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Influence of Cd substitution on structural, electrical and magnetic properties of M-type barium hexaferrites co-precipitated nanomaterials
Journal of Alloys and Compounds 584 (2014) 646–651
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Influence of Cd substitution on structural, electrical and magnetic properties of M-type barium hexaferrites co-precipitated nanomaterials Muhammad F. Din a, Ishtiaq Ahmad a, Mukhtar Ahmad a, M.T. Farid a, M. Asif Iqbal a,g, G. Murtaza b, Majid Niaz Akhtar c, Imran Shakir d, Muhammad Farooq Warsi e, Muhammad Azhar Khan f,⇑ a
Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan Centre for Advanced Studies in Physics, G.C. University, Lahore 54000, Pakistan Department of Physics, COMSATS Institute of Information Technology, Lahore, Pakistan d Deanship of Scientific Research, College of Engineering, PO Box 800, King Saud University, Riyadh 11421, Saudi Arabia e Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan f Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan g NUST College of Electrical and Mechanical Engineering, Islamabad, Pakistan b c
a r t i c l e
i n f o
Article history: Received 8 August 2013 Received in revised form 2 September 2013 Accepted 9 September 2013 Available online 19 September 2013 Keywords: Nanomaterials Magnetic properties High frequency Electrical resistivity
a b s t r a c t Nanocrystalline M-type hexagonal ferrites with the nominal chemical composition Ba0.5Co0.5xCdxFe12O19 (where x = 0–0.5) have been synthesized by the co-precipitation method and sintered at high annealing temperature (1250 °C) to study their structural, electrical and magnetic properties. The aim of the present work is to increase the DC electrical resistivity and coercivity of these M-type hexaferrites nanomaterials by the substitution of cadmium (Cd2+) ions at Co2+ site. The analysis of X-ray diffraction (XRD) patterns indicates single M-type hexaferrite phase. The parameters such as lattice constants (a and c), cell volume (V), X-xay density (Dx), bulk density (Db), crystallite size (D) and percentage porosity (%P) were calculated from XRD data. The crystallite size is found in the range of 26–47 nm and this size is small enough to obtain a suitable signal-to-noise ratio for application in the magnetic recording media. The room temperature DC electrical resistivity increases from 2.31 109 X cm to 6.42 109 X cm with the increased Cd contents. The magnetic properties such as saturation magnetization (Ms), coercivity (Hc), remanence (Mr) and squareness ratio (Mr/Ms) were calculated from hysteresis loops. The coercivity increases from 155 Oe to 1852 Oe while the saturation magnetization decreases from 33.5 to 9.2 emu g1 as the concentration of Cd increases. The electrical and magnetic properties such as DC electrical resistivity, coercivity and remanence suggested the synthesized materials suitable candidate for high frequency applications and recording media. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide nanomaterials have broad range of applications due their versatile behavior that is attributed to their variable oxidation states. Among many types of metal oxides, ferrites are the materials that contain more than 50% iron content and are under extensive study these days due their very important technological applications such as in telecommunication, and high frequency devices fabrications [1]. Ferrites are further classified depending upon their structure. The M-type barium ferrite, BaFe12O19, is one 2þ 3þ of the several hexaferrites with a general formula Ba2þ l Mm Fe2n 2+ Olþmþ3n , where l, m and n are integers and M is a divalent metal ion. M-type barium hexaferrite is very important component in permanent magnets, magnetic recording media and high fre-
⇑ Corresponding author. Tel.: +92 62 9255461; fax: +92 62 9255474. E-mail address: [email protected] (M.A. Khan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.043
quency devices. The attractive properties of these materials are saturation magnetization, remanence, coercivity and electrical resistivity. Nanocrystalline barium ferrite with small crystallite size is desirable to increase the capacity of information storage as well as to reduce the medium noise [2,3]. Understanding the effects of various diamagnetic and paramagnetic cations on the magnetic and electrical properties of substituted M-type barium hexaferrites is one of the most important tasks associated with the use of these materials in variety of modern technological applications [4–8]. Many methods have been developed and effectively used to synthesize the hexagonal ferrites, such as ceramic method [9,10], solution combustion technique [11], sol–gel auto-combustion process [12,13], ball-milling [14] and co-precipitation technique [15,16]. Co-precipitation is one of the simplest techniques for the preparation of hexaferrite nanomaterials. In order to enhance the electrical and magnetic properties such as electrical resistivity, remanence and coercivity,
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many researchers have attempted to substitute divalent and trivalent cations such as Ni [17], Co [18], Mn [19], La [20] and Tb [21]. In the present study we have reported the synthesis of Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) nano-hexaferrites by co-precipitation method. The influence of Cd on the electrical and magnetic properties of Ba0.5Co0.5xCdxFe12O19 nanomaterials have been investigated. These nanohexaferrites exhibited an increase in dc electrical resistivity and coercivity with increase in cadmium concentration which are required conditions for the materials to be useful in high frequency applications and magnetic recording media [22,23].
