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Effect of calcination temperature on microstructure, dielectric, magnetic and optical properties of Ba0.7La0.3Fe11.7Co0.3O19 hexaferrites
Physica B 456 (2015) 206–212
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Physica B journal homepage: www.elsevier.com/locate/physb
Effect of calcination temperature on microstructure, dielectric, magnetic and optical properties of Ba0.7La0.3Fe11.7Co0.3O19 hexaferrites Talwinder Kaur a, Barjinder Kaur b, Bilal H. Bhat c, Sachin Kumar d, A.K. Srivastava a,n a
Department of Physics, Lovely Professional University, Phagwara, Punjab 144411, India Department of Physics, National Institute of Technology, Jalandhar 144011, India c Department of Physics, University of Kashmir, 190006, India d Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 July 2014 Received in revised form 29 August 2014 Accepted 2 September 2014 Available online 16 September 2014
M-type barium hexaferrite Ba0.7La0.3Fe11.7Co0.3O19 (BaLCM) powder, synthesized using sol gel auto combustion method, heat treated at 700, 900, 1100 and 1200 1C. X ray diffraction (XRD) powder patterns of heat treated samples show the formation of pure phase of M-type hexaferrite after 700 1C. Thermo gravimetric analysis (TGA) reveals that the weight loss of BaLCM becomes constant after 680 1C. The presence of two prominent peaks, at 432 cm 1 and 586 cm 1 in Fourier Transform Infrared Spectroscopy (FT-IR) spectra, gives the idea of formation of M-type hexaferrites. The M–H curve obtained from Vibrating Sample Magnetometer (VSM) were used to calculate saturation magnetization (MS), retentivity (Mr), squareness ration (SR) and coercivity (Hc). The maximum value of coercivity (5602 Oe) is found at 900 1C. The band gap dependency on temperature was studied using UV–vis NIR spectroscopy. The dielectric constant has been found to be high at low frequency but it decreases with increase in frequency. Such kind of dielectric behavior is explained on the basis of Koop's phenomenological theory and Maxwell Wagner theory. & 2014 Elsevier B.V. All rights reserved.
Keywords: Hexaferrites Microstructure Optical properties Magnetic properties
1. Introduction Recently M-type hexaferrites (BaM) have attracted much attention due to their unique magnetic and dielectric properties. These materials can be used in supercapacitors, sound devices, magnetic hose (to transport magnetic field due to ferromagnetic nature), microwave devices (mobile technology), perpendicular magnetic recording devices and fridge magnets [1–5]. The family of hexaferrites like M, U, W, X, Y and Z-type is well known for its magnetic properties. Among all hexaferrites, M-type barium hexaferrites, (abbreviated as BaM) having magneto-plumbite structure with space group P63/mmc, exhibits better properties than others. They have high coercivity (6700 Oe) [6], high saturation magnetization (72 emu/g) [7], and high curie temperature (TC ¼502 1C) [8]. Structure of BaM has dual layer containing 2 molecules of BaM having RRnSSn stacks. S and Sn have spinal structure with 2 oxygen layers, while R and Rn consists of three oxygen layers with hexagonal structure. There are 38 O2 , 2 Ba2 þ ions, 24 Fe3 þ ions in a unit cell of hexagonal ferrite. The Fe3 þ ions are situated in
n
Corresponding author. Tel.: þ 919915029377. E-mail address: [email protected] (A.K. Srivastava).
http://dx.doi.org/10.1016/j.physb.2014.09.003 0921-4526/& 2014 Elsevier B.V. All rights reserved.
trigonal bi-pyramidal site (2b), tetrahedral site (4f1) and octahedral sites (12 k, 2a and 4f2). Due to presence of 24 Fe3 þ ions, it shows good magnetic properties, but out of 5 crystallographic sites of Fe3 þ , two sites have spin down ion (4f1 and 4f2) causes reduction in average magnetic moment of the molecule [9,10]. Cations (like transition metals) with magnetic moment can be substituted at spin down site to change the magnetic properties. Several authors have tried to substitute cations like Co–Ti, La–Na [11], Ca–Cr [12], Al [13], Co–Ti, Bi–Ti [14], Eu [15], Bi [16], Mg–Mn–Co–Ti [17], Co–Zn–Sn [18].Due to small ionic radii, rare earth metal ions and transition metal ions may replace the barium ions and ferric ions, respectively. To make BaM suitable for different applications, its properties are required to be tailored which can be accomplished by substituting cations and heat treatment. There are many routes to synthesize BaM such as sol gel method [19], citrate precursor method [20], micro emulsion method [21], milling method [22], and micro wave induction method [23]. Of these, the sol gel method offers low temperature, uniform grain size and low cost. In this paper we have studied the M-type barium hexaferrite Ba0.7La0.3Fe11.7Co0.3O19 (BaLCM) synthesized via sol–gel auto combustion method. The aim of this work is to study the effect of heat treatment on structural, optical, dielectric and magnetic properties of the as synthesized samples.
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2. Experimental method M-type barium hexaferrites Ba1 xLaxCoxFe12 xO19 (x ¼0.3) powders have been synthesized via sol gel combustion method using AR grade Ba(NO3)2 (LOBA Chemie., 99% purity), La (NO3)3·6H2O (LOBA Chemie., 99% purity), Co(NO3)2·6H2O (LOBA Chemie., 99% purity), Fe(NO3)3·9H2O (LOBA Chemie., 98% purity) and citric acid(LOBA Chemie., 99.5% purity). Aqueous solution of iron and metal salts are prepared separately in stoichiometric proportions by dissolving them in distilled water and then making a mixture by adding solutions with constant magnetic stirring. Aqueous solution of citric acid is added to the salt solution with cations to citric acid molar ratio of 1:1.5.Then ammonium hydroxide solution is added drop by drop to attain the 6.8 pH value. Then the solution is heated at 80 1C–85 1C for 4–6 h with continuous stirring using magnetic stirrer. ð1 xÞBaðNO3 Þ2 þ xLaðNO3 Þ3 þ xCoðNO3 Þ2 6H 2 O NH 4 OH Ba1 x Lax Cox Fe12 x O19 þ ð12 xÞFeðNO3 Þ3 9H 2 O Heat After evaporation of water, the liquid gets converted into a homogenous brown colored gel. The viscous solution is dried over hot plate at 280–300 1C for 3 h to form the precursor material. Presintering is done at 500 1C for 2 h. Then the precursor material is calcined at 700 1C, 900 1C, 1100 1C and 1200 1C for 5 h. The whole process is carried out at room temperature as illustrated in the flow chart (Fig. 1).
3. Characterization techniques Structural properties of heat treated samples have been investigated using X-ray powder diffraction patterns obtained from Bruker AXS D8 Advance X-ray diffractometer in the range 20˚ to 80˚ using Cu-kα radiation operating at 40 kV and 35 mA having step size of 0.021. To analyse the attached functional groups, Fourier transform infrared spectra (FT-IR interferometer IR prestige-21 FT-IR (model-8400 S)) are taken in the range 400–4000 cm 1. Thermo gravimetric analyzer (TGA) under N2 atmosphere 2+
Metal Nitrates (Ba ,La3+, Co 2+, Fe3+)
207
(NETZSCH TG 209F1 Libra TGA209F1D-0105-L) with a heating rate of 30 1C/10 min from 0 to 1000 1C is used for thermal analysis. Magnetic properties have been investigated with vibrating sample magnetometer (PAR-155 Princeton Applied Research, USA) at room temperature. Dielectric analysis has been performed with LCR meter (Model: 6440B) at 20 Hz- 3 MHz. Surface features have been taken from FE-SEM (TESCAN MIRA II) at an operating voltage of 5– 25 kV and at a scale of 2 mm. Band gap study has been carried out using UV–vis–NIR (Model: Varian Carry 5000 at room temperature with 0.2 nm resolution).
