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Effect of composition on structural and magnetic properties of nanocrystalline Ni0.8−xZn0.2MgxFe2O4 ferrite
Polyhedron 29 (2010) 2569–2573
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Effect of composition on structural and magnetic properties of nanocrystalline Ni0.8 xZn0.2MgxFe2O4 ferrite M.A. Gabal a,b,*, W.A. Bayoumy b a b
Chemistry Department, Faculty of Science, King Abdul Aziz University, Jeddah, Saudi Arabia Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
a r t i c l e
i n f o
Article history: Received 16 February 2010 Accepted 17 April 2010 Available online 27 April 2010 Keywords: Ferrite Mg-substitution Egg-white XRD VSM
a b s t r a c t Nanocrystalline magnetic particles of Ni0.8 xZn0.2MgxFe2O4 ferrites with x lying between 0.0 and 0.8 were synthesized using metal nitrates and freshly extracted egg-white. The synthesized powders were characterized using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and transmission electron microscopy (TEM). With increasing magnesium concentration, the lattice constant increases while Xray density decreases. The average crystallite size determined from XRD data using Scherrer formula lie in the range of 35–59 nm. TEM image shows spherically agglomerated particles with average crystallite size agreed well with that obtained from XRD. Magnetic properties measured at room temperature by vibrating sample magnetometer (VSM) reveal a decrease in saturation magnetization up to Mg content of 0.6. In agreement with FT-IR results, the unexpected increase in the magnetization at Mg content of 0.8 can be attributed to the tendency of Mg2+ ions to occupy the tetrahedral site. The decrease in the value of coercivity with increasing magnesium content can be explained based on the magneto-crystalline anisotropy. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Spinel ferrites nanocrystals have been widely investigated in the recent years due to their remarkable electrical and magnetic properties and wide practical applications in information storage system, ferrofluid technology, magneto-caloric refrigeration and medical diagnosis [1]. To meet the demand of high performance devices, an important step is to synthesize ferrite crystals in nanoscale forms with narrow particle size distribution and minimum particles agglomeration. Below the critical size these crystals exist in a single domain state so the domain wall resonance is avoided and the material can work at higher frequencies [2]. Many techniques have been provided for the synthesis of the nano-sized ferrites. These methods include sol–gel [3], organic precursors [4], hydrothermal [5], co-precipitation [6], cathodic electrophoretic deposition (EPD) [7], mechanochemical synthesis [8], reverse micelle [9], and electrochemical deposition [10]. More recently, a simple, cost effective and environmentally friendly method utilizing egg-white is also used [11–13]. Zinc substitution plays a decisive role in determining the ferrite properties. Mixed Zn ferrites and especially Ni–Zn ferrites are the most important magnetic materials, which offer a broad range of * Corresponding author at: Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. Tel.: +966 557071572. E-mail address: [email protected] (M.A. Gabal). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.04.019
frequency suppression in the MHz band frequency. They posses a unique combination of desirable properties such as large magnetic permeability at high frequencies, electrical resistivity, mechanical hardness, chemical stability in addition to the reasonable cost [14]. They have a wide range of applications [15] in microwave absorbance, electronic devices such as radio and TV sets, integrated nonreciprocal circuits, high frequency transformers, memory core devices, rod antennas, read-write heads for high-speed digital tape or disk recording, telecommunication applications, excellent catalysts for alkylation of aromatics and in gas sensing. MgFe2O4 [16] is a partially inverse cubic spinel. It can be considered as a collinear ferrimagnet whose degree of inversion is sensitive to the sample preparation history. Magnetic properties of ferrites are strongly dependent on their chemical compositions and additives or substitutions. Small amount of foreign ions in the ferrite can dramatically change the properties of ferrites. In the literature very few studies on the Mg substituted Ni–Zn ferrites are present. El Hiti [17] studied the dc conductivity of ZnxMg0.8 xNi0.2Fe204 system, with x = 0.0, 0.2, 0.4, 0.6 and 0.8, as a function of temperature and composition. The dc conductivity was found to increase with increasing temperature, while it decreases with increasing Zn content. The Curie transition temperature decreases, while the activation energy for conduction increases by increasing Zn content. Ni0.2ZnxMg0.8 xFe2O4 ferrites; 0 6 x P 0.8 were studied using X-ray diffraction and Mössbauer spectroscopy [18]. The samples proved to have a single-phase cubic spinel structure. The
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dependence of the lattice constants and intercationic distances on Zn content was studied. According to the Mössbauer studies, the samples with x = 0.0–0.4 are magnetically ordered while others are paramagnetic. The cation distributions were deduced and supported by X-ray studies. Ni1 xMgxFe2O4 ferrites (0 6 x 6 1) were prepared by the co-precipitation method [19]. The samples were characterized by XRD. The crystallite size variation is within the range 10–13 nm. The Curie temperature was determined using AC magnetic susceptibility data and the observed variation is explained based on cations distribution among tetrahedral and octahedral sites. The properties of ferrites are highly sensitive to the cation distribution, which in turn is controlled by preparation conditions and substitution of different metals. Accordingly, the present work is aimed at the preparation of Ni0.8 xZn0.2MgxFe2O4 ferrites with x = 0.0–0.8 using egg-white method. The produced nano-sized ferrites were characterized using thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and transmission electron microscopy (TEM) techniques. The change in the magnetic properties of the investigated system was measured using vibrating sample magnetometer (VSM). To the best of our knowledge, the influences of magnesium substitution on the structural and magnetic properties of Ni–Zn ferrites have seldom been reported.