for all the peaks are compared with the standard pattern for hexagonal barium ferrite JCPDS Card No. (84-0757). The analysis of XRD patterns indicates that all the peaks are related to the M-type hexagonal structure with no extra peaks. This also reveals that the crystallization of samples in the single phase M-type hexagonal structure occurred to very high degree of perfection, with no evidence of impurity and crystal deformation. The lattice parameters (a and c) and cell volume (Vcell) were calculated by using the following equations [27]:
1 2. Experimental procedure Ba0.5Co0.5xCdxFe12O19 hexaferrite samples with x = 0.0–0.5 were synthesized by chemical co-precipitation method [24,25]. The reagents Ba(CH3COO)2, Cd(CH3COO)2, FeCl36H2O, Co(CH3COO)24H2O NaOH and Na2CO3 of analytical grade having 99.9% pure supplied by GmMH (E. Merck, Germany) and de-ionized water was used to make the precursor. Carbonates and salts of metals with stoichiometric composition of (Ba0.5Co0.5xCdxFe12O19) were mixed in 25 mL of de-ionized water and stirred constantly with the help of magnetic stirrer until a homogeneous solution was obtained. These metal chlorides were present in the solution in a stoichiometric ratio with (1:2:16). The precipitating agent was prepared by mixing NaOH and Na2CO3 in 100 mL of de-ionized water. The required amounts of NaOH (1.66 g) and Na2CO3 (4.42 g) were taken to ensure the pH value higher than 10, required for chemical reaction to take place. Dilute precipitated solution was used for the formation of uniform precipitates. The precipitating agent was added slowly until the coprecipitation occurs. When the precipitation completed, the solution was thoroughly washed with distilled water for several times until the removal of NaCl is confirmed. Adding the last drop of washing from the beaker into AgNO3, confirm the removal of NaCl, by precipitation technique. If no precipitation occurs that confirm the removal of NaCl. The solution was then filtered and the obtained precipitates were dried in an oven at temperature 110 °C for 12 h. The dried powder was mixed homogenously in an agate mortar and pestle by grinding the constituents for 2 h. The ground powders were then pressed into pellets at a pressure of 35 KN for about 5 min using Paul-Otto Weber hydraulic press. The pellets were pre-sintered in a digital electric furnace at temperature 1000 °C for 25 h in an ambient atmosphere. Final sintering was done at 1250 °C for 16 h. After each heat treatment, the samples were quenched in air to obtain the equilibrium position of the cations. X-ray diffraction patterns were obtained by a Bruker D8 diffractometer, using Cu Ka radiation, and they were identified by relevant JCPDS-ICDD powder diffraction files. The dc electrical resistivity was measured by two point probe method [26]. Magnetic properties were measured by the Lake Shore 7400 Vibrating Sample Magnetometer (VSM) with a maximum applied magnetic field of 4 KOe.
3. Results and discussion 3.1. Structural analysis X-ray diffraction (XRD) patterns of the sintered powder of Ba0.5Co0.5xCdxFe12O19 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) ferrites are shown in Fig. 1. All the XRD patterns were indexed and observed d-values
Fig. 1. X-ray diffraction patterns for Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites.
2 dhkl
2
¼
2
2
4ðh þ hk þ k Þ l þ 2 3a2 c
V cell ¼ 0:8666a2 c
ð3:1Þ
ð3:2Þ
where dhkl is the d-spacing and h, k, l are the miller indices. Fig. 2 depicts the variation of lattice parameters (‘a’ and ‘c’) as a function of Cd concentration (x). The lattice parameter ‘a’ reflects small variation than the lattice parameter ‘c’ which is in agreement with the fact that all hexagonal type structure exhibit constant lattice parameter ‘a’ and variable lattice parameter ‘c’ [28]. Similar results have been reported for M-type hexagonal ferrites by Popa et al. [29,30] and Townes et al. [31]. The value of lattice parameter ‘a’ remains almost constant while ‘c’ increases with the increase in Cd contents. The cell volume also increased with the increase of Cd concentration. The observed variation in the cell dimensions may be explained on the basis of relative ionic radii of Cd2+, Co2+ and Fe 3+. The increase in the lattice parameter ‘c’ and cell volume may be due to larger ionic radius of Cd2+(0.97 Å) compared to that of Co2+ (0.74 Å). The increase in cell dimensions indicates the solubility of Cd2+ ions in the M-type hexaferrite lattice which is in agreement with the reported literature [32]. The inter-atomic distance increased due to elongation in lattice parameters with the increase of Cd contents. The lattice parameters ratio c/a and cell volume are given in Table 1. The lattice parameters ratio lies in the expected range from 3.988 to 3.998 and exhibited the formation of M-type hexagonal structure. It has been reported that the lattice parameters ratio ‘c/a’ may be used to quantify the structure type and M-type hexaferrite structure may be assumed if the lattice parameters ratio is observed to be lower than 3.98 [33]. The variations in lattice parameters and cell volume revealed that the cadmium ion is entering in the lattice of M-type hexaferrites. The X-ray density, bulk density and percentage porosity were calculated from the XRD data by using the appropriate equations (Table 1) [27]. The X-ray density decreased from 5.377 to 5.279 g/cm3 that may be due to smaller mass of doped samples than that of the un-doped one as the X-ray density is corresponds
Fig. 2. Variation of lattice constants (a, c) for Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites.
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Table 1 The values of lattice parameters ratio, cell volume, X-ray density, bulk density and porosity calculated for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents. x
c/a
Volume (Å3)
Dx (g/cm3)
Db (g/cm3)
%P
0 0.1 0.2 0.3 0.4 0.5
3.988 3.989 3.991 3.994 3.997 3.998
674.48 675.10 675.98 676.85 677.77 678.64
5.377 5.352 5.339 5.320 5.299 5.279
4.942 4.886 4.793 4.699 4.612 4.599
6.419 8.168 11.141 12.086 14.019 14.165
to the atomic mass. The decrease in X-ray density may be due to the increase in cell volume because X-ray density is inversely related to cell volume. The bulk density found to be in the range from 4.942 to 4.599 g/cm3. The X-ray density is higher than the bulk density which might be due to the presence of pores occurred during the sintering process. The porosity of the doped samples is higher than that of the pure barium hexaferrite (Table 1). The crystallite size was calculated by using the Scherrer formula [34].
