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Structural and magnetic Properties of TbZn-substituted calcium barium M-type nano-structured hexa-ferrites
Journal of Alloys and Compounds 589 (2014) 258–262
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structural and magnetic Properties of TbZn-substituted calcium barium M-type nano-structured hexa-ferrites Hasan M. Khan a,b, M.U. Islam a,⇑, Yongbing Xu b,c, M. Asif Iqbal a,d, Irshad Ali a a
Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan Department of Electronics, University of York, York YO10 5DD, UK c Nanjing–York International Centre of Spintronics and Nano-Engineering, Nanjing University, Nanjing 210093, China d National University of Science and Technology, College of E & ME, Islamabad, Pakistan b
a r t i c l e
i n f o
Article history: Received 15 October 2013 Received in revised form 15 November 2013 Accepted 15 November 2013 Available online 25 November 2013 Keywords: Sol–gel auto combustion Nanoparticles Hexagonal ferrites Magnetization
a b s t r a c t Effect of TbZn substitution on the structural and magnetic properties of Ca0.5Ba0.5xTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00–1.00) ferrites prepared by sol–gel auto combustion is reported. The synthesized samples were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Vibrating Sample magnetometery. The X-ray diffraction analysis confirmed single phase M-type hexa-ferrite structure. The lattice parameters were found to increase as TbZn contents increases, which is attributed to the ionic sizes of the implicated cations. The TbZn seems to be completely soluble in the lattice. The results of scanning electron microscopy and transmission electron microscopy shows that the grain size decreases with increase of TbZn substitution. The coercivity values (1277–2025 Oe) of all samples lies in the range of M-type hexa-ferrite and indicate that an increase of anisotropy was achieved by substitution of TbZn, while the size of nanoparticles was drastically reduced between 18 and 25 nm. The increased anisotropy and fine particle size are useful for many applications, such as improving signal noise ratio of recording devices. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Hexa-ferrites are very attractive materials for technological applications due to their unique electrical and magnetic properties. Most of the investigations carried out on ferrites with the M-type hexagonal crystalline structure. Hexagonal ferrites have many applications, such as recording, magneto-optical devices, and permanent magnets. Its unique magneto-dielectric property is particularly interesting in microwave and radio frequency applications. Several studies on the magnetic properties of hexagonal ferrites have been reported [1–4]. Moreover the low cost of ferrite materials and easy manufacturing has greatly enhanced their production and commercial effectiveness. The magnetic properties of BaFe12O19 strongly depend on the mean grain size and morphology of the powders synthesized [5–8]. Nano-structured hexagonal BaFe12O19 has a single magnetic domain and high anisotropy and because of that it offers excellent magnetic properties [9]. The intrinsic magnetic properties resulted from the specific site occupancy of the Fe ions and rare earth (RE) elements along with divalent cations can drastically affect the electromagnetic properties of ferrites [10,11]. For these reasons manipulation ⇑ Corresponding author. E-mail address: [email protected] (M.U. Islam). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.107
of Fe and RE in the lattices is a promising way to improve magnetic properties of hexagonal ferrites. In this work the sol–gel method was used to fabricate calcium barium hexa-ferrites. Very few investigations are available on Ca-based M-type hexa-ferrites [12,13]. Calcium as a base is chosen for its intrinsic properties like the reduction in sintering temperature and enhancement in the properties. The aim of the present study is to investigate the effect of TbZn substitution on the magnetic properties of Ca-based M-type hexa-ferrites.
2. Experimental details 2.1. Sample preparation M-type hexa-ferrites with a chemical composition of Ca0.5Ba0.5xTbxZnyFe12yO19 (x = 0.00–0.10; y = 0.00–1.00) were synthesized by sol–gel auto combustion method using analytical grade citric acid, iron chloride (FeCl3.6H2O), calcium chloride (CaCl2), barium nitrate Ba(NO3)2, zinc nitrate Zn(NO3)2 and terbium oxide (Tb2O3). The reagents were stirred in 100 ml water (1:1) for 20 min and finally mixed into the citric acid solution. Tb2O3 was first dissolved in nitric acid at 80 °C for converting it into nitrate and then mixed with the prepared solution as the Tb2O3 is insoluble in water. To maintain the pH value at 7 an aqueous ammonia solution at a ratio of 40:60 was added drop-wise, while stirring at 60 °C. The sols were allowed to gel at 60 °C after 7 h and the gel was then dried at 200 °C for 1 h. The dried gel was ground homogenously in an agate mortar and pestle for 20 min. Finally, a homogenized powder was formed and sintered at 1100 °C for 8 h.
