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Effect of zinc substitution on magnetic and electrical properties of nanocrystalline nickel ferrite synthesized by refluxing method
Physica B 407 (2012) 1104–1107
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Physica B journal homepage: www.elsevier.com/locate/physb
Effect of zinc substitution on magnetic and electrical properties of nanocrystalline nickel ferrite synthesized by refluxing method A.I. Nandapure a, S.B. Kondawar b,n, P.S. Sawadh a, B.I. Nandapure b a b
B.D. College of Engineering, Sevagram, Wardha 442001, India Department of Physics, R.T.M. Nagpur University, Nagpur 440033, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2011 Received in revised form 4 January 2012 Accepted 5 January 2012 Available online 17 January 2012
Nanocrystalline Nickel ferrite (NiFe2O4) and Zn substituted nickel ferrite (NiZnFe2O4) have been synthesized by the refluxing method. These ferrites were characterized by XRD, TEM, Mossbauer spectroscopy and VSM in order to study the effect of zinc substitution in nickel ferrite. XRD diffraction results confirm the spinel structure for the prepared nanocrystalline ferrites with an average crystallite size of 14–16 nm. Lattice parameter was found to increase with the substitution of Zn2 þ ions from ˚ TEM images confirmed average particle size of about 20 nm and indicates nanocrystal8.40 A˚ to 8.42 A. line nature of the compounds. A shift in isomeric deviation with the doublet was observed due to the influence of Zn substitution in the nickel ferrite. The Zn content has a significant influence on the magnetic behavior and electrical conductivity of NiFe2O4. Saturation magnetization drastically increased whereas room temperature electrical conductivity decreased due to the addition of Zn content in NiFe2O4, indicating super magnetic material with lesser coercivity. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nickel ferrite Nickel zinc ferrite Reflux method Mossbauer spectroscopy VSM
1. Introduction Recently, nanocrystalline magnetic materials have been receiving more attention due to their novel material properties, which are significantly different from those of their bulk counterparts. Spinel-type oxides (MFe2O4, where M is a divalent metal) which include the magnetic ferrites, are often denoted by the formula AB2O4, where A and B refer to tetrahedral and octahedral sites, respectively, in the face centered cubic lattice. These materials are technologically important and have been used in many applications including magnetic recording media and magnetic fluids for the storage and/or retrieval of information, magnetic resonance imaging (MRI) enhancement, catalysis, magnetically guided drug delivery, sensors and pigments [1]. Among all ferrites, NiFe2O4 has been synthesized because it has promising applications like ferro-fluids, gas sensor, catalyst, microwave absorber, information storage devices, magnetic fluids, drug delivery [2]. Nickel ferrite (NiFe2O4) is basically an inverse spinel ferrite in which the tetrahedral (A) sites are occupied by ferric ions and the octahedral (B) sites by ferric and nickel ions. Thus, the compound can be represented as (Fe3 þ )A[Ni2 þ Fe3 þ ]BO2 4. The magnetic properties and cation distribution are found to be different in nanocrystalline spinel ferrites, when compared to
n
Corresponding author. Tel.: þ91 0712 2042086; fax: þ 91 0712 2500736. E-mail address: [email protected] (S.B. Kondawar).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2012.01.081
the bulk counterpart [3]. Various methods have been developed to synthesize nanocrystalline NiFe2O4 including the refluxing method [4], citrate precursor techniques [5], radio-frequency thermal plasma torch [2], self-combustion method [6], solid state reaction [7], sol–gel [8], precipitation route [9], sol–gel auto combustion [10], reactive milling [11], doping with niobium ions [12], thermal plasma [13], pulsed wire discharge [14] and hydrothermal route [15]. Wan and Qu prepared soft magnetic spinel ferrites of the form A2 þ B32 þ O4 such as NiFe2O4 and ZnFe2O4 materials with very low coercive force (Hc 0) and relatively high saturation magnetization (Ms 72 emu/g) are widely used in microwave devices due to their high saturation magnetization, high permeability, high electrical resistivity and low eddy current losses [16–18]. Doping of NiFe2O4 ferrite with non-magnetic Zn2 þ ion is a simple route to drastically enhance electrical and magnetic properties of NiFe2O4. Zinc substituted ferrites are technologically important materials because of their high magnetic permeability and low core losses. The investigations of various characteristic properties of Ni–Zn ferrites such as their structure, electrical conductivity and dielectric behavior showed that the grain size and grain boundary plays a major role in the conduction mechanism in these materials [19]. Resulting Zn2 þ substituted NiFe2O4 ferrites have spinel configuration based on a face centered cubic lattice of the oxygen ions with the unit cell consisting of 8 formula units of the type (Zn2 þ Fe3 þ )A [Ni2 þ Fe3 þ ]BO2 4. Metallic cations in ( ) occupy the tetrahedral sites (A) and the metallic cations in [ ] occupy the octahedral sites
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(B). Whatever the method of synthesizing the nano-sized ferrites, it is very important to consider their stability in terms of their size because the various properties begin to change significantly as we go to smaller size. Amongst all the methods used for synthesizing the nanocrystalline ferrites, refluxing method is a cheap and lowtemperature technique that allows the fine control of the product’s chemical composition as well as densification and high productivity is often achieved at lower temperature. Kim et al. studied Ag-coated Ni–Zn ferrite microspheres prepared by electroless plating [20], Sharma et al. [21] prepared NiZnFe2O4 of average particle size 45 nm, using citrate precursor method and studied the influence of Zn substitution on dielectric properties of nanocrystalline nickel ferrites only. In the present study, we report the synthesis of NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites of average particle size 20 nm, their characterizations and the effect of Zn2 þ substitution on electrical and magnetic properties of the nickel ferrite.