D¼
Kk b cos h
ð3:3Þ
where k is X-ray wavelength, b is the full width at half maximum, h is the Bragg’s angle and the value of shape factor K is 0.89 for hexagonal ferrites [35]. The crystallite size is found to be in the range of 26–47 nm. The crystallite size decreases with the increase in Cd concentration. The variation of crystallite size with Cd concentration is shown in Fig. 3. This behavior is attributed to the fact that higher the porosity smaller is the crystallite size and Cd doping impedes the grain growth. The decreasing trend in crystallite size shows that Cd is entering into the hexaferrite lattice which is confirmed by XRD analysis. This result agrees well with XRD data because the values of lattice parameters ‘a’ and ‘c’ increase with increasing Cd contents, which results in the increase of cell volume. The crystallite size observed in the present study is much smaller as compared to the already reported literature such as 60 nm, 70 nm, 83 nm and 151 nm for the M-type hexagonal ferrites [35–38]. The crystallite size of 50 nm is desirable for a suitable low-signal-tonoise ratio [39]. Therefore, the crystallite size 26–47 nm for the Cd doped Ba-hexaferrite nanomaterials synthesized in the present work is believed to be suitable for use in the recording media. The factor that decides the performance limits of the recording media is the media noise which comes into play from the coupling between the magnetic grains. It is generally believed that the interaction between the small grains does not play any significant role.
Fig. 3. The dependence of crystallite size and percentage porosity of Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites on various Cd contents.
3.2. Electrical properties Electrical conduction in ferrites is strongly affected by impurities at room temperature, whereas at high temperature it is due to polaron hopping [40]. The variation of room temperature electrical resistivity may be explained by Verwey’s hopping mechanism [41]. According to Verwey, the electronic conduction in ferrites is mainly due to hopping of electrons between ions of the same element present in more than one valence state, distributed randomly over crystallographically equivalent lattice sites. The crystal structure of ferrites shows that the cations either in tetrahedral or octahedral sites are surrounded by the oxygen anions and to first approximation can be treated as isolated from each other. Thus the localized electron model is more appreciable to discuss the conduction mechanism in ferrites rather than the band model. DC-electrical resistivity of the samples containing different Cd contents is measured at 393 K. The resistivity values have registered an increase of almost three times, i.e. from 2.31 109 X cm to 6.42 109 X cm with the increase of Cd2+ substitution from 0.0 to 0.5 (Fig. 4). The room temperature resistivity values of M-type hexaferrites reported by various authors are found to be in the range of 106–108 X cm [7,42]. Our results of resistivity fall in the reported range. The observed variations can be explained in terms of the site occupancy in the crystal lattice and the nature of substituted ions. The Co2+ ions partially occupy tetrahedral as well as octahedral sites. The magnetic iron ions partially occupy tetrahedral as well as octahedral sites [43]. Non-magnetic Cd ions strongly prefer the tetrahedral sites. As the Cd2+ ions substitution increase in tetrahedral sites, some of the Fe3+ ions at tetrahedral sites migrate to octahedral sites. As a consequence, the chance of electron generation decreases. The formation of Fe2+ and Fe3+ diffusion to octahedral sites decreases so this leads to the increase in resistivity for Cd2+ substituted hexaferrites. Moreover, the electrical resistivity is increased due to the increased number of grain boundaries caused by the smaller crystallite size [44]. Due to high resistivity, the power and eddy current losses decrease remarkably and consequently make the synthesized nanomaterials useful in high frequency devices fabrications. 3.3. Magnetic properties The hysteresis loops for Ba0.5Co0.5xCdxFe12O19 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) ferrites are shown in Fig. 5. The magnetic properties such as coercivity (Hc), saturation magnetization (Ms), remanence (Mr), and squareness ratio (Mr/Ms) are calculated from the hysteresis loops (Table 2). The variation of magnetic parameters such as Ms and Mr with Cd substitution is shown in Fig. 6. The behavior of
Fig. 4. Room temperature DC electrical resistivity for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents (x).
M.F. Din et al. / Journal of Alloys and Compounds 584 (2014) 646–651
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Fig. 5. Hysteresis loops for Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites.
Table 2 The values of different magnetic parameters calculated for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents. Cd concentration (x)
Magnetization, Ms (emu/g)
Retentivity, Mr (emu/g)
Coercivity, Hc (Oe)
Squareness ratio (Mr/ Ms)
0 0.1 0.2 0.3 0.4 0.5
33.5 32 27.2 22.5 13.5 9.2
7 11.5 16.5 9 4.5 5.4
155 370 585 615 962 1852
0.21 0.36 0.61 0.4 0.33 0.59
these magnetic properties can be explained on the basis of the occupation of doped cations at different sites in the hexagonal ferrite structure. Fig. 6 indicates that Ms decreases gradually while Mr increased up to a doping level of x = 0.2 and then starts to decrease by the Cd incorporation. This is likely due to the site occupancy of the doped metal ions at different lattice sites. M-type hexaferrite carries the total magnetic moment of 20 lB per unit cell [28]. Generally, the magnetic behavior of the ferromagnetic hexaferrite materials is largely governed by the distribution of iron ions on the crystallographic lattice sites and therefore the Fe3+–Fe2+ exchange interactions. The magnetization of the hexaferrite material
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Fig. 6. Variation of saturation magnetization (Ms), and remanence (Mr) for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents (x).
