Investigations on Mn x Zn 1-x Fe 2 O 4 (x = 0.1, 0.3 and 0.5) nanoparticles synthesized by sol-gel and co-precipitation methods

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1 Indian Journal of Engineering & Materials Sciences Vol. 18, October 2011, pp Investigations on Mn x Zn 1-x Fe 2 O 4 (x = 0.1, 0.3 and 0.5) nanoparticles synthesized by sol-gel and co-precipitation methods Mahesh Chand, Arvind Kumar, Annveer, Sandeep Kumar, Ajay Shankar & R P Pant* CSIR-National Physical Laboratory, Dr K S Krishnan Marg, New Delhi , India Received 9 March 2011; accepted 20 September 2011 Mn 2+ substituted Zn ferrite nanoparticles have been synthesized using two different wet methods, sol-gel and coprecipitation. The doping effect of Mn 2+ ion concentration on physical properties like structural and magnetic properties is investigated. The crystallite size and lattice parameter increases with increasing Mn 2+ concentration. EPR study reveals that the line width and g-factor increases with increasing Mn 2+ concentration for co-precipitated particles whereas the reverse trend for sol-gel technique is observed. However, in both the processes the saturation magnetization (M s ) and coercivity (H c ) increase with increasing Mn 2+ concentration. In a comparative study, the nanoparticles synthesized by sol-gel are more crystalline in nature as compared to co-precipitated particles. Keywords: Magnetic materials, Sol-gel, Co-precipitation, EPR spectroscopy Nanomagnetic particles have attracted much interest in recent years for both, academic and technological fronts 1. The Mn-Zn ferrites are widely used in various applications, such as magnetic recording heads, transformers, choke coils, noise filters, electromagnetic gadgets, memory or data storage devices, etc 2,3. The interests in soft magnetic materials are therefore turned into their nano crystallization and substitution for expecting to improve their combination properties 4,5. The magnetic properties in these ferrites can be tuned by choosing various kinds of M 2+ among divalent cations (Zn 2+, Mg 2+, Co 2+, Ni 2+, Cu 2+ ). Some of them can be used as a temperature-sensitive magnetic fluid with increased saturation magnetization Synthesis of various ferrite nanoparticles is of great importance for the investigation and exploration of their specific properties including the Mn-Zn ferrite nanoparticles Mn-Zn ferrites have the excellent magnetic properties such as high initial permeability and low magnetic losses 15. Physical properties of these nano ferrites depend on the synthesis parameters such as co-precipitation time, sintering temperature and rate of heating and cooling. Various methods have been used for the synthesis of fine particles of ferrites, which exhibit novel properties when compared to their properties in bulk. Non-conventional methods *Corresponding author ( rppant@nplindia.org) such as co-precipitation, hydrothermal process, oxidation, high energy ball milling and sol-gel synthesis In the present investigations, we have synthesized Mn substituted Zn ferrite nanoparticles (where x = 0.1, 0.3 and 0.5) sol-gel and coprecipitation method. A comparative study on structural properties like crystalline phase, crystalline size, lattice strain and magnetic properties like saturation magnetization, coercivity and spin concentration in both the synthesis processes has been evaluated. Experimental Procedure Synthesis of materials Mn substituted Zn ferrite nanoparticles (where x = 0.1, 0.3, 0.5) were prepared by two different methods sol-gel and co-precipitation. The materials used in both processes were Mn(NO 3 ).6H 2 O (99.99%, Sigma- Aldrich), Zn(NO 3 ).6H 2 O (98%, Sigma-Aldrich), and Fe(NO 3 ).9H 2 O(98%, Merck) as received. In coprecipitation method, all the materials were taken in required stoichiometric ratio and a homogeneous solution was prepared. For example, 0.1M Mn 2+, 0.9M Zn 2+ and 2M Fe 3+ aqueous solutions are required for preparing Mn 0.1 Zn 0.9 Fe 2 O 4 composition. Similarly, by adjusting initial ratios we synthesized other compositions. The resulting mixture was continuously stirred for half an hour at 343 K, 25% ammonia solution was added drop by drop to