2. Experimental Ferric nitrate [Fe(NO3)3 9H2O], nickel nitrate [Ni(NO3)2 6H2O] and zinc nitrate [Zn(NO3)2 6H2O] with the purity 99.5% were purchased from Hi-media and used as received. NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites have been synthesized by simple approach of refluxing method. In a typical procedure, specific molar concentration of nickel nitrate and ferric nitrate as precursors were mixed in a starch solution and stirred for half an hour for the synthesis of NiFe2O4. Under reflux condition, NaOH was added drop by drop for 4 h to provide a net negative surface charge to the nuclei limiting their further growth and aggregation. After refluxing, the solution was kept overnight and then filtered and dried in hot air oven at 80 1C for 12 h. Dried sample was treated at different temperatures 800 1C, 900 1C, 1000 1C, 1100 1C in order to maintain the stability of compound giving stoichiometric ferrite structure according to the following reaction, 2Fe3 þ þ M2 þ þ 8OH -Fe2 MðOHÞ8 -MFe2 O4 þ4H2 O: The nucleation rate is quite high at the beginning of the precipitation process whereas the excess of OH ions provides a net negative surface charge to the nuclei avoiding aggregation. For synthesis of Zn2þ substituted NiFe2O4 ferrite, specific molar concentration of nickel nitrate, zinc nitrate and ferric nitrate as precursor were mixed in a starch solution and stirred for half an hour. Under reflux condition, NaOH was added drop by drop for 4 h. After refluxing, the solution was kept overnight and then filtered and dried in hot air oven at 80 1C for 12 h. Finally sample was treated at different temperatures 800 1C, 900 1C, 1000 1C, 1100 1C resulting into the Zn2þ substituted nanocrystalline NiFe2O4. These ferrites have been abbreviated as NF for NiFe2O4 and NZF for Zn2þ substituted NiFe2O4, respectively. Ferrites have been characterized by XRD (Philips PW 1730 automatic X-ray diffractometer with Cu-Ka radiation of ˚ l ¼1.5428 A), TEM (Hitachi H-7000 operated at 100 kV and 30 mA), Mossbauer spectroscopy (4.2–300 K, 5 T) using a commercial oxford cryostat. Magnetic measurement using EG & G PAR model 4500 vibrating sample magnetometer and electrical resistivity measurement using four point probe technique was done to study the effect of zinc substitution in nickel ferrite.
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD pattern of (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4 treated at 1100 1C in air. Most of the XRD
Fig. 1. XRD pattern of (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4.
