conditions [40]. In the present work we have prepared BaFe12O19/Fe3O4 nanocomposite ceramics by a novel and simple method to understand exchange spring behavior in ferrite composites.
Respective oxides with suitable microstructure were prepared using plain chemical methods, succeeded by mixing and proper heat treatment of the individual single phases. The simultaneous enhancements in remanence and in (BH)max compared with the parent hard ferrite (BaFe12O19) has been achieved.
2. Experimental 2.1 Synthesis of BaFe12O19/Fe3O4nanocomposite The precursors used for the present investigation includes, barium nitrate, ferric nitrate and ferric chloride, i.e., Ba(NO3)2, Fe(NO3)3.9H2O, FeCl3.6H2O (purity > 99%, sdFine), sodium hydroxide (NaOH) and urea were used as base medium and ethylene glycol and deionized water were the solvent medium. All the chemicals are analytically graded and used without further purification. The ferrites were synthesized separately by simple hydrothermal/solvothermal methods. The synthesis of the hard ferrite, barium ferrite nanoparticles (BaFe12O19) has been carried out by dissolving 1:12 ratio of Ba(NO3)2 and Fe(NO3)3.9H2O respectively, in distilled water followed by the addition of NaOH drop wise for the precipitation to occur. After stirring for 30 min, the resulting suspension was transferred to an autoclave having capacity of 70 ml and heated to 200 °C for 12 h. The obtained hydrothermal product was washed several times with water and ethanol and dried overnight at 70 °C. The synthesis of the soft ferrite, magnetite nanoparticles (Fe3O4) was done by solvothermal method. In a typical process 0.5mol FeCl3.6H2O was dissolved in ethylene glycol followed by the addition of required amount of NaOH and urea. The resultant suspension was vigorously stirred for 30 min and transferred to an autoclave which was heated at 200 °C for 12 h. The black product was washed many times with water and ethanol and dried overnight at 70 °C. The preparation of BaFe12O19/Fe3O4 nanocomposite has been done by mixing the soft and hard ferrite in different weight ratios such as 3:1, 6:1 and 9:1 respectively. The samples were divided into three batches which were heat treated at three different temperatures to realize the effect of temperature on the magnetic property of the composite. The samples were heat treated at 100, 300 and 600 °C for a time period of 2 hours.
2.2 Characterizations
The samples were characterized by means of X-ray diffraction using X-ray difractometer with high intensity Cu Kα radiation (Rigaku D/MAX -2400). The surface morphology of the samples was determined by using field-emission scanning electron microscope (FESEM, FEI quanta-250) equipped with energy dispersive X-ray spectroscopy (EDS). Magnetic hysteresis loops were measured by using vibrating sample magnetometer (VSM, EG&G Parc, Model 4500) at room temperature.
3. Results and Discussion 3.1 XRD Analysis The XRD patterns of pristine BaFe12O19 (BaM), Fe3O4
nanoparticles and the
nanocomposite of BaFe12O19/Fe3O4 with different ratios such as 9:1, 6:1 and 3:1 heat treated at 600 °C for 2 h are shown in Fig. 1. Figure 1(a) shows the crystalline structure of pure Fe3O4 nanoparticles. The observed patterns are indexed with the standard JCPDS # 89-2355. The diffraction peaks of (220), (311), (400), (422), (511) and (440) reflect the magnetite crystal with cubic spinel structure [42]. The average grain size of the prepared magnetite nanoparticles estimated from the full width at half maximum (FWHM) using Scherrer formula is found to be 59 nm.
Fig.1. X-ray diffraction pattern of (a) Fe3O4 nanoparticles, (b) BaFe12O19, BaFe12O19/Fe3O4 nanocomposite mixed in ratio (c) 3:1, (d) 6:1 and (e) 9:1 respectively and heat treated at 600 °C for 2 h. The XRD pattern in Fig. 1b indicates a pure hexaferrite phase with all the major diffraction peaks matching the crystalline BaFe12O19 corresponds to JCPDS # 27-1029. All peaks belong to the phase of M type barium ferrite (magnetoplumbite) and no other impurity was observed. The average grain size estimated for the nanostructured hard ferrite is found to be 78 nm. Figure 1(c, d and e) shows the diffraction patterns for the nanocomposite with the BaM/Fe3O4 ratios of 9:1, 6:1, 3:1 respectively. All these diffraction patterns clearly show the characteristic diffraction peaks for BaM (*) and Fe3O4 (o) and no other impurity peak or change in peak position has been observed even after the samples are heat treated. The XRD patterns of nanocomposite also shows the peaks correspond to the soft ferrite phase are broader when compared to those of barium ferrite confirming the small size of soft magnetic phase in the nanocomposite. So it can be confirmed that the composite consists of two independent phases irrespective of chemical treatment. 3.2 Morphological analysis
