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Effect of cation distribution on the structural and magnetic properties of nickel substituted nanosized Mn–Zn ferrites prepared by co-precipitation method
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 230–233
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of cation distribution on the structural and magnetic properties of nickel substituted nanosized Mn–Zn ferrites prepared by co-precipitation method C. Venkataraju a,, G. Sathishkumar b, K. Sivakumar c a
Department of Physics, A.M.A. College of Engineering, Vadamavandal, Kancheepuram 604 401, India Department of Physics, Sri Sairam Engineering College, West Tambaram, Chennai 600 044, India c Department of Physics, Anna University Chennai, Chennai 600 025, India b
a r t i c l e in f o
a b s t r a c t
Article history: Received 19 May 2009 Received in revised form 21 July 2009 Available online 4 September 2009
Nano particles of Mn(0.5–x)NixZn0.5Fe2O4 (x =0.0, 0.1, 0.2, 0.3) have been synthesized by chemical coprecipitation method. The lattice constant and distribution of cation in the tetrahedral and octahedral ˚ for all sites have been deduced through X-ray diffraction (XRD) data analysis. The lattice constant (A) Mn/Ni concentration is found to be less than that for the corresponding bulk values. X-ray intensity calculations indicate that there is deviation in the normal cation distribution. Magnetization decreases with increasing Ni concentration except for x=0.3, where it shows increasing trend. This is due to migration of Fe3 + ions from B-site to A-site, which reduces the B–B coupling and there by the spin canting in the B sublattice. The Curie temperature was found to decrease with increase in nickel concentration except for x =0.3, where it shows a rise. Coercivity is very low and is found to be inversely proportional to the grain size. & 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetic Material Chemical synthesis X-ray diffraction Magnetic property
1. Introduction Ferrites are superior magnetic materials widely used in microwave and electrical industries. They exhibit high electrical resistivity combined with useful ferromagnetic behavior. In recent years, nanocrystalline ferrites have attracted much interest because of their unusual magnetic properties and their promising technological applications. The bulk properties of ferrites changes as its dimensions are changed to nanoscale. With improvement in synthesis and characterization techniques in nanometric range, there is a tremendous growth in the field of ferrites. Super paramagnetisim, spin canting, core/shell structure, metastable cation distribution, etc. are some of the phenomena, which have been observed in nanoparticles of various ferrites. These phenomena depend on number of factors such as composition, grain size, surface morphology, anisotropy and interparticle interactions [1–3]. The magnetic and the electrical properties of ferrites are reported to be highly sensitive to the cation distribution, which in turn depend on the material of synthesis and sintering conditions. Various preparation techniques have been used for synthesis of fine particles of ferrites, which exhibit novel properties when compared to their properties in bulk. Physical methods like mechanical milling present a high degree of crystalline defects. Goya and Rechenberg [4] have reported that oxygen ions escape Corresponding author.
E-mail address: [email protected] (C. Venkataraju). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.08.043
from the spinel structure, thereby creating anion vacancies during milling. Non-conventional methods such as co-precipitation, thermal decomposition, sol gel and hydrothermal methods have been widely used. Co-precipitation is an attractive method of producing ferrites because of increased homogeneity, purity and reactivity. They are relatively simple, low cost and particle size control can be easily achieved. Bueno et al. [5] have reported the influence of Mn substitution on the magnetic properties and microstructure of Ni0.5 x Zn0.5 xMn2xFe2O4 ferrites, synthesized by nitrate–citrate precursor method. Verma et al. [6] have reported the development of a new power ferrite with low power loss based on manganese– nickel–zinc ferrite composition for switched mode power supplies. Singh et al. [7] have reported the effect of cation distribution on the properties of Mn0.2Ni0.8xZnxFe2O4. The present investigation reports the dependence on cationic stoichiometry of structural and magnetic properties of nanosized Mn(0.5 x)Nix Zn0.5Fe2O4 (x= 0.0, 0.1, 0.2 and 0.3) synthesized by chemical coprecipitation method. The starting materials used were metal chlorides which contribute to environmental safety.