2. Experimental Precursors of the mixed ferrite samples were prepared, using stoichiometric ratios of the metal nitrate and freshly extracted egg-white, according to egg-white method as previously described [12,13]. The dried precursors were ground and calcined in a muffle furnace at 550 °C for 2 h. Thermogravimetric analysis (TG) was carried out, using Perkin– Elmer thermal analyzer on precursors up to 600 °C at a heating rate of 5 °C min 1 in air. X-ray powder diffraction analysis was conducted on D8 Advance diffractometer using Cu Ka radiation (operated at 40 kV and 35 mA). Fourier transform infrared spectroscopic analysis, using KBr pellets, was carried out in the range 200–4000 cm 1 using a Jasco model FT-IR 310 spectrometer.
Transmission electron microscopy was performed using Jeol 2010 transmission electron microscope with an accelerating voltage of 100 kV. A drop of diluted sample in alcohol was dripped on the TEM grid and dried, which was then used to examine the grain size and morphology of synthesized sample. The characteristic hysteresis loops of the system were measured at room temperature, up to a maximum external field of ±8 kOe, by using vibrating sample magnetometer (VSM; Lake Shore 7404). Parameters like specific saturation magnetization (Ms), coercive force (Hc) and remanence (Mr) were evaluated.
3. Results and discussion The TG analysis was conducted to determine the temperature at which the egg-white content can decomposed completely and to identify the proper calcination temperature for obtaining ferrites. Fig. 1 shows the thermogravimetric (TG) and differential thermogravimetric (DTG) curves of the dried egg-white precursor with x = 0.4. The thermal decomposition follows five major steps. These steps are attributed to the dehydration followed by the decomposition of the anhydrous precursor to form the spinel ferrite. The first step starts with the removal of the adsorbed water and ends at about 100 °C. This is followed by the decomposition of the anhydrous precursor through successive steps which are finished at 500 °C giving an overall weight loss of 73%. No weight loss can be observed after this temperature. Accordingly, the egg-white precursors were calcined at 550 °C. Fig. 2 shows XRD patterns of the investigated egg-white precursors annealed at 550 °C. The observed diffraction peaks for all the samples are perfectly indexed to cubic spinel phase (JCPDS card No. 88-1940 and 08-0234), and no impurities are detected in the XRD patterns. The diffraction peaks can be indexed to the planes of (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1) and (4 4 0). The observed broadening of diffraction peaks indicates the nano-crystallinity of the samples. The particle size of the synthesized ferrite samples was estimated from X-ray peak broadening of diffraction peaks using Scherrer formula [20]. The values of the particle size, lattice constant and X-ray density as deduced from the X-ray data are given in Table 1. The average particle size for Ni0.8 xZn0.2MgxFe2O4 gradually increases with increasing Mg content. A slight decrease was observed
Fig. 1. TG–DTG curves in air of precursor with Mg content of 0.4. Heating rate = 5 °C min
1
.
M.A. Gabal, W.A. Bayoumy / Polyhedron 29 (2010) 2569–2573
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Fig. 3. TEM image of the sample with Mg content of 0.6.
Fig. 2. Characteristic parts of XRD patterns of Ni0.8 xZn0.2MgxFe2O4 system.