Fig. 7. Variation of coercivity Hc for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents (x).
varies with the factors influencing the strength of these exchange interactions. It has been reported that Co2+ occupy the octahedral 4f2 sites [45]. The Co2+ has three unpaired electrons i.e. the magnetic moment of 3 lB while Cd2+ has 0 lB. The Co2+ ions are located in the Ba2+ site, whose nearest neighbor is the 2b lattice site because it is known that the nearest neighbors of Ba2+ in the Ba-layer are 12 k, 4f2and 2b, where the 2b site is closer to the Ba2+ site at a distance of 0.340 nm as compared to the 4f2 site which is at a distance of d = 0.366 nm [46]. The substitution of Cd2+ (0.97 Å) to the Co2+ (0.74 Å) at Ba2+ site induces a perturbation in both electron-density and symmetry around the 2b lattice site that could weaken the exchange interactions and hence reduced saturation magnetization. In addition, the presence of non-magnetic Cd2+ ion in the vicinity of Fe3+ (5 lB) ion would dilute the strength of the exchange interactions and reduces the total magnetic moment to result in the reduction of the saturation magnetization. In present work, the squareness ratio is found in the range of 0.21–0.61 and is given in Table 2. The coercivity is the measure of magnetic field strength required for overcoming the magnetocrystalline anisotropy to flip the magnetic moments, is also affected by the dopant concentration. From Fig. 5, it can be observed that the width of the hysteresis loop increases with the increase in dopant concentration indicating an increase in the coercivity of the synthesized materials. The coercivity is found to increase from 155 to 1852 Oe with the addition of Cd contents up to x = 0.5 (Fig. 7). It has been reported [47] that the small crystallite size increases the coercivity (Hc inversely proportional to the r). In the present studies the crystallite size decreases (Fig. 3) with the increase in Cd concentration which is responsible for the enhancement in the coercivity. The increase in coercivity may also be explained due to the fact that Hc increases with the increase in porosity [48]. Further higher porosity samples contain smaller crystallite size which leads to high coercivity values. To maintain the recording information for long time, the magnetic recording media depends on its magnetic parameters. The longitudinal magnetic recording media requires high enough coercivity of 600 Oe and for the perpendicular magnetic recording media coercivity requires above 1200 Oe [23]. The ferrite materials which have low coercivity are useful for magnetic recording applications such as hard disks and video tapes [49]. In the present investigations, the coercivity of the synthesized samples lie in the range 155–1852 Oe. The samples with nominal compositions Ba0.5Co0.2Cd0.3Fe12O19 and Ba0.5Co0.1Cd0.4Fe12O19 are useful for longitudinal magnetic recording media while the composition Ba0.5Cd0.5Fe12O19 is beneficial for perpendicular magnetic recording media. If Hc >
Mr/2, the materials are hard magnets and such materials may also be useful for devices fabrications working at high frequencies [50]. In the present investigations, the synthesized hexaferrite nanomaterials have Hc > Mr/2, so these materials are hard magnets and such materials might be useful for high frequency applications. 4. Conclusions Cadmium substituted barium hexaferrite nanomaterials have been synthesized successfully by the simple chemical co-precipitation route. The X-ray diffraction patterns reveal the formation of M-type hexagonal structure with an average crystallite size in the range of 26–47 nm. This crystallite size is small enough to obtain the suitable signal-to-noise ratio in the recording media. The squareness ratio values lie in the range of 0.21–0.61 for M-type hexaferrites. Room temperature DC electrical resistivity increases with the increase of Cd concentration, which may be due to large number of grain boundaries caused by the smaller crystallite size. The coercivity is enhanced while saturation magnetization and remanence is decreased with increasing the Cd2+ addition which may be due to magnetic dilution, particle size effects and hence reduction of super-exchange interactions. The magnetic properties such as coercivity and remanence and DC electrical resistivity make these nanomaterials useful for applications in the recording media and high frequency applications. Acknowledgements The authors would like extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project no RGP-VPP-312 and also to the Bahauddin Zakaryia University Multan, Pakistan for financial support. References [1] K.C. Patil, T. Rattan, S.T. Aruna, Chemistry of Nanocrystalline Oxide Materials: Combustion Synthesis, Properties and Applications, World Scientific Publishing, Co. Pte. Ltd. (2008) 154–161. [2] T. González-Carreño, M.P. Morales, C.J. Serna, Mater. Lett. 43 (2000) 97–101. [3] S. Yamamoto, X. Liu, A. Morisako, J. Magn. Magn. Mater. 316 (2007) e152– e154. [4] H. Sözeri, J. Magn. Magn. Mater. 321 (2009) 2717–2722. [5] U. Topal, H.I. Bakan, J.Eu. Ceram, Soc. 30 (2010) 3167–3171. [6] M.J. Iqbal, M.N. Ashiq, P.H. Gomez, J. Alloys Comp. 478 (2009) 736–740. [7] S. Hussain, A. Maqsood, J. Magn. Magn. Mater. 316 (2007) 73–80. [8] J. Lee, M. Fuger, J. Fidler, D. Suess, T. Schrefl, O. Shimizu, J. Magn. Magn. Mater. 322 (2010) 3869–3875. [9] G.M. Rai, M.A. Iqbal, K.T. Kubra, J. Alloys Comp. 495 (2010) 229–233.