4. Results and discussions 4.1. Structural properties 4.1.1. Phase analysis of BaLCM The synthesized sample of BaLCM, heat treated at 700 1C, 900 1C, 1100 1C and 1200 1C for 5 h, have been investigated by well-known non-destructive X-ray powder diffractometer technique and presented in Fig. 2. All the samples are in crystalline phase and show absence of impurities above 700 1C. The presence of apparent peaks having hkl values 110, 008, 107, 114, 203, 205, 217, 2011 and 220 confirms the hexagonal structure of BaLCM powders and these peaks are similar to the peaks in standard pattern (JCPDS-391433). While comparing the peaks of BaLCM with standard data of un-substituted barium hexaferrite, it has been observed that these peaks for the samples calcined at 700 1C, 900 1C, 1100 1C and 1200 1C appear at the same diffraction angle position. All the samples except the one sample calcined at 700 1C exhibit single phase crystallinity. This proves that the substituted ions have occupied the crystallographic sites. Very small peak of α-Fe2O3 appears in sample calcined at 700 1C (shown as n between 107 and 114 hkl peaks), but it disappears above 700 1C. No undesired phases are observed above 700 1C. This indicates the complete crystallization, formation of homogeneous mixture of starting compound solutions, and synthesis of pure substituted BaM. 4.1.2. Crystallite size of BaLCM Crystallite size (D) has been estimated from the Scherer formula [24]: D ¼ kλ=β cos θ, where λ is X ray wavelength (1.54056 Å), β is full width at half maxima (in radian) and θ is the Bragg angle and k is the shape factor having unit value for hexagonal structure. The crystallite size increases with increase in
Citric acid
Mixture solution Ammonia solution Sol Stirring at 80 - 85˚C Gel Heated at 280 - 300˚C Precursor material Heat treatment BaLCM Fig. 1. Flow Chart for the synthesis of BaLCM powders.
Fig. 2. X ray powder patterns for Ba0.7La0.3Fe11.7Co0.3O19 at (a) T ¼ 700 1C, (b) T ¼ 900 1C, (c) T ¼ 1100 1C and (d) T ¼ 1200 1C.
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calcination temperature (Table 1(a)); minimum size (17.53 nm) appears at 700 1C. On the basis of obtained results, it can be interpreted that the agglomeration may happen with increase in calcination temperature. 4.1.3. Surface area Surface area calculated from the following expression has been shown in Table 1(b) [9]: S ¼ 6000=DDx , where D is crystallite size and Dx is X-ray density. The calculated surface area shows a decrease in its value from 64.971 m2/g to 21.251 m2/g. Crystallite size is the dominant factor for variation in surface area. As the crystallite size increases, lesser number of atoms appear at the surface. 4.1.4. Strain The strain in the BaLCM has been evaluated using following relation [9]
β cos θ ¼
0:9λ þ 2η sin θ D
Fig. 3. Variation of lattice constants with temperature for BaLCM.
where η is the strain. The strain shows a decrement as the calcination temperature increases. Strain is the distortion that is induced during synthesis process and heat treatment process. But the reason for decrease in value may attribute to the formation and enhancement towards pure hexagonal structural properties. 4.1.5. Lattice constants, densities and porosity Lattice constants (a and c) obtained from X-ray data using following equation have been tabulated in Table 1(a) [25]: " # 2 2 2 1 4 h þ hk þ k l ¼ þ 2 2 2 a c dhkl 3 where dhkl is d spacing value, hkl are miller indices for peaks in XRD powder patterns. It has been found that at 700 1C, lattice constant ‘a’ has highest value (5.9034 Å), above that it becomes constant and ‘c’ continuously decreases with increase in temperature from 23.2245 Å to 23.0870 Å as shown in Fig. 3.The main reason for contraction may be due to presence of strain, microstructural defects and interaction between substituted cations. Volume of cell can be calculated from [26]: Vcell ¼ 0.8666a2c. It has Table 1 (a) Diffraction angle (2θ), d spacing (d), full width at half maxima (β), lattice constants (a and c) and volume of cell (Vcell) with variation of calcination temperature for BaLCM. T (1C)
700 900 1100 1200
2θ (1)
34.016 34.137 34.137 34.184
d (Å)
2.63342 2.62436 2.62436 2.62088
β (1)
0.525 0.195 0.193 0.174
Lattice constants a(Å)
c(Å)
5.9034 5.8844 5.8844 5.8844
23.2245 23.1552 23.1211 23.0870
D (nm)
Vcell (Å3)
17.53 47.22 47.71 52.93
701.4060 694.8185 693.7962 692.7735
been seen that with increase in temperature, crystallite size increases and volume of the unit cell decreases. This may be attributed to the presence of a rare-earth element, Lanthanum, which has ability to form strong lanthanide–oxygen bonds. The binding energy of lanthanide oxygen octahedral in rare-earth substituted oxide materials is known to be much higher than the transition metal ion-oxygen octahedral. This may be the factor for shrinkage of crystal lattice. X ray density can be calculated from following expression [26]: Dx ¼
ZM NA V cell
where Z is number of molecule per unit cell, M is molecular weight, NA is Avogadro's number (6.023 1023), and Vcell is cell volume. X ray density increases with calcination temperature from 5.268 g/cm3 to 5.334 g/cm3.This may be ascribed to the lattice contraction. 4.1.6. Porosity Porosity of the sample can be calculated using relation [26]: Dx Db P¼ 100 Dx Db ¼
m ∏r 2 d
where Dx is X ray density, Db is bulk density and can be calculated from mass (m) and volume of palette having radius (r) and thickness (d). Due to difference between bulk density and X-ray density, the samples exhibit porosity. Heat treatment or sample preparation parameters may induce shape defects in grains which may cause porosity in BaLCM. 4.2. Thermo gravimetric analysis (TGA)
Table 1 (b) X-ray density (DX), bulk density (Db), porosity (P), crystallite size (D), phase present, surface area (S) and strain induced (η) with variation of calcination temperature for BaLCM. T (1C) DX g/cm3 Db g/cm3 P (%)
D (nm) Phase
700
5.268
1.4187
73.06 17.53
900 1100 1200
5.318 5.326 5.334
1.2809 1.4182 3.04179
75.91 47.22 73.37 47.71 42.97 52.93
Hexa α-Fe2O3 Hexa Hexa Hexa
S 103 (m2/g) η 10 3 64.971
15.01
23.893 23.612 21.251
5.55 5.50 4.95
Thermal gravimetric analysis was performed to investigate the effect of heat on mass of sample. The resultant TGA curve of M-type hexaferrite Ba0.7La0.3Fe11.7Co0.3O19 (BaLCM) powder has been shown in Fig. 4. The reduction of weight (9.21%) between 0 1C to 425 1C is due to decomposition of remaining organic matter. Oxidation reduction reaction between the metal nitrates and citric acid is the reason for decomposition. Between 425 1C and 680 1C, there is weight loss of 19.97% due to decomposition of precursor and conversion from hematite to hexaferrite structure [27]. After 680 1C weight loss becomes constant which indicates the formation of BaLCM. From XRD study, it has been confirmed
T. Kaur et al. / Physica B 456 (2015) 206–212
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Fig. 4. Thermo gravimetric diagram for Ba0.7La0.3Fe11.7Co0.3O19 precursor.