Table 1 Lattice parameters, X-ray densities, average crystallite size, FT-IR spectral data and magnetic data of Ni0.8 xZn0.2MgxFe2O4 system. Parameters
x = 0.0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
Lattice parameter (a) (Å) X-ray density (Dx) (g cm 3) Average crystallite size (D) (nm) Tetrahedral vibration (m1) (cm 1) Octahedral vibration (m2) (cm 1) Saturation magnetization (Ms) (emu g 1) Magnetic moment (gB) (lB) Remanent magnetization (Mr) (emu g 1) Coercivity (Hc) (emu g 1)
8.368 5.34 36
8.378 5.17 41
8.383 5.01 45
8.384 4.85 35
8.404 4.66 59
584
583
581
580
550
417
415
413
412
408
43.1
41.7
41.0
30.4
36.1
1.82 8.2
1.71 8.3
1.63 8.5
1.17 4.2
1.35 7.9
65.8
57.0
35.0
17.4
11.9
at x = 0.6. The lattice constant increases with increasing magnesium concentration, which can be explained based on the relative ionic radius. The ionic radius (oct: 0.72 Å) of Mg2+ ions is larger than the ionic radius (oct: 0.69 Å) of Ni2+ ions. Replacement of smaller Ni2+ cations with larger Mg2+ cations causes an increase in lattice constant. The X-ray density is observed to decrease with increasing magnesium content. This can be attributed to the substitution of heavier nickel atom by the lighter magnesium atom. This decrease in weight with increasing size causes a decrease in the X-ray density. The representative TEM image of Ni0.2Zn0.2Mg0.6Fe2O4 is shown in Fig. 3. It indicates that the ferrite particles obtained are spherical in shape. The particles are clearly agglomerated due to the interaction between magnetic particles. The average size of the particles is predominantly about 36 nm, which is in close agreement with that obtained from XRD studies. The FT-IR spectra of the investigated ferrites (Fig. 4) show two strong absorption bands in the range 580–616 and 409– 418 cm 1. These bands (m1 and m2) are assigned to the vibrations of the metal ion–oxygen complexes in the tetrahedral and octahedral sites, respectively [21]. The vibrational frequencies of the bands corresponding to tetrahedral and octahedral sites are given in Table 1.
Fig. 4. Magnetic hysteresis loops for Ni0.8 xZn0.2MgxFe2O4 system.
From the table it is clear that, the value of m2 slightly decreases with increasing Mg content. This can be attributed to the comparatively larger ionic radius of Mg ions, which prefer octahedral site occupation, than that of Ni ions. The values of m1 are slightly changed with the increase in the Mg content up to concentration of 0.6 after which a drastic shift in its value to lower wave numbers was observed. Pradeep et al. [22] suggests a new cation distribution for MgFe2O4 in which Mg2+ ions were found to prefer both tetrahedral and octahedral sites occupation. They attributed this to the synthesis method and the impact of nanoregime. Thus the decrease in the m1 frequency value at x = 0.8 can be attributed to the tendency of magnesium ions to occupy tetrahedral site with a corresponding
M.A. Gabal, W.A. Bayoumy / Polyhedron 29 (2010) 2569–2573
70
50 Ms Hc
40
60 50
30
40 20
30
10
Coercivity (Oe)
migration of the iron ions from tetrahedral to octahedral site. This arrangement results in a decrease in the vibrational frequency corresponding to the tetrahedral sublattice as the Mg2+ ions have a higher ionic radii (tet: 0.57 Å) than that of Fe3+ ions (tet: 0.49 Å) which they replace. Moreover, the presence of a small shoulder in the octahedral band at x = 0.8 confirms the cation exchange between the sites [23]. The spectra also show that, the intensity of the absorption band (m2) decreases with increasing Mg concentration. It is known that the intensity ratio is a function of the change of dipole moment with the internuclear distance (dl/dr). This value represents the contribution of the ionic bond Ni–O in the lattice. So, the observed decrease in the absorption band (m2) intensity with increasing Mg content is presumably due to the perturbation occurring in Ni–O bonds by substitution with the Mg2+ ions. Fig. 5 represents typical room temperature magnetic hysteresis of the Ni0.8 xZn0.2MgxFe2O4 nanocrystals over the field range 8 kOe to 8 kOe. The value of the magnetization sharply increases with the external magnetic field strength at low field region. The value of saturation magnetization (Ms), coercivity (Hc), remanent magnetization (Mr) and magnetic moment (gB) are listed in Table 1. Fig. 6 plots the saturation magnetization (Ms) and the coercivity (Hc) as a function of magnesium content. The decrease in the saturation magnetization and the coercivity of the nanocrystals by increasing the Mg content can be attributed to the magnetic character and the anisotropic nature of magnesium, respectively. This decrease suggest the decrease of strongly interacted magnetic state towards a paramagnetic state [24]. The changes in magnetic properties such as Ms, Hc, Mr and gB are due to the influence of the cationic stoichiometry and their occupancy in the specific sites. The magnetic order in the cubic system of ferromagnetic spinels is due to super-exchange interaction mechanism occurring between the metal ions in the tetrahedral A-sites and octahedral B-sites [24]. The replacement of Ni2+ ions (with magnetic moment of 2.3 lB) by Mg2+ ion (with zero magnetic moment) which has preferential octahedral site occupancy results in the reduction of super-exchange interaction between A and B-sites. In other meaning, as the magnesium concentration
Saturation magnetization (emu/g)
2572
20
0
10 0
0.2
0.4
0.6
0.8
Mg Content Fig. 6. Variation of the saturation magnetization and the coercivity with x in Ni0.8 xZn0.2MgxFe2O4 system.
