Structural and Magnetic Properties of Mn0.50-xZn0.50CuxFe2O4


Mn0.50-xZn0.50CuxFe2O4 (where x = 0.0 - 0.3) ferrites have been synthesized by auto combustion method. X-ray diffraction patterns reveal that all compositions are of single phase cubic spinel structure. The lattice parameter decreases with the increase in Cu2+ content obeying the Vegard’s law. The bulk density, average grain size, initial permeability, Néel temperature and saturation magnetic induction of Mn0.50-xZn0.50CuxFe2O4 increased with increasing Cu2+content. It is observed that both density and initial permeability increase with increasing sintering temperature. The maximum initial permeability is found to be 1061 which is almost four times greater than that of the parent composition. The resonance frequency of all the samples shifts towards the lower frequency as the permeability increases with Cu2+ content. It is observed from B-H loops of Mn0.50-xZn0.50CuxFe2O4 that coercivity decreases and retentivity increases with Cu2+ content. Possible explanations for the observed magnetic properties with various Cu2+ contents are discussed.

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F. Alam, M. Khan, H. Das and A. Hossain, "Structural and Magnetic Properties of Mn0.50-xZn0.50CuxFe2O4," Materials Sciences and Applications, Vol. 4 No. 12, 2013, pp. 831-838. doi: 10.4236/msa.2013.412106.

1. Introduction

Polycrystalline spinel ferrites are technologically very important materials having potential applications and interesting physical properties. Mn-Zn and substituted Mn-Zn ferrites are pertinent magnetic materials due to their high permeability, high magnetization, relatively high Néel temperature, low losses, low cost and environmental stability. These ferrites have been widely used in electrical and magnetic devices for high frequency applications [1-4]. The physical and magnetic properties can be controlled by the preparation condition, chemical composition, sintering temperature and the amount of substitutions. Several investigations on the properties of Ni-Mn-Zn [5], Ni-Cu-Zn [6], Mg-Cu-Zn [7], Co-Mn-Zn [8] ferrites have been reported. It was found that the poor densification and slow grain growth rate can be remarkably improved and consequently initial permeability can be enhanced by the substitution of Cu [9]. No report has been found in the literature regarding the magnetic properties of Cu substituted Mn-Zn ferrites for Mn prepared by combustion method. In the present work, the influence of Cu2+ in place of Mn2+ on the properties of Mn-Zn ferrites has been investigated by studying the structure and some magnetic properties.

2. Experimental

The chemical compositions of Mn0.50-xZn0.50CuxFe2O4 (with x = 0.0 - 0.3 at a step of 0.1) were prepared by combustion method. The stoichiometric amounts of commercially available analytical grade powders of MnCl2·4H2O, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O and Fe (NO3)3·9H2O were dissolved in ethanol to obtain a mixed homogenous solution. Ammonia solution was slowly added to adjust the pH at level 7. The solution was placed at constant temperature bath (70˚C) followed by an ignition and formed a fluffy loose powders of the desired composition. The resultant powders were calcined at 700˚C for five hours in air. The grounded fine powders were then pressed into discand toroid-shaped samples. The samples prepared from each composition were sintered at 1200˚C, 1250˚C and 1300˚C for five hours in air. During sintering, temperature ramps were 10˚C/min for heating and 5˚C/min for cooling. The structural characterization was carried out with an X-ray diffractometer using CuKα radiation (λ = 1.54178Å). The lattice parameter was determined by using the Nelson-Riley extrapolation method [10]. The lattice parameters obtained from each reflected plane were plotted against NelsonRiley function,