that BaLCM formation happens at 700 1C. The reaction interval for the BaLCM is 255 1C. 4.3. FT-IR study From FT-IR spectra analysis, the information regarding attached molecular bands or presence of functional groups, which are remains of chemicals used in the synthesis process, can be identified. Two prominent peaks that arise at 432 cm 1 and 586 cm 1, indicate the formation of hexaferrite (Fig. 5). The stretching vibrations of metal–oxygen bond are the reason for these peaks. The bands near 420–480 cm 1 and 550–590 cm 1 are attributed to the vibration of ferric crystallographic site (octahedral and tetrahedral) [28]. Absence of hydroxyl group carboxylic group (at 3200–3700 cm 1) in samples calcined above 700 1C reveals the completion of the reaction. There is no peaks associated to the nitrate ions near 1300 cm 1 [29]. Small band near 2900 cm 1 occurs due to presence of moisture absorbed by the sample [30]. The spectra for 700 1C and 900 1C show less transmittance area with relatively more prominent peaks. This shows that the cations have been substituted themselves at the ferric sites accompanied with strong interaction as compared to other samples. 4.4. Surface features FE-SEM micrographs of BaLCM has been recorded at room temperature and depicted in Fig. 6. Micrographs show hexagonal structure, uneven distribution, and grain size deformation. Crystallites are not closely packed throughout the whole sample but traces of their cluster can be visualized. During the synthesis, lots of gases evolved during combustion and decomposition of organic matter may cause porosity. 4.5. Dielectric properties Dielectric data been has recorded on LCR meter in a range 20 Hz–3 MHz using a pellet (with 8 t pressure) of BaLCM coated with silver metal powder. Though dielectric data have been calculated in 20 Hz to 3 MHz, but for clear analysis, the graphs have been considered for different range of frequencies because of the prominent effect shown by samples in small regions. It is clear that BaLCM gives a normal ferrite response towards frequency i. e. dielectric constant is high for low frequency but decreases at high frequency (Fig. 7). This behavior can be explained on the basis of Koop's phenomenological theory and Maxwell Wagner theory [31]. This may be attributed to an electron exchange between Fe3 þ and Fe2 þ .This results in the displacement of the ionic charges and is also responsible for polarization.
Fig. 5. FT-IR spectra for Ba0.7La0.3Fe11.7Co0.3O19 at different temperatures.
Dielectric constant has been calculated from parallel plate capacitor equation [31]: K¼
Cd
ε0 A
where K is dielectric constant, C is capacitance of the palette, d is thickness of sample palette and A is cross section area and εo is the permittivity of free space (8.85 10–12 F/m). Variation of dielectric constant can be explained on the basis of interfacial polarization of heterogeneous structure of barium hexaferrite which arises due to porosity and presence of grains. The effective contribution may also come from grain boundaries at low frequencies. The structure of material is supposed to be made of conducting grains separated by less conducting grain boundaries [32]. Heat treatment may be the cause of their creation. The hopping between Fe2 þ and Fe3 þ causes dispersion at low frequency. The electrons are displaced by the applied field and causes polarization but displacement happens only when hopping time is less than half of the alternating field. So the dielectric constant will decrease at high frequency [33]. Because of low eddy currents and high dielectric constant, the prepared material can play a significant role in electronics market having application as radiation absorber or high frequency component. Dielectric loss can be calculated by using expression [34]:
ε″ ¼ ε0 tan δ
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Fig. 8. Variation of dielectric loss with frequency of BaLCM at (a) 700 1C (b) 900 1C and (c) 1200 1C.
Fig. 6. FE-SEM micrographs for Ba0.7La0.3Fe11.7Co0.3O19 (a) at 900 1C (b) at 1100 1C.
Fig. 9. Effect of frequency on dielectric loss tangent at (a) 700 1C (b) 900 1C and (c) 1200 1C.
Fig. 7. Influence of frequency on dielectric constant of BaLCM at (a) 700 1C (b) 900 1C and (c) 1200 1C.
where ε0 is dielectric constant and ε″ is dielectric loss factor. Dielectric losses are also high at low frequency (Fig. 8).The behavior of dielectric loss tan δ with frequency is shown in Fig. 9. It becomes clear that after attaining a maximum value, dielectric loss tanδ decreases with increase in frequency. As described earlier in this section, hopping is the reason for high dielectric constant. Loss tangent behavior can similarly be explained on this basis. Maximum of loss tangent may be observed when the hopping is nearly equal to that of the externally applied electric field. After the maxima, hopping time becomes more than the applied signal causing decrease in loss tangent.
Fig. 10. Hysteresis temperatures.
loop
for
Ba0.7La0.3Fe11.7Co0.3O19
powder
at
different
4.6. Magnetic properties Coercivity, saturation magnetization, remnant magnetization and magnetic moment have been calculated from M–H curves
(Fig. 10). The highest value for coercivity (5602 Oe) was found in a sample calcined at 900 1C (Fig. 11) and saturation magnetization shows a continuous increase with increase in temperature.
T. Kaur et al. / Physica B 456 (2015) 206–212
Fig. 11. Variation in magnetic properties with temperature for BaLCM.
211
Fig. 12. Optical band gap for (a) BaM at 900 1C and (b) BaLCM at 900 1C.
Table 2 Summary of Coercivity (Hc), saturation Magnetization (Ms), Retentivity (Mr), Squareness Ratio (SR) Anisotropy constant (K) and magneton number (ηB) for Ba0.7La0.3Fe11.7Co0.3O19 with variation of calcination temperature.
absorber [35], circulators [36], multi-layer chip inductor [37], and electrical filters [36]. 4.7. Optical properties
T (1C)
Hc (Oe)
Ms(emu/g)
Mr(emu/g)
SR(Mr/Ms)
K(HA2/kg)
ηB(μB)
700 900 1100 1200
5210 5602 3493 1752
47.22 49.34 51.06 52.52
28.79 30.61 32 28.63
0.6096 0.6203 0.6267 0.5451
12.30 13.82 8.92 4.60
9.408 9.830 10.173 10.464
Calcined product at 900 1C show highest porosity (75.91%) and low bulk density (1.2809 g/cm3). These parameters affect the magnetic properties of material. The following relation (Eq. (1)) has been employed to find out anisotropic constant (K) [28]: Hc ¼
2K
μ0 MS
ð1Þ
where μ0 is permeability in vacuum (4π 107 H/m), MS is saturation magnetization and Hc is coercivity. Presence of pores impedes the movement of domains walls that needs the application of high magnetic field to make alignment, so cause high coercivity. Magneton number can be found out from the equation [13]:
ηB ¼
Molecular weight M S 5585
Saturation magnetization of BaLCM is lower than that of the MS of BaM. It has been observed that with increase in temperature there is increase in saturation magnetization of BaLCM (Fig. 11). This is due to increase in grain size, reduction of grain boundary, and enhancement of the superexchange interaction between Fe–O–Fe. Reduction of porosity may also be responsible for increment in MS because grains come closer and more and more magnetic moment align themselves with decrease in porosity resulting in increased agglomeration. This has been revealed in FE–SEM micrographs. Magnetic parameters have been affected by, cationic distribution, strain, ionic radii, valency of ferric ion, and lattice distortion. As magneton number depends on saturation magnetization, so it increases with increase in Ms. Due to anisotropic morphology of barium hexaferrite, anisotropic constant has highest value at 900 1C. This can also be understood in terms of coercivity. At 900 1C, coercivity is highest and according to the relation (Eq. (1)), anisotropic constant is also maximum (Table 2). By controlling the temperature, the magnetic properties can be enhanced to meet the criteria for hyper frequency application as micro wave
The optical properties of BaLCM have been investigated by using UV–Vis–NIR absorption spectra. In the optical region, incidence of electromagnetic waves on the material causes electrons in the valence band to absorb the energy, raising its energy level. This absorbed band gap energy has been calculated from the spectrum from following relation [38]:
α¼
Aðhν Eg Þ1=2 hν
where E is energy band gap and A is a constant. The band gap has been obtained by plotting the graph from fundamental absorbance edges. Fig. 12 shows the spectra for energy band gap and the absorbance spectra (inset). The band gap values concluded from graph are: 3.92 eV (pure barium hexaferrite at 900 1C), 4.1 eV (for Ba0.7La0.3Fe11.7Co0.3O19 at 900 1C). The graph shows that with substitution the band gap increases. BaM shows low value of band gap than BaLCM. Band gap of BaLCM is higher than those of barium hexaferrite (3.18 eV) and thin films (2.32 eV) [38,39]. Quantum confinement at nano scale plays important role in increasing the band gap. From UV–NIR absorption spectra (Inset) it has been concluded that the absorption region for hexaferrites is 200–600 nm. The literature available for UV–vis study for barium hexaferrites is very less.