increases, the magnetization of the B-site (MB) decreases while that of A-site (MA) remains constant. As the net magnetization (Ms) equals (MB–MA), the net magnetization decreases. The unexpected increase in the value of saturation magnetization at x = 0.8 strengthen the tendency of magnesium ions to occupy tetrahedral site with a corresponding migration of the iron ions (with magnetic moment of 5 lB) from tetrahedral to octahedral site, as revealed from FT-IR results. The decrease in the value of coercivity with increasing magnesium concentration can be explained on the basis of magneto-crystalline anisotropy. The magneto-crystalline anisotropy is dependent on the different distributions of magnetic moments of the ions on the surface of the nanoparticles. The anisotropy constant value of Ni-ferrite is greater than that of Mg-ferrite [12]. Therefore, the replacement of Ni2+ by Mg2+ ions leads to a decrease in the coercivity values. 4. Conclusions Nickel magnesium ferrites Ni0.8 xZn0.2MgxFe2O4 nanoparticles have been successfully synthesized using the metal nitrate and freshly extracted egg-white. The X-ray diffraction confirmed the single-phase cubic spinel structure of the samples. FT-IR spectra confirmed the ferrites formation. TEM revealed an average crystallite size of 36 nm for the sample with Mg content of 0.6. The decrease in the saturation magnetization and the coercivity of the nanocrystals by increasing the Mg content is attributed to the magnetic character and the anisotropic nature of magnesium. Acknowledgement The authors are grateful to King Abdul Aziz University for providing financial support for this work. References
Fig. 5. FT-IR spectra of Ni0.8 xZn0.2MgxFe2O4 system.
[1] S. Kumar, V. Singh, S. Aggarwal, U. Mandal, R. Kotnala, Mater. Sci. Eng. B 166 (2010) 76. [2] B.P. Rao, A.M. Kumar, K.H. Rao, Y.L.N. Murthy, O.F. Caltun, I. Dumitru, L. Spinu, J. Optoelectric. Adv. Mater. 8 (2006) 1703. [3] A. Kumar, M.C. Varma, C.L. Dube, K.H. Rao, S.C. Kashyap, J. Magn. Magn. Mater. 320 (2008) e370. [4] U.R. Lima, M.C. Nasar, R.S. Nasar, M.C. Rezende, J.H. Arajo, J. Magn. Magn. Mater. 320 (2008) 1666. [5] X. Li, G. Wang, J. Magn. Magn. Mater. 321 (2009) 1276. [6] I.H. Gul, W. Ahmed, A. Maqsood, J. Magn. Magn. Mater. 320 (2008) 270. [7] C. Washburn, J. Jorne, S. Kurinec, Key Eng. Mater. 314 (2006) 127. [8] M. Jalaly, M.H. Enayati, F. Karimazadeh, P. Kameli, Powder Technol. 193 (2009) 150.