259
H.M. Khan et al. / Journal of Alloys and Compounds 589 (2014) 258–262 2.2. Characterization
qXray ¼
Fourier transform infrared (FTIR) spectrophotometer (BioRad Merlin FTS 3000MX) was used to obtain optical spectra of the prepared samples. X-ray diffraction analysis was performed using a shimadzu X-ray diffractometer with Cu Ka radiation (k = 1.5406 Å). The crystallite size was determined by Scherrer formula [14].
D¼
kk b Cos h
1 2
2.2.1. Electron microscopy TEM was used for the estimation of the crystallite size and aggregation. The analysis of the samples were investigated using a TEM:JEOL Japan 2011 (with LaB6 filament). Specimens were prepared by sonicating the powder particles in the ethanol for 1 h. Few drops of the suspension were put onto carbon-coated copper TEM grids before allowing them to dry in air.SEM analysis of the samples were performed using an FEI SIRION SEM operating at 20 kV to analyze the surface morphology and particles size of the samples.
ð1Þ
2
¼
2
2
4ðh þ hk þ k Þ l þ 2 c 3a2
ð2Þ
V cell ¼ 0:8666a2 P¼
3.1. FTIR spectroscopy
ð4Þ
qXray
qm ¼
3. Results and discussions
ð3Þ
1 qm
m
Fig. 1 shows FTIR profile of three sintered samples with nominal compositions Ca0.5Ba0.5XTbxZnyFe12yO19 (x = 0.02, 0.06 and 0.10; y = 0.20, 0.60 and 1.00). Three absorption bands at 554, 545 and 522 cm1 respectively were observed [16,17]. The occurrence of absorption bands in FTIR spectra are due to stretching vibrations to the metal–oxygen in the lattice and confirmed the formation of hexagonal structure [18].
ð5Þ
phr2
0
Transmittance %
10 20 30
ð6Þ
where M is the molar mass, a and c are lattice constants, m the mass of pellets, r radius of the pellets, NA Avogadro’s number and Vcell is the unit cell volume. The vibrating sample magnetometer (VSM) Lake Shore (7400, USA), was used to measure M–H loops at room temperature.
where K is shape constant, k is the wavelength, b is FWHM intensity and h is the Bragg angle. The values of X-ray density (qX-ray), bulk density (qm), porosity (P) and the unit cell volumes (Vcell) were calculated by following equations [15]:
d
2M N A V cell
3.2. X-ray diffraction analysis
x=0.02 x=0.06 x=0.10
Fig. 2 shows XRD analysis of the Ca0.5Ba0.5XTbxZnyFe12yO19 (x = 0.00, 0.02, 0.06 and 0.10; y = 0.00, 0.20, 0.60 and 1.00) hexaferrite materials. The XRD patterns reveals that all the peaks identified match well with the standard M-type barium hexa-ferrites JCPDS card No. ICDD 00–051-1879 [19]. The crystallite size calculated using scherrer formula is in the range of (15–46 nm) as listed in Table 1 which is less than 50 nm [32]. Fig. 3 shows variation of lattice parameters calculated using X-ray data. The lattice constants of Ca0.5Ba0.5xTbxZnyFe12yO19 hexa-ferrites varies as a = (5.842 – 5.874) Å and c= (23.281 – 23.792) Å, which are slightly different than that of BaFe12O19. These slight changes in the lattice constants may have
40 50 60 70 4000 3500 3000 2500 2000 1500 1000 500
0
-1
Wave Number (cm ) Fig. 1. FTIR analysis of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.02, 0.06 and 0.10).
X=0.00
221 006
220
201 110 201 008 114 205 110 203 301 206
140
214
300 264 281 218
20 13 300 11 14
214
304
Intensity (arb. units)
X=0.02
X=0.04
X=0.06
X=0.08
X=0.10 20
30
40
50
60
70
2 Theta (Degrees) Fig. 2. XRD analysis of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00–1.00).
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3.3. Magnetic measurements
Zn.Contents 0.0
6.00
0.2
0.4
0.6
0.8
1.0 24.4
a( ) C( )
5.95
24.2
5.90 5.85
24.0
)
a(
5.70 5.65
23.6 23.4
5.60
23.2
5.55 5.50
23.8
)
5.75
C(
5.80
0.00
0.02
0.04
0.06
0.08
0.10
23.0
Tb. Contents Fig. 3. Lattice parameters for Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00– 1.00).
been caused by the difference between the ionic radii of Tb3+ (0.92 Å) and Fe3+ (0.645 Å). An increase in the lattice constants by substitution of rare earth element has also been observed by other researchers [18,19]. Hence the variation of lattice parameters and grain size indicates that Tb3+ has entered into the lattice of the hexa-ferrite made. It was reported earlier in the literature that the ‘c/a’ ratio can be used to quantify the structure of the hexa-ferrite [20]. The range of lattice parameters in our samples lies near to expected range from 3.98–4.05 Å and manifested the M-type hexaferrite.