where is the Bragg’s angle. The exact values of lattice constant, a0 were estimated from the extrapolation of the best fitted line to or = 90˚. The bulk density, was determined using the expression , where W and are the weight of the sample in air and water, respectively and ρ is the density of water at room temperature. The theoretical density, was calculated using the relation: , where NA is Avogadro’s number, M is the molecular weight of the corresponding composition and Z is the number of molecules per unit cell, which is 8 for the spinel cubic structure. The porosity, P was calculated from the relation. The micrographs of Mn0.50-xZn0.50CuxFe2O4 were taken by a Scanning Electron Microscope (SEM). From these micrographs, the average grain size (D) was estimated by the linear intercept method. The frequency and temperature dependent complex initial permeability were measured using Wayne Kerr Impedance Analyzer (Model No.6500B) in the frequency range 100 Hz - 120 MHz. The real part () and the imaginary part () of the complex initial permeability were calculated using the following relations,and, where is the self inductance of the sample core and, is derived geometrically. is the inductance of the winding coil without the sample core, N is the number of turns of the coil (N = 4), h is the thickness, is the outer radius and is the inner radius of the toroid-shaped sample. The relative quality factor (RQF) was calculated from the relation:, where tand is the loss factor. B-H loops were measured at room temperature using an Automatic Magnetic Hysteresis Graph Tracer (Model no. AMH-300, Laboratorio Electrofisico).

3. Results and Discussion

3.1. Structural Analysis of Mn0.50-xZn0.50CuxFe2O4

Figure 1 shows the X-ray diffraction (XRD) patterns for various Mn0.50-xZn0.50CuxFe2O4 (with x = 0.0 - 0.3 at a step of 0.1) sintered at 1250˚C. The positions of the peaks for the various compositions indicate a single phase cubic spinel crystal structure. It is seen that a0 decreases linearly with increasing Cu2+ content and obeys Vegard’s law [11].

The variation of a0 as a function of Cu2+ content is shown in Figure 2. The values of a0 are shown in Table 1. The decreasing a0 with Cu2+content may be explained in terms of ionic radii. As the ionic radius of Mn2+ (0.80 Å) ion is larger than that of Cu2+ (0.72 Å), a0 decreases. [8,9,12]. As the smaller Cu2+ ions replace the bigger Mn2+ ions in Mn0.50-xZn0.50CuxFe2O4, the unit cell compresses without disturbing the symmetry of lattice [13].

A similar linear variation has been reported by Ravinder et al. [14]. The r-variant for Mn0.50-xZn0.50CuxFe2O4 was calculated by the expression: r-variant = (0.50-x)rMn + xrCu ,where rMn and rCu are the radii of the Mn2+ and Cu2+ respectively. It is found that the r-variant decreases with increase in Cu2+ content which is also shown in Figure 2.

Figure 3 shows the effect of Cu2+ substitutions on and in Mn0.50-xZn0.50CuxFe2O4 sintered at various sintering temperatures, Ts. It is found that is larger in magnitude than corresponding. This is because

Figure 1. XRD patterns of Mn0.50-xZn0.50CuxFe2O4 sintered at 1250˚C.

Figure 2. Variation of lattice parameter and r-variant of Mn0.50-xZn0.50CuxFe2O4 with Cu2+ content.

Table 1. Lattice parameter, density, porosity, average grain size, initial permeability, resonance frequency, residual and saturation inductions, coercive field and Néel temperature of various Mn0.50-xZn0.50CuxFe2O4 sintered at different sintering temperatures.

some pores may be developed in the samples during sintering process. It is also found that both ρth and ρB increase with increase in Cu2+ content while the P follows the opposite trend. The increase in ρB can be attributed to the difference in atomic weight and specific gravity of the Cu and Mn. The atomic weight and specific gravity of Cu (63.55 amu and 8.94 g·cm3) is greater than that of Mn (55.94 amu and 7.21 g·cm3) [15]. Also Cu2+ promotes densification of Mn0.50-xZn0.50CuxFe2O4 during low sintering temperature. These combined factors may contribute to the enhancement of ρB for various Mn0.50-xZn0.50CuxFe2O4 [16,17]. Similar results have been reported by Haque et al. [7]. On the other hand, P of Mn0.50-xZn0.50CuxFe2O4 reduces due to increasing ρB. It is revealed from Figures 3(a) and (b) that ρB increases and P decreases with increasing Ts for various Mn0.50-xZn0.50CuxFe2O4. This is because during the sintering process, the thermal energy generates a force that drives the grain boundaries to grow over pores, thereby decreasing the pore volume and increases the density of the compositions. These results agree well with the reported value [7,18]. The increase in ρB and decrease in P for various Mn0.50-xZn0.50CuxFe2O4 with the addition of Cu2+ for various Ts are tabulated in Table 1.