5. Conclusion The formation of BaLCM with no impurities has been revealed from XRD, FT-IR and FE-SEM. TGA shows that BaLCM are formed at 680 1C. UV–vis–NIR exhibits that the band gap of samples increases with substitution. From VSM analysis techniques, it was observed that the saturation magnetization, retentivity, squareness ratio increases with increase in temperature but coercivity and anisotropy constant is the highest for the sample calcined at 9001C. BaLCM shows normal ferrite dielectric behavior.
Acknowledgments We are thankful to STIC-Kochi (Ernakulum) for XRD and UV–vis– NIR and IIT Roorkee for FE-SEM and VSM facilities. We are highly
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thankful to Lovely Professional University for providing financial support (No. LPU/DRD/IPF/Sac/004) for this research work. References [1] P. Bhattacharya, S. Dhibar, G. Hatui, A. Mandal, T. Das, C.K. Das, RSC Adv. 4 (2014) 17039. [2] C. Navau, J. Prat-Camps, O. Romero-Isart, J.I. Cirac, A. Sanchez, Phys. Rev. Lett. 112 (2014) 253901. [3] M.N. Ashiq, J.M. Iqbal, M. Najam-ul-Haq., P.H. Gomez, A.M. Qureshi, J. Magn. Magn. Mater. 324 (2012) 15. [4] H.A. Patel, J. Byun, C.T. Yavuz, J. Nanopart. Res. 14 (2012) 881. [5] V. Anbarasu, P.M. Md Gazzali., T. Karthik, A. Manigandan, K. Sivakumar, J. Mater. Sci-Mater. El. 24 (2013) 916. [6] M. Radwan, M.M. Rashad, M.M. Hessien, J. Mater. Process. Technol. 181 (2007) 106. [7] T. Gonzalez-Carreno, M.P. Morales, C.J. Serna, Mater. Lett. 43 (2000) 97. [8] M. Cernea, S.G. Sandu, C. Galassi, R. Radu, V. Kuncser, J. Alloys Compd. 561 (2013) 121. [9] A. Thakur, R.R. Singh, P.B. Barman, Mater. Chem. Phys. 141 (2013) 562. [10] P.B. Braun, PB. Philos. Res. Rep. 12 (1957) 491. [11] M.L. Gregori, M.S. Pinho, R.C. Lima, J.C.S. Leandro, T. Ogasawara, Key Eng. Mater. 264–268 (2004) 1229. [12] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, M.A. Iqbal, J. Supercond. Nov. Magn. 26 (2013) 3277. [13] V.N. Dhage, M.L. Mane, A.P. Keche, C.T. Birajdar, K.M. Jadhav, Physica B 406 (2011) 789. [14] A.G. Belous, O.I. V'yunov, E.V. Pashkova, V.P. Ivanitskii, O.N. Gavrilenko, J. Phys. Chem. B 110 (2006) 26477. [15] F. Khademi, A. Poorbafrani, P. Kameli, H. Salamati, J. Supercond. Nov. Magn. 25 (2012) 525. [16] P. Winotai, S. Thongmee, I.M. Tang, Mater. Res. Bull. 35 (2000) 1747. [17] M.K. Tehrani, A. Ghasemi, M. Moradi, R.S. Alam, J. Alloys Compds. 509 (2011) 8398.
[18] Y. Liu, M.G.B. Drew, Y. Liu, J. Wang, M. Zhang, J. Magn. Magn. Mater. 322 (2010) 814. [19] H. Li, J. Huang, Q. Li, X. Su, J. Sol–Gel Sci. Technol. 52 (2009) 309. [20] M.C. Dimri, S.C. Kashyap, D.C. Dube, Ceram. Int. 30 (2004) 1623. [21] X. Liu, J. Wang, L.M. Gan, S.C. Ng, J. Ding, J. Magn. Magn. Mater. 184 (1998) 344. [22] A. Gonzalez-Angeles, G. Mendoza-Suarez, A. Gruskova, I. Toth, V. Jancarik, M. Papanova, J.I. Escalante-Garcı., J. Magn. Magn. Mater. 270 (2004) 77. [23] A.P. Safronov, O.M. Samatov, A.I. Medvedev, I.V. Beketov, A.M. Murzakaev, Nanotechnol. Russ. 7 (2012) 486. [24] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, M.A. Iqbal, J. Supercond. Nov. Magn. 26 (2013) 3315. [25] W.Y. Zhao, P. Wei, H.B. Cheng, X.F. Tang, Q.J. Zhang, J. Am. Ceram. Soc. 90 (2007) 2095. [26] M.J. Iqbal, M.N. Ashiq, Chem. Eng. J. 136 (2008) 383. [27] A. Poorbafrani, P. Kameli, H. Salamati, J. Mater. Sci. 48 (2013) 186. [28] F. Song, X. Shen, J. Xiang, H. Song, Mater. Chem. Phys. 120 (2010) 213. [29] F. Shahbaz Tehrani, V. Daadmehr, A.T. Rezakhani, R. Hosseini Akbarnejad, S. Gholipour, J. Supercond. Nov. Magn. 25 (2012) 2443. [30] J.P. Singh, G. Dixit, R.C. Srivastava, H.M. Agrawal, K. Asokan, J. Phys. D Appl. Phys. 44 (2011) 435306. [31] A. Singh, S. Bindra Narang, K. Singh, O.P. Pandey, R.K. Kotnala, J. Ceram. Process. Res. 11 (2010) 241. [32] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, J. Electron. Mater. 43 (2014) 512. [33] S.B. Narang, I.S. Hudiara, J. Ceram. Process. Res. 7 (2006) 113. [34] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, J. Mater. Eng. Perform. 22 (2013) 2104. [35] M.R. Meshram, Nawal K. Agrawal, B. Sinha, P.S. Misra, J. Magn. Magn. Mater. 271 (2004) 207. [36] M.J. Iqbal, S. Farooq, Mater. Res. Bull. 46 (2011) 662. [37] A. Haq, M. Anis-ur-Rehman, Physica B 407 (2012) 822. [38] M. Karmakar, B. Mondal, M. Pal, K. Mukherjee, Sens. Actuator B—Chem. 190 (2014) 627. [39] S. Anjum, M.S. Rafique, M. Khaleeq-ur-Rahman, K. Siraj, Arslan Usman, S.I. Hussain, S. Naseem, J. Magn. Magn. Mater. 324 (2012) 711.