2. Experimental details Nano particles of Mn(0.5 x)NixZn0.5Fe2O4 with x varying from 0.0 to 0.3 were prepared by co-precipitation method. Aqueous solutions of MnCl2, ZnSO4, NiCl2 and FeCl3 in their respective
ARTICLE IN PRESS C. Venkataraju et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 230–233
stoichiometry (100 ml of solution containing (0.5 x) M MnCl2, (x) M NiCl2, 0.5 M ZnSO4 and 100 ml of 2 M FeCl3) were mixed thoroughly at 80 1C and this mixture was added to the boiling solution of NaOH (0.55 M dissolved in 1600 ml of distilled water) within 10 s under constant stirring and a pH of 11 was maintained throughout the reaction. Conversion of metal salts into hydroxides and subsequent transformation of metal hydroxide into nanoferrites takes place upon heating to 100 1C and maintained for 60 min. The nanoparticles thus formed were isolated by centrifugation and washed several times with deionized water followed by acetone and then dried at room temperature. The dried powder was grounded thoroughly in a clean agate mortar. The structure and crystallite size were determined from the X-ray diffraction ˚ radiation. Data (XRD) measurements using CuKa (l = 1.5406 A) were collected every 0.021 in the angle range 20–701 in 2y. The counting time was fixed at 15 s per step for all samples. The particle size was determined by subjecting the samples to transmission electron microscopy (TEM) using a Philips CM 20 microscope. The magnetization measurements were done at room temperature up to a maximum field of 12 kOe and the tempera-
231
ture dependence of magnetization in a field of 1 kOe were done using Vibrational Sample Magnetometer (Lakeshore Model 7404).
3. Results and discussion 3.1. XRD analysis The X-ray diffraction pattern for Mn(0.5 x)NixZn0.5Fe2O4 (with x=0.0, 0.1, 0.2, 0.3) is shown in the Fig. 1. These diffraction lines provide clear evidence of the formation of ferrite phase in all the samples. The broad XRD line indicates that the ferrite particles are of nanosize. All the peaks in the diffraction pattern have been indexed and the refinement of the lattice parameter was done using PowderX software. The average particle size for each composition was calculated from the XRD line width of the (3 11) peak using Scherrer formula [8]. The values of the particle size and lattice constant as deduced from the X-ray data are given in the Table 1. The average particle size for Mn0.4Zn0.5Ni0.1Fe2O4 is found to be 16 nm. The lattice constant decreases with increasing nickel concentration, which can be explained based on the relative ionic ˚ of Ni2 + ions is smaller than the radius. The ionic radius (0.69 A) ˚ of Mn2 + ions. Replacement of smaller Ni2 + ionic radius (0.82 A) cations for larger Mn2 + cations in the manganese zinc ferrite causes a decrease in lattice constant. However the lattice constant of the samples with nickel concentration up to x =0.3 was found to be less than that of bulk. A significant fraction of Mn2 + and Zn2 + occupies the octahedral sites and forces Fe3 + to the tetrahedral sites against their chemical preferences. Since Fe3 + ions have ˚ occupying the tetrahedral sites in smaller ionic radius (0.64 A), place of larger divalent ions leads to contraction in lattice parameter as observed. In order to determine the cation distribution and its variation with composition, X-ray intensity calculations were carried out using formula suggested by Buerger [9]. IðhklÞ ¼ jFhkl j2 P Lp
ð1Þ
where I(hkl) is the relative integral intensity, Fhkl the structure factor, P the multiplication factor and Lp is the Lorentz polarization factor. The distribution of divalent and trivalent cation amongst octahedral and tetrahedral sites in the Mn(0.5 x)NixZn0.5Fe2O4 was determined from the ratio of X-ray diffraction lines I(220)/I(440), I(220)/I(400) and I(4 0 0)/I(4 4 0). These ratios are considered to be sensitive to the cation distribution [10]. The absorption and temperature factors are not taken into account in our calculation because these factors do not affect the relative intensity calculations for spinels at room temperature. The structure factor formula for the plane (h k l) is taken from those reported by Furuhashi [11]. The multiplicity and Loretnz polarization factors are taken from literature [12]. The cation distribution for which the experimental ratios agree well with observed intensity ratios is taken as the correct one. It is known that there is correlation between the ionic radius and lattice constant. The lattice constant can be calculated theoretically by the relation suggested by Mazen et al. [13]. 8 ath ¼ pffiffiffi ½ðrA þ RO Þ þ3ðrB þ RO Þ 3 3
Fig. 1. XRD pattern for the system Mn(0.5 x)NixZn0.5Fe2O4 (with x= 0.0, 0.1, 0.2, 0.3).