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Influence of Cd substitution on structural, electrical and magnetic properties of M-type barium hexaferrites co-precipitated nanomaterials Muhammad F. Din a, Ishtiaq Ahmad a, Mukhtar Ahmad a, M.T. Farid a, M. Asif Iqbal a,g, G. Murtaza b, Majid Niaz Akhtar c, Imran Shakir d, Muhammad Farooq Warsi e, Muhammad Azhar Khan f,⇑ a
Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan Centre for Advanced Studies in Physics, G.C. University, Lahore 54000, Pakistan Department of Physics, COMSATS Institute of Information Technology, Lahore, Pakistan d Deanship of Scientific Research, College of Engineering, PO Box 800, King Saud University, Riyadh 11421, Saudi Arabia e Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan f Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan g NUST College of Electrical and Mechanical Engineering, Islamabad, Pakistan b c
a r t i c l e
i n f o
Article history: Received 8 August 2013 Received in revised form 2 September 2013 Accepted 9 September 2013 Available online 19 September 2013 Keywords: Nanomaterials Magnetic properties High frequency Electrical resistivity
a b s t r a c t Nanocrystalline M-type hexagonal ferrites with the nominal chemical composition Ba0.5Co0.5xCdxFe12O19 (where x = 0–0.5) have been synthesized by the co-precipitation method and sintered at high annealing temperature (1250 °C) to study their structural, electrical and magnetic properties. The aim of the present work is to increase the DC electrical resistivity and coercivity of these M-type hexaferrites nanomaterials by the substitution of cadmium (Cd2+) ions at Co2+ site. The analysis of X-ray diffraction (XRD) patterns indicates single M-type hexaferrite phase. The parameters such as lattice constants (a and c), cell volume (V), X-xay density (Dx), bulk density (Db), crystallite size (D) and percentage porosity (%P) were calculated from XRD data. The crystallite size is found in the range of 26–47 nm and this size is small enough to obtain a suitable signal-to-noise ratio for application in the magnetic recording media. The room temperature DC electrical resistivity increases from 2.31 109 X cm to 6.42 109 X cm with the increased Cd contents. The magnetic properties such as saturation magnetization (Ms), coercivity (Hc), remanence (Mr) and squareness ratio (Mr/Ms) were calculated from hysteresis loops. The coercivity increases from 155 Oe to 1852 Oe while the saturation magnetization decreases from 33.5 to 9.2 emu g1 as the concentration of Cd increases. The electrical and magnetic properties such as DC electrical resistivity, coercivity and remanence suggested the synthesized materials suitable candidate for high frequency applications and recording media. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide nanomaterials have broad range of applications due their versatile behavior that is attributed to their variable oxidation states. Among many types of metal oxides, ferrites are the materials that contain more than 50% iron content and are under extensive study these days due their very important technological applications such as in telecommunication, and high frequency devices fabrications [1]. Ferrites are further classified depending upon their structure. The M-type barium ferrite, BaFe12O19, is one 2þ 3þ of the several hexaferrites with a general formula Ba2þ l Mm Fe2n 2+ Olþmþ3n , where l, m and n are integers and M is a divalent metal ion. M-type barium hexaferrite is very important component in permanent magnets, magnetic recording media and high fre-
⇑ Corresponding author. Tel.: +92 62 9255461; fax: +92 62 9255474. E-mail address: [email protected] (M.A. Khan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.043
quency devices. The attractive properties of these materials are saturation magnetization, remanence, coercivity and electrical resistivity. Nanocrystalline barium ferrite with small crystallite size is desirable to increase the capacity of information storage as well as to reduce the medium noise [2,3]. Understanding the effects of various diamagnetic and paramagnetic cations on the magnetic and electrical properties of substituted M-type barium hexaferrites is one of the most important tasks associated with the use of these materials in variety of modern technological applications [4–8]. Many methods have been developed and effectively used to synthesize the hexagonal ferrites, such as ceramic method [9,10], solution combustion technique [11], sol–gel auto-combustion process [12,13], ball-milling [14] and co-precipitation technique [15,16]. Co-precipitation is one of the simplest techniques for the preparation of hexaferrite nanomaterials. In order to enhance the electrical and magnetic properties such as electrical resistivity, remanence and coercivity,
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many researchers have attempted to substitute divalent and trivalent cations such as Ni [17], Co [18], Mn [19], La [20] and Tb [21]. In the present study we have reported the synthesis of Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) nano-hexaferrites by co-precipitation method. The influence of Cd on the electrical and magnetic properties of Ba0.5Co0.5xCdxFe12O19 nanomaterials have been investigated. These nanohexaferrites exhibited an increase in dc electrical resistivity and coercivity with increase in cadmium concentration which are required conditions for the materials to be useful in high frequency applications and magnetic recording media [22,23].