Fig. 4 shows the M-H loops for Ca0.5Ba0.5XTbxZnyFe12yO19 (x = 0.00–0.10; y = 0.00–1.00) measured by VSM at a maximum applied field of 10,000 Oe at room temperature. The effect of different doping contents of TbZn on the magnetic properties of the calcium barium ferrite powders are listed in Table 1. Fig. 5 shows the coercivity (HC) rapidly falls from 1635 Oe to 1277 Oe with substitution of dopants at x = 0.02 and y = 0.20, which is due to the replacement of Fe3+ ions at 4f2 site. It has been reported [21] that 4f2 and 2b sites contribute towards large anisotropy field but due to simultaneous grain growth the coercivity as a whole decreases [22,23]. At higher substitution level at x = 0.10, y = 1.00, the value of HC increases to its maximum value of 2025 Oe due to the presence of Fe2+ ions at 2a site which positively contributes for enhancement of the magneto-crystalline anisotropy. Similar variation is also observed by Wang in La substituted hexagonal ferrite. This indicates that the introduction of TbZn can be used to tune the anisotropy/coercivity of the materials. Fig. 6 show the saturation magnetization (Ms) and the remanent magnetization (Mr) increase to their maximum to 36.76 and 20.95 (emu/g) respectively at concentration x = 0.04 of the rare earth element Tb3+, at x P 0.04 there is a decrease in both Mr and Ms for all the samples. The magnetic behavior of the samples may depend upon the factors like ionic radii and strength of the magnetic interactions. The ionic radii of Tb3+ and Fe3+ is not comparable and this difference in ionic radii introduces a local strain which causes the disorder and modifications of local electronic states. The variation of rare earth concentration
Zn. Contents 0.0
0.2
0.4
0.6
0.8
1.0
40
2200 2000
20
Hc (Oe)
M (emu/g)
2400 x=0.00,y=0.00 x=0.02,y=0.20 x=0.04,y=0.40 x=0.06,y=0.60 x=0.08,y=0.80 x=0.10,y=1.00
0 -10000
-5000
0
5000
10000
1800 1600 1400
H (Oe)
1200
-20
1000 -0.02 0.00
-40
0.02
0.04
0.06
0.08
0.10
0.12
Tb. Contents
Fig. 4. VSM analysis of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00–1.00).
Fig. 5. Variation of coercivity of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10).
Table 1 Magnetic and lattice parameter values for Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10). Contents of (TbZn)
X = 0.0 Y = 0.0
X = 0.02 Y = 0.20
X = 0.04 Y = 0.40
X = 0.06 Y = 0.60
X = 0.08 Y = 0.80
X = 0.10 Y = 1.0
Magnetization Ms (emu/g) Remanence Mr (emu/g) Mr/Ms Coercivity Hc (Oe) a (Å) c (Å) Ratio c/a Vcell D (nm) ± (0.01) P (%) D (nm), SEM D (nm), TEM
37.08 23.6 0.63 1645 5.73 23.19 4.04 456.21 17.21 26 48 19
40.2 25.7 0.63 1577 5.81 23.27 4.00 453.62 22.81 42
41.7 26.5 0.63 1511 5.87 23.31 3.97 445.15 12.16 47
35.8 21.4 0.59 1925 5.91 23.58 3.98 451.47 14.19 28 32 15
38.2 23.9 0.62 1786 5.86 23.67 4.03 449.34 21.27 27
37.8 22.9 0.60 1805 5.95 23.81 4.00 452.13 24.12 21 27 23
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Zn. Contents 50
0.0
0.2
0.4
0.6
0.8
1.0
3.4. Scanning electron microscopy, EDX and transmission electron microscopy
Ms&Mr (emu/g)
45 40 35
Ms
30 25 20 15
influences the super exchange interactions between magnetic cations and hence on the magnetization [24–27].
Mr
0.00
0.02
0.04
0.06
0.08
0.10
Tb. Contents Fig. 6. Variation in Ms and Mr of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10).