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Physica B journal homepage: www.elsevier.com/locate/physb
Effect of calcination temperature on microstructure, dielectric, magnetic and optical properties of Ba0.7La0.3Fe11.7Co0.3O19 hexaferrites Talwinder Kaur a, Barjinder Kaur b, Bilal H. Bhat c, Sachin Kumar d, A.K. Srivastava a,n a
Department of Physics, Lovely Professional University, Phagwara, Punjab 144411, India Department of Physics, National Institute of Technology, Jalandhar 144011, India c Department of Physics, University of Kashmir, 190006, India d Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 July 2014 Received in revised form 29 August 2014 Accepted 2 September 2014 Available online 16 September 2014
M-type barium hexaferrite Ba0.7La0.3Fe11.7Co0.3O19 (BaLCM) powder, synthesized using sol gel auto combustion method, heat treated at 700, 900, 1100 and 1200 1C. X ray diffraction (XRD) powder patterns of heat treated samples show the formation of pure phase of M-type hexaferrite after 700 1C. Thermo gravimetric analysis (TGA) reveals that the weight loss of BaLCM becomes constant after 680 1C. The presence of two prominent peaks, at 432 cm 1 and 586 cm 1 in Fourier Transform Infrared Spectroscopy (FT-IR) spectra, gives the idea of formation of M-type hexaferrites. The M–H curve obtained from Vibrating Sample Magnetometer (VSM) were used to calculate saturation magnetization (MS), retentivity (Mr), squareness ration (SR) and coercivity (Hc). The maximum value of coercivity (5602 Oe) is found at 900 1C. The band gap dependency on temperature was studied using UV–vis NIR spectroscopy. The dielectric constant has been found to be high at low frequency but it decreases with increase in frequency. Such kind of dielectric behavior is explained on the basis of Koop's phenomenological theory and Maxwell Wagner theory. & 2014 Elsevier B.V. All rights reserved.
Keywords: Hexaferrites Microstructure Optical properties Magnetic properties
1. Introduction Recently M-type hexaferrites (BaM) have attracted much attention due to their unique magnetic and dielectric properties. These materials can be used in supercapacitors, sound devices, magnetic hose (to transport magnetic field due to ferromagnetic nature), microwave devices (mobile technology), perpendicular magnetic recording devices and fridge magnets [1–5]. The family of hexaferrites like M, U, W, X, Y and Z-type is well known for its magnetic properties. Among all hexaferrites, M-type barium hexaferrites, (abbreviated as BaM) having magneto-plumbite structure with space group P63/mmc, exhibits better properties than others. They have high coercivity (6700 Oe) [6], high saturation magnetization (72 emu/g) [7], and high curie temperature (TC ¼502 1C) [8]. Structure of BaM has dual layer containing 2 molecules of BaM having RRnSSn stacks. S and Sn have spinal structure with 2 oxygen layers, while R and Rn consists of three oxygen layers with hexagonal structure. There are 38 O2 , 2 Ba2 þ ions, 24 Fe3 þ ions in a unit cell of hexagonal ferrite. The Fe3 þ ions are situated in
n
Corresponding author. Tel.: þ 919915029377. E-mail address: [email protected] (A.K. Srivastava).
http://dx.doi.org/10.1016/j.physb.2014.09.003 0921-4526/& 2014 Elsevier B.V. All rights reserved.
trigonal bi-pyramidal site (2b), tetrahedral site (4f1) and octahedral sites (12 k, 2a and 4f2). Due to presence of 24 Fe3 þ ions, it shows good magnetic properties, but out of 5 crystallographic sites of Fe3 þ , two sites have spin down ion (4f1 and 4f2) causes reduction in average magnetic moment of the molecule [9,10]. Cations (like transition metals) with magnetic moment can be substituted at spin down site to change the magnetic properties. Several authors have tried to substitute cations like Co–Ti, La–Na [11], Ca–Cr [12], Al [13], Co–Ti, Bi–Ti [14], Eu [15], Bi [16], Mg–Mn–Co–Ti [17], Co–Zn–Sn [18].Due to small ionic radii, rare earth metal ions and transition metal ions may replace the barium ions and ferric ions, respectively. To make BaM suitable for different applications, its properties are required to be tailored which can be accomplished by substituting cations and heat treatment. There are many routes to synthesize BaM such as sol gel method [19], citrate precursor method [20], micro emulsion method [21], milling method [22], and micro wave induction method [23]. Of these, the sol gel method offers low temperature, uniform grain size and low cost. In this paper we have studied the M-type barium hexaferrite Ba0.7La0.3Fe11.7Co0.3O19 (BaLCM) synthesized via sol–gel auto combustion method. The aim of this work is to study the effect of heat treatment on structural, optical, dielectric and magnetic properties of the as synthesized samples.
T. Kaur et al. / Physica B 456 (2015) 206–212
2. Experimental method M-type barium hexaferrites Ba1 xLaxCoxFe12 xO19 (x ¼0.3) powders have been synthesized via sol gel combustion method using AR grade Ba(NO3)2 (LOBA Chemie., 99% purity), La (NO3)3·6H2O (LOBA Chemie., 99% purity), Co(NO3)2·6H2O (LOBA Chemie., 99% purity), Fe(NO3)3·9H2O (LOBA Chemie., 98% purity) and citric acid(LOBA Chemie., 99.5% purity). Aqueous solution of iron and metal salts are prepared separately in stoichiometric proportions by dissolving them in distilled water and then making a mixture by adding solutions with constant magnetic stirring. Aqueous solution of citric acid is added to the salt solution with cations to citric acid molar ratio of 1:1.5.Then ammonium hydroxide solution is added drop by drop to attain the 6.8 pH value. Then the solution is heated at 80 1C–85 1C for 4–6 h with continuous stirring using magnetic stirrer. ð1 xÞBaðNO3 Þ2 þ xLaðNO3 Þ3 þ xCoðNO3 Þ2 6H 2 O NH 4 OH Ba1 x Lax Cox Fe12 x O19 þ ð12 xÞFeðNO3 Þ3 9H 2 O Heat After evaporation of water, the liquid gets converted into a homogenous brown colored gel. The viscous solution is dried over hot plate at 280–300 1C for 3 h to form the precursor material. Presintering is done at 500 1C for 2 h. Then the precursor material is calcined at 700 1C, 900 1C, 1100 1C and 1200 1C for 5 h. The whole process is carried out at room temperature as illustrated in the flow chart (Fig. 1).
3. Characterization techniques Structural properties of heat treated samples have been investigated using X-ray powder diffraction patterns obtained from Bruker AXS D8 Advance X-ray diffractometer in the range 20˚ to 80˚ using Cu-kα radiation operating at 40 kV and 35 mA having step size of 0.021. To analyse the attached functional groups, Fourier transform infrared spectra (FT-IR interferometer IR prestige-21 FT-IR (model-8400 S)) are taken in the range 400–4000 cm 1. Thermo gravimetric analyzer (TGA) under N2 atmosphere 2+
Metal Nitrates (Ba ,La3+, Co 2+, Fe3+)
207
(NETZSCH TG 209F1 Libra TGA209F1D-0105-L) with a heating rate of 30 1C/10 min from 0 to 1000 1C is used for thermal analysis. Magnetic properties have been investigated with vibrating sample magnetometer (PAR-155 Princeton Applied Research, USA) at room temperature. Dielectric analysis has been performed with LCR meter (Model: 6440B) at 20 Hz- 3 MHz. Surface features have been taken from FE-SEM (TESCAN MIRA II) at an operating voltage of 5– 25 kV and at a scale of 2 mm. Band gap study has been carried out using UV–vis–NIR (Model: Varian Carry 5000 at room temperature with 0.2 nm resolution).