ð2Þ
Table 1 Structural and magnetic parameters of Mn(0.5–x)NixZn0.5Fe2O4 (x = 0.0, 0.1, 0.2, 0.3). x
Composition
˚ Lattice constant aExp(A)
˚ Lattice constant aCal(A)
Particle size (nm)
MMax (emu/g)
Coercivity Hc(Oe)
Curie temperature (Tc 1C)
0.0 0.1 0.2 0.3
Mn0.5Zn0.5Fe2O4 Mn0.4Ni0.1Zn0.5Fe2O4 Mn0.3Ni0.2Zn0.5Fe2O4 Mn0.2Ni0.3Zn0.5Fe2O4
8.401 8.390 8.374 8.368
8.419 8.404 8.388 8.373
12 16 13 16
14 12 11 15
0.65 0.39 0.29 0.19
190 185 170 220
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˚ rA and rB are the ionic where Ro is the radius of oxygen ion (1.32 A), radii of tetrahedral (A-Site) and octahedral (B-Site), respectively. The ionic radius for each site was calculated according to rA =[CAMnr(Mn2 + )+CANir(Ni2 + ) +CAZnr(Zn2 + )+CAFer(Fe3 + )] rB = [CBMnr(Mn2 + )+CBNir(Ni2 + ) +CBZnr(Zn2 + )+CBFer(Fe3 + ) where rMn2 + , rNi2 + , rZn2 + and rFe3 + are the + CBFer(Fe2 + )]/2 cationic radius of Mn, Ni, Zn and Fe, respectively. The agreement between ath and aexp obtained from X-ray data indirectly supports the cation distribution deduced from X-ray intensity calculations. It is seen that there is a reasonable agreement between experimental and theoretically found values of lattice constant suggesting that the estimated cation distribution is in agreement with real distribution. The cation distribution parameter is shown in the Table 2. It is seen that there is a deviation in cation preferences in nanosize particles leading to a metastable state and statistically disordered cation distribution.
Smaller grain have larger surface to volume ratio. Spin disorder from the surface of the nanoparticles increase especially when the surface/volume is large. According to core-shell morphology of nanoparticles, the core consists of ferrimagnetically aligned spins and a surface with disorder spins. The core magnetic moments align with the applied magnetic field and get saturated upto a particular magnetic field. Any further increase of magnetic field has effect only on the surface causing a slow down of magnetization. This is the reason all our samples does not reach saturation under an applied field of 12,000 G. There is a rise in the maximum magnetization value of the sample with composition x= 0.3 due to migration of Fe3 + ions from B-site to A-site. The decrease in the number of Fe3 + ions at the B-site reduces the
3.2. Transmission electron microscopy analysis The particle size and morphology of the sample with x= 0.1 is shown in the Fig. 2. The average particle size is around 15 nm. TEM analysis revealed that the particles are nearly spherical and are non-agglomerated. The particle size determined from TEM was found to be in close agreement with that obtained from XRD studies. 3.3. Magnetic studies The variation of maximum magnetization M with nickel concentration is shown in Fig. 3. As in normal behavior, the magnetization of all the samples increases with increasing applied magnetic field and attains its maximum value for fields higher than 12,000 Gauss. It is clear that as the Ni ion concentration increases, the maximum magnetization M decreases up to x = 0.2. This decrease in maximum magnetization can be explained according to cation distribution. As the manganese concentration decreases, the magnetization of the B-site (MB) decreases while that of A-site (MA) increases. As the net magnetization (Ms) equals (MB–MA), the net magnetization deceases. However, these values are low for all compositions than their corresponding values for bulk ferrites. In addition to deviation in cation distribution, the other factors reported in the literature by other researchers may contribute to the reduction of Ms in nanoparticles. Coey [14] attributed the smaller value of Ms in nanoparticles to the existence of random canting of particles surface spins caused by competing antiferromagnetic exchange interactions due to asymmetry in the environment of these spins. Pankhurst [15] claimed that the lower magnetization values are due to non-saturation effects because of random distribution of the small particles with enhanced values of magneto-crystalline anisotropy. Size effect in nanoparticles can cause a reduction in the magnetization value in comparison with the bulk counterpart.
Fig. 2. TEM images of Mn0.4Ni0.1Zn0.5Fe2O4.
Fig. 3. Hysteresis curve for the system Mn(0.5 x)NixZn0.5Fe2O4 (with x = 0.0, 0.1, 0.2, 0.3) at room temperature.