SEM and TEM analysis was performed for three selected samples with nominal composition Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 and Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19. Fig. 7(a–c) show the SEM analysis of Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19, Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19 samples. It is clear from the SEM profiles that the particles have well defined shape and boundaries. Fig. 7(d–f) show the TEM analysis of Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 and Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19 samples. The values of particle sizes calculated by TEM were found to be in the range of 18– 25 nm, 16–23 nm and 14–21 nm. The grain size calculated by SEM ranges from 40–80 nm, 30–60 nm and 20–45 nm, for Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 and Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19 samples respectively. The particle size
Fig. 7. (a–c) SEM analysis of Ca0.5Ba0.5XTbxZnyFe12yO19 i.e (x = 0.02, 0.06 and 0.10) (d–f) TEM analysis of Ca0.5Ba0.5XTbxZnyFe12yO19 i.e (x = 0.02, 0.06 and 0.10). Table 2 EDS analysis of M-type hexa-ferrite Ca0.5Ba0.5XTbxZnyFe12yO19 powders, (x = 0.02, 0.06 and 1.00). Sr. No. 1 2 3
Formula of Sample Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19 Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19
Zn 2.40 4.95 5.67
Fe 79.7 72.09 68.74
Observed
Weight
%
Ca 4.11 4.13 4.21
Ba 8.60 7.53 6.47
Tb 5.16 11.29 14.73
Total 100 100 100
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H.M. Khan et al. / Journal of Alloys and Compounds 589 (2014) 258–262
of the SEM samples are much smaller compared to those of the reported earlier 83 nm, 151–200 nm, 70–100 nm and 60–300 nm ranges for the M-type hexa-ferrite [28–33]. These particles are small in size and suitable for improving signal-to-noise ratio in high density recoding media. The elemental composition of the synthesized samples was estimated by energy dispersive X-ray spectroscopy analysis. It is evident from the analysis that stoichiometric amounts of Ba2+ and Fe3+ decreases while Tb3+ and Zn2+ increases. This signifies that Tb3+ is being substituted by Ba2+ site and Zn2+ is being substituted by Fe3+ site as listed in Table 2. 4. Conclusions All the TbZn substituted Ca0.5Ba0.5XTbxZnyFe12yO19 samples exhibit a single phase magnetoplumbite structure with slightly increasing values of lattice parameters a and c. Both anisotropy and coercivity can be tuned at certain substitution level. The magnetization behavior has been characterized by taking into account the preferential site occupancy of sub-lattice sites by substituted cations, magnetic dilution and spin canting. The crystallites size was found in the range 14–25 nm and 16–28 nm calculated by TEM and Scherrer equation, respectively. The increased anisotropy and reduction of particle size are beneficial for many applications, such as improving signal noise ratio and stability of recording devices.
References [1] Vaishali V. Soman, V.M. Nanoti, D.K. Kulkarni, Dielectric and magnetic properties of Mg–Ti substituted barium hexa-ferrite, Ceram. Int. 39 (2013) 5713–5723. [2] Y. Ebrahimi, A.A. Sabbagh Alvani, A.A. Sarabi, H. Sameie, R. Salimi, M. Sabbagh Alvani, S. Moosakhani, ‘‘A comprehensive study on the magnetic properties of nanocrystalline SrCo0.2Fe11.8O19 ceramics synthesized via diverse routes’’, Ceram. Int. 38 (2012) 885–3892. [3] E. Naiden, V. Maltsen, G. Ryabtsen, Phys. Stat. Sol. A 120 (1990) 209. [4] K. Yoon, D. Lee, H. Jung, S. Yoon, J. Mater. Sci. 27 (1992) 2941. [5] J. Ding, D. Maurice, W.F. Miao, P.G. McCormick, R. Street, Hexa-ferrite magnetic materials prepared by mechanical alloying, J. Magn. Magn. Mater. 150 (1995) 417–420.
[6] G. Mendoza-Suarez, J.A. Matutes-Aquino, J.I. Escalante-Garcıa, H. ManchaMolinar, D. Rıos-Jara, K.K. Johal, Magnetic properties and microstructure of Ba– ferrite powders prepared by ball milling, J. Magn. Magn. Mater. 223 (2001) 55– 56. [7] S.M. Abbas, A.K. Dixit, R. Chatterjee, T.C. Goel, J. Magn. Magn. Mater. 309 (2007) 20. [8] J. Kulikowski, J. Magn. Magn. Mater. 41 (1984) 56. [9] S. Ruan, B. Xu, H. Suo, F. Wu, S. Xiang, M. Zhao, Microwave absorptive behavior of ZnCo-substituted W-type Ba hexa-ferrite nanocrystalline composite material, J. Magn. Magn. Mater. 212 (2000) 175–177. [10] D. Lisjak, K. Bobzin, K. Richardt, M. Bégard, G. Bolelli, L. Lusvarghi, A. Hujanen, P. Lintunen, M. Pasquale, E. Olivetti, J. Eur. Ceram. Soc. (2009) 9. [11] W. Jing, Z. Hong, B. Shuxin, C. Ke, Z. Changrui, Microwave absorbing properties of rare-earth elements substituted W-type barium ferrite, J. Magn. Magn. Mater. 312 (2007) 310–313. [12] Muhammad Javed Iqbal, Muhammad Naeem Ashiq, Iftikhar Hussain Gul, ‘‘Physical, Electrical and dielectric properties of Ca-substituted strontium hexa-ferrite (SrFe12O19) nanoparticles synthesized by co-precipitation method’’, J. Magn. Magn. Mater. 322 (2010) 1720–1726. [13] Shahid Hussain, A. Maqsood, J. Magn. Magn. Mater. 316 (2007) 73–80. [14] M.J. Iqbal, M.N. Ashiq, Chem. Eng. J. 136 (2008) 383. [15] P.A.M. Catellanos, J.C.S. Jarque, J.A. Rivera, Phys. B 362 (2005) 95–102. [16] A. Mali, A. Ataei, Scr. Mater. 53 (2005) 1065. [17] Muhammad Javed Iqbal, saima farooq, Mater. Res. Bull. 46 (2011) 662–667. [18] M.N. Ashiq, M.J. Iqbal, I.H. Gul, J. Alloys Comp. 487 (2009) 341–345. [19] N. Rezlescu, C. Doroftei, E. Rezlescu, P.D. Popa, Phys. Status Solidi 15 (2006) 3844–3851. [20] T.R. Wagner, J. Solid State Chem. 136 (1998) 120. [21] Sun. Chang, Sun. Kangning, Chui. Pengfei, Microwave absorption properties of Ce- substituted M-type barium ferrite, J. Magn. Magn. Mater. 