4. Results and discussions 4.1. Structural properties 4.1.1. Phase analysis of BaLCM The synthesized sample of BaLCM, heat treated at 700 1C, 900 1C, 1100 1C and 1200 1C for 5 h, have been investigated by well-known non-destructive X-ray powder diffractometer technique and presented in Fig. 2. All the samples are in crystalline phase and show absence of impurities above 700 1C. The presence of apparent peaks having hkl values 110, 008, 107, 114, 203, 205, 217, 2011 and 220 confirms the hexagonal structure of BaLCM powders and these peaks are similar to the peaks in standard pattern (JCPDS-391433). While comparing the peaks of BaLCM with standard data of un-substituted barium hexaferrite, it has been observed that these peaks for the samples calcined at 700 1C, 900 1C, 1100 1C and 1200 1C appear at the same diffraction angle position. All the samples except the one sample calcined at 700 1C exhibit single phase crystallinity. This proves that the substituted ions have occupied the crystallographic sites. Very small peak of α-Fe2O3 appears in sample calcined at 700 1C (shown as n between 107 and 114 hkl peaks), but it disappears above 700 1C. No undesired phases are observed above 700 1C. This indicates the complete crystallization, formation of homogeneous mixture of starting compound solutions, and synthesis of pure substituted BaM. 4.1.2. Crystallite size of BaLCM Crystallite size (D) has been estimated from the Scherer formula [24]: D ¼ kλ=β cos θ, where λ is X ray wavelength (1.54056 Å), β is full width at half maxima (in radian) and θ is the Bragg angle and k is the shape factor having unit value for hexagonal structure. The crystallite size increases with increase in
Citric acid
Mixture solution Ammonia solution Sol Stirring at 80 - 85˚C Gel Heated at 280 - 300˚C Precursor material Heat treatment BaLCM Fig. 1. Flow Chart for the synthesis of BaLCM powders.
Fig. 2. X ray powder patterns for Ba0.7La0.3Fe11.7Co0.3O19 at (a) T ¼ 700 1C, (b) T ¼ 900 1C, (c) T ¼ 1100 1C and (d) T ¼ 1200 1C.
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calcination temperature (Table 1(a)); minimum size (17.53 nm) appears at 700 1C. On the basis of obtained results, it can be interpreted that the agglomeration may happen with increase in calcination temperature. 4.1.3. Surface area Surface area calculated from the following expression has been shown in Table 1(b) [9]: S ¼ 6000=DDx , where D is crystallite size and Dx is X-ray density. The calculated surface area shows a decrease in its value from 64.971 m2/g to 21.251 m2/g. Crystallite size is the dominant factor for variation in surface area. As the crystallite size increases, lesser number of atoms appear at the surface. 4.1.4. Strain The strain in the BaLCM has been evaluated using following relation [9]
β cos θ ¼
0:9λ þ 2η sin θ D
Fig. 3. Variation of lattice constants with temperature for BaLCM.
where η is the strain. The strain shows a decrement as the calcination temperature increases. Strain is the distortion that is induced during synthesis process and heat treatment process. But the reason for decrease in value may attribute to the formation and enhancement towards pure hexagonal structural properties. 4.1.5. Lattice constants, densities and porosity Lattice constants (a and c) obtained from X-ray data using following equation have been tabulated in Table 1(a) [25]: " # 2 2 2 1 4 h þ hk þ k l ¼ þ 2 2 2 a c dhkl 3 where dhkl is d spacing value, hkl are miller indices for peaks in XRD powder patterns. It has been found that at 700 1C, lattice constant ‘a’ has highest value (5.9034 Å), above that it becomes constant and ‘c’ continuously decreases with increase in temperature from 23.2245 Å to 23.0870 Å as shown in Fig. 3.The main reason for contraction may be due to presence of strain, microstructural defects and interaction between substituted cations. Volume of cell can be calculated from [26]: Vcell ¼ 0.8666a2c. It has Table 1 (a) Diffraction angle (2θ), d spacing (d), full width at half maxima (β), lattice constants (a and c) and volume of cell (Vcell) with variation of calcination temperature for BaLCM. T (1C)
700 900 1100 1200
2θ (1)
34.016 34.137 34.137 34.184
d (Å)
2.63342 2.62436 2.62436 2.62088
β (1)
0.525 0.195 0.193 0.174
Lattice constants a(Å)
c(Å)
5.9034 5.8844 5.8844 5.8844
23.2245 23.1552 23.1211 23.0870
D (nm)
Vcell (Å3)
17.53 47.22 47.71 52.93
701.4060 694.8185 693.7962 692.7735
been seen that with increase in temperature, crystallite size increases and volume of the unit cell decreases. This may be attributed to the presence of a rare-earth element, Lanthanum, which has ability to form strong lanthanide–oxygen bonds. The binding energy of lanthanide oxygen octahedral in rare-earth substituted oxide materials is known to be much higher than the transition metal ion-oxygen octahedral. This may be the factor for shrinkage of crystal lattice. X ray density can be calculated from following expression [26]: Dx ¼
ZM NA V cell
where Z is number of molecule per unit cell, M is molecular weight, NA is Avogadro's number (6.023 1023), and Vcell is cell volume. X ray density increases with calcination temperature from 5.268 g/cm3 to 5.334 g/cm3.This may be ascribed to the lattice contraction. 4.1.6. Porosity Porosity of the sample can be calculated using relation [26]: Dx Db P¼ 100 Dx Db ¼
m ∏r 2 d
where Dx is X ray density, Db is bulk density and can be calculated from mass (m) and volume of palette having radius (r) and thickness (d). Due to difference between bulk density and X-ray density, the samples exhibit porosity. Heat treatment or sample preparation parameters may induce shape defects in grains which may cause porosity in BaLCM. 4.2. Thermo gravimetric analysis (TGA)
Table 1 (b) X-ray density (DX), bulk density (Db), porosity (P), crystallite size (D), phase present, surface area (S) and strain induced (η) with variation of calcination temperature for BaLCM. T (1C) DX g/cm3 Db g/cm3 P (%)
D (nm) Phase
700
5.268
1.4187
73.06 17.53
900 1100 1200
5.318 5.326 5.334
1.2809 1.4182 3.04179
75.91 47.22 73.37 47.71 42.97 52.93
Hexa α-Fe2O3 Hexa Hexa Hexa
S 103 (m2/g) η 10 3 64.971
15.01
23.893 23.612 21.251
5.55 5.50 4.95
Thermal gravimetric analysis was performed to investigate the effect of heat on mass of sample. The resultant TGA curve of M-type hexaferrite Ba0.7La0.3Fe11.7Co0.3O19 (BaLCM) powder has been shown in Fig. 4. The reduction of weight (9.21%) between 0 1C to 425 1C is due to decomposition of remaining organic matter. Oxidation reduction reaction between the metal nitrates and citric acid is the reason for decomposition. Between 425 1C and 680 1C, there is weight loss of 19.97% due to decomposition of precursor and conversion from hematite to hexaferrite structure [27]. After 680 1C weight loss becomes constant which indicates the formation of BaLCM. From XRD study, it has been confirmed
T. Kaur et al. / Physica B 456 (2015) 206–212
209
Fig. 4. Thermo gravimetric diagram for Ba0.7La0.3Fe11.7Co0.3O19 precursor.