Table 2 Comparison of X-ray intensity ratios for estimating cation distribution. S. No
1 2 3 4
A-site
+ Mn0.22Zn0.20Fe30.58 + Mn0.19Zn0.22Fe30.59 + Mn0.16Zn0.24Fe30.6 + Mn0.12Zn0.25Fe30.63
B-site
+ Mn0.28Zn0.30Fe31.42 + Mn0.21Zn0.28Ni0.1Fe31.410 + Mn0.14Zn0.26Ni0.2Fe31.40 + Mn0.08Zn0.25Ni0.3Fe31.37
I(2 2 0)/I(4 4 0)
I(2 2 0)/I(4 0 0)
I(4 0 0)/I(4 4 0)
Obs
Cal
Obs
Cal
Obs
Cal
0.57 0.54 0.55 0.56
0.55 0.56 0.56 0.55
1.2 1.2 1.3 1.2
1.3 1.4 1.3 1.3
0.47 0.45 0.44 0.46
0.46 0.46 0.47 0.47
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Arulmurugan [17] for Mn0.5Zn0.5Fe2O4 nanoparticles prepared by co-precipitation method. All our samples do not show any cusp like feature.
4. Conclusion Nano sized Mn Ni Zn Ferrites were prepared by co-precipitation method. X-ray intensity calculation reveals that there is a deviation in the normal cation distribution between A-sites and Bsites. The magnetization of the nanoferrites was less than that of the bulk value and decreases with increase in Ni concentration except for x = 0.3 where there is a rise. This is due to deviation in normal cation distribution and significant amount of canting existing in B sublattice for lower Ni concentration. Coercivity is low for all the samples.
References Fig. 4. M3–T curves for the system Mn(0.5 x)NixZn0.5Fe2O4 (with x= 0.0, 0.1, 0.2, 0.3).
antiferromagnetic B–B exchange interaction and as a consequence reduces the spin canting in the B-Site. This increases the B sublattice moments with the effect of an overall increase in the net magnetization. Mohammad Javad [16] has reported saturation magnetization value of 87.2 emu/g for Mn0.5Zn0.5Fe2O4 prepared through mechanical milling method. Arulmurugan [17] has reported saturation magnetization value of 33.5 emu/g for Mn0.5Zn0.5Fe2O4 prepared through co-precipitation method. The experimental results show low coercivity for all the samples. The temperature dependence of magnetization is shown in Fig. 4. The Curie temperature Tc was determined from the M3 versus T plots. The Tc values of the samples were deduced from the relation of the spontaneous magnetization Ma(Tc T)b with b =1/3 [18,19]. It is observed that the Curie temperature Tc decreases with nickel substitution upto x= 0.2 and increases for x= 0.3 as shown in Table 1. The decrease in the Curie temperature may be due to the weakening of A–B interaction as a result of spin canting existing in B sublattice. However for the sample x =0.3, the interaction among the iron ions at A-site and B-site increases thereby increasing the Curie temperature. Wolski et al. [20] have observed a cusp like feature in the magnetization versus temperature (M versus T) curve of MnFe2O4 fine particles prepared by a hydrothermal precipitation method. A similar cusp has been reported by
[1] J.M.D. Coey, K. Khalafella, Phys. Status Solidi A 11 (1972) 225. [2] W.A. De Heer, Nanomagnetism Characterization of Nanophase Materials, in: Z.L. Wang (ed.), Wiley-VCH, Germany, 2000. [3] H.S. Nalwa (Ed.), Magnetic Nanostructure, American Scientific Publishers, USA, 2002 (chapter 1–4). [4] G.F. Goya, H.R. Rechenberg, J. Magn. Magn. Mater. 203 (1999) 141. [5] A.R. Bueno, Maria L. Gregori, Maria, C.S. Nobrega, Mater. Chem. Phys. 105 (2007) 229–233. [6] A. Verma, M.I. Alam, Ratnamala Chatterjee, T.C. Goel, R.G. Mendiratta, J. Magn. Magn. Mater. 300 (2006) 500–505. [7] A.k. Singh, A. Verma, O.P. Thakur, Chandra Prakash, T.C. Goel, R.G. Mendiratta, Solid State Commun. 125 (2003) 121–125. [8] B.D. Cullity, Elements of X-ray Diffraction, Addison–Wesley Pub.Co., INC, 1966, p. 42; Chandana Rath, S. Anand, R.P. Das, K.K. Sahoo, S.D. Kulkarni, S.K. Date, N.C. Mishra, J. Appl. Phys. 91 (2002) 2211. [9] M.J. Buerger, Crystal Structure Analysis, Wiley, New York, 1960. [10] H. Ohnishi, T. Teranishi, J. Phys. Soc. Jpn. 16 (1961) 36. [11] H. Furuhashi, M. Inagaki, S. Naka, J. Inorg. Nucl. Chem. 35 (1973) 3009–3014. [12] C. Otero Arean, J.L. Rodriguez Blanco, J.M. Rubio Gonzalez, M.C. Trobajoa Fernandez, J. Mater. Sci. Lett. 9 (1990) 229. [13] S.A. Mazen, M.H. Abdallah, B.A. Sabrah, H.A.M. Hashem, Phys. Status Solidi A 134 (1992) 263–271. [14] J.M.D. Coey, Can. J. Phys. 65 (1987) 1210. [15] Q.A. Pankhurst, R.J. Pollard, Phys. Rev. Lett. 67 (1991) 248. [16] Mohammad Javad Nasr Isfahani, J. Magn. Magn. Mater. 