324 (2012) 802. [22] A.L. Stuijts, Trans. Br. Ceram. Soc. 55 (1956) 57. [23] J. Topfera, S. Schwarzer, S. Senz, D. Hesse, Influence of SiO2 and CaO additions on the microstructure and magnetic properties of sintered Sr-hexa-ferrite, J. Eur. Ceram. Soc. 25 (2005) 1681–1688. [24] G. Albanese, J. Phys. 38 (1977) C1–C85. [25] S. Ounnunkad, Solid State Commun. 138 (2006) 472–475. [26] Y. Wang, L. Li, H. Liu, H. Qiu, F. Xu, Mater. Lett. 62 (2008) 2060–2062. [27] E. Rezlescu, N. Rezlescu, C. Pasnicu, M.L. Craus, P.D. Popa, Cryst. Res. Technol. 31 (1996) 343–352. [28] A. Deschamps, F. Bertaut, Comp. Rend. Acad. Sci. 17 (1957) 3069. [29] S.W. Lee, S.Y. An, I. Shim, C.S. Kim, J. Magn. Magn. Mater. 290–291 (2005) 231. [30] M.M. Rashad, M. Radwan, M.M. Hessien, J. Alloy Comp. 453 (2008) 304. [31] C. Mu, N. Chen, X. Pan, X. Shen, X. Gu, Mater. Lett. 62 (2008) 840. [32] H. Khan, M. Islam, I. Ali, M.U. Rana, Electrical transport properties of Bi2O3doped CoFe2O4 and CoHo0.02Fe1.98O4 Ferrites, Mater. Sci. Appl. 2 (8) (2011) 1083–1089. [33] Muhammad Javed Iqbal, Muhammad Naeem Ashiq, Iftikhar Hussain Gul ‘‘Physical, Electrical and dielectric properties of Ca-substituted strontium hexa-ferrite (SrFe12O19) nanoparticles synthesized by co-precipitation method’’, J. Magn. Magn. Mater. 322 (2010) 1720–1726.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structural and magnetic Properties of TbZn-substituted calcium barium M-type nano-structured hexa-ferrites Hasan M. Khan a,b, M.U. Islam a,⇑, Yongbing Xu b,c, M. Asif Iqbal a,d, Irshad Ali a a
Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan Department of Electronics, University of York, York YO10 5DD, UK c Nanjing–York International Centre of Spintronics and Nano-Engineering, Nanjing University, Nanjing 210093, China d National University of Science and Technology, College of E & ME, Islamabad, Pakistan b
a r t i c l e
i n f o
Article history: Received 15 October 2013 Received in revised form 15 November 2013 Accepted 15 November 2013 Available online 25 November 2013 Keywords: Sol–gel auto combustion Nanoparticles Hexagonal ferrites Magnetization
a b s t r a c t Effect of TbZn substitution on the structural and magnetic properties of Ca0.5Ba0.5xTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00–1.00) ferrites prepared by sol–gel auto combustion is reported. The synthesized samples were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Vibrating Sample magnetometery. The X-ray diffraction analysis confirmed single phase M-type hexa-ferrite structure. The lattice parameters were found to increase as TbZn contents increases, which is attributed to the ionic sizes of the implicated cations. The TbZn seems to be completely soluble in the lattice. The results of scanning electron microscopy and transmission electron microscopy shows that the grain size decreases with increase of TbZn substitution. The coercivity values (1277–2025 Oe) of all samples lies in the range of M-type hexa-ferrite and indicate that an increase of anisotropy was achieved by substitution of TbZn, while the size of nanoparticles was drastically reduced between 18 and 25 nm. The increased anisotropy and fine particle size are useful for many applications, such as improving signal noise ratio of recording devices. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Hexa-ferrites are very attractive materials for technological applications due to their unique electrical and magnetic properties. Most of the investigations carried out on ferrites with the M-type hexagonal crystalline structure. Hexagonal ferrites have many applications, such as recording, magneto-optical devices, and permanent magnets. Its unique magneto-dielectric property is particularly interesting in microwave and radio frequency applications. Several studies on the magnetic properties of hexagonal ferrites have been reported [1–4]. Moreover the low cost of ferrite materials and easy manufacturing has greatly enhanced their production and commercial effectiveness. The magnetic properties of BaFe12O19 strongly depend on the mean grain size and morphology of the powders synthesized [5–8]. Nano-structured hexagonal BaFe12O19 has a single magnetic domain and high anisotropy and because of that it offers excellent magnetic properties [9]. The intrinsic magnetic properties resulted from the specific site occupancy of the Fe ions and rare earth (RE) elements along with divalent cations can drastically affect the electromagnetic properties of ferrites [10,11]. For these reasons manipulation ⇑ Corresponding author. E-mail address: [email protected] (M.U. Islam). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.107
of Fe and RE in the lattices is a promising way to improve magnetic properties of hexagonal ferrites. In this work the sol–gel method was used to fabricate calcium barium hexa-ferrites. Very few investigations are available on Ca-based M-type hexa-ferrites [12,13]. Calcium as a base is chosen for its intrinsic properties like the reduction in sintering temperature and enhancement in the properties. The aim of the present study is to investigate the effect of TbZn substitution on the magnetic properties of Ca-based M-type hexa-ferrites.