characteristic peaks of NiFe2O4 belong to the spinel ferrite. No other separate phase oxides could be identified by XRD. All (h k l) reflections are in good agreement with JCPDS file no. 10-325. Average crystallite size from the broadening of the (3 1 1) peak using Scherrer equation [22] and lattice parameter using the formula [23] of as-synthesized ferrite found to be 14 nm and ˚ respectively. NiFe2O4 belongs to the class of ferrites with 8.40 A, inverse spinel structure having structural formula Fe3 þ [Ni2 þ Fe3 þ ]O4. The metal ions given in square bracket are called octahedral (sites B) ions and that of outside square bracket called tetrahedral (sites A) ions. The nickel ions Ni2 þ together with half of iron ions Fe3 þ occupy B site and remaining half of iron ions reside in A site. XRD pattern of Zn2 þ substituted NiFe2O4 showing pure spinel phase as all the peaks matched well with JCPDS file no. 8-234. Average crystallite size from the broadening of the (3 1 1) peak and lattice constant of NiZnFe2O4 were found to be ˚ respectively. Zn2 þ substituted NiFe2O4 is a soft 16 nm and 8.42 A, magnetic material having structural formula [Zn2 þ Fe3 þ ]A[Ni2 þ Fe3 þ ]BO4, where the subscript A and B denote tetrahedral sites and octahedral sites in spinel AB2O4. Ni2 þ is stabilized in the octahedral crystal field whereas Zn2 þ preferred tetrahedral sites because of its facility to form covalent bonds involving sp3 hybrid orbital. The average crystallite size and lattice constant were found to increase with the substitution of Zn2 þ ions. This increase can be attributed to the substitution of the larger Zn2 þ ions for the smaller Ni2 þ ions [24,25]. 3.2. Transmission electron microscopy The morphology and structure of NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites were investigated by TEM. It is clearly seen from the TEM bright-field images Figs. 2(a) and (b) that the particles of the synthesized ferrites are mostly in the form of square and paralelloids as well as their truncated forms. Large number of small scattered grains with the strongest spotty patterns as observed in TEM, indicating a highly crystalline spinel structure [1] with the particle size about 20 nm for both the ferrites. By comparing TEM images of both the ferrites, it can be seen that the particle size and morphology of NiFe2O4 have been
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Fig. 2. (a) TEM image of NiFe2O4. (b) TEM image of Zn2 þ substituted NiFe2O4.
affected by Zn2 þ substitution which also confirmed from their XRD.
3.3. Mossbauer spectroscopy Figs. 3(a) and (b) shows Mossbauer spectrum of NiFe2O4 and Zn2 þ substituted NiFe2O4, respectively, taken at room temperature. The spectrum of NiFe2O4 consists of two sextets corresponding to magnetic scattering. The isomeric shift (d) of 0.36 mm/s and 0.25 mm/s were congruent with the presence of Fe(III) in two different states [26]. This ferrite displays two different interaction of super-exchange between Fe3 þ –O–Fe3 þ and Fe3 þ –O–Ni2 þ in NiFe2O4 lattice, where Fe3 þ ions occupy the center of the octahedral oxygen and tetrahedral oxygen in equal proportion while Ni2 þ ions occupy mainly octahedral sites [27]. Spectra of Zn2 þ substituted NiFe2O4 reveals the presence of iron atoms in tetrahedral (A) and octahedral (B) sites. The value of quadrupole splitting close to zero indicates the cubic symmetry attributed to spinel phase. The values of isomer shift (d) between 0.28 to 0.37 mm/s are consistent with iron ions in trivalent state [8]. Spectrum shows a presence of doublet with two sextets indicating presence of Zn2 þ ions in NiZnFe2O4. Mossbauer spectra of Zn2 þ substituted NiFe2O4 shows a central doublet superimposed on two sextets indicates the increased saturation magnetization towards super paramagnetic, which also supported from magnetic behavior of the materials using VSM.
Fig. 3. (a) Mossbauer spectra of NiFe2O4. (b) Mossbauer spectra of Zn2 þ substituted NiFe2O4.