321 (3) (2009) 152–156. [17] R. Arulmurugan, B. Jeyadevan, G. Vaidyanathan, S. Sendhilnathan, J. Magn. Magn. Mater. 288 (2008) 470–477. [18] A. Belayachi, J.L. Dormann, M. Nogues, J. Phys. Condens. Mater. 10 (1998) 1599. [19] R.N. Bhowmik, R. Ranganathan, R. Nagarajan, J. Magn. Magn. Mater. 299 (2006) 327. [20] W. Wolski, E. Wolska, J. Kaczmarek, P. Piszora, Phys. Status Solidi A 152 (1995).
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of cation distribution on the structural and magnetic properties of nickel substituted nanosized Mn–Zn ferrites prepared by co-precipitation method C. Venkataraju a,, G. Sathishkumar b, K. Sivakumar c a
Department of Physics, A.M.A. College of Engineering, Vadamavandal, Kancheepuram 604 401, India Department of Physics, Sri Sairam Engineering College, West Tambaram, Chennai 600 044, India c Department of Physics, Anna University Chennai, Chennai 600 025, India b
a r t i c l e in f o
a b s t r a c t
Article history: Received 19 May 2009 Received in revised form 21 July 2009 Available online 4 September 2009
Nano particles of Mn(0.5–x)NixZn0.5Fe2O4 (x =0.0, 0.1, 0.2, 0.3) have been synthesized by chemical coprecipitation method. The lattice constant and distribution of cation in the tetrahedral and octahedral ˚ for all sites have been deduced through X-ray diffraction (XRD) data analysis. The lattice constant (A) Mn/Ni concentration is found to be less than that for the corresponding bulk values. X-ray intensity calculations indicate that there is deviation in the normal cation distribution. Magnetization decreases with increasing Ni concentration except for x=0.3, where it shows increasing trend. This is due to migration of Fe3 + ions from B-site to A-site, which reduces the B–B coupling and there by the spin canting in the B sublattice. The Curie temperature was found to decrease with increase in nickel concentration except for x =0.3, where it shows a rise. Coercivity is very low and is found to be inversely proportional to the grain size. & 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetic Material Chemical synthesis X-ray diffraction Magnetic property
1. Introduction Ferrites are superior magnetic materials widely used in microwave and electrical industries. They exhibit high electrical resistivity combined with useful ferromagnetic behavior. In recent years, nanocrystalline ferrites have attracted much interest because of their unusual magnetic properties and their promising technological applications. The bulk properties of ferrites changes as its dimensions are changed to nanoscale. With improvement in synthesis and characterization techniques in nanometric range, there is a tremendous growth in the field of ferrites. Super paramagnetisim, spin canting, core/shell structure, metastable cation distribution, etc. are some of the phenomena, which have been observed in nanoparticles of various ferrites. These phenomena depend on number of factors such as composition, grain size, surface morphology, anisotropy and interparticle interactions [1–3]. The magnetic and the electrical properties of ferrites are reported to be highly sensitive to the cation distribution, which in turn depend on the material of synthesis and sintering conditions. Various preparation techniques have been used for synthesis of fine particles of ferrites, which exhibit novel properties when compared to their properties in bulk. Physical methods like mechanical milling present a high degree of crystalline defects. Goya and Rechenberg [4] have reported that oxygen ions escape Corresponding author.