2. Experimental details 2.1. Sample preparation M-type hexa-ferrites with a chemical composition of Ca0.5Ba0.5xTbxZnyFe12yO19 (x = 0.00–0.10; y = 0.00–1.00) were synthesized by sol–gel auto combustion method using analytical grade citric acid, iron chloride (FeCl3.6H2O), calcium chloride (CaCl2), barium nitrate Ba(NO3)2, zinc nitrate Zn(NO3)2 and terbium oxide (Tb2O3). The reagents were stirred in 100 ml water (1:1) for 20 min and finally mixed into the citric acid solution. Tb2O3 was first dissolved in nitric acid at 80 °C for converting it into nitrate and then mixed with the prepared solution as the Tb2O3 is insoluble in water. To maintain the pH value at 7 an aqueous ammonia solution at a ratio of 40:60 was added drop-wise, while stirring at 60 °C. The sols were allowed to gel at 60 °C after 7 h and the gel was then dried at 200 °C for 1 h. The dried gel was ground homogenously in an agate mortar and pestle for 20 min. Finally, a homogenized powder was formed and sintered at 1100 °C for 8 h.
259
H.M. Khan et al. / Journal of Alloys and Compounds 589 (2014) 258–262 2.2. Characterization
qXray ¼
Fourier transform infrared (FTIR) spectrophotometer (BioRad Merlin FTS 3000MX) was used to obtain optical spectra of the prepared samples. X-ray diffraction analysis was performed using a shimadzu X-ray diffractometer with Cu Ka radiation (k = 1.5406 Å). The crystallite size was determined by Scherrer formula [14].
D¼
kk b Cos h
1 2
2.2.1. Electron microscopy TEM was used for the estimation of the crystallite size and aggregation. The analysis of the samples were investigated using a TEM:JEOL Japan 2011 (with LaB6 filament). Specimens were prepared by sonicating the powder particles in the ethanol for 1 h. Few drops of the suspension were put onto carbon-coated copper TEM grids before allowing them to dry in air.SEM analysis of the samples were performed using an FEI SIRION SEM operating at 20 kV to analyze the surface morphology and particles size of the samples.
ð1Þ
2
¼
2
2
4ðh þ hk þ k Þ l þ 2 c 3a2
ð2Þ
V cell ¼ 0:8666a2 P¼
3.1. FTIR spectroscopy
ð4Þ
qXray
qm ¼
3. Results and discussions
ð3Þ
1 qm
m
Fig. 1 shows FTIR profile of three sintered samples with nominal compositions Ca0.5Ba0.5XTbxZnyFe12yO19 (x = 0.02, 0.06 and 0.10; y = 0.20, 0.60 and 1.00). Three absorption bands at 554, 545 and 522 cm1 respectively were observed [16,17]. The occurrence of absorption bands in FTIR spectra are due to stretching vibrations to the metal–oxygen in the lattice and confirmed the formation of hexagonal structure [18].