M.A. Gabal, W.A. Bayoumy / Polyhedron 29 (2010) 2569–2573 [9] S. Thakar, S.C. Katyal, M. Singh, J. Magn. Magn. Mater. 321 (2009) 1. [10] Q. Tian, J. Li, Q. Wang, S. Wang, X. Zhang, Thin Solid Film, 2009, doi:10.1016/ j.tsf.2009.06.042. [11] S. Maensiri, C. Masingboon, B. Boonchom, S. Seraphin, Scripta Mater. 56 (2007) 797. [12] M.A. Gabal, J. Magn. Magn. Mater. 321 (2009) 3144. [13] M.A. Gabal, Y.M. Al Angary, S.S. Al-Juaid, J. Alloy. Compd., 2009, doi:10.1016/ j.jallcom.2009.11.124. [14] M.M. Mallapur, B.K. Chougule, Mater. Lett. 64 (2010) 231. [15] M.M. Rashad, E.M. Elsayed, M.M. Moharam, R.M. Abou-Shahba, A.E. Saba, J. Alloy. Compd. 486 (2009) 759.
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M.M. Haque, M. Huq, M.A. Hakim, Physica B 404 (2009) 3915. M.A. E1 Hiti, J. Magn. Magn. Mater. 136 (1994) 138. M.A. Amer, M. El Hiti, J. Magn. Magn. Mater. 234 (2001) 118. M. Naeema, N.A. Shahb, I.H. Gul, A. Maqsood, J. Alloy. Compd. 487 (2009) 739. M.A. Gabal, Chem. Phys. 118 (2009) 153. R.D. Waldron, Phys. Rev. 99 (1955) 1727. A. Pradeep, P. Priyadharsini, G. Chandrasekaran, J. Magn. Magn. Mater. 320 (2008) 2774. [23] P. Priyadharsini, A. Pradeep, P.S. Rao, G. Chandrasekaran, Chem. Phys. 116 (2009) 207. [24] Y. Kseoglu, A. Baykal, F. Gzüak, H. Kavas, Polyhedron 28 (2009) 2887.
Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Effect of composition on structural and magnetic properties of nanocrystalline Ni0.8 xZn0.2MgxFe2O4 ferrite M.A. Gabal a,b,*, W.A. Bayoumy b a b
Chemistry Department, Faculty of Science, King Abdul Aziz University, Jeddah, Saudi Arabia Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
a r t i c l e
i n f o
Article history: Received 16 February 2010 Accepted 17 April 2010 Available online 27 April 2010 Keywords: Ferrite Mg-substitution Egg-white XRD VSM
a b s t r a c t Nanocrystalline magnetic particles of Ni0.8 xZn0.2MgxFe2O4 ferrites with x lying between 0.0 and 0.8 were synthesized using metal nitrates and freshly extracted egg-white. The synthesized powders were characterized using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and transmission electron microscopy (TEM). With increasing magnesium concentration, the lattice constant increases while Xray density decreases. The average crystallite size determined from XRD data using Scherrer formula lie in the range of 35–59 nm. TEM image shows spherically agglomerated particles with average crystallite size agreed well with that obtained from XRD. Magnetic properties measured at room temperature by vibrating sample magnetometer (VSM) reveal a decrease in saturation magnetization up to Mg content of 0.6. In agreement with FT-IR results, the unexpected increase in the magnetization at Mg content of 0.8 can be attributed to the tendency of Mg2+ ions to occupy the tetrahedral site. The decrease in the value of coercivity with increasing magnesium content can be explained based on the magneto-crystalline anisotropy. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Spinel ferrites nanocrystals have been widely investigated in the recent years due to their remarkable electrical and magnetic properties and wide practical applications in information storage system, ferrofluid technology, magneto-caloric refrigeration and medical diagnosis [1]. To meet the demand of high performance devices, an important step is to synthesize ferrite crystals in nanoscale forms with narrow particle size distribution and minimum particles agglomeration. Below the critical size these crystals exist in a single domain state so the domain wall resonance is avoided and the material can work at higher frequencies [2]. Many techniques have been provided for the synthesis of the nano-sized ferrites. These methods include sol–gel [3], organic precursors [4], hydrothermal [5], co-precipitation [6], cathodic electrophoretic deposition (EPD) [7], mechanochemical synthesis [8], reverse micelle [9], and electrochemical deposition [10]. More recently, a simple, cost effective and environmentally friendly method utilizing egg-white is also used [11–13]. Zinc substitution plays a decisive role in determining the ferrite properties. Mixed Zn ferrites and especially Ni–Zn ferrites are the most important magnetic materials, which offer a broad range of * Corresponding author at: Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. Tel.: +966 557071572. E-mail address: [email protected] (M.A. Gabal). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.04.019