that BaLCM formation happens at 700 1C. The reaction interval for the BaLCM is 255 1C. 4.3. FT-IR study From FT-IR spectra analysis, the information regarding attached molecular bands or presence of functional groups, which are remains of chemicals used in the synthesis process, can be identified. Two prominent peaks that arise at 432 cm 1 and 586 cm 1, indicate the formation of hexaferrite (Fig. 5). The stretching vibrations of metal–oxygen bond are the reason for these peaks. The bands near 420–480 cm 1 and 550–590 cm 1 are attributed to the vibration of ferric crystallographic site (octahedral and tetrahedral) [28]. Absence of hydroxyl group carboxylic group (at 3200–3700 cm 1) in samples calcined above 700 1C reveals the completion of the reaction. There is no peaks associated to the nitrate ions near 1300 cm 1 [29]. Small band near 2900 cm 1 occurs due to presence of moisture absorbed by the sample [30]. The spectra for 700 1C and 900 1C show less transmittance area with relatively more prominent peaks. This shows that the cations have been substituted themselves at the ferric sites accompanied with strong interaction as compared to other samples. 4.4. Surface features FE-SEM micrographs of BaLCM has been recorded at room temperature and depicted in Fig. 6. Micrographs show hexagonal structure, uneven distribution, and grain size deformation. Crystallites are not closely packed throughout the whole sample but traces of their cluster can be visualized. During the synthesis, lots of gases evolved during combustion and decomposition of organic matter may cause porosity. 4.5. Dielectric properties Dielectric data been has recorded on LCR meter in a range 20 Hz–3 MHz using a pellet (with 8 t pressure) of BaLCM coated with silver metal powder. Though dielectric data have been calculated in 20 Hz to 3 MHz, but for clear analysis, the graphs have been considered for different range of frequencies because of the prominent effect shown by samples in small regions. It is clear that BaLCM gives a normal ferrite response towards frequency i. e. dielectric constant is high for low frequency but decreases at high frequency (Fig. 7). This behavior can be explained on the basis of Koop's phenomenological theory and Maxwell Wagner theory [31]. This may be attributed to an electron exchange between Fe3 þ and Fe2 þ .This results in the displacement of the ionic charges and is also responsible for polarization.
Fig. 5. FT-IR spectra for Ba0.7La0.3Fe11.7Co0.3O19 at different temperatures.
Dielectric constant has been calculated from parallel plate capacitor equation [31]: K¼
Cd
ε0 A
where K is dielectric constant, C is capacitance of the palette, d is thickness of sample palette and A is cross section area and εo is the permittivity of free space (8.85 10–12 F/m). Variation of dielectric constant can be explained on the basis of interfacial polarization of heterogeneous structure of barium hexaferrite which arises due to porosity and presence of grains. The effective contribution may also come from grain boundaries at low frequencies. The structure of material is supposed to be made of conducting grains separated by less conducting grain boundaries [32]. Heat treatment may be the cause of their creation. The hopping between Fe2 þ and Fe3 þ causes dispersion at low frequency. The electrons are displaced by the applied field and causes polarization but displacement happens only when hopping time is less than half of the alternating field. So the dielectric constant will decrease at high frequency [33]. Because of low eddy currents and high dielectric constant, the prepared material can play a significant role in electronics market having application as radiation absorber or high frequency component. Dielectric loss can be calculated by using expression [34]:
ε″ ¼ ε0 tan δ
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Fig. 8. Variation of dielectric loss with frequency of BaLCM at (a) 700 1C (b) 900 1C and (c) 1200 1C.
Fig. 6. FE-SEM micrographs for Ba0.7La0.3Fe11.7Co0.3O19 (a) at 900 1C (b) at 1100 1C.
Fig. 9. Effect of frequency on dielectric loss tangent at (a) 700 1C (b) 900 1C and (c) 1200 1C.
Fig. 7. Influence of frequency on dielectric constant of BaLCM at (a) 700 1C (b) 900 1C and (c) 1200 1C.
where ε0 is dielectric constant and ε″ is dielectric loss factor. Dielectric losses are also high at low frequency (Fig. 8).The behavior of dielectric loss tan δ with frequency is shown in Fig. 9. It becomes clear that after attaining a maximum value, dielectric loss tanδ decreases with increase in frequency. As described earlier in this section, hopping is the reason for high dielectric constant. Loss tangent behavior can similarly be explained on this basis. Maximum of loss tangent may be observed when the hopping is nearly equal to that of the externally applied electric field. After the maxima, hopping time becomes more than the applied signal causing decrease in loss tangent.
Fig. 10. Hysteresis temperatures.
loop
for
Ba0.7La0.3Fe11.7Co0.3O19
powder
at
different
4.6. Magnetic properties Coercivity, saturation magnetization, remnant magnetization and magnetic moment have been calculated from M–H curves
(Fig. 10). The highest value for coercivity (5602 Oe) was found in a sample calcined at 900 1C (Fig. 11) and saturation magnetization shows a continuous increase with increase in temperature.
T. Kaur et al. / Physica B 456 (2015) 206–212
Fig. 11. Variation in magnetic properties with temperature for BaLCM.
211
Fig. 12. Optical band gap for (a) BaM at 900 1C and (b) BaLCM at 900 1C.
Table 2 Summary of Coercivity (Hc), saturation Magnetization (Ms), Retentivity (Mr), Squareness Ratio (SR) Anisotropy constant (K) and magneton number (ηB) for Ba0.7La0.3Fe11.7Co0.3O19 with variation of calcination temperature.
absorber [35], circulators [36], multi-layer chip inductor [37], and electrical filters [36]. 4.7. Optical properties
T (1C)
Hc (Oe)
Ms(emu/g)
Mr(emu/g)
SR(Mr/Ms)
K(HA2/kg)
ηB(μB)
700 900 1100 1200
5210 5602 3493 1752
47.22 49.34 51.06 52.52
28.79 30.61 32 28.63
0.6096 0.6203 0.6267 0.5451
12.30 13.82 8.92 4.60
9.408 9.830 10.173 10.464
Calcined product at 900 1C show highest porosity (75.91%) and low bulk density (1.2809 g/cm3). These parameters affect the magnetic properties of material. The following relation (Eq. (1)) has been employed to find out anisotropic constant (K) [28]: Hc ¼
2K
μ0 MS
ð1Þ
where μ0 is permeability in vacuum (4π 107 H/m), MS is saturation magnetization and Hc is coercivity. Presence of pores impedes the movement of domains walls that needs the application of high magnetic field to make alignment, so cause high coercivity. Magneton number can be found out from the equation [13]:
ηB ¼
Molecular weight M S 5585
Saturation magnetization of BaLCM is lower than that of the MS of BaM. It has been observed that with increase in temperature there is increase in saturation magnetization of BaLCM (Fig. 11). This is due to increase in grain size, reduction of grain boundary, and enhancement of the superexchange interaction between Fe–O–Fe. Reduction of porosity may also be responsible for increment in MS because grains come closer and more and more magnetic moment align themselves with decrease in porosity resulting in increased agglomeration. This has been revealed in FE–SEM micrographs. Magnetic parameters have been affected by, cationic distribution, strain, ionic radii, valency of ferric ion, and lattice distortion. As magneton number depends on saturation magnetization, so it increases with increase in Ms. Due to anisotropic morphology of barium hexaferrite, anisotropic constant has highest value at 900 1C. This can also be understood in terms of coercivity. At 900 1C, coercivity is highest and according to the relation (Eq. (1)), anisotropic constant is also maximum (Table 2). By controlling the temperature, the magnetic properties can be enhanced to meet the criteria for hyper frequency application as micro wave
The optical properties of BaLCM have been investigated by using UV–Vis–NIR absorption spectra. In the optical region, incidence of electromagnetic waves on the material causes electrons in the valence band to absorb the energy, raising its energy level. This absorbed band gap energy has been calculated from the spectrum from following relation [38]:
α¼
Aðhν Eg Þ1=2 hν
where E is energy band gap and A is a constant. The band gap has been obtained by plotting the graph from fundamental absorbance edges. Fig. 12 shows the spectra for energy band gap and the absorbance spectra (inset). The band gap values concluded from graph are: 3.92 eV (pure barium hexaferrite at 900 1C), 4.1 eV (for Ba0.7La0.3Fe11.7Co0.3O19 at 900 1C). The graph shows that with substitution the band gap increases. BaM shows low value of band gap than BaLCM. Band gap of BaLCM is higher than those of barium hexaferrite (3.18 eV) and thin films (2.32 eV) [38,39]. Quantum confinement at nano scale plays important role in increasing the band gap. From UV–NIR absorption spectra (Inset) it has been concluded that the absorption region for hexaferrites is 200–600 nm. The literature available for UV–vis study for barium hexaferrites is very less.