E-mail address: [email protected] (C. Venkataraju). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.08.043
from the spinel structure, thereby creating anion vacancies during milling. Non-conventional methods such as co-precipitation, thermal decomposition, sol gel and hydrothermal methods have been widely used. Co-precipitation is an attractive method of producing ferrites because of increased homogeneity, purity and reactivity. They are relatively simple, low cost and particle size control can be easily achieved. Bueno et al. [5] have reported the influence of Mn substitution on the magnetic properties and microstructure of Ni0.5 x Zn0.5 xMn2xFe2O4 ferrites, synthesized by nitrate–citrate precursor method. Verma et al. [6] have reported the development of a new power ferrite with low power loss based on manganese– nickel–zinc ferrite composition for switched mode power supplies. Singh et al. [7] have reported the effect of cation distribution on the properties of Mn0.2Ni0.8xZnxFe2O4. The present investigation reports the dependence on cationic stoichiometry of structural and magnetic properties of nanosized Mn(0.5 x)Nix Zn0.5Fe2O4 (x= 0.0, 0.1, 0.2 and 0.3) synthesized by chemical coprecipitation method. The starting materials used were metal chlorides which contribute to environmental safety.
2. Experimental details Nano particles of Mn(0.5 x)NixZn0.5Fe2O4 with x varying from 0.0 to 0.3 were prepared by co-precipitation method. Aqueous solutions of MnCl2, ZnSO4, NiCl2 and FeCl3 in their respective
ARTICLE IN PRESS C. Venkataraju et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 230–233
stoichiometry (100 ml of solution containing (0.5 x) M MnCl2, (x) M NiCl2, 0.5 M ZnSO4 and 100 ml of 2 M FeCl3) were mixed thoroughly at 80 1C and this mixture was added to the boiling solution of NaOH (0.55 M dissolved in 1600 ml of distilled water) within 10 s under constant stirring and a pH of 11 was maintained throughout the reaction. Conversion of metal salts into hydroxides and subsequent transformation of metal hydroxide into nanoferrites takes place upon heating to 100 1C and maintained for 60 min. The nanoparticles thus formed were isolated by centrifugation and washed several times with deionized water followed by acetone and then dried at room temperature. The dried powder was grounded thoroughly in a clean agate mortar. The structure and crystallite size were determined from the X-ray diffraction ˚ radiation. Data (XRD) measurements using CuKa (l = 1.5406 A) were collected every 0.021 in the angle range 20–701 in 2y. The counting time was fixed at 15 s per step for all samples. The particle size was determined by subjecting the samples to transmission electron microscopy (TEM) using a Philips CM 20 microscope. The magnetization measurements were done at room temperature up to a maximum field of 12 kOe and the tempera-
231
ture dependence of magnetization in a field of 1 kOe were done using Vibrational Sample Magnetometer (Lakeshore Model 7404).
3. Results and discussion 3.1. XRD analysis The X-ray diffraction pattern for Mn(0.5 x)NixZn0.5Fe2O4 (with x=0.0, 0.1, 0.2, 0.3) is shown in the Fig. 1. These diffraction lines provide clear evidence of the formation of ferrite phase in all the samples. The broad XRD line indicates that the ferrite particles are of nanosize. All the peaks in the diffraction pattern have been indexed and the refinement of the lattice parameter was done using PowderX software. The average particle size for each composition was calculated from the XRD line width of the (3 11) peak using Scherrer formula [8]. The values of the particle size and lattice constant as deduced from the X-ray data are given in the Table 1. The average particle size for Mn0.4Zn0.5Ni0.1Fe2O4 is found to be 16 nm. The lattice constant decreases with increasing nickel concentration, which can be explained based on the relative ionic ˚ of Ni2 + ions is smaller than the radius. The ionic radius (0.69 A) ˚ of Mn2 + ions. Replacement of smaller Ni2 + ionic radius (0.82 A) cations for larger Mn2 + cations in the manganese zinc ferrite causes a decrease in lattice constant. However the lattice constant of the samples with nickel concentration up to x =0.3 was found to be less than that of bulk. A significant fraction of Mn2 + and Zn2 + occupies the octahedral sites and forces Fe3 + to the tetrahedral sites against their chemical preferences. Since Fe3 + ions have ˚ occupying the tetrahedral sites in smaller ionic radius (0.64 A), place of larger divalent ions leads to contraction in lattice parameter as observed. In order to determine the cation distribution and its variation with composition, X-ray intensity calculations were carried out using formula suggested by Buerger [9]. IðhklÞ ¼ jFhkl j2 P Lp
ð1Þ
where I(hkl) is the relative integral intensity, Fhkl the structure factor, P the multiplication factor and Lp is the Lorentz polarization factor. The distribution of divalent and trivalent cation amongst octahedral and tetrahedral sites in the Mn(0.5 x)NixZn0.5Fe2O4 was determined from the ratio of X-ray diffraction lines I(220)/I(440), I(220)/I(400) and I(4 0 0)/I(4 4 0). These ratios are considered to be sensitive to the cation distribution [10]. The absorption and temperature factors are not taken into account in our calculation because these factors do not affect the relative intensity calculations for spinels at room temperature. The structure factor formula for the plane (h k l) is taken from those reported by Furuhashi [11]. The multiplicity and Loretnz polarization factors are taken from literature [12]. The cation distribution for which the experimental ratios agree well with observed intensity ratios is taken as the correct one. It is known that there is correlation between the ionic radius and lattice constant. The lattice constant can be calculated theoretically by the relation suggested by Mazen et al. [13]. 8 ath ¼ pffiffiffi ½ðrA þ RO Þ þ3ðrB þ RO Þ 3 3
Fig. 1. XRD pattern for the system Mn(0.5 x)NixZn0.5Fe2O4 (with x= 0.0, 0.1, 0.2, 0.3).