3.4. Magnetization measurements Magnetization measurements of NiFe2O4 and Zn2 þ substituted NiFe2O4 were performed using VSM technique. M–H curves of NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites at room temperature are shown in Fig. 4. It can be seen from the figure that the saturation magnetization (Ms), coercive field (Hc) and remenant magnetization (Mr) for NiFe2O4 were found to be 36.77 emu/g, 120.48 Oe and 5.64 emu/g, respectively. In comparison with Fe3O4, these ferrites have low coercive field because of the nonmagnetic Ni2 þ ion presence in the lattice. The nickel ions are located between the grains and increase the effect of the grains on each other. The value of Ms in the crystals is attributed to the greater fraction of surface spins in these crystals that tend to be in a canted or a spin glass-like state with a smaller net moment [28]. Saturation magnetization (Ms), coercive field (Hc) and remenant magnetization (Mr) for Zn2 þ substituted NiFe2O4 were found to be 62.23 emu/g, 23.92 Oe and 1.42 emu/g, respectively. Saturation magnetization was found to be increased due to the addition of Zn2 þ content. The increase in saturation magnetization in NiZnFe2O4 is due to preference of non-magnetic Zn2 þ ion in A site encourage the migration of Fe3 þ ion into B site which gives rise antiparallel spin coupling and spin canting resulting in the weakening of A–B exchange interaction [29]. The effect of Zn2 þ substitution on magnetic parameters in NiFe2O4 as decreased in Hc, Mr and increased in Ms is an indication of NiZnFe2O4 ferrite towards super magnetic nature due to central
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4. Conclusions The synthesized NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites are crystalline, having average particle size of 20 nm as revealed by TEM. Lattice parameter was found to increase with the substitution of Zn2 þ ions in NiFe2O4. VSM results show that saturation magnetization of NiFe2O4 drastically increased due to the addition of Zn2 þ content indicating super magnetic material with lesser remanance as supported from its Mossbauer spectroscopy. The substitution of Zn2 þ in NiFe2O4 lowered the electrical conductivity due to widening of band gap as a result of size dependent property.
Acknowledgment The authors are grateful for the financial support of University Grants Commission, New Delhi, India (F No.39-540/2010 SR). Fig. 4. M–H curves for (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4.
Fig. 5. Electrical conductivity of (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4.
doublet superimposed on two sextets as also seen in its Mossbauer spectrum. 3.5. Electrical conductivity measurements The DC electrical conductivity measurements were carried out on pressed pellets of the synthesized ferrites using four probe technique. The pellets were prepared in hydraulic press by applying pressure of 5 T. The pellets had diameter of 1.0 cm and thickness 0.005 cm. DC electrical conductivity was determined from resistance measurement. Fig. 5 shows the variation of DC electrical conductivity of NiFe2O4 and Zn2 þ substituted NiFe2O4 with temperature in the range from 303 to 383 K. The room temperature conductivity of NiFe2O4 and Zn2 þ substituted NiFe2O4 is almost same but difference of conductivity at higher temperature was noticed. The substitution of Zn2 þ ions in NiFe2O4 may increase the band gap of NiFe2O4 which affects the conductivity and is in good agreement of increased in average crystallite size as determined from XRD and confirmed from TEM.
