IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 8, AUGUST 2014
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Structural, Morphological, and Magnetic Characterization of Sol-Gel Synthesized MnCuZn Ferrites Shahid Mahmood Ramay1 , Hafiz Muhammad Rafique2, Sana Aslam2 , Saadat Anwar Siddiqi3 , Shahid Atiq4 , Murtaza Saleem4,5 , Shahzad Naseem4, and Muhammad Ali Shar6 1 Department
of Physics and Astronomy, College of Science, King Saud University, Riyadh 11421, Saudi Arabia 2 Department of Physics, University of the Punjab, Lahore 54590, Pakistan 3 Interdisciplinary Research Centre in Biomedical Materials (IRCBM), COMSATS Institute of Information Technology, Defence road, Off Raiwind, Lahore 53700, Pakistan 4 Centre of Excellence in Solid State Physics, University of the Punjab, Lahore 54590, Pakistan 5 Department of Physics, School of Science and Engineering, Lahore University of Management Sciences, Lahore 54792, Pakistan 6 Center of Excellence for Research in Engineering Materials, Department of Mechanical Engineering, King Saud University, Riyadh 11421, Saudi Arabia Manganese copper ferrites belong to a family of ferrites with specific importance due to their high permeability values and low losses at low frequencies. In this paper, a series of ferrite samples has been synthesized using a novel and low cost sol-gel autocombustion route in order to systematically investigate the structural, compositional, morphological, and magnetic properties when zinc is substituted by copper in the series. X-ray diffraction reveals that all the samples have characteristic cubic spinel structure. The effect of zinc substitution at the copper site on the chemical bonding of the ferrite samples was investigated by Fourier transform infrared spectroscopy. Energy dispersive X-ray analysis was performed to determine the homogeneity and stoichiometric composition of elements present in the samples. Nanosized, uniformly shaped grains were evident from the images obtained using a scanning electron microscopy, which indirectly confirms the significance of the autocombustion synthesis technique employed in this paper. Magnetic properties determined using a vibrating sample magnetometer exhibited that zinc substitution could enhance the saturation magnetization of the samples, attributed to the substitution of a paramagnetic element (zinc) by a diamagnetic one (copper). Index Terms— Ferrites, magnetic properties, sol-gel autocombustion, spinel structure.
I. I NTRODUCTION
S
OFT ferrimagnetic materials have received extensive attention from the viewpoint of their high permeability, low loss, and high performance at high frequencies [1]. Ni-Cu-Zn ferrites are usually used for multi-layer chip ferrite inductor (MLCI) that is considered as an imperative surface mounting device for electronic applications. The performance of MLCI depends on the quality of sintered powder and Ni-Cu-Zn ferrites are used as magnetic materials for MLCIs due to their excellent properties at high frequencies and low sintering temperature [2], [3]. In addition, Mn-Cu-Zn ferrites have largely the same properties as that of Ni-Cu-Zn ferrites but later are considered as low cost materials that can be easily synthesized in pure form by autocombustion techniques. In the past, numerous techniques like ceramic route, coprecipitation, hydrothermal, and mechanical alloying have been developed to synthesize these ferrites but these techniques have complicated steps (sometimes) and require long processing time [4]. Especially in ceramic technique, the obtained particles are large and non-uniform in size, which produce voids on compaction and subsequently lower the density. In order to control these difficulties, wet sol-gel method has been used
Manuscript received December 16, 2013; revised March 27, 2014; accepted April 4, 2014. Date of current version August 15, 2014. Corresponding author: S. M. Ramay (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2317111
for the production of homogenous, fine, and grained ferrites. Among all chemical techniques, sol-gel combustion method is considered low cost and excellent for single-phase ferrites. The process is based on gelling the aqueous salts of desired metals and organic fuels, which give voluminous and fluffy powder with large surface area [5]. In addition, combustion reaction is self-propagating and produces an adiabatic temperature in the range of 1500–3000 K [6], [7] that is sufficient for the required phase of ferrites within a short time. In this paper, we have synthesized Mn-Cu-Zn ferrites by combustion method in order to investigate its structural properties and its subsequent effect on the electrical and magnetic properties at room temperature. II. E XPERIMENTAL P ROCEDURES Sol-gel autocombustion method was employed to synthesize a series of ferrite samples with general formula Mn0.5 Cu0.5−x Znx Fe2 O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5). This method combines the process of preparing sol, converting it into gel and combusting in a distinctive manner. To prepare the samples, analytical grade reagents, Mn-nitrate [Mn(NO3)2 .4H2 O], Cu-nitrate [Cu(NO3 )2 . 3H2 O], Zn-nitrate [Zn(NO3)2 .6H2 O], and Fe-nitrate [Fe(NO3 )2 .9H2 O] were weighed in a stoichiometric composition. The nitrate precursors were fully dissolved in deionized water by constant stirring. Citric acid (C6 H8 O7 ), a reducing agent [7], was dissolved in this nitrate solution
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 8, AUGUST 2014
Fig. 1. XRD patterns of Mn0.5 Cu0.5−x Znx Fe2 O4 with, x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5.