3.2. Microstructure of Mn0.50-xZn0.50CuxFe2O4

The scanning electron micrographs as shown in Figure 4 reveal that the grain size is influenced by the Cu2+ substitution, where the grain size increases with increase in Cu2+ content in Mn0.50-xZn0.50CuxFe2O4 sintered at 1200˚C. The D of Mn0.50-xZn0.50CuxFe2O4 varies from 2 μm to 4 μm at constant Ts.


Figure 3. Variation of (a) density and (b) porosity of Mn0.50-xZn0.50CuxFe2O4 as a function of Cu2+ content.

The values of D are presented in Table 1. Figure 5 shows the micrographs of Mn0.40Zn0.50Cu0.10Fe2O4 at different Ts. It is seen that the D increases with increasing Ts. The grain size increases with increase in Cu2+ due to the melting point of copper (1084˚C) is less than that of manganese (1246˚C). During sintering Cu2+ influences the microstructure by the formation of liquid phase. It facilitates the grain growth and grain growth reflects the competition between the driving force for grain boundary movement and the retarding force exerted by pores [19, 20]. D increases with Ts because of homogeneous grain growth.

3.3. Magnetic Properties of Mn0.50-xZn0.50CuxFe2O4

3.3.1. Frequency Dependent Complex Initial Permeability

Figure 6 shows the complex initial permeability spectra

Figure 4. Scanning electron micrographs of Mn0.50-xZn0.50CuxFe2O4 sintered at 1200˚C.

Figure 5. Scanning electron micrographs of Mn0.40Zn0.50Cu0.10Fe2O4 for various sintering temperatures.

for all Mn0.50-xZn0.50CuxFe2O4 (where x = 0.0 to 0.3 at a step of 0.1) sintered at 1200˚C, 1250˚C and 1300˚C, respectively as a function of frequency.

The complex permeability is given by. Here describes the stored energy expressing the component of magnetic induction B in phase and describes the dissipation of energy expressing the component 90˚ out of phase with the alternating magnetic field H. It is found that increases with increasing Cu2+ content for various Mn0.50-xZn0.50CuxFe2O4 at constant Ts. It is also observed that increases with increasing Ts. Highest was obtained 1061 for Mn0.20Zn0.50Cu0.30Fe2O4 at frequency 103 Hz sintered at 1300˚C. remains almost constant in the frequency range up to a certain frequency, which is called the resonance frequency, fr. There is a decrease in and increase in above fr observed for various Mn0.50-xZn0.50CuxFe2O4. The fr is the range of frequency of the compositions up to which these can be used efficiently. The values of and fr for all samples sintered at various Ts are presented in Table 1.

It is observed from Figure 7 that the increases and fr shifted towards the lower frequency with Cu2+ in Mn0.50-xZn0.50CuxFe2O4 at different Ts. The increasing and shifting fr to lower frequency at constant Ts

Figure 6. Frequency dependent Complex initial permeability of Mn0.50-xZn0.50CuxFe2O4 at various Ts.

follows the Snoek’s relation [21]. The of ferrites depends on many factors like reversible domain wall displacement, the amount and the type of dopant ions, D and intragranular porosity, etc [22-24]. It is well known that of polycrystalline ferrite is related to two magnetizing mechanisms: domain wall motion and spin rotation [25-27]. Globus et al. [22] studied several Ni-Zn ferrites and found a linear relationship between and D. Kakaktar et al. [28] studied the effect of D on and found that µD. The can be expressed by Globus-Duplex relation, where Ms is the saturation magnetization; K1 is the anisotropy constant [22]. In our present study of microstructure, it is seen that the D increases significantly with Cu2+ content. Therefore, the increase of with increasing Cu2+ content is justified. The increasing with Ts is due to increase in both and D.

Figure 8 shows the variation of with Cu2+ content at different frequencies for various Mn0.50-xZn0.50CuxFe2O4 sintered at 1200˚C, 1250˚C and 1300˚C. It is found that decreases at higher frequencies. This is due to the fact that at higher frequencies impurities between grains and intragranular pores act as pinning points and increasingly hinder the motion of spin and domain walls thereby decreasing their contribution to permeability and