for all the peaks are compared with the standard pattern for hexagonal barium ferrite JCPDS Card No. (84-0757). The analysis of XRD patterns indicates that all the peaks are related to the M-type hexagonal structure with no extra peaks. This also reveals that the crystallization of samples in the single phase M-type hexagonal structure occurred to very high degree of perfection, with no evidence of impurity and crystal deformation. The lattice parameters (a and c) and cell volume (Vcell) were calculated by using the following equations [27]:
1 2. Experimental procedure Ba0.5Co0.5xCdxFe12O19 hexaferrite samples with x = 0.0–0.5 were synthesized by chemical co-precipitation method [24,25]. The reagents Ba(CH3COO)2, Cd(CH3COO)2, FeCl36H2O, Co(CH3COO)24H2O NaOH and Na2CO3 of analytical grade having 99.9% pure supplied by GmMH (E. Merck, Germany) and de-ionized water was used to make the precursor. Carbonates and salts of metals with stoichiometric composition of (Ba0.5Co0.5xCdxFe12O19) were mixed in 25 mL of de-ionized water and stirred constantly with the help of magnetic stirrer until a homogeneous solution was obtained. These metal chlorides were present in the solution in a stoichiometric ratio with (1:2:16). The precipitating agent was prepared by mixing NaOH and Na2CO3 in 100 mL of de-ionized water. The required amounts of NaOH (1.66 g) and Na2CO3 (4.42 g) were taken to ensure the pH value higher than 10, required for chemical reaction to take place. Dilute precipitated solution was used for the formation of uniform precipitates. The precipitating agent was added slowly until the coprecipitation occurs. When the precipitation completed, the solution was thoroughly washed with distilled water for several times until the removal of NaCl is confirmed. Adding the last drop of washing from the beaker into AgNO3, confirm the removal of NaCl, by precipitation technique. If no precipitation occurs that confirm the removal of NaCl. The solution was then filtered and the obtained precipitates were dried in an oven at temperature 110 °C for 12 h. The dried powder was mixed homogenously in an agate mortar and pestle by grinding the constituents for 2 h. The ground powders were then pressed into pellets at a pressure of 35 KN for about 5 min using Paul-Otto Weber hydraulic press. The pellets were pre-sintered in a digital electric furnace at temperature 1000 °C for 25 h in an ambient atmosphere. Final sintering was done at 1250 °C for 16 h. After each heat treatment, the samples were quenched in air to obtain the equilibrium position of the cations. X-ray diffraction patterns were obtained by a Bruker D8 diffractometer, using Cu Ka radiation, and they were identified by relevant JCPDS-ICDD powder diffraction files. The dc electrical resistivity was measured by two point probe method [26]. Magnetic properties were measured by the Lake Shore 7400 Vibrating Sample Magnetometer (VSM) with a maximum applied magnetic field of 4 KOe.
3. Results and discussion 3.1. Structural analysis X-ray diffraction (XRD) patterns of the sintered powder of Ba0.5Co0.5xCdxFe12O19 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) ferrites are shown in Fig. 1. All the XRD patterns were indexed and observed d-values
Fig. 1. X-ray diffraction patterns for Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites.
2 dhkl
2
¼
2
2
4ðh þ hk þ k Þ l þ 2 3a2 c
V cell ¼ 0:8666a2 c
ð3:1Þ
ð3:2Þ
where dhkl is the d-spacing and h, k, l are the miller indices. Fig. 2 depicts the variation of lattice parameters (‘a’ and ‘c’) as a function of Cd concentration (x). The lattice parameter ‘a’ reflects small variation than the lattice parameter ‘c’ which is in agreement with the fact that all hexagonal type structure exhibit constant lattice parameter ‘a’ and variable lattice parameter ‘c’ [28]. Similar results have been reported for M-type hexagonal ferrites by Popa et al. [29,30] and Townes et al. [31]. The value of lattice parameter ‘a’ remains almost constant while ‘c’ increases with the increase in Cd contents. The cell volume also increased with the increase of Cd concentration. The observed variation in the cell dimensions may be explained on the basis of relative ionic radii of Cd2+, Co2+ and Fe 3+. The increase in the lattice parameter ‘c’ and cell volume may be due to larger ionic radius of Cd2+(0.97 Å) compared to that of Co2+ (0.74 Å). The increase in cell dimensions indicates the solubility of Cd2+ ions in the M-type hexaferrite lattice which is in agreement with the reported literature [32]. The inter-atomic distance increased due to elongation in lattice parameters with the increase of Cd contents. The lattice parameters ratio c/a and cell volume are given in Table 1. The lattice parameters ratio lies in the expected range from 3.988 to 3.998 and exhibited the formation of M-type hexagonal structure. It has been reported that the lattice parameters ratio ‘c/a’ may be used to quantify the structure type and M-type hexaferrite structure may be assumed if the lattice parameters ratio is observed to be lower than 3.98 [33]. The variations in lattice parameters and cell volume revealed that the cadmium ion is entering in the lattice of M-type hexaferrites. The X-ray density, bulk density and percentage porosity were calculated from the XRD data by using the appropriate equations (Table 1) [27]. The X-ray density decreased from 5.377 to 5.279 g/cm3 that may be due to smaller mass of doped samples than that of the un-doped one as the X-ray density is corresponds
Fig. 2. Variation of lattice constants (a, c) for Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites.
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Table 1 The values of lattice parameters ratio, cell volume, X-ray density, bulk density and porosity calculated for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents. x
c/a
Volume (Å3)
Dx (g/cm3)
Db (g/cm3)
%P
0 0.1 0.2 0.3 0.4 0.5
3.988 3.989 3.991 3.994 3.997 3.998
674.48 675.10 675.98 676.85 677.77 678.64
5.377 5.352 5.339 5.320 5.299 5.279
4.942 4.886 4.793 4.699 4.612 4.599
6.419 8.168 11.141 12.086 14.019 14.165
to the atomic mass. The decrease in X-ray density may be due to the increase in cell volume because X-ray density is inversely related to cell volume. The bulk density found to be in the range from 4.942 to 4.599 g/cm3. The X-ray density is higher than the bulk density which might be due to the presence of pores occurred during the sintering process. The porosity of the doped samples is higher than that of the pure barium hexaferrite (Table 1). The crystallite size was calculated by using the Scherrer formula [34].