Fig.2. FESEM micrographs of (a) pure Fe3O4 nanoparticles, (b) pure BaFe12O19 nanoflakes, (c) BaFe12O19/Fe3O4 nanocomposite mixed in the ratio 6:1 and (d) BaFe12O19/Fe3O4 nanocomposite mixed in the ratio 9:1, heat treated at 600 °C for 2 h, (e) energy dispersive Xray analysis of BaFe12O19/Fe3O4 nanocomposite. Microscopic studies of both pure and nanocomposites were done by using field emission scanning electron microscopy. The morphological images of pure Fe3O4 and BaM are shown in Fig. 2a and 2b respectively. It clearly shows the formation of uniformly distributed spherical nanoparticles of Fe3O4 with the average diameter of ~50 nm whereas the hydrothermally synthesized BaM nanoparticles show the formation of nanoflakes having randomly oriented hexagonal shapes. Figure 2(c and d) corresponds to the FESEM images of the nanocomposite of hard and soft magnetic ferrites mixed in different ratios of 6:1 and 9:1 and heat treated at 600 °C for 2h. The presence of both nanoflakes and nanospheres suggests the existence of both the magnetic phases in all the samples. As the ratio between BaM and Fe3O4 changes from 9:1 to 3:1, the spherical nanoparticles of Fe3O4 in the sample becomes abundant (image corresponding to 3:1 not shown here). The two phases in the nanocomposites are well distributed. The presence of Ba, Fe, O elements were further confirmed by the EDAX of both pure ferrites and the nanocomposite as shown in Fig.2e. 3.3 Magnetic Properties Figure 3 depicts the room temperature hysteresis loops for the as-prepared soft and hard magnetic nanoparticles and their nanocomposites heat treated at 100, 300 and 600 °C for 2 h respectively. Fe3O4 nanoparticles exhibit a superparamagnetic hysteresis loop with the saturation magnetization (Ms) of 75.3 emu/g and a very small coercivity of 100 Oe confirming the soft magnetic behavior of the nanoparticles. Hysteresis loop for the hard magnetic BaFe12O19 shows the intrinsic coercivity (Hci) of 3.135 kOe and the saturation magnetization of 33 emu/g. It has been reported that the theoretical value of saturation
magnetization for single crystalline barium ferrite is 72 emu/g [43]. The particle size effect of the hydrothermally synthesized BaFe12O19 nanoflakes and the effect of calcination temperature [44] can be accounted for the lower magnetization and coercivity compared to their bulk counterparts. The hysteresis loops of the nanocomposite BaFe12O19/Fe3O4 samples heat treated at 100 °C for 2 h (Set A) with different mass ratios of 3:1, 6:1, 9:1, show an increase in the saturation magnetization value with a decrease in the hard ferrite content in the nanocomposite. The samples do not exhibit a smooth hysteresis loop and they show the presence of two phases in the hysteresis loop instead of a single phase. They exhibit a dualmagnetic-phase phenomenon, and hence the formation of exchange spring nanocomposite is not successful. Figure 3b shows the magnetic hysteresis loop for the nanocomposite samples heat treated at 300 °C for 2 h (Set B). Though all the samples present a smooth hysteresis loop with an enhancement in saturation magnetization and changes in coercivity, the “beewaist” type hysteresis loop is observed for all the nanocomposites. It indicates the hard and soft magnetic phases are switching individually due to the in-complete exchange-coupling.
Fig.3. Magnetic hysteresis loops of nanocomposite samples prepared by mixing the hard and soft magnetic phases in three different ratios of 9:1, 6:1 and 3:1 heat treated at (a) 100, (b) 300 and (c) 600 °C for 2 h. The hysteresis loops for the nanocomposite heat treated at 600 °C for 2 h (Set C) are shown in Fig. 3c. It shows that similar to the pure-phase hysteresis loops of BaM and Fe3O4 at room temperature the nanocomposites also show a single loop for all the different ratios of hard and soft magnetic phases. This single-shaped hysteresis loop indicates the magnetization of both magnetic phases reverses conjointly. All the nanocomposites present the typical smooth single-phase hysteresis loop indicating the exchange coupling between the hard (BaM) and soft (Fe3O4) magnetic phases. A superposition of the two individual peaks might occur in the absence of exchange coupling. The nanocomposites heat treated at 600 °C for 2 h show the smooth hysteresis loop which confirms that the crystallographically two different magnetic phases in the nanocomposites are well coupled [45, 46, 47, 48]. The saturation magnetization of the nanocomposite increases with increase in Fe3O4 content. It can be seen from Table 1 that the magnetization increases from 37 to 56 emu/g and it is close to that of pure Fe3O4 when the ratio of the hard and soft magnetic phase reaches 3:1. The remanence of the BaM/Fe3O4 nanocomposite is larger than those of pure BaFe12O19 and Fe3O4 nanostructures which also can be inferred from the Table 1. This enhancement in remanence for the nanocomposite occurs due to the excellent exchange coupling between the hard and soft magnetic phases. Similar observations have already been reported in thin films as well as composite ceramics [49, 50]. The ratio of the remanent magnetization with the saturation magnetization provides the magnetization squareness (S=Mr/Ms) which is found to be 0.34 for the nanocomposite sample mixed in the ratio 9:1 which also decreases with decrease in the hard magnetic content. Sample BaFe12O19 Fe3O4 BaM/Fe3O4(9:1) BaM/Fe3O4 (6:1) BaM/Fe3O4 (3:1)
Ms (emu/g) 32.7 75.2 36.7 41.1 55.6
Mr(emu /g) 11.60 6.16 12.30 13.30 14.60
Hc (kOe) 3.1 0.1 3.17 2.50 0.67
S= Mr/Ms 0.35 0.08 0.34 0.32 0.26
(BH)max (MGOe)
4.70 1.03 5.04 3.03 2.20
Table 1. Magnetic parameters of the prepared nanocomposites heat treated at 600 °C for 2h.