frequency suppression in the MHz band frequency. They posses a unique combination of desirable properties such as large magnetic permeability at high frequencies, electrical resistivity, mechanical hardness, chemical stability in addition to the reasonable cost [14]. They have a wide range of applications [15] in microwave absorbance, electronic devices such as radio and TV sets, integrated nonreciprocal circuits, high frequency transformers, memory core devices, rod antennas, read-write heads for high-speed digital tape or disk recording, telecommunication applications, excellent catalysts for alkylation of aromatics and in gas sensing. MgFe2O4 [16] is a partially inverse cubic spinel. It can be considered as a collinear ferrimagnet whose degree of inversion is sensitive to the sample preparation history. Magnetic properties of ferrites are strongly dependent on their chemical compositions and additives or substitutions. Small amount of foreign ions in the ferrite can dramatically change the properties of ferrites. In the literature very few studies on the Mg substituted Ni–Zn ferrites are present. El Hiti [17] studied the dc conductivity of ZnxMg0.8 xNi0.2Fe204 system, with x = 0.0, 0.2, 0.4, 0.6 and 0.8, as a function of temperature and composition. The dc conductivity was found to increase with increasing temperature, while it decreases with increasing Zn content. The Curie transition temperature decreases, while the activation energy for conduction increases by increasing Zn content. Ni0.2ZnxMg0.8 xFe2O4 ferrites; 0 6 x P 0.8 were studied using X-ray diffraction and Mössbauer spectroscopy [18]. The samples proved to have a single-phase cubic spinel structure. The
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M.A. Gabal, W.A. Bayoumy / Polyhedron 29 (2010) 2569–2573
dependence of the lattice constants and intercationic distances on Zn content was studied. According to the Mössbauer studies, the samples with x = 0.0–0.4 are magnetically ordered while others are paramagnetic. The cation distributions were deduced and supported by X-ray studies. Ni1 xMgxFe2O4 ferrites (0 6 x 6 1) were prepared by the co-precipitation method [19]. The samples were characterized by XRD. The crystallite size variation is within the range 10–13 nm. The Curie temperature was determined using AC magnetic susceptibility data and the observed variation is explained based on cations distribution among tetrahedral and octahedral sites. The properties of ferrites are highly sensitive to the cation distribution, which in turn is controlled by preparation conditions and substitution of different metals. Accordingly, the present work is aimed at the preparation of Ni0.8 xZn0.2MgxFe2O4 ferrites with x = 0.0–0.8 using egg-white method. The produced nano-sized ferrites were characterized using thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and transmission electron microscopy (TEM) techniques. The change in the magnetic properties of the investigated system was measured using vibrating sample magnetometer (VSM). To the best of our knowledge, the influences of magnesium substitution on the structural and magnetic properties of Ni–Zn ferrites have seldom been reported.
2. Experimental Precursors of the mixed ferrite samples were prepared, using stoichiometric ratios of the metal nitrate and freshly extracted egg-white, according to egg-white method as previously described [12,13]. The dried precursors were ground and calcined in a muffle furnace at 550 °C for 2 h. Thermogravimetric analysis (TG) was carried out, using Perkin– Elmer thermal analyzer on precursors up to 600 °C at a heating rate of 5 °C min 1 in air. X-ray powder diffraction analysis was conducted on D8 Advance diffractometer using Cu Ka radiation (operated at 40 kV and 35 mA). Fourier transform infrared spectroscopic analysis, using KBr pellets, was carried out in the range 200–4000 cm 1 using a Jasco model FT-IR 310 spectrometer.
Transmission electron microscopy was performed using Jeol 2010 transmission electron microscope with an accelerating voltage of 100 kV. A drop of diluted sample in alcohol was dripped on the TEM grid and dried, which was then used to examine the grain size and morphology of synthesized sample. The characteristic hysteresis loops of the system were measured at room temperature, up to a maximum external field of ±8 kOe, by using vibrating sample magnetometer (VSM; Lake Shore 7404). Parameters like specific saturation magnetization (Ms), coercive force (Hc) and remanence (Mr) were evaluated.