References [1] S. Maensiri, C. Masingboon, B. Boonchom, S. SEraphin, Scripta Mater. 56 (2007) 797. [2] S. Son, M. Taheri, E. Carpenter, V.G. Harris, M.E. McHenry, J. Appl. Phys. 91 (2002) 7589. [3] A.T. Raghavender, R.G. Kulkarni, K.M. Jadhav, J. Phys. 46 (2008) 366. [4] Z. Zhong, Q. Li, Y. Zhang, H. Zhong, M. Cheng, Y. Zhang, Powder Technol. 155 (2005) 193. [5] A. Verma, O.P. Thakur, C. Prakash, T.C. Goel, R.G. Mendiratta, Mater. Sci. Eng. B 116 (2005) 1. [6] N. Iftimie, E. Rezlescu, P.D. Popa, N. Rezlescu, J. Optoelect. Adv. Mater. 8 (2006) 1016. [7] T.K. Kundu, S. Mishra, Bull. Mater. Sci. 31 (2008) 507. [8] M. Popovici, C. Savii, D. Niznanskya, J. Subrta, J. Bohaceka, D. Becherescub, C. Caizerc, C. Enache, C. Ionescu, J. Optoelect. Adv. Mater. 5 (2003) 251. [9] S.K. Date, P.A. Joy, P.S. Anilkumar, B. Sahoo, W. Keune, Phys. Stat. Sol. (c) 1 (2004) 3495. [10] Z. Yue, J. Zhou, H. Zhang, Z. Gui, L. Li, Chi. J. Mater. Res. 13 (1999) 483. [11] T.F. Marinca, I. Chicinas-, O. Isnard, V. Pop, F. Popa, J. Alloy Comp. 509 (2011) 7931. [12] S. Mishra, N. Karak, T.K. Kundu, D. Das, N. Maity, D. Chakravorty, Mater. Lett. 60 (2006) 1111. [13] A.B. Nawale, N.S. Kanhe, K.R. Patil, S.V. Bhoraskar, V.L. Mathe, A.K. Das, J. Alloy. Comp. 509 (2011) 4404. [14] Y. Kinemuchi, K. Ishizaka, H. Suematsu, W. Jiang, K. Yatsui, Thin Solid Films 407 (2002) 109. [15] A. Baykal, N. Kasapoglu, H. Kavas, M.S. Toprak, Y. Koseoglu, Turk. J. Chem. 33 (2009) 33. [16] M. Wan, J. Fan, Polym. Sci. part A: Polym. Chem. 36 (1998) 2749. [17] Y. Qu, H. Yang, N. Yang, Y. Fan, H. Zhu, G. Zou, Mater. Lett. 60 (2006) 3548. [18] M.P. Gonzalez-Sandoval, A.M. Beesley, M.M. Yoshida, L.F. Cobas, J.A.M. Aquino, J. Alloy Comp. 369 (2004) 190. [19] J.J. Thomas, S. Krishnan, N. Kaldrikkal, Procesdings ICNM-2009, Appl. Sci. Inno. 2009. [20] J.H. Kim, S.S. Kim, J. Alloy. Comp. 509 (2011) 4399. [21] S. Sharma, K. Verma, U. Chaubeya, V. Singh, B.R. Mehta, Mater. Sci. Eng. B 167 (2010) 187. [22] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedure, Second ed., Wiley, New York, 1974. [23] C.C. Hwang, J.S. Tsai, T.H. Huang, Mater. Chem. Phys. 93 (2005) 330. [24] S.M. Haque, M.A. Choudhury, M.F. Islam, J. Magn. Magn. Mater. 251 (2002) 292. [25] K.B. Modi, P.V. Tanna, S.S. Laghate, H.H. Joshi, J. Mater. Sci. Lett. 19 (2000) 1111. [26] Y. Sui, W.H. Su, F.L. Zheng, D.P. Xu, Mater. Sci. Eng. A 286 (2000) 115. [27] M.A.F. Ramalho, L Gama, S.G Antonio, J. Mater. Sci. 42 (2007) 3603. [28] M.S. Niasari, F. Davar, T. Mahmoudi, Polyhedron 28 (2009) 1455. [29] N. Yahya, A. Aripin, A.A. Aziz, H. Daud, H. Zaid, L.K. Pah, N. Maarof, Am. J. Eng. Appl. Sci. 1 (2008) 53.
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effect of zinc substitution on magnetic and electrical properties of nanocrystalline nickel ferrite synthesized by refluxing method A.I. Nandapure a, S.B. Kondawar b,n, P.S. Sawadh a, B.I. Nandapure b a b
B.D. College of Engineering, Sevagram, Wardha 442001, India Department of Physics, R.T.M. Nagpur University, Nagpur 440033, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2011 Received in revised form 4 January 2012 Accepted 5 January 2012 Available online 17 January 2012
Nanocrystalline Nickel ferrite (NiFe2O4) and Zn substituted nickel ferrite (NiZnFe2O4) have been synthesized by the refluxing method. These ferrites were characterized by XRD, TEM, Mossbauer spectroscopy and VSM in order to study the effect of zinc substitution in nickel ferrite. XRD diffraction results confirm the spinel structure for the prepared nanocrystalline ferrites with an average crystallite size of 14–16 nm. Lattice parameter was found to increase with the substitution of Zn2 þ ions from ˚ TEM images confirmed average particle size of about 20 nm and indicates nanocrystal8.40 A˚ to 8.42 A. line nature of the compounds. A shift in isomeric deviation with the doublet was observed due to the influence of Zn substitution in the nickel ferrite. The Zn content has a significant influence on the magnetic behavior and electrical conductivity of NiFe2O4. Saturation magnetization drastically increased whereas room temperature electrical conductivity decreased due to the addition of Zn content in NiFe2O4, indicating super magnetic material with lesser coercivity. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nickel ferrite Nickel zinc ferrite Reflux method Mossbauer spectroscopy VSM