2 386 INDIAN J. ENG. MATER. SCI., OCTOBER 2011 maintain 9.5 ph and the particles were coated by oleic acid. The precipitated particles were washed several times with double distilled water to remove the salt residues and other impurities. It was further dried at 323 K to obtain the powder. In case of sol-gel method, Mn(NO 3 ).6H 2 O, Zn(NO 3 ).6H 2 O, and Fe(NO 3 ).9H 2 O were dissolved in 20 ml of glycol and mixed to make the sol. The entire sol was thoroughly stirred for one hour at 343 K which turns into gel form. Then gel form was burnt out completely to form a loose powder at 473 K. All prepared samples were characterized by XRD, HRTEM, EPR and magnetic measurement using search coil method. Characterization The XRD patterns of all the samples were recorded by Rigaku powder X-ray diffractometer (Model- XRG 2KW) at scanning rate of 0.02 /s in 2θ range from 20 to 70 at 40 kv, 30 ma using CuKα radiation (λ= å). The shape, size and microstructure of the particles were analyzed by High Resolution Transmission Electron Microscope (HRTEM, Tecnai GZ F30 STWIN FEG) operated at 300 kv electron accelerating voltage. The concentration of uncompensated spins, g-value and peak-to-peak line width of the samples were calculated at ambient temperature using X-band Brukar Biospin make model A300 EPR spectrometer operated at 100 khz modulation frequency and 23 db microwave power. The magnetization measurements were carried out by search coil method. For this, a polytronic power supply (Model-BCS-1000), electromagnet (Type Hem-100) and flux meter (Model FM109) were used. Results and Discussion X-ray diffraction The sharp lines and absence of impurity peaks in the XRD patterns indicate the formation of single crystalline phase in both the synthesis processes and are shown in Fig. 1. All the diffraction peaks match with the spinel structure of MnZnFe 2 O 4 (JCPDS No ). The crystallite size of all the samples was calculated using Scherrer formula. The full width at half maximum (FWHM) of (311) peak was determined by fitting Lorentzian curves using the peak fit software. The crystallite size (D), lattice constant (a) and strain (ε) are shown in Table 1. The estimated crystallite size and lattice parameter increase with increasing Mn 2+ concentration in both the synthesis methods. This increase is attributed to large ionic radii of Mn 2+ (0.93 Å) replacing Zn 2+ (0.74 Å) in tetrahedral site which in resulting the Table 1 The XRD results for sol-gel and co-precipitation methods D (nm) a (Å) Strain [(d 0 - d s )/ d s ] D (nm) a (Å) Strain [(d 0 - d s )/ d s ] Mn 0.1 Zn 0.9 Fe 2 O Mn 0.3 Zn 0.7 Fe 2 O Mn 0.5 Zn 0.5 Fe 2 O Fig. 1 The XRD patterns of Mn x Zn 1-x Fe 2 O 4 for (a) co-precipitation method and (b) sol-gel method

3 CHAND et al.: Mn 2+ SUBSTITUTED ZN FERRITE NANOPARTICLES 387 distortion in the lattice. The nanoparticles synthesized by sol-gel are more crystalline as compared to coprecipitated particles. This may be attributed to that in co-precipitation we used a surfactant to prevent the agglomeration of the particles and controlled the size by maintaining ph value constant. However, in solgel uncontrolled agglomeration and layer thickness of glycol may lead to formation of large particles. The induced strain is determined by using experimental value and JCPDS data ( ) which is given in Table 1. High resolution transmission electron microscope The morphology of the particles formed was examined by direct observation via high-resolution transmission electron microscope for all the collected particles. As an example, the micrographs of Mn 0.5 Zn 0.5 Fe 2 O 4 synthesized by co-precipitation and sol-gel methods are shown in Figs 2a and 2b. It is clear from these figures that particles are spherical in shape. The average particle size varies in the range of 8-10 nm for co-precipitation and nm for sol-gel method. These values are in well agreement with those calculated from XRD patterns. Electron paramagnetic resonance In order to understand the information about the spin related phenomena the electron paramagnetic resonance (EPR) measurements have been carried out. The EPR spectra of all samples were recorded at room temperature and shown in Fig. 3. Figure 3a indicates the narrow signal is mainly due to superexchange interaction between Mn 2+ and Fe 3+ ions. The broad resonance EPR signal in Fig. 3b is due to dipoledipole interaction and indicates the superexchange interaction between Mn 2+ and Fe 3+ ions. The peak-topeak line width ( H pp ), g value and spin concentration (N S ) are analyzed using Lorentzian Fig. 2 The HRTEM image of Mn 0.5 Zn 0.5 Fe 2 O 4 for (a) co-precipitation method and (b) sol-gel method Fig. 3 The EPR spectra of Mn x Zn 1-x Fe 2 O 4 for (a) co-precipitation method and (b) sol-gel method