3. Results and discussion The TG analysis was conducted to determine the temperature at which the egg-white content can decomposed completely and to identify the proper calcination temperature for obtaining ferrites. Fig. 1 shows the thermogravimetric (TG) and differential thermogravimetric (DTG) curves of the dried egg-white precursor with x = 0.4. The thermal decomposition follows five major steps. These steps are attributed to the dehydration followed by the decomposition of the anhydrous precursor to form the spinel ferrite. The first step starts with the removal of the adsorbed water and ends at about 100 °C. This is followed by the decomposition of the anhydrous precursor through successive steps which are finished at 500 °C giving an overall weight loss of 73%. No weight loss can be observed after this temperature. Accordingly, the egg-white precursors were calcined at 550 °C. Fig. 2 shows XRD patterns of the investigated egg-white precursors annealed at 550 °C. The observed diffraction peaks for all the samples are perfectly indexed to cubic spinel phase (JCPDS card No. 88-1940 and 08-0234), and no impurities are detected in the XRD patterns. The diffraction peaks can be indexed to the planes of (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1) and (4 4 0). The observed broadening of diffraction peaks indicates the nano-crystallinity of the samples. The particle size of the synthesized ferrite samples was estimated from X-ray peak broadening of diffraction peaks using Scherrer formula [20]. The values of the particle size, lattice constant and X-ray density as deduced from the X-ray data are given in Table 1. The average particle size for Ni0.8 xZn0.2MgxFe2O4 gradually increases with increasing Mg content. A slight decrease was observed
Fig. 1. TG–DTG curves in air of precursor with Mg content of 0.4. Heating rate = 5 °C min
1
.
M.A. Gabal, W.A. Bayoumy / Polyhedron 29 (2010) 2569–2573
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Fig. 3. TEM image of the sample with Mg content of 0.6.
Fig. 2. Characteristic parts of XRD patterns of Ni0.8 xZn0.2MgxFe2O4 system.
Table 1 Lattice parameters, X-ray densities, average crystallite size, FT-IR spectral data and magnetic data of Ni0.8 xZn0.2MgxFe2O4 system. Parameters
x = 0.0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
Lattice parameter (a) (Å) X-ray density (Dx) (g cm 3) Average crystallite size (D) (nm) Tetrahedral vibration (m1) (cm 1) Octahedral vibration (m2) (cm 1) Saturation magnetization (Ms) (emu g 1) Magnetic moment (gB) (lB) Remanent magnetization (Mr) (emu g 1) Coercivity (Hc) (emu g 1)
8.368 5.34 36
8.378 5.17 41
8.383 5.01 45
8.384 4.85 35
8.404 4.66 59
584
583
581
580
550
417
415
413
412
408
43.1
41.7
41.0
30.4
36.1
1.82 8.2
1.71 8.3
1.63 8.5
1.17 4.2
1.35 7.9
65.8
57.0
35.0
17.4
11.9
at x = 0.6. The lattice constant increases with increasing magnesium concentration, which can be explained based on the relative ionic radius. The ionic radius (oct: 0.72 Å) of Mg2+ ions is larger than the ionic radius (oct: 0.69 Å) of Ni2+ ions. Replacement of smaller Ni2+ cations with larger Mg2+ cations causes an increase in lattice constant. The X-ray density is observed to decrease with increasing magnesium content. This can be attributed to the substitution of heavier nickel atom by the lighter magnesium atom. This decrease in weight with increasing size causes a decrease in the X-ray density. The representative TEM image of Ni0.2Zn0.2Mg0.6Fe2O4 is shown in Fig. 3. It indicates that the ferrite particles obtained are spherical in shape. The particles are clearly agglomerated due to the interaction between magnetic particles. The average size of the particles is predominantly about 36 nm, which is in close agreement with that obtained from XRD studies. The FT-IR spectra of the investigated ferrites (Fig. 4) show two strong absorption bands in the range 580–616 and 409– 418 cm 1. These bands (m1 and m2) are assigned to the vibrations of the metal ion–oxygen complexes in the tetrahedral and octahedral sites, respectively [21]. The vibrational frequencies of the bands corresponding to tetrahedral and octahedral sites are given in Table 1.
Fig. 4. Magnetic hysteresis loops for Ni0.8 xZn0.2MgxFe2O4 system.