ð2Þ
Table 1 Structural and magnetic parameters of Mn(0.5–x)NixZn0.5Fe2O4 (x = 0.0, 0.1, 0.2, 0.3). x
Composition
˚ Lattice constant aExp(A)
˚ Lattice constant aCal(A)
Particle size (nm)
MMax (emu/g)
Coercivity Hc(Oe)
Curie temperature (Tc 1C)
0.0 0.1 0.2 0.3
Mn0.5Zn0.5Fe2O4 Mn0.4Ni0.1Zn0.5Fe2O4 Mn0.3Ni0.2Zn0.5Fe2O4 Mn0.2Ni0.3Zn0.5Fe2O4
8.401 8.390 8.374 8.368
8.419 8.404 8.388 8.373
12 16 13 16
14 12 11 15
0.65 0.39 0.29 0.19
190 185 170 220
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˚ rA and rB are the ionic where Ro is the radius of oxygen ion (1.32 A), radii of tetrahedral (A-Site) and octahedral (B-Site), respectively. The ionic radius for each site was calculated according to rA =[CAMnr(Mn2 + )+CANir(Ni2 + ) +CAZnr(Zn2 + )+CAFer(Fe3 + )] rB = [CBMnr(Mn2 + )+CBNir(Ni2 + ) +CBZnr(Zn2 + )+CBFer(Fe3 + ) where rMn2 + , rNi2 + , rZn2 + and rFe3 + are the + CBFer(Fe2 + )]/2 cationic radius of Mn, Ni, Zn and Fe, respectively. The agreement between ath and aexp obtained from X-ray data indirectly supports the cation distribution deduced from X-ray intensity calculations. It is seen that there is a reasonable agreement between experimental and theoretically found values of lattice constant suggesting that the estimated cation distribution is in agreement with real distribution. The cation distribution parameter is shown in the Table 2. It is seen that there is a deviation in cation preferences in nanosize particles leading to a metastable state and statistically disordered cation distribution.
Smaller grain have larger surface to volume ratio. Spin disorder from the surface of the nanoparticles increase especially when the surface/volume is large. According to core-shell morphology of nanoparticles, the core consists of ferrimagnetically aligned spins and a surface with disorder spins. The core magnetic moments align with the applied magnetic field and get saturated upto a particular magnetic field. Any further increase of magnetic field has effect only on the surface causing a slow down of magnetization. This is the reason all our samples does not reach saturation under an applied field of 12,000 G. There is a rise in the maximum magnetization value of the sample with composition x= 0.3 due to migration of Fe3 + ions from B-site to A-site. The decrease in the number of Fe3 + ions at the B-site reduces the
3.2. Transmission electron microscopy analysis The particle size and morphology of the sample with x= 0.1 is shown in the Fig. 2. The average particle size is around 15 nm. TEM analysis revealed that the particles are nearly spherical and are non-agglomerated. The particle size determined from TEM was found to be in close agreement with that obtained from XRD studies. 3.3. Magnetic studies The variation of maximum magnetization M with nickel concentration is shown in Fig. 3. As in normal behavior, the magnetization of all the samples increases with increasing applied magnetic field and attains its maximum value for fields higher than 12,000 Gauss. It is clear that as the Ni ion concentration increases, the maximum magnetization M decreases up to x = 0.2. This decrease in maximum magnetization can be explained according to cation distribution. As the manganese concentration decreases, the magnetization of the B-site (MB) decreases while that of A-site (MA) increases. As the net magnetization (Ms) equals (MB–MA), the net magnetization deceases. However, these values are low for all compositions than their corresponding values for bulk ferrites. In addition to deviation in cation distribution, the other factors reported in the literature by other researchers may contribute to the reduction of Ms in nanoparticles. Coey [14] attributed the smaller value of Ms in nanoparticles to the existence of random canting of particles surface spins caused by competing antiferromagnetic exchange interactions due to asymmetry in the environment of these spins. Pankhurst [15] claimed that the lower magnetization values are due to non-saturation effects because of random distribution of the small particles with enhanced values of magneto-crystalline anisotropy. Size effect in nanoparticles can cause a reduction in the magnetization value in comparison with the bulk counterpart.