4 388 INDIAN J. ENG. MATER. SCI., OCTOBER 2011 Fig. 4 The magnetization curves of Mn x Zn 1-x Fe 2 O 4 for (a) co-precipitation method and (b) sol-gel method distribution function and are given in Table 2. The g-value and H PP increase with increasing Mn 2+ concentration for co-precipitated particles and decrease in sol-gel method. This may be due to large particle size formation which leads to less number of spins at surface of the particles. Magnetic measurement Magnetic measurements were carried out for all the samples using the search coil method at room temperature. From these measurements saturation magnetization (M s ), remanence (M r ) and coercivity (H c ) were evaluated and are given in Table 3. Figure 4a and 4b exhibit the magnetization curve for coprecipitation and sol-gel methods, respectively. The saturation magnetization (M s ) and coercivity (H c ) increase with increasing Mn 2+ concentration. This increase is may be due to the large ionic radii of Mn 2+ (0.93 Å) replacing Zn 2+ (0.74 Å) in tetrahedral site which in resulting the increase in particle size. The reason of high coercivity (>127 Oe) can be attributed to the presence of some larger size other Table 2 The EPR results for sol-gel and co-precipitation methods H pp (G) g-value N s ( spins/g) H pp (G) g-value N s ( spins/g) Mn 0.1 Zn 0.9 Fe 2 O Mn 0.3 Zn 0.7 Fe 2 O Mn 0.5 Zn 0.5 Fe 2 O Table 3 The magnetic measurement results for sol-gel and co-precipitation methods M s (emu/g) M r ( emu/g) H c (Oe) M s (emu/g) M r ( emu/g) H c (Oe) Mn 0.1 Zn 0.9 Fe 2 O Mn 0.3 Zn 0.7 Fe 2 O Mn 0.5 Zn 0.5 Fe 2 O then superparamagnetic size particles in the material. Maximum saturation magnetization of and emu/g for sol-gel and co-precipitation is obtained, respectively. For sol-gel, the less value of M s may be due to the thick layer of glycol on the surface of the particles which hampers the net magnetic moments. Figure 4a shows a typical super paramagnetic (SP) behaviour with nearly zero coercivity and zero remanence for samples x=0.1, 0.3 synthesized by co-precipitation. The SP behaviour is seen for the particles which have zero coercivity and zero remanence. The particles exhibit SP behaviour when KV<kT, where K is anisotropy constant, V is volume, k is the Boltzmann constant and T is temperature. Thermal energy dominates on the anisotropy energy of the particles then SP behaviour occurs. Conclusions Mn 2+ substituted zinc ferrite (Mn x Zn 1-x Fe 2 O 4, where x=0.1, 0.3 and 0.5) nanoparticles synthesized by co-precipitation and sol-gel methods have been

5 CHAND et al.: Mn 2+ SUBSTITUTED ZN FERRITE NANOPARTICLES 389 investigated for their physical properties like morphology, structural and magnetic properties. Structural analysis confirms the formation of single phase spinel structure. EPR parameters, g-factor and H pp increase with increasing Mn 2+ concentration in co-precipitated method, whereas their values decrease in of case sol-gel method. The saturation magnetization and coercivity increase with increasing Mn concentration. The comparative study reveals that the nanoparticles synthesized by sol-gel are more crystalline as compared to co-precipitated particles. This may be attributed to that in coprecipitation we used a surfactant to prevent the agglomeration of the particles and controlled the size by maintaining ph value constant. However, in solgel uncontrolled agglomeration and layer thickness of glycol may lead to formation of large particles. The co-precipitated particles have moderate magnetization in smaller in size and are well suited for ferrofluid preparation. Acknowledgement The authors would like to thank Director, National Physical Laboratory, for his continuous encouragement to carry out the work. We are also thankful to Council of Scientific and Industrial Research (CSIR) for funding this work. References 1 Chen J P, Sorensen C M, Klabunde K J, Hadjipanayis G C, Devlin E & Kostikas A, Phys B, 54 (1996) Dasgupta S, Kim K B, Ellrich J, Eckert J & Manna, J Alloys Comp, 424(1) (2006) Son S, Swaminathan R & McHenry M E, J Appl Phys, 93(10) (2003) Makovec D, Kosak A, Znidarsic A & Drofenik M, J Magn Magn Mater, 289(1) (2005) Ott G, Wrba J & Lucke R, J Magn Magn Mater, 254 (2003) Jeyadevan B, Chinnasamy C N, Shinoda K & Tohji K, J Appl Phys, 93(10) (2003) Lebourgeois R & Coillot C, J Appl Phys, 103(7) (2008) 07E Drofenik M, Znidarsic A & Zajc, J Appl Phys, 82(1) (1997) Lisjak D & Drofenik M, J Eur Ceram Soc, 24 (2004) Fiorillo F, Beatrice C, Bottauscio O, Manzin A & Chiampi M, Appl Phys Lett, 89(12) (2006) Zhao J H, Zang R B & Fan Z Y, Electron Compon Mater, 26(12) (2006) Richardi J, Pileni M P & Weis J J, Phys Rev E, 77(6) (2008) Kosak A, Makovec D, Drofenik M & Znidarsic A, J Magn Magn Mater, 272(2) (2004) Mandala K, Chakraverty S, Mandal S P, Agudo P, Pal M & Chakravorty D, J Appl Phys, 92(1) (2002) Iyer R, Desai R & Upadhyay R V, Bull Mater Sci, 32 (2) (2009) Yan S, Ling W & Zhou E, J Cryst Growth, 273 (2004) Giri J, Sriharsha T, Asthana S, Gundu Rao T K, Nigam A K & Bahadur D, J Magn Magn Mater, 293 (2005) Zheng Z G, Zhong X C, Zhang Y H, Yu H Y & Zeng D C, J Alloys Compd, 466 (2008) 377.