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Indian Journal of Materials Science

Volume 2013 (2013), Article ID 910762, 7 pages

http://dx.doi.org/10.1155/2013/910762

## Synthesis and Characterization of Lithium-Substituted Cu-Mn Ferrite Nanoparticles

^{1}Department of Physics, Manarat Dhaka International College, Dhaka 1212, Bangladesh^{2}Department of Arts and Sciences, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh

Received 5 July 2013; Accepted 29 August 2013

Academic Editors: H. Leiste and D. L. Sales

Copyright © 2013 M. A. Mohshin Quraishi and M. H. R. Khan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The effect of Li substitution on the structural and magnetic properties of Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} (*x* = 0.00, 0.10, 0.20, 0.30, 0.40, and 0.44) ferrite nanoparticles prepared by combustion technique has been investigated. Structural and surface morphology have been studied by X-ray diffractometer (XRD) and high-resolution optical microscope, respectively. The observed particle size of various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} is found to be in the range of 9 nm to 30 nm. XRD result confirms single-phase spinel structure for each composition. The lattice constant increases with increasing Li content. The bulk density shows a decreasing trend with Li substitution. The real part of initial permeability () and the grain size (*D*) increase with increasing Li content. It has been observed that the higher the is, the lower the resonance frequency in Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites is.

#### 1. Introduction

Ferrite nanoparticles have attracted a growing interest due to their potential applications such as magnetic recording [1], storage [2], and biotechnology [3]. In the most recent years, the interest in the use of nanoparticles in biomedical applications has greatly increased [4, 5]. The size and composition of nanoparticles influence the bio-application of the magnetic nanoparticles [6]. It is well known that the physical and chemical properties of the nanosized magnetic materials are quite different from those of the bulk ones due to their surface effect and quantum confinement effects. These nanoparticles can be obtained through precipitation of metallic salts in different media as polymers [7], organic acid or alcohol [8], sugars [9], and so forth. In particular, sol-gel, autocombustion, thermal decomposition, hydrothermal, ball milling, reverse micelle synthesis, solid-phase reaction, thermally activated solid state reaction, and pulsed laser deposition have been developed to prepare the single-domain MnFe_{2}O_{4 }nanoparticles [10–23]. Manganese ferrite (MnFe_{2}O_{4}) nanoparticles have become very popular due to their wide range of magnetic applications, such as recording devices, drug delivery, ferrofluid, biosensors, and catalysis [10, 24–27]. Recently, Deraz and Alarifi [28] have studied structural and magnetic properties of MnFe_{2}O_{4} nanoparticles by combustion route. Till now, no other report has been found in the literature for Li-doped Cu-Mn ferrite. Lithium ferrites are low-cost materials which are attractive for microwave device applications. Hence, there has been a growing interest in Li-substituted Cu-Mn ferrite for microwave applications and high permeability with low magnetic loss. Therefore, this paper is devoted to study the effect of Li^{+} substitution on the physical and magnetic properties of Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites prepared by combustion technique.

#### 2. Experimental

##### 2.1. Sample Preparation and Characterization

The Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites were prepared by autocombustion technique. The analytical grade of Li(NO_{3})_{2,} MnCl_{2}·4H_{2}O, Cu(NO_{3})_{2}·3H_{2}O, and Fe(NO_{3})_{3}·9H_{2}O was taken as raw material and weighted according to the stoichiometric amount and then dissolved in ethanol. The mixture was placed in a magnetic heating stirrer at 80°C, followed by an ignition, the combustion takes place within a few seconds, and fine nanosized powders were precipitated. These powders were crushed and ground thoroughly. The fine powders of the composition were then calcined at 900°C for 5 h for the final formation of Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites nanoparticles. Then, the fine powders were granulated using polyvinyl alcohol (PVA) as a binder and pressed uniaxially into disk-shaped (about 13 mm outer diameter, 1.5 mm–2.0 mm thickness) and toroid-shaped (about 13 mm outer diameter, about 6.5 mm inner diameter and 2 mm thickness) samples. The samples prepared from each composition were sintered at 1200°C for 1 hour in air. The temperature ranges for sintering was maintained at 5°C/min for heating and 10°C/min for cooling. All sintered samples were polished and thermal etching was performed. X-ray diffraction was carried out with an X-ray diffractometer (Model: D8 Advance, Bruker AXS) for each sample. For this purpose, monochromatic Cu- radiation was used. The lattice parameter for each peak of each sample was calculated by using the formula
where , , and are the indices of the crystal planes. To determine the exact lattice parameter for each sample, Nelson-Riley method was used. The Nelson-Riley function is given as

The values of lattice constant “” of all the peaks for a sample are plotted against . Then, using a least-square fit method exact lattice parameter “” was determined. The point where the least-square fit straight line cuts the -axis (i.e., at or ) is the actual lattice parameter of the sample.