D¼
Kk b cos h
ð3:3Þ
where k is X-ray wavelength, b is the full width at half maximum, h is the Bragg’s angle and the value of shape factor K is 0.89 for hexagonal ferrites [35]. The crystallite size is found to be in the range of 26–47 nm. The crystallite size decreases with the increase in Cd concentration. The variation of crystallite size with Cd concentration is shown in Fig. 3. This behavior is attributed to the fact that higher the porosity smaller is the crystallite size and Cd doping impedes the grain growth. The decreasing trend in crystallite size shows that Cd is entering into the hexaferrite lattice which is confirmed by XRD analysis. This result agrees well with XRD data because the values of lattice parameters ‘a’ and ‘c’ increase with increasing Cd contents, which results in the increase of cell volume. The crystallite size observed in the present study is much smaller as compared to the already reported literature such as 60 nm, 70 nm, 83 nm and 151 nm for the M-type hexagonal ferrites [35–38]. The crystallite size of 50 nm is desirable for a suitable low-signal-tonoise ratio [39]. Therefore, the crystallite size 26–47 nm for the Cd doped Ba-hexaferrite nanomaterials synthesized in the present work is believed to be suitable for use in the recording media. The factor that decides the performance limits of the recording media is the media noise which comes into play from the coupling between the magnetic grains. It is generally believed that the interaction between the small grains does not play any significant role.
Fig. 3. The dependence of crystallite size and percentage porosity of Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites on various Cd contents.
3.2. Electrical properties Electrical conduction in ferrites is strongly affected by impurities at room temperature, whereas at high temperature it is due to polaron hopping [40]. The variation of room temperature electrical resistivity may be explained by Verwey’s hopping mechanism [41]. According to Verwey, the electronic conduction in ferrites is mainly due to hopping of electrons between ions of the same element present in more than one valence state, distributed randomly over crystallographically equivalent lattice sites. The crystal structure of ferrites shows that the cations either in tetrahedral or octahedral sites are surrounded by the oxygen anions and to first approximation can be treated as isolated from each other. Thus the localized electron model is more appreciable to discuss the conduction mechanism in ferrites rather than the band model. DC-electrical resistivity of the samples containing different Cd contents is measured at 393 K. The resistivity values have registered an increase of almost three times, i.e. from 2.31 109 X cm to 6.42 109 X cm with the increase of Cd2+ substitution from 0.0 to 0.5 (Fig. 4). The room temperature resistivity values of M-type hexaferrites reported by various authors are found to be in the range of 106–108 X cm [7,42]. Our results of resistivity fall in the reported range. The observed variations can be explained in terms of the site occupancy in the crystal lattice and the nature of substituted ions. The Co2+ ions partially occupy tetrahedral as well as octahedral sites. The magnetic iron ions partially occupy tetrahedral as well as octahedral sites [43]. Non-magnetic Cd ions strongly prefer the tetrahedral sites. As the Cd2+ ions substitution increase in tetrahedral sites, some of the Fe3+ ions at tetrahedral sites migrate to octahedral sites. As a consequence, the chance of electron generation decreases. The formation of Fe2+ and Fe3+ diffusion to octahedral sites decreases so this leads to the increase in resistivity for Cd2+ substituted hexaferrites. Moreover, the electrical resistivity is increased due to the increased number of grain boundaries caused by the smaller crystallite size [44]. Due to high resistivity, the power and eddy current losses decrease remarkably and consequently make the synthesized nanomaterials useful in high frequency devices fabrications. 3.3. Magnetic properties The hysteresis loops for Ba0.5Co0.5xCdxFe12O19 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) ferrites are shown in Fig. 5. The magnetic properties such as coercivity (Hc), saturation magnetization (Ms), remanence (Mr), and squareness ratio (Mr/Ms) are calculated from the hysteresis loops (Table 2). The variation of magnetic parameters such as Ms and Mr with Cd substitution is shown in Fig. 6. The behavior of
Fig. 4. Room temperature DC electrical resistivity for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents (x).
M.F. Din et al. / Journal of Alloys and Compounds 584 (2014) 646–651
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Fig. 5. Hysteresis loops for Ba0.5Co0.5xCdxFe12O19 (0.0 6 x 6 0.5) ferrites.
Table 2 The values of different magnetic parameters calculated for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents. Cd concentration (x)
Magnetization, Ms (emu/g)
Retentivity, Mr (emu/g)
Coercivity, Hc (Oe)
Squareness ratio (Mr/ Ms)
0 0.1 0.2 0.3 0.4 0.5
33.5 32 27.2 22.5 13.5 9.2
7 11.5 16.5 9 4.5 5.4
155 370 585 615 962 1852
0.21 0.36 0.61 0.4 0.33 0.59
these magnetic properties can be explained on the basis of the occupation of doped cations at different sites in the hexagonal ferrite structure. Fig. 6 indicates that Ms decreases gradually while Mr increased up to a doping level of x = 0.2 and then starts to decrease by the Cd incorporation. This is likely due to the site occupancy of the doped metal ions at different lattice sites. M-type hexaferrite carries the total magnetic moment of 20 lB per unit cell [28]. Generally, the magnetic behavior of the ferromagnetic hexaferrite materials is largely governed by the distribution of iron ions on the crystallographic lattice sites and therefore the Fe3+–Fe2+ exchange interactions. The magnetization of the hexaferrite material
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Fig. 6. Variation of saturation magnetization (Ms), and remanence (Mr) for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents (x).