5. Conclusion The formation of BaLCM with no impurities has been revealed from XRD, FT-IR and FE-SEM. TGA shows that BaLCM are formed at 680 1C. UV–vis–NIR exhibits that the band gap of samples increases with substitution. From VSM analysis techniques, it was observed that the saturation magnetization, retentivity, squareness ratio increases with increase in temperature but coercivity and anisotropy constant is the highest for the sample calcined at 9001C. BaLCM shows normal ferrite dielectric behavior.
Acknowledgments We are thankful to STIC-Kochi (Ernakulum) for XRD and UV–vis– NIR and IIT Roorkee for FE-SEM and VSM facilities. We are highly
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thankful to Lovely Professional University for providing financial support (No. LPU/DRD/IPF/Sac/004) for this research work. References [1] P. Bhattacharya, S. Dhibar, G. Hatui, A. Mandal, T. Das, C.K. Das, RSC Adv. 4 (2014) 17039. [2] C. Navau, J. Prat-Camps, O. Romero-Isart, J.I. Cirac, A. Sanchez, Phys. Rev. Lett. 112 (2014) 253901. [3] M.N. Ashiq, J.M. Iqbal, M. Najam-ul-Haq., P.H. Gomez, A.M. Qureshi, J. Magn. Magn. Mater. 324 (2012) 15. [4] H.A. Patel, J. Byun, C.T. Yavuz, J. Nanopart. Res. 14 (2012) 881. [5] V. Anbarasu, P.M. Md Gazzali., T. Karthik, A. Manigandan, K. Sivakumar, J. Mater. Sci-Mater. El. 24 (2013) 916. [6] M. Radwan, M.M. Rashad, M.M. Hessien, J. Mater. Process. Technol. 181 (2007) 106. [7] T. Gonzalez-Carreno, M.P. Morales, C.J. Serna, Mater. Lett. 43 (2000) 97. [8] M. Cernea, S.G. Sandu, C. Galassi, R. Radu, V. Kuncser, J. Alloys Compd. 561 (2013) 121. [9] A. Thakur, R.R. Singh, P.B. Barman, Mater. Chem. Phys. 141 (2013) 562. [10] P.B. Braun, PB. Philos. Res. Rep. 12 (1957) 491. [11] M.L. Gregori, M.S. Pinho, R.C. Lima, J.C.S. Leandro, T. Ogasawara, Key Eng. Mater. 264–268 (2004) 1229. [12] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, M.A. Iqbal, J. Supercond. Nov. Magn. 26 (2013) 3277. [13] V.N. Dhage, M.L. Mane, A.P. Keche, C.T. Birajdar, K.M. Jadhav, Physica B 406 (2011) 789. [14] A.G. Belous, O.I. V'yunov, E.V. Pashkova, V.P. Ivanitskii, O.N. Gavrilenko, J. Phys. Chem. B 110 (2006) 26477. [15] F. Khademi, A. Poorbafrani, P. Kameli, H. Salamati, J. Supercond. Nov. Magn. 25 (2012) 525. [16] P. Winotai, S. Thongmee, I.M. Tang, Mater. Res. Bull. 35 (2000) 1747. [17] M.K. Tehrani, A. Ghasemi, M. Moradi, R.S. Alam, J. Alloys Compds. 509 (2011) 8398.
[18] Y. Liu, M.G.B. Drew, Y. Liu, J. Wang, M. Zhang, J. Magn. Magn. Mater. 322 (2010) 814. [19] H. Li, J. Huang, Q. Li, X. Su, J. Sol–Gel Sci. Technol. 52 (2009) 309. [20] M.C. Dimri, S.C. Kashyap, D.C. Dube, Ceram. Int. 30 (2004) 1623. [21] X. Liu, J. Wang, L.M. Gan, S.C. Ng, J. Ding, J. Magn. Magn. Mater. 184 (1998) 344. [22] A. Gonzalez-Angeles, G. Mendoza-Suarez, A. Gruskova, I. Toth, V. Jancarik, M. Papanova, J.I. Escalante-Garcı., J. Magn. Magn. Mater. 270 (2004) 77. [23] A.P. Safronov, O.M. Samatov, A.I. Medvedev, I.V. Beketov, A.M. Murzakaev, Nanotechnol. Russ. 7 (2012) 486. [24] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, M.A. Iqbal, J. Supercond. Nov. Magn. 26 (2013) 3315. [25] W.Y. Zhao, P. Wei, H.B. Cheng, X.F. Tang, Q.J. Zhang, J. Am. Ceram. Soc. 90 (2007) 2095. [26] M.J. Iqbal, M.N. Ashiq, Chem. Eng. J. 136 (2008) 383. [27] A. Poorbafrani, P. Kameli, H. Salamati, J. Mater. Sci. 48 (2013) 186. [28] F. Song, X. Shen, J. Xiang, H. Song, Mater. Chem. Phys. 120 (2010) 213. [29] F. Shahbaz Tehrani, V. Daadmehr, A.T. Rezakhani, R. Hosseini Akbarnejad, S. Gholipour, J. Supercond. Nov. Magn. 25 (2012) 2443. [30] J.P. Singh, G. Dixit, R.C. Srivastava, H.M. Agrawal, K. Asokan, J. Phys. D Appl. Phys. 44 (2011) 435306. [31] A. Singh, S. Bindra Narang, K. Singh, O.P. Pandey, R.K. Kotnala, J. Ceram. Process. Res. 11 (2010) 241. [32] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, J. Electron. Mater. 43 (2014) 512. [33] S.B. Narang, I.S. Hudiara, J. Ceram. Process. Res. 7 (2006) 113. [34] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, J. Mater. Eng. Perform. 22 (2013) 2104. [35] M.R. Meshram, Nawal K. Agrawal, B. Sinha, P.S. Misra, J. Magn. Magn. Mater. 271 (2004) 207. [36] M.J. Iqbal, S. Farooq, Mater. Res. Bull. 46 (2011) 662. [37] A. Haq, M. Anis-ur-Rehman, Physica B 407 (2012) 822. [38] M. Karmakar, B. Mondal, M. Pal, K. Mukherjee, Sens. Actuator B—Chem. 190 (2014) 627. [39] S. Anjum, M.S. Rafique, M. Khaleeq-ur-Rahman, K. Siraj, Arslan Usman, S.I. Hussain, S. Naseem, J. Magn. Magn. Mater. 324 (2012) 711.