Fig. 2. TEM images of Mn0.4Ni0.1Zn0.5Fe2O4.
Fig. 3. Hysteresis curve for the system Mn(0.5 x)NixZn0.5Fe2O4 (with x = 0.0, 0.1, 0.2, 0.3) at room temperature.
Table 2 Comparison of X-ray intensity ratios for estimating cation distribution. S. No
1 2 3 4
A-site
+ Mn0.22Zn0.20Fe30.58 + Mn0.19Zn0.22Fe30.59 + Mn0.16Zn0.24Fe30.6 + Mn0.12Zn0.25Fe30.63
B-site
+ Mn0.28Zn0.30Fe31.42 + Mn0.21Zn0.28Ni0.1Fe31.410 + Mn0.14Zn0.26Ni0.2Fe31.40 + Mn0.08Zn0.25Ni0.3Fe31.37
I(2 2 0)/I(4 4 0)
I(2 2 0)/I(4 0 0)
I(4 0 0)/I(4 4 0)
Obs
Cal
Obs
Cal
Obs
Cal
0.57 0.54 0.55 0.56
0.55 0.56 0.56 0.55
1.2 1.2 1.3 1.2
1.3 1.4 1.3 1.3
0.47 0.45 0.44 0.46
0.46 0.46 0.47 0.47
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Arulmurugan [17] for Mn0.5Zn0.5Fe2O4 nanoparticles prepared by co-precipitation method. All our samples do not show any cusp like feature.
4. Conclusion Nano sized Mn Ni Zn Ferrites were prepared by co-precipitation method. X-ray intensity calculation reveals that there is a deviation in the normal cation distribution between A-sites and Bsites. The magnetization of the nanoferrites was less than that of the bulk value and decreases with increase in Ni concentration except for x = 0.3 where there is a rise. This is due to deviation in normal cation distribution and significant amount of canting existing in B sublattice for lower Ni concentration. Coercivity is low for all the samples.
References Fig. 4. M3–T curves for the system Mn(0.5 x)NixZn0.5Fe2O4 (with x= 0.0, 0.1, 0.2, 0.3).
antiferromagnetic B–B exchange interaction and as a consequence reduces the spin canting in the B-Site. This increases the B sublattice moments with the effect of an overall increase in the net magnetization. Mohammad Javad [16] has reported saturation magnetization value of 87.2 emu/g for Mn0.5Zn0.5Fe2O4 prepared through mechanical milling method. Arulmurugan [17] has reported saturation magnetization value of 33.5 emu/g for Mn0.5Zn0.5Fe2O4 prepared through co-precipitation method. The experimental results show low coercivity for all the samples. The temperature dependence of magnetization is shown in Fig. 4. The Curie temperature Tc was determined from the M3 versus T plots. The Tc values of the samples were deduced from the relation of the spontaneous magnetization Ma(Tc T)b with b =1/3 [18,19]. It is observed that the Curie temperature Tc decreases with nickel substitution upto x= 0.2 and increases for x= 0.3 as shown in Table 1. The decrease in the Curie temperature may be due to the weakening of A–B interaction as a result of spin canting existing in B sublattice. However for the sample x =0.3, the interaction among the iron ions at A-site and B-site increases thereby increasing the Curie temperature. Wolski et al. [20] have observed a cusp like feature in the magnetization versus temperature (M versus T) curve of MnFe2O4 fine particles prepared by a hydrothermal precipitation method. A similar cusp has been reported by
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