ð5Þ
phr2
0
Transmittance %
10 20 30
ð6Þ
where M is the molar mass, a and c are lattice constants, m the mass of pellets, r radius of the pellets, NA Avogadro’s number and Vcell is the unit cell volume. The vibrating sample magnetometer (VSM) Lake Shore (7400, USA), was used to measure M–H loops at room temperature.
where K is shape constant, k is the wavelength, b is FWHM intensity and h is the Bragg angle. The values of X-ray density (qX-ray), bulk density (qm), porosity (P) and the unit cell volumes (Vcell) were calculated by following equations [15]:
d
2M N A V cell
3.2. X-ray diffraction analysis
x=0.02 x=0.06 x=0.10
Fig. 2 shows XRD analysis of the Ca0.5Ba0.5XTbxZnyFe12yO19 (x = 0.00, 0.02, 0.06 and 0.10; y = 0.00, 0.20, 0.60 and 1.00) hexaferrite materials. The XRD patterns reveals that all the peaks identified match well with the standard M-type barium hexa-ferrites JCPDS card No. ICDD 00–051-1879 [19]. The crystallite size calculated using scherrer formula is in the range of (15–46 nm) as listed in Table 1 which is less than 50 nm [32]. Fig. 3 shows variation of lattice parameters calculated using X-ray data. The lattice constants of Ca0.5Ba0.5xTbxZnyFe12yO19 hexa-ferrites varies as a = (5.842 – 5.874) Å and c= (23.281 – 23.792) Å, which are slightly different than that of BaFe12O19. These slight changes in the lattice constants may have
40 50 60 70 4000 3500 3000 2500 2000 1500 1000 500
0
-1
Wave Number (cm ) Fig. 1. FTIR analysis of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.02, 0.06 and 0.10).
X=0.00
221 006
220
201 110 201 008 114 205 110 203 301 206
140
214
300 264 281 218
20 13 300 11 14
214
304
Intensity (arb. units)
X=0.02
X=0.04
X=0.06
X=0.08
X=0.10 20
30
40
50
60
70
2 Theta (Degrees) Fig. 2. XRD analysis of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00–1.00).
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3.3. Magnetic measurements
Zn.Contents 0.0
6.00
0.2
0.4
0.6
0.8
1.0 24.4
a( ) C( )
5.95
24.2
5.90 5.85
24.0
)
a(
5.70 5.65
23.6 23.4
5.60
23.2
5.55 5.50
23.8
)
5.75
C(
5.80
0.00
0.02
0.04
0.06
0.08
0.10
23.0
Tb. Contents Fig. 3. Lattice parameters for Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00– 1.00).
been caused by the difference between the ionic radii of Tb3+ (0.92 Å) and Fe3+ (0.645 Å). An increase in the lattice constants by substitution of rare earth element has also been observed by other researchers [18,19]. Hence the variation of lattice parameters and grain size indicates that Tb3+ has entered into the lattice of the hexa-ferrite made. It was reported earlier in the literature that the ‘c/a’ ratio can be used to quantify the structure of the hexa-ferrite [20]. The range of lattice parameters in our samples lies near to expected range from 3.98–4.05 Å and manifested the M-type hexaferrite.
Fig. 4 shows the M-H loops for Ca0.5Ba0.5XTbxZnyFe12yO19 (x = 0.00–0.10; y = 0.00–1.00) measured by VSM at a maximum applied field of 10,000 Oe at room temperature. The effect of different doping contents of TbZn on the magnetic properties of the calcium barium ferrite powders are listed in Table 1. Fig. 5 shows the coercivity (HC) rapidly falls from 1635 Oe to 1277 Oe with substitution of dopants at x = 0.02 and y = 0.20, which is due to the replacement of Fe3+ ions at 4f2 site. It has been reported [21] that 4f2 and 2b sites contribute towards large anisotropy field but due to simultaneous grain growth the coercivity as a whole decreases [22,23]. At higher substitution level at x = 0.10, y = 1.00, the value of HC increases to its maximum value of 2025 Oe due to the presence of Fe2+ ions at 2a site which positively contributes for enhancement of the magneto-crystalline anisotropy. Similar variation is also observed by Wang in La substituted hexagonal ferrite. This indicates that the introduction of TbZn can be used to tune the anisotropy/coercivity of the materials. Fig. 6 show the saturation magnetization (Ms) and the remanent magnetization (Mr) increase to their maximum to 36.76 and 20.95 (emu/g) respectively at concentration x = 0.04 of the rare earth element Tb3+, at x P 0.04 there is a decrease in both Mr and Ms for all the samples. The magnetic behavior of the samples may depend upon the factors like ionic radii and strength of the magnetic interactions. The ionic radii of Tb3+ and Fe3+ is not comparable and this difference in ionic radii introduces a local strain which causes the disorder and modifications of local electronic states. The variation of rare earth concentration
Zn. Contents 0.0
0.2
0.4
0.6
0.8
1.0
40
2200 2000
20
Hc (Oe)
M (emu/g)
2400 x=0.00,y=0.00 x=0.02,y=0.20 x=0.04,y=0.40 x=0.06,y=0.60 x=0.08,y=0.80 x=0.10,y=1.00
0 -10000
-5000
0
5000
10000
1800 1600 1400
H (Oe)
1200
-20
1000 -0.02 0.00
-40
0.02
0.04
0.06
0.08
0.10
0.12
Tb. Contents
Fig. 4. VSM analysis of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10; y = 0.00–1.00).