1. Introduction Recently, nanocrystalline magnetic materials have been receiving more attention due to their novel material properties, which are significantly different from those of their bulk counterparts. Spinel-type oxides (MFe2O4, where M is a divalent metal) which include the magnetic ferrites, are often denoted by the formula AB2O4, where A and B refer to tetrahedral and octahedral sites, respectively, in the face centered cubic lattice. These materials are technologically important and have been used in many applications including magnetic recording media and magnetic fluids for the storage and/or retrieval of information, magnetic resonance imaging (MRI) enhancement, catalysis, magnetically guided drug delivery, sensors and pigments [1]. Among all ferrites, NiFe2O4 has been synthesized because it has promising applications like ferro-fluids, gas sensor, catalyst, microwave absorber, information storage devices, magnetic fluids, drug delivery [2]. Nickel ferrite (NiFe2O4) is basically an inverse spinel ferrite in which the tetrahedral (A) sites are occupied by ferric ions and the octahedral (B) sites by ferric and nickel ions. Thus, the compound can be represented as (Fe3 þ )A[Ni2 þ Fe3 þ ]BO2 4. The magnetic properties and cation distribution are found to be different in nanocrystalline spinel ferrites, when compared to
n
Corresponding author. Tel.: þ91 0712 2042086; fax: þ 91 0712 2500736. E-mail address: [email protected] (S.B. Kondawar).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2012.01.081
the bulk counterpart [3]. Various methods have been developed to synthesize nanocrystalline NiFe2O4 including the refluxing method [4], citrate precursor techniques [5], radio-frequency thermal plasma torch [2], self-combustion method [6], solid state reaction [7], sol–gel [8], precipitation route [9], sol–gel auto combustion [10], reactive milling [11], doping with niobium ions [12], thermal plasma [13], pulsed wire discharge [14] and hydrothermal route [15]. Wan and Qu prepared soft magnetic spinel ferrites of the form A2 þ B32 þ O4 such as NiFe2O4 and ZnFe2O4 materials with very low coercive force (Hc 0) and relatively high saturation magnetization (Ms 72 emu/g) are widely used in microwave devices due to their high saturation magnetization, high permeability, high electrical resistivity and low eddy current losses [16–18]. Doping of NiFe2O4 ferrite with non-magnetic Zn2 þ ion is a simple route to drastically enhance electrical and magnetic properties of NiFe2O4. Zinc substituted ferrites are technologically important materials because of their high magnetic permeability and low core losses. The investigations of various characteristic properties of Ni–Zn ferrites such as their structure, electrical conductivity and dielectric behavior showed that the grain size and grain boundary plays a major role in the conduction mechanism in these materials [19]. Resulting Zn2 þ substituted NiFe2O4 ferrites have spinel configuration based on a face centered cubic lattice of the oxygen ions with the unit cell consisting of 8 formula units of the type (Zn2 þ Fe3 þ )A [Ni2 þ Fe3 þ ]BO2 4. Metallic cations in ( ) occupy the tetrahedral sites (A) and the metallic cations in [ ] occupy the octahedral sites
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(B). Whatever the method of synthesizing the nano-sized ferrites, it is very important to consider their stability in terms of their size because the various properties begin to change significantly as we go to smaller size. Amongst all the methods used for synthesizing the nanocrystalline ferrites, refluxing method is a cheap and lowtemperature technique that allows the fine control of the product’s chemical composition as well as densification and high productivity is often achieved at lower temperature. Kim et al. studied Ag-coated Ni–Zn ferrite microspheres prepared by electroless plating [20], Sharma et al. [21] prepared NiZnFe2O4 of average particle size 45 nm, using citrate precursor method and studied the influence of Zn substitution on dielectric properties of nanocrystalline nickel ferrites only. In the present study, we report the synthesis of NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites of average particle size 20 nm, their characterizations and the effect of Zn2 þ substitution on electrical and magnetic properties of the nickel ferrite.