Fig. 7. Variation of coercivity Hc for Ba0.5Co0.5xCdxFe12O19 ferrites containing different Cd contents (x).
varies with the factors influencing the strength of these exchange interactions. It has been reported that Co2+ occupy the octahedral 4f2 sites [45]. The Co2+ has three unpaired electrons i.e. the magnetic moment of 3 lB while Cd2+ has 0 lB. The Co2+ ions are located in the Ba2+ site, whose nearest neighbor is the 2b lattice site because it is known that the nearest neighbors of Ba2+ in the Ba-layer are 12 k, 4f2and 2b, where the 2b site is closer to the Ba2+ site at a distance of 0.340 nm as compared to the 4f2 site which is at a distance of d = 0.366 nm [46]. The substitution of Cd2+ (0.97 Å) to the Co2+ (0.74 Å) at Ba2+ site induces a perturbation in both electron-density and symmetry around the 2b lattice site that could weaken the exchange interactions and hence reduced saturation magnetization. In addition, the presence of non-magnetic Cd2+ ion in the vicinity of Fe3+ (5 lB) ion would dilute the strength of the exchange interactions and reduces the total magnetic moment to result in the reduction of the saturation magnetization. In present work, the squareness ratio is found in the range of 0.21–0.61 and is given in Table 2. The coercivity is the measure of magnetic field strength required for overcoming the magnetocrystalline anisotropy to flip the magnetic moments, is also affected by the dopant concentration. From Fig. 5, it can be observed that the width of the hysteresis loop increases with the increase in dopant concentration indicating an increase in the coercivity of the synthesized materials. The coercivity is found to increase from 155 to 1852 Oe with the addition of Cd contents up to x = 0.5 (Fig. 7). It has been reported [47] that the small crystallite size increases the coercivity (Hc inversely proportional to the r). In the present studies the crystallite size decreases (Fig. 3) with the increase in Cd concentration which is responsible for the enhancement in the coercivity. The increase in coercivity may also be explained due to the fact that Hc increases with the increase in porosity [48]. Further higher porosity samples contain smaller crystallite size which leads to high coercivity values. To maintain the recording information for long time, the magnetic recording media depends on its magnetic parameters. The longitudinal magnetic recording media requires high enough coercivity of 600 Oe and for the perpendicular magnetic recording media coercivity requires above 1200 Oe [23]. The ferrite materials which have low coercivity are useful for magnetic recording applications such as hard disks and video tapes [49]. In the present investigations, the coercivity of the synthesized samples lie in the range 155–1852 Oe. The samples with nominal compositions Ba0.5Co0.2Cd0.3Fe12O19 and Ba0.5Co0.1Cd0.4Fe12O19 are useful for longitudinal magnetic recording media while the composition Ba0.5Cd0.5Fe12O19 is beneficial for perpendicular magnetic recording media. If Hc >
Mr/2, the materials are hard magnets and such materials may also be useful for devices fabrications working at high frequencies [50]. In the present investigations, the synthesized hexaferrite nanomaterials have Hc > Mr/2, so these materials are hard magnets and such materials might be useful for high frequency applications. 4. Conclusions Cadmium substituted barium hexaferrite nanomaterials have been synthesized successfully by the simple chemical co-precipitation route. The X-ray diffraction patterns reveal the formation of M-type hexagonal structure with an average crystallite size in the range of 26–47 nm. This crystallite size is small enough to obtain the suitable signal-to-noise ratio in the recording media. The squareness ratio values lie in the range of 0.21–0.61 for M-type hexaferrites. Room temperature DC electrical resistivity increases with the increase of Cd concentration, which may be due to large number of grain boundaries caused by the smaller crystallite size. The coercivity is enhanced while saturation magnetization and remanence is decreased with increasing the Cd2+ addition which may be due to magnetic dilution, particle size effects and hence reduction of super-exchange interactions. The magnetic properties such as coercivity and remanence and DC electrical resistivity make these nanomaterials useful for applications in the recording media and high frequency applications. Acknowledgements The authors would like extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project no RGP-VPP-312 and also to the Bahauddin Zakaryia University Multan, Pakistan for financial support. References [1] K.C. Patil, T. Rattan, S.T. Aruna, Chemistry of Nanocrystalline Oxide Materials: Combustion Synthesis, Properties and Applications, World Scientific Publishing, Co. Pte. Ltd. (2008) 154–161. [2] T. González-Carreño, M.P. Morales, C.J. Serna, Mater. Lett. 43 (2000) 97–101. [3] S. Yamamoto, X. Liu, A. Morisako, J. Magn. Magn. Mater. 316 (2007) e152– e154. [4] H. Sözeri, J. Magn. Magn. Mater. 321 (2009) 2717–2722. [5] U. Topal, H.I. Bakan, J.Eu. Ceram, Soc. 30 (2010) 3167–3171. [6] M.J. Iqbal, M.N. Ashiq, P.H. Gomez, J. Alloys Comp. 478 (2009) 736–740. [7] S. Hussain, A. Maqsood, J. Magn. Magn. Mater. 316 (2007) 73–80. [8] J. Lee, M. Fuger, J. Fidler, D. Suess, T. Schrefl, O. Shimizu, J. Magn. Magn. Mater. 322 (2010) 3869–3875. [9] G.M. Rai, M.A. Iqbal, K.T. Kubra, J. Alloys Comp. 495 (2010) 229–233.
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