Fig. 5. Variation of coercivity of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10).
Table 1 Magnetic and lattice parameter values for Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10). Contents of (TbZn)
X = 0.0 Y = 0.0
X = 0.02 Y = 0.20
X = 0.04 Y = 0.40
X = 0.06 Y = 0.60
X = 0.08 Y = 0.80
X = 0.10 Y = 1.0
Magnetization Ms (emu/g) Remanence Mr (emu/g) Mr/Ms Coercivity Hc (Oe) a (Å) c (Å) Ratio c/a Vcell D (nm) ± (0.01) P (%) D (nm), SEM D (nm), TEM
37.08 23.6 0.63 1645 5.73 23.19 4.04 456.21 17.21 26 48 19
40.2 25.7 0.63 1577 5.81 23.27 4.00 453.62 22.81 42
41.7 26.5 0.63 1511 5.87 23.31 3.97 445.15 12.16 47
35.8 21.4 0.59 1925 5.91 23.58 3.98 451.47 14.19 28 32 15
38.2 23.9 0.62 1786 5.86 23.67 4.03 449.34 21.27 27
37.8 22.9 0.60 1805 5.95 23.81 4.00 452.13 24.12 21 27 23
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Zn. Contents 50
0.0
0.2
0.4
0.6
0.8
1.0
3.4. Scanning electron microscopy, EDX and transmission electron microscopy
Ms&Mr (emu/g)
45 40 35
Ms
30 25 20 15
influences the super exchange interactions between magnetic cations and hence on the magnetization [24–27].
Mr
0.00
0.02
0.04
0.06
0.08
0.10
Tb. Contents Fig. 6. Variation in Ms and Mr of Ca0.5Ba0.5XTbxZnyFe12yO19, (x = 0.00–0.10).
SEM and TEM analysis was performed for three selected samples with nominal composition Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 and Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19. Fig. 7(a–c) show the SEM analysis of Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19, Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19 samples. It is clear from the SEM profiles that the particles have well defined shape and boundaries. Fig. 7(d–f) show the TEM analysis of Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 and Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19 samples. The values of particle sizes calculated by TEM were found to be in the range of 18– 25 nm, 16–23 nm and 14–21 nm. The grain size calculated by SEM ranges from 40–80 nm, 30–60 nm and 20–45 nm, for Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19, Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 and Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19 samples respectively. The particle size
Fig. 7. (a–c) SEM analysis of Ca0.5Ba0.5XTbxZnyFe12yO19 i.e (x = 0.02, 0.06 and 0.10) (d–f) TEM analysis of Ca0.5Ba0.5XTbxZnyFe12yO19 i.e (x = 0.02, 0.06 and 0.10). Table 2 EDS analysis of M-type hexa-ferrite Ca0.5Ba0.5XTbxZnyFe12yO19 powders, (x = 0.02, 0.06 and 1.00). Sr. No. 1 2 3
Formula of Sample Ca0.5Ba0.48Tb0.02Zn0.20Fe11.8O19 Ca0.5Ba0.44Tb0.06Zn0.60Fe11.4O19 Ca0.5Ba0.4Tb0.10Zn1.00Fe11O19
Zn 2.40 4.95 5.67
Fe 79.7 72.09 68.74
Observed
Weight
%
Ca 4.11 4.13 4.21
Ba 8.60 7.53 6.47
Tb 5.16 11.29 14.73
Total 100 100 100
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of the SEM samples are much smaller compared to those of the reported earlier 83 nm, 151–200 nm, 70–100 nm and 60–300 nm ranges for the M-type hexa-ferrite [28–33]. These particles are small in size and suitable for improving signal-to-noise ratio in high density recoding media. The elemental composition of the synthesized samples was estimated by energy dispersive X-ray spectroscopy analysis. It is evident from the analysis that stoichiometric amounts of Ba2+ and Fe3+ decreases while Tb3+ and Zn2+ increases. This signifies that Tb3+ is being substituted by Ba2+ site and Zn2+ is being substituted by Fe3+ site as listed in Table 2. 4. Conclusions All the TbZn substituted Ca0.5Ba0.5XTbxZnyFe12yO19 samples exhibit a single phase magnetoplumbite structure with slightly increasing values of lattice parameters a and c. Both anisotropy and coercivity can be tuned at certain substitution level. The magnetization behavior has been characterized by taking into account the preferential site occupancy of sub-lattice sites by substituted cations, magnetic dilution and spin canting. The crystallites size was found in the range 14–25 nm and 16–28 nm calculated by TEM and Scherrer equation, respectively. The increased anisotropy and reduction of particle size are beneficial for many applications, such as improving signal noise ratio and stability of recording devices.
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