2. Experimental Ferric nitrate [Fe(NO3)3 9H2O], nickel nitrate [Ni(NO3)2 6H2O] and zinc nitrate [Zn(NO3)2 6H2O] with the purity 99.5% were purchased from Hi-media and used as received. NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites have been synthesized by simple approach of refluxing method. In a typical procedure, specific molar concentration of nickel nitrate and ferric nitrate as precursors were mixed in a starch solution and stirred for half an hour for the synthesis of NiFe2O4. Under reflux condition, NaOH was added drop by drop for 4 h to provide a net negative surface charge to the nuclei limiting their further growth and aggregation. After refluxing, the solution was kept overnight and then filtered and dried in hot air oven at 80 1C for 12 h. Dried sample was treated at different temperatures 800 1C, 900 1C, 1000 1C, 1100 1C in order to maintain the stability of compound giving stoichiometric ferrite structure according to the following reaction, 2Fe3 þ þ M2 þ þ 8OH -Fe2 MðOHÞ8 -MFe2 O4 þ4H2 O: The nucleation rate is quite high at the beginning of the precipitation process whereas the excess of OH ions provides a net negative surface charge to the nuclei avoiding aggregation. For synthesis of Zn2þ substituted NiFe2O4 ferrite, specific molar concentration of nickel nitrate, zinc nitrate and ferric nitrate as precursor were mixed in a starch solution and stirred for half an hour. Under reflux condition, NaOH was added drop by drop for 4 h. After refluxing, the solution was kept overnight and then filtered and dried in hot air oven at 80 1C for 12 h. Finally sample was treated at different temperatures 800 1C, 900 1C, 1000 1C, 1100 1C resulting into the Zn2þ substituted nanocrystalline NiFe2O4. These ferrites have been abbreviated as NF for NiFe2O4 and NZF for Zn2þ substituted NiFe2O4, respectively. Ferrites have been characterized by XRD (Philips PW 1730 automatic X-ray diffractometer with Cu-Ka radiation of ˚ l ¼1.5428 A), TEM (Hitachi H-7000 operated at 100 kV and 30 mA), Mossbauer spectroscopy (4.2–300 K, 5 T) using a commercial oxford cryostat. Magnetic measurement using EG & G PAR model 4500 vibrating sample magnetometer and electrical resistivity measurement using four point probe technique was done to study the effect of zinc substitution in nickel ferrite.
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD pattern of (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4 treated at 1100 1C in air. Most of the XRD
Fig. 1. XRD pattern of (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4.
characteristic peaks of NiFe2O4 belong to the spinel ferrite. No other separate phase oxides could be identified by XRD. All (h k l) reflections are in good agreement with JCPDS file no. 10-325. Average crystallite size from the broadening of the (3 1 1) peak using Scherrer equation [22] and lattice parameter using the formula [23] of as-synthesized ferrite found to be 14 nm and ˚ respectively. NiFe2O4 belongs to the class of ferrites with 8.40 A, inverse spinel structure having structural formula Fe3 þ [Ni2 þ Fe3 þ ]O4. The metal ions given in square bracket are called octahedral (sites B) ions and that of outside square bracket called tetrahedral (sites A) ions. The nickel ions Ni2 þ together with half of iron ions Fe3 þ occupy B site and remaining half of iron ions reside in A site. XRD pattern of Zn2 þ substituted NiFe2O4 showing pure spinel phase as all the peaks matched well with JCPDS file no. 8-234. Average crystallite size from the broadening of the (3 1 1) peak and lattice constant of NiZnFe2O4 were found to be ˚ respectively. Zn2 þ substituted NiFe2O4 is a soft 16 nm and 8.42 A, magnetic material having structural formula [Zn2 þ Fe3 þ ]A[Ni2 þ Fe3 þ ]BO4, where the subscript A and B denote tetrahedral sites and octahedral sites in spinel AB2O4. Ni2 þ is stabilized in the octahedral crystal field whereas Zn2 þ preferred tetrahedral sites because of its facility to form covalent bonds involving sp3 hybrid orbital. The average crystallite size and lattice constant were found to increase with the substitution of Zn2 þ ions. This increase can be attributed to the substitution of the larger Zn2 þ ions for the smaller Ni2 þ ions [24,25]. 3.2. Transmission electron microscopy The morphology and structure of NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites were investigated by TEM. It is clearly seen from the TEM bright-field images Figs. 2(a) and (b) that the particles of the synthesized ferrites are mostly in the form of square and paralelloids as well as their truncated forms. Large number of small scattered grains with the strongest spotty patterns as observed in TEM, indicating a highly crystalline spinel structure [1] with the particle size about 20 nm for both the ferrites. By comparing TEM images of both the ferrites, it can be seen that the particle size and morphology of NiFe2O4 have been
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Fig. 2. (a) TEM image of NiFe2O4. (b) TEM image of Zn2 þ substituted NiFe2O4.
affected by Zn2 þ substitution which also confirmed from their XRD.