Fig. 2. FTIR spectra of Mn0.5 Cu0.5−x Znx Fe2 O4 with, x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5.
by keeping the metal nitrate to citric acid molar ratio as 1:2. To neutralize (pH = 7), the as-prepared acidic solution, ammonia solution (NH4 OH) was added drop by drop with continuous stirring. Ammonia solution identifies the cation present in the solution and provides nanocrystalline powder. The mixed solution so obtained was heated at 150 °C with continuous stirring using a magnetic agitator. The heating was completely carried in a fume hood to have a toxic-gas-free environment. The temperature was kept constant till the phase change of the solution to xero-gel. The magnetic capsule was then removed and the temperature was raised up to 400 °C. After a while, the xero-gel changed into dried gel, which ultimately burnt in a self-propagating combustion process. The autocombustion behavior prevailed as long as the dried gel was reduced completely to a loose and fluffy powder by giving off nitric oxide, oxygen, and carbon dioxide. The powder was ground in a mortar and pestle to achieve uniform sized homogenous particles. The crystal structure was investigated using X-ray diffraction (XRD). The relevant chemical bonding in the ferrite samples was confirmed by Fourier transform infrared spectroscopy (FTIR). Surface morphology and chemical composition were determined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Magnetic characterization was performed by using a vibrating sample magnetometer (VSM).
no. 01-074-2072. A slight increase in the lattice constant was noted from 8.414 to 8.446 Å, as the Zn contents were increased. The trend could be attributed to the relative larger ionic radius of Zn2+ (0.74 Å) as compared with that of Cu2+ (0.73 Å). The crystallite was evaluated using the well known Scherrer’s formula utilizing the most intense diffraction peak (311). The crystallite size was first increased from 17.5 to 24.2 nm from x = 0.0–0.4, while a sharp decrease in crystallite size (14.2 nm) was observed for x = 0.5. The non-linear trend in crystallite size could be ascribed to the degree of combustion process, which progressed during the exothermic reaction. FTIR spectra of Mn0.5 Cu0.5−x Znx Fe2 O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) samples recorded at room temperature have been provided in Fig. 2. The wavenumber region from 4000 to 1300 cm−1 is the functional group region and finger print region lies in 1300–909 cm−1 . The functional groups, such as OH, NH, C=O, and so on, reside in high frequency region. The sample does not contain any of the functional group if there is no absorption peak in high frequency region [9]. There is only one downward absorption peak in the finger print region for the series of samples from 527 to 553 cm−1 . There were nitrates and hydroxides in the ferrites solution but the IR spectra for Mn0.5 Cu0.5−x Znx Fe2 O4 did not show any peak for these groups. During autocombustion process, all these components were burnt away, leaving pure ferrites. Sol-gel autocombustion is an exothermic process in which nitrate ions cause oxidation and citrate ions cause reduction. In addition, the higher frequency band near ∼560 cm−1 is attributed to the tetrahedral vibration due to stretching vibration of the tetrahedral groups. It is inferred that normal mode of vibration due to tetrahedral cluster is dominant as opposed to any octahedral clustering, attributed to the shorter bond lengths. The values of these bond lengths are an indirect validation of experimental values of ferrite lattice constants [10]. Fig. 3 shows the microstructural analysis of all the samples viewed by SEM in the pellet form. Fig. 3(a) shows the micrograph of Mn0.5 Cu0.5 Fe2 O4 sample. As evident, no specific grains were visible and no boundaries could be marked as the grain clusters were densely packed. As the Zn contents were substituted by some of the Cu atoms, the ferrite powder
III. R ESULTS AND D ISCUSSION Fig. 1 shows the XRD patterns of all the six samples prepared by varying the Zn contents at Cu-site. All the compositions revealed single phase spinel structure, characteristic of the spinel ferrite, implying that the temperature produced during self-burning was sufficient for the reaction of constituents to form the desired spinel ferrites, with well oriented texture. The intensities of the diffraction peaks were seemed to remain almost same as the Zn contents were substituted at the Cu-sites. It could be inferred that, sufficiently high temperature produced during self-combustion process, was sufficient to synthesize any composition that could be prepared by substituting Zn at the Cu-site. The lattice parameters were evaluated by the procedure as explained in [8] and were matched with the JCPD reference
RAMAY et al.: STRUCTURAL, MORPHOLOGICAL, AND MAGNETIC CHARACTERIZATION OF SOL-GEL SYNTHESIZED MnCuZn FERRITES
Fig. 3. SEM images of Mn0.5 Cu0.5−x Znx Fe2 O4 with x = (a) 0.0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.5. Inset: relevant EDX spectra.