The physical or bulk densities of the samples were determined by Archimedes principle with water medium using the following expression: where is the weight of the sample in air, is the weight of the sample in the water, and is the density of water in room temperature.

The theoretical density was calculated using the following expression:
where is Avogadro's number (6.02 × 10^{23} mol^{−1}) and is the molecular weight.

The optical micrographs for various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites have been taken by using high-resolution optical microscope (Model: NMM-800TRF). Average grain sizes of all samples were determined from optical micrographs by linear intercept technique [29]. The frequency-dependent initial permeability for each sample was measured by using a Wayne Kerr Impedance Analyzer (Model: 6500B). The complex permeability measurement on toroid-shaped samples was carried out at room temperature in frequency range 10 KHz–100 MHz. Both the and of the complex permeability were calculated using the following relations:
where is the self-inductance of the sample core and is derived geometrically. Here, is the inductance of the winding coil without the sample core, is the number of turns of the coil , and is the area of cross-section of the toroidal sample as follows:
where , = inner diameter, = outer diameter, = Height and is the mean diameter of the toroidal sample as follows:
The Loss factor, , was determined from the ratio = /.

#### 3. Results and Discussion

##### 3.1. X-Ray Diffraction Analysis

The XRD analysis was performed to verify the formation of spinel structure of various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites, in which Mn^{2+} is replaced with Li^{+ }and Fe^{3+}. The XRD patterns of these Li^{+}-substituted Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} (with = 0.00, 0.10, 0.20, 0.30, 0.40, and 0.44) ferrites sintered at 1200°C in air for 1 h are shown in Figure 1. The patterns indicated that these materials have a well-defined single crystalline phase and formation of cubic spinel structure for each composition. Analyzing the XRD patterns, it is observed that the positions of the peaks comply with the reported value [30] and some traces of raw materials were found for = 0.00, = 0.10 and = 0.20 and = 0.30).

##### 3.2. Lattice Constant

The values of lattice constant obtained from each plane are plotted against Nelson-Riley function [31]. The values of lattice constant were estimated from the extrapolation of these lines to or °. It is noticed from Figure 2 that increases with the increase of Li^{+} content in Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} (with = 0.00, 0.10, 0.20, 0.30, 0.40, and 0.44) ferrites. Values of for various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites are presented in Table 2. The increase in with Li content indicates that the present system obeys Vegard’s law [32]. This increase of can be attributed to the ionic size. The ionic radius of Li^{+} (0.76 Å) is greater than that of Mn^{2+} (0.67 Å) [29, 33]. When the larger Li^{+}and Fe^{3+} ions enter the lattice, the unit cell expands while preserving the overall cubic symmetry.

##### 3.3. Average Particle Size

The average particle size was estimated by using Debye-Scherrer [34] formula from the broadening of the highest intensity peaks (311) of XRD patterns:
where is the average particle size, is the wavelength of the radiation used as the primary beam of Cu-K_{α} ( Å), is the angle of the incident beam in degree, and * β* is the full width at half maximum (FWHM) of the fundamental reflection (311) in radian of the FCC ferrites phase. Debye-Scherer formula assumes approximation and gives the average particle size if the grain size distribution is narrow and strain-induced effects are quite negligible.

Figure 3 shows the XRD patterns of Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites sintered at 1200°C for 1 h, where (311) peak is shown in expanded form to understand the variation of FWHM of the Bragg peaks with the Li content. From Figure 3, it is seen that the value of FWHM decreases with the increase of lithium content. The particle size of the sample is inversely proportional to FWHM according to Debye-Scherrer formula. The observed particle size is in the range from 9 to 30 nm which has been listed in Table 1.

##### 3.4. Theoretical and Bulk Density

The values of and for the various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites (with = 0.00, 0.10, 0.20, 0.30, 0.40, and 0.44) are tabulated in Table 2. It is noticed from Figure 4 that both and decrease with the increase of Li substitution in Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites for constant sintering temperature. This phenomenon could be explained in terms of the atomic weight.The atomic weight of Mn (54.94 amu) is greater than that of combined atomic weight of the Li (6.941 amu) and Fe (55.845 amu) [33].