The variation of remanence and coercivity for the nanocomposite with increasing hard magnetic content is shown in Fig. 4a. When the concentration of hard magnetic BaM is 90 wt % and that of the soft magnetic phase is 10 %, the coercivity increases and reaches a value of 3.2 kOe but it decreases with the increasing concentration of Fe3O4. When there is no soft magnetic phase, the direct coupling among the hard grains influences the magnetization of the sample, which decreases with the incorporation of soft magnetic phase to hard magnetic phase. In a nanocomposite where hard and soft magnetic phases are exchange coupled, the magnetic moments of both the phases interact parallelly in the absence of a reverse field. When a reverse field is applied, there will be a rotation of the magnetic moment of the soft phase along the direction of field. As the field increases progressively, the domain wall of the soft phase reaches the interface between the hard and soft phases. Hence, at lower concentration of soft magnetic phase, the hard magnetic phase exerts more exchange interaction on the soft phase and as a result the coercive field increases. However, the increase in the concentration of soft magnetic phase further decreases the coercivity and reached a minimum value of 0.67 kOe. This may be due to the increased interaction among the soft grains rather than that between the soft and hard phases. Similar results have been already reported for the nanocomposite of BaFe12O19 with CoFe2O4 and Ni0.8Zn0.2Fe2O4 [50, 51]. Demagnetization curves of the nanocomposite heat treated at 600 °C for 2h along with the parent hard (BaFe12O19) and soft magnet (Fe3O4) are shown in Fig.4b. The maximum energy product of the BaFe12O19/Fe3O4 nanocomposite were calculated and the maximum value of (BH)max is found for the nanocomposite mixed in the ratio 9:1 attaining a value of 5.04 MGOe as given in Table. 1. This enhancement in (BH)max also concludes the presence of well exchange coupling between the hard magnetic (BaFe12O19) and soft magnetic (Fe3O4) phases. The exchange coupling of 600 °C heat treated nanocomposites were further evaluated by plotting their switching field distribution curves (dM/dH of the demagnetization curves) as shown in Fig. 4c. The switching field distribution shows one single peak for hard magnetic BaFe12O19, indicating that there is one-step completion of magnetic reversal in the hard magnetic phase. The dM/dH curves in Fig.4c, for the BaFe12O19/Fe3O4 nanocomposite in three different ratios exhibits a broad single peak suggesting the strong exchange coupling in the prepared samples. Generally, if the hard and soft magnetic phases are uncoupled or coupling between the phases is weak two distinct peaks can be observed in the switching field distribution [52]. The intensities of the distribution peaks decreases with the increasing hard magnetic content (3:1>6:1>9:1), which indicates that better exchange coupling occurs
for nanocomposite mixed
in 9:1 ratio of hard /soft magnetic phases. The magnetic
orientation at the interface between the hard and soft grains is maximum when there is 10% soft magnetic phase in the nanocomposite. When we increase the soft magnetic content, the interaction at the interface gradually decreases due to the decrease in exchange coupling.
Fig.4. (a) The variation in coercive field and magnetic remanence with increasing hard magnetic content, (b) demagnetization curve of the samples and (c) switching field distribution curves of parent hard ferrite and nanocomposite 4. Conclusion In summary, we have synthesized and observed the exchange spring action between hard BaFe12O19 and soft Fe3O4 nanostructures after suitable heat treatment adapting the microstructural distribution of the two phases. They are mixed together in three different ratios and heat treated at three different temperatures and their corresponding magnetic properties comfirms the exchange spring action. The nanocomposites heat treated at 600 °C show better performance than the parent materials, having well exchange coupling between the components. Though more optimizations have to be carried out for improving the exchange spring magnetism in the nanocomposite, this study shows the stable formation of
BaFe12O19/Fe3O4 nanocomposite under mild conditions without high temperature sintering and the possibility of increasing magnetic properties of permanent magnets. Acknowledgements The authors are thankful to UGC-DAE CSR, Indore and Kalpakkam Node for financial support through collaborative research project. The authors are also thankful to DST-FIST and DST-PURSE for the characterization of the samples.
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Highlights
Hard/Soft magnetic nanocomposite was prepared by simple mixing and heat treatment.
Simple chemical method (hydrothermal/solvothermal) was employed for the synthesis of pristine hard and soft magnetic nanostructures.
Microscopic studies show the presence of both phases.
Exchange-spring behavior was observed in BaFe12O19/Fe3O4 nanocomposite, further evaluated using switching field distribution curves.