Fig. 4. M–H loops of Mn0.5 Cu0.5−x Znx Fe2 O4 with, x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5.
samples were seemed to form grains with well defined and sharp boundaries as can be observed in Fig. 3(d). The grain clusters were measured mostly in the micrometer range due to low resolution. The grain structure was again disturbed as the Cu atoms were completely replaced by the Zn atoms. As far as porosity is concerned, largely the images show a well compact and densified surface, as quite a few pores are visible. The insets of all the micrographs show EDX spectra of the relevant samples. These spectra confirmed the stoichiometric amounts of all the elements present in the ferrite samples according to the specific amounts of these element present in the corresponding empirical formula of the composition. Fig. 4 shows the magnetic hysteresis (M–H ) loops of the Mn0.5 Cu0.5−x Znx Fe2 O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) obtained at room temperature by an applied field of ±10 kOe. The loops clearly show the ferromagnetic nature of the samples. The area bounded by the M–H loops gives the energy loss. The small area under the M–H curves is an
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Fig. 5. Saturation magnetization and coercivity plotted as a function of Zn contents in Mn0.5 Cu0.5−x Znx Fe2 O4 with x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5.
indication of low energy losses, an important characteristic from application view point. These low energy losses make these ferrite compositions, an excellent choice as core materials for power transformers in electronics, recording heads, loading coils, and in telecommunication applications [11]. The values of saturation magnetization (Ms ) and coercivity (Hc ), evaluated from the loops have been plotted in Fig. 5. As can be observed, Hc decreases first (from 185 to 116 Oe) as Cu is replaced by Zn atoms in the ferrite samples and then a sharp increase in Hc (∼270 Oe) is observed as Cu is completely replaced by Zn. However, these values of Hc are well within the range as attributed to the ferrite samples [12]. Sharp rise in Hc value at x = 0.5 could be attributed to the particle size effect. It has been reported that Hc of magnetic materials strongly depends upon the particle size and below a critical value of particle size, a sharp rise in Hc could be expected [13]. In the present series, crystallite size also showed a sharp decrease at x = 0.5, as explained earlier. In addition, maximum value of Hc has also been reported previously, for x = 0.5, when Cu is replaced by Zn in Cu1−x Znx Fe2 O4 [14]. The value of Ms was observed to increase from 42.2 to 61.24 emu/g as the Cu contents were replaced by Zn in the ferrite matrix. This trend could be attributed to the substitution of a diamagnetic (Cu) ion by a paramagnetic one (Zn) [9]. IV. C ONCLUSION A series of ferrite samples with general formula, Mn0.5 Cu0.5−x Znx Fe2 O4 , x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5, were synthesized by sol-gel autocombustion method to study the effect of Zn doping on its structural, morphological, and magnetic properties. XRD, SEM, FTIR spectroscopy, and VSM analysis were conducted to characterize these materials. XRD analysis reveals the cubic spinel structure of all the samples with a small increase in lattice constant as Cu was replaced by Zn ions, attributed to the ionic radii effect. FTIR spectra of these samples show that sol-gel autocombustion technique yields ferrites in the finger print region.
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 8, AUGUST 2014
Absorption peak falls downward only once in each IR spectra, which infers that al nitrates and hydroxides present in citratenitrate solution burnt away during autocombustion and solgel formation. The hysteresis loops obtained from VSM for all Mn0.5 Cu0.5−x Znx Fe2 O4 (x = 0.0–0.5) samples have small areas under the curves and hence low energy losses, making these ferrite compositions favorable for applications in high frequency regions. ACKNOWLEDGEMENT The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project No. RG 1435–004. R EFERENCES [1] R. Valenzuela, Magnetic Ceramics. Cambridge, U.K.: Cambridge Univ. Press, 1994. [2] A. Thakur and M. Singh, “Preparation and characterization of nanosize Mn0.4 Mn0.6 Fe2 O4 ferrite by citrate precursor method,” Ceram. Int., vol. 29, no. 5, pp. 505–511, 2003. [3] A. Lucas, R. Lebourgeois, F. Mazaleyrat, and E. Leboure, “Temperature dependence of core loss in cobalt substituted Ni-Zn-Cu ferrite,” J. Magn. Magn. Mater., vol. 323, no. 9, pp. 735–739, Mar. 2011. [4] M. R. Syue, F. J. Wei, C. S. Chou, and C. M. Fu, “Magnetic, dielectric, and complex impedance properties of nanocrystalline Mn–Zn ferrites prepared by novel combustion method,” Thin Solid Films, vol. 519, no. 23, pp. 8303–8306, Sep. 2011.
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