##### 3.5. Microstructure

The optical micrographs of Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites (where = 0.00, = 0.10, = 0.20, = 0.30, = 0.40, and = 0.44) are shown in Figure 5 sintered at 1200°C. The grain size is significantly dependent on Li substitution. The increases with increasing Li substitution for fixed sintering temperature which is shown in Figure 5. This is probably due to the lower melting temperature of Li (180°C) compared to Mn (1245°C). The values of for various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites are presented in Table 2.

##### 3.6. Complex Initial Permeability

The compositional variations of complex initial permeability spectra for the various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} samples sintered at 1200°C are shown in Figure 6. It is observed that the remains fairly constant in the frequency range up to some critical frequency which is called resonance frequency, . A sharp decrease in and increase in are observed above the . The increases with the increase of Li^{+} content for various Li_{x}Cu_{0.12} Mn_{0.88−2x}Fe_{2+x}O_{4}. On the other hand, was found to decrease with Li substitution. It was observed that of Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites sintered at 1200°C increases from 18 to 55. Figure 7 shows both and as a function of Li content for various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites. According to Globus and Duplex model [35], the can be explained as , where is the saturation magnetization and is the magnetocrystalline anisotropy constant. This increase in permeability is expected, because grain size of all samples increases with Li content. It is known that the mobility of domain walls is greatly affected by the microstructure of ferrites. Therefore, in the present case, variation of the initial permeability may be influenced by its grain size.

The variation of loss factor, with frequency for all samples, has been studied. The variation of initial loss with frequency for the various Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} samples sintered at 1200°C is shown in Figure 8. At lower frequencies magnetic loss is observed and remains constant up to, a certain frequency, 9 MHz; this frequency limit depends upon the sintering temperatures. The lag of domain wall motion with respect to the applied magnetic field is responsible for magnetic loss and this is accredited to lattice imperfections [36]. At higher frequencies, a rapid increase in loss factor is observed. A resonance loss peak is shown in this rapid increase of magnetic loss. At the resonance, maximum energy transfer occurs from the applied field to the lattice which results in the rapid increases in loss factor.

#### 4. Conclusion

The Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ( = 0.00 to = 0.44) nanoparticles have been successfully synthesized by the combustion technique. The observed particle size is in the range from 9 nm to 30 nm. The XRD patterns confirm that the compositions are single phase and form cubic spinel structure. The lattice parameter increases linearly with increasing Li content and obeys Vegard’s law. The study of microstructure shows that grain size increases with increasing Li content. The bulk density decreases with increasing Li substitution in Li_{x}Cu_{0.12}Mn_{0.88−2x}Fe_{2+x}O_{4} ferrites. The real part of initial permeability increases with increase of Li content for a fixed sintering temperature. This result may be explained with the help of average grain size. The highest was found 55 for = 0.44 which is three times greater than that of parent composition. It was also observed that the resonance frequency, , and real part of initial permeability, , are inversely proportional which confirms Snoek’s relation, = constant.

#### Acknowledgments

The authors are grateful to the BUET authority for providing financial support for this research. The authors are also thankful to the authority of BCSIR for using their equipment.