From the table it is clear that, the value of m2 slightly decreases with increasing Mg content. This can be attributed to the comparatively larger ionic radius of Mg ions, which prefer octahedral site occupation, than that of Ni ions. The values of m1 are slightly changed with the increase in the Mg content up to concentration of 0.6 after which a drastic shift in its value to lower wave numbers was observed. Pradeep et al. [22] suggests a new cation distribution for MgFe2O4 in which Mg2+ ions were found to prefer both tetrahedral and octahedral sites occupation. They attributed this to the synthesis method and the impact of nanoregime. Thus the decrease in the m1 frequency value at x = 0.8 can be attributed to the tendency of magnesium ions to occupy tetrahedral site with a corresponding
M.A. Gabal, W.A. Bayoumy / Polyhedron 29 (2010) 2569–2573
70
50 Ms Hc
40
60 50
30
40 20
30
10
Coercivity (Oe)
migration of the iron ions from tetrahedral to octahedral site. This arrangement results in a decrease in the vibrational frequency corresponding to the tetrahedral sublattice as the Mg2+ ions have a higher ionic radii (tet: 0.57 Å) than that of Fe3+ ions (tet: 0.49 Å) which they replace. Moreover, the presence of a small shoulder in the octahedral band at x = 0.8 confirms the cation exchange between the sites [23]. The spectra also show that, the intensity of the absorption band (m2) decreases with increasing Mg concentration. It is known that the intensity ratio is a function of the change of dipole moment with the internuclear distance (dl/dr). This value represents the contribution of the ionic bond Ni–O in the lattice. So, the observed decrease in the absorption band (m2) intensity with increasing Mg content is presumably due to the perturbation occurring in Ni–O bonds by substitution with the Mg2+ ions. Fig. 5 represents typical room temperature magnetic hysteresis of the Ni0.8 xZn0.2MgxFe2O4 nanocrystals over the field range 8 kOe to 8 kOe. The value of the magnetization sharply increases with the external magnetic field strength at low field region. The value of saturation magnetization (Ms), coercivity (Hc), remanent magnetization (Mr) and magnetic moment (gB) are listed in Table 1. Fig. 6 plots the saturation magnetization (Ms) and the coercivity (Hc) as a function of magnesium content. The decrease in the saturation magnetization and the coercivity of the nanocrystals by increasing the Mg content can be attributed to the magnetic character and the anisotropic nature of magnesium, respectively. This decrease suggest the decrease of strongly interacted magnetic state towards a paramagnetic state [24]. The changes in magnetic properties such as Ms, Hc, Mr and gB are due to the influence of the cationic stoichiometry and their occupancy in the specific sites. The magnetic order in the cubic system of ferromagnetic spinels is due to super-exchange interaction mechanism occurring between the metal ions in the tetrahedral A-sites and octahedral B-sites [24]. The replacement of Ni2+ ions (with magnetic moment of 2.3 lB) by Mg2+ ion (with zero magnetic moment) which has preferential octahedral site occupancy results in the reduction of super-exchange interaction between A and B-sites. In other meaning, as the magnesium concentration
Saturation magnetization (emu/g)
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0
10 0
0.2
0.4
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Mg Content Fig. 6. Variation of the saturation magnetization and the coercivity with x in Ni0.8 xZn0.2MgxFe2O4 system.
increases, the magnetization of the B-site (MB) decreases while that of A-site (MA) remains constant. As the net magnetization (Ms) equals (MB–MA), the net magnetization decreases. The unexpected increase in the value of saturation magnetization at x = 0.8 strengthen the tendency of magnesium ions to occupy tetrahedral site with a corresponding migration of the iron ions (with magnetic moment of 5 lB) from tetrahedral to octahedral site, as revealed from FT-IR results. The decrease in the value of coercivity with increasing magnesium concentration can be explained on the basis of magneto-crystalline anisotropy. The magneto-crystalline anisotropy is dependent on the different distributions of magnetic moments of the ions on the surface of the nanoparticles. The anisotropy constant value of Ni-ferrite is greater than that of Mg-ferrite [12]. Therefore, the replacement of Ni2+ by Mg2+ ions leads to a decrease in the coercivity values. 4. Conclusions Nickel magnesium ferrites Ni0.8 xZn0.2MgxFe2O4 nanoparticles have been successfully synthesized using the metal nitrate and freshly extracted egg-white. The X-ray diffraction confirmed the single-phase cubic spinel structure of the samples. FT-IR spectra confirmed the ferrites formation. TEM revealed an average crystallite size of 36 nm for the sample with Mg content of 0.6. The decrease in the saturation magnetization and the coercivity of the nanocrystals by increasing the Mg content is attributed to the magnetic character and the anisotropic nature of magnesium. Acknowledgement The authors are grateful to King Abdul Aziz University for providing financial support for this work. References
Fig. 5. FT-IR spectra of Ni0.8 xZn0.2MgxFe2O4 system.
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