3.3. Mossbauer spectroscopy Figs. 3(a) and (b) shows Mossbauer spectrum of NiFe2O4 and Zn2 þ substituted NiFe2O4, respectively, taken at room temperature. The spectrum of NiFe2O4 consists of two sextets corresponding to magnetic scattering. The isomeric shift (d) of 0.36 mm/s and 0.25 mm/s were congruent with the presence of Fe(III) in two different states [26]. This ferrite displays two different interaction of super-exchange between Fe3 þ –O–Fe3 þ and Fe3 þ –O–Ni2 þ in NiFe2O4 lattice, where Fe3 þ ions occupy the center of the octahedral oxygen and tetrahedral oxygen in equal proportion while Ni2 þ ions occupy mainly octahedral sites [27]. Spectra of Zn2 þ substituted NiFe2O4 reveals the presence of iron atoms in tetrahedral (A) and octahedral (B) sites. The value of quadrupole splitting close to zero indicates the cubic symmetry attributed to spinel phase. The values of isomer shift (d) between 0.28 to 0.37 mm/s are consistent with iron ions in trivalent state [8]. Spectrum shows a presence of doublet with two sextets indicating presence of Zn2 þ ions in NiZnFe2O4. Mossbauer spectra of Zn2 þ substituted NiFe2O4 shows a central doublet superimposed on two sextets indicates the increased saturation magnetization towards super paramagnetic, which also supported from magnetic behavior of the materials using VSM.
Fig. 3. (a) Mossbauer spectra of NiFe2O4. (b) Mossbauer spectra of Zn2 þ substituted NiFe2O4.
3.4. Magnetization measurements Magnetization measurements of NiFe2O4 and Zn2 þ substituted NiFe2O4 were performed using VSM technique. M–H curves of NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites at room temperature are shown in Fig. 4. It can be seen from the figure that the saturation magnetization (Ms), coercive field (Hc) and remenant magnetization (Mr) for NiFe2O4 were found to be 36.77 emu/g, 120.48 Oe and 5.64 emu/g, respectively. In comparison with Fe3O4, these ferrites have low coercive field because of the nonmagnetic Ni2 þ ion presence in the lattice. The nickel ions are located between the grains and increase the effect of the grains on each other. The value of Ms in the crystals is attributed to the greater fraction of surface spins in these crystals that tend to be in a canted or a spin glass-like state with a smaller net moment [28]. Saturation magnetization (Ms), coercive field (Hc) and remenant magnetization (Mr) for Zn2 þ substituted NiFe2O4 were found to be 62.23 emu/g, 23.92 Oe and 1.42 emu/g, respectively. Saturation magnetization was found to be increased due to the addition of Zn2 þ content. The increase in saturation magnetization in NiZnFe2O4 is due to preference of non-magnetic Zn2 þ ion in A site encourage the migration of Fe3 þ ion into B site which gives rise antiparallel spin coupling and spin canting resulting in the weakening of A–B exchange interaction [29]. The effect of Zn2 þ substitution on magnetic parameters in NiFe2O4 as decreased in Hc, Mr and increased in Ms is an indication of NiZnFe2O4 ferrite towards super magnetic nature due to central
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4. Conclusions The synthesized NiFe2O4 and Zn2 þ substituted NiFe2O4 ferrites are crystalline, having average particle size of 20 nm as revealed by TEM. Lattice parameter was found to increase with the substitution of Zn2 þ ions in NiFe2O4. VSM results show that saturation magnetization of NiFe2O4 drastically increased due to the addition of Zn2 þ content indicating super magnetic material with lesser remanance as supported from its Mossbauer spectroscopy. The substitution of Zn2 þ in NiFe2O4 lowered the electrical conductivity due to widening of band gap as a result of size dependent property.
Acknowledgment The authors are grateful for the financial support of University Grants Commission, New Delhi, India (F No.39-540/2010 SR). Fig. 4. M–H curves for (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4.
Fig. 5. Electrical conductivity of (a) NiFe2O4 and (b) Zn2 þ substituted NiFe2O4.
doublet superimposed on two sextets as also seen in its Mossbauer spectrum. 3.5. Electrical conductivity measurements The DC electrical conductivity measurements were carried out on pressed pellets of the synthesized ferrites using four probe technique. The pellets were prepared in hydraulic press by applying pressure of 5 T. The pellets had diameter of 1.0 cm and thickness 0.005 cm. DC electrical conductivity was determined from resistance measurement. Fig. 5 shows the variation of DC electrical conductivity of NiFe2O4 and Zn2 þ substituted NiFe2O4 with temperature in the range from 303 to 383 K. The room temperature conductivity of NiFe2O4 and Zn2 þ substituted NiFe2O4 is almost same but difference of conductivity at higher temperature was noticed. The substitution of Zn2 þ ions in NiFe2O4 may increase the band gap of NiFe2O4 which affects the conductivity and is in good agreement of increased in average crystallite size as determined from XRD and confirmed from TEM.
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