#### References

- M. Suda, M. Nakagawa, T. Iyoda, and Y. Einaga, “Reversible photoswitching of ferromagnetic FePt nanoparticles at room temperature,”
*Journal of the American Chemical Society*, vol. 129, no. 17, pp. 5538–5543, 2007. View at Publisher · View at Google Scholar · View at Scopus - B. O. Regan and M. Gratzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO
_{2}films,”*Nature*, vol. 353, pp. 737–740, 1991. View at Publisher · View at Google Scholar - M. Shinkai, “Functional magnetic particles for medical application,”
*Journal of Bioscience and Bioengineering*, vol. 94, no. 6, pp. 606–613, 2002. View at Google Scholar - C. C. Berry and A. S. G. Curtis, “Functionalisation of magnetic nanoparticles for applications in biomedicine,”
*Journal of Physics D*, vol. 36, no. 13, article R198, 2003. View at Publisher · View at Google Scholar - S. Mornet, S. Vasseur, F. Grasset, and E. Duguet, “Magnetic nanoparticle design for medical diagnosis and therapy,”
*Journal of Materials Chemistry*, vol. 14, no. 14, pp. 2161–2175, 2004. View at Publisher · View at Google Scholar · View at Scopus - C. Corot, P. Robert, J. M. Ideé, and M. Port, “Recent advances in iron oxide nanocrystal technology for medical imaging,”
*Advanced Drug Delivery Reviews*, vol. 58, no. 14, pp. 1471–1504, 2006. View at Google Scholar - J.-F. Berret, N. Schonbeck, F. Gazeau et al., “Controlled clustering of superparamagnetic nanoparticles using block copolymers: design of new contrast agents for magnetic resonance imaging,”
*Journal of the American Chemical Society*, vol. 128, no. 5, pp. 1755–1761, 2006. View at Publisher · View at Google Scholar · View at Scopus - C. Sun, R. Size, and M. Zhang, “Folic acid-PEG conjugated superparamagnetic nanoparticles for targeted cellular uptake and detection by MRI,”
*Journal of Biomedical Materials Research A*, vol. 78, no. 3, pp. 550–557, 2006. View at Google Scholar - R. Y. Hong, B. Feng, L. L. Chen, G. H. Li, Y. Zeng, and D. G. Wei, “Synthesis, characterization and MRI application of dextran-coated Fe
_{3}O_{4}magnetic nanoparticles,”*Biochemical Engineering Journal*, vol. 42, no. 3, pp. 290–300, 2008. View at Google Scholar - N. M. Deraz and S. Shaban, “Optimization of catalytic, surface and magnetic properties of nanocrystalline manganese ferrite,”
*Journal of Analytical and Applied Pyrolysis*, vol. 86, pp. 173–179, 2009. View at Google Scholar - M. A. Ahmed, N. Okasha, and M. M. El-Sayed, “Enhancement of the physical properties of rare-earth-substituted Mn-Zn ferrites prepared by flash method,”
*Ceramics International*, vol. 33, no. 1, pp. 49–58, 2007. View at Publisher · View at Google Scholar · View at Scopus - Q. M. Wei, J.-B. Li, Y.-J. Chen, and Y.-S. Han, “X-ray study of cation distribution in NiMn
_{1−x}Fe_{2−x}O_{4}ferrites,”*Materials Characterization*, vol. 47, no. 3-4, pp. 247–252, 2001. View at Google Scholar - M. H. Mahmoud, H. H. Hamdeh, J. C. Ho, M. J. O'Shea, and J. C. Walker, “Moessbauer studies of manganese ferrite fine particles processed by ball-milling,”
*Journal of Magnetism and Magnetic Materials*, vol. 220, no. 2, pp. 139–146, 2000. View at Publisher · View at Google Scholar · View at Scopus - M. Muroi, R. Street, P. G. McCormick, and J. Amighian, “Magnetic properties of ultrafine MnFe
_{2}O_{4}powders prepared by mechanochemical processing,”*Physical Review B*, vol. 63, no. 18, Article ID 184414, 2001. View at Google Scholar · View at Scopus - C. Li and Z. J. Zhang, “Size-dependent superparamagnetic properties of Mn spinel ferrite nanoparticles synthesized from reverse micelles,”
*Chemistry of Materials*, vol. 13, no. 6, pp. 2092–2096, 2001. View at Google Scholar - M. H. Mahmoud, C. M. Williams, J. Cai, I. Siu, and J. C. Walker, “Investigation of Mn-ferrite films produced by pulsed laser deposition,”
*Journal of Magnetism and Magnetic Materials*, vol. 261, no. 3, pp. 314–318, 2003. View at Google Scholar - C. Alvani, G. Ennas, A. La Barbera, G. Marongiu, F. Padella, and F. Varsano, “Synthesis and characterization of nanocrystalline MnFe
_{2}O_{4}: advances in thermochemical water splitting,”*International Journal of Hydrogen Energy*, vol. 30, no. 13-14, pp. 1407–1411, 2005. View at Publisher · View at Google Scholar · View at Scopus - D. Carta, M. F. Casula, A. Falqui et al., “A structural and magnetic investigation of the inversion degree in ferrite nanocrystals MFe
_{2}O_{4}(M = Mn, Co, Ni),”*Journal of Physical Chemistry C*, vol. 113, no. 20, pp. 8606–8615, 2009. View at Publisher · View at Google Scholar · View at Scopus - Y. Liu, Y. Zhang, J. D. Feng, C. F. Li, J. Shi, and R. Xiong, “Dependence of magnetic properties on crystallite size of CoFe
_{2}O_{4}nanoparticles synthesised by auto-combustion method,”*Journal of Experimental Nanoscience*, vol. 4, no. 2, pp. 159–168, 2009. View at Publisher · View at Google Scholar · View at Scopus - C. Cannas, A. Musinu, D. Peddis, and G. Piccaluga, “Synthesis and characterization of CoFe
_{2}O_{4}nanoparticles dispersed in a silica matrix by a sol-gel autocombustion method,”*Chemistry of Materials*, vol. 18, no. 16, pp. 3835–3842, 2006. View at Publisher · View at Google Scholar · View at Scopus - C. Cannas, A. Falqui, A. Musinu, D. Peddis, and G. Piccaluga, “CoFe
_{2}O_{4}nanocrystalline powders prepared by citrate-gel methods: synthesis, structure and magnetic properties,”*Journal of Nanoparticle Research*, vol. 8, no. 2, pp. 255–267, 2006. View at Publisher · View at Google Scholar · View at Scopus - L.J. Zhao, H.J. Zhang, Y. Xing et al., “Studies on the magnetism of cobalt ferrite nanocrystals synthesized by hydrothermal method,”
*Journal of Solid State Chemistry*, vol. 181, no. 2, pp. 245–252, 2008. View at Publisher · View at Google Scholar · View at Scopus - Q. Liu, J.H. Sun, H.R. Long, X.Q. Sun, X.J. Zhong, and Z. Xu, “Hydrothermal synthesis of CoFe
_{2}O_{4}nanoplatelets and nanoparticles,”*Materials Chemistry and Physics*, vol. 108, no. 2-3, pp. 269–273, 2008. View at Publisher · View at Google Scholar · View at Scopus - S. R. Ahmed, S. B. Ogale, G. C. Papaefthymiou, R. Ramesh, and P. Kofinas, “Magnetic properties of CoFe
_{2}O_{4}nanoparticles synthesized through a block copolymer nanoreactor route,”*Applied Physics Letters*, vol. 80, no. 9, pp. 1616–1618, 2002. View at Publisher · View at Google Scholar · View at Scopus - I. Brigger, C. Dubernet, and P. Couvreur, “Nanoparticles in cancer therapy and diagnosis,”
*Advanced Drug Delivery Reviews*, vol. 54, no. 5, pp. 631–651, 2002. View at Publisher · View at Google Scholar · View at Scopus - R. Arulmurugan, G. Vaidyanathan, S. Sendhilnathan, and B. Jeyadevan, “Mn-Zn ferrite nanoparticles for ferrofluid preparation: study on thermal-magnetic properties,”
*Journal of Magnetism and Magnetic Materials*, vol. 298, no. 2, pp. 83–94, 2006. View at Publisher · View at Google Scholar · View at Scopus - J. B. Haun, T. J. Yoon, H. Lee, and R. Weissleder, “Magnetic nanoparticle biosensors,”
*Wiley Interdisciplinary Reviews*, vol. 2, no. 3, pp. 291–304, 2010. View at Publisher · View at Google Scholar · View at Scopus - N. M. Deraz and A. Alarifi, “Controlled synthesis, physicochemical and magnetic properties of nano-crystalline Mn ferrite system,”
*International Journal of Electrochemical Science*, vol. 7, pp. 5534–5543, 2012. View at Google Scholar - M. I. Mendelson, “Average grain size in polycrystalline ceramics,”
*Journal of the American Ceramic Society*, vol. 52, no. 8, pp. 443–446, 1969. View at Google Scholar · View at Scopus - C. Rath, S. Anand, R. P. Das et al., “Dependence on cation distribution of particle size, lattice parameter, and magnetic properties in nanosize Mn-Zn ferrite,”
*Journal of Applied Physics*, vol. 91, no. 4, article 2211, 2002. View at Publisher · View at Google Scholar · View at Scopus - J. B. Nelson and D. P. Riley, “An experimental investigation of extrapolation methods in the derivation of accurate unit-cell dimensions of crystals,”
*Proceedings of the Physical Society*, vol. 57, no. 3, pp. 160–177, 1945. View at Publisher · View at Google Scholar · View at Scopus - L. Vegard, “The constitution of mixed crystal and the space occupied by atom,”
*Zeitschrift für Physik*, no. 17, pp. 17–26, 1921. View at Google Scholar - M. J. Winter, University of Sheffield, Yorkshire, UK, 1995–2006, http://www.webelements.com/.
- B. D. Cullity,
*Elements of X-Ray Diffraction*, Addison-Wesley, Reading, Mass, USA, 3rd edition, 1972. - A. Globus, P. Duplex, and G. M. Guyot, “Determination of initial magnetization curve from crystallites size and effective anisotropy field,”
*IEEE Transactions on Magnetics*, vol. 7, no. 3, pp. 617–622, 1971. View at Google Scholar · View at Scopus - J. L. Snoek, “Dispersion and absorption in magnetic ferrites at frequencies above one Mc/s,”
*Physica*, vol. 17, no. 4, pp. 207–217, 1948. View at Google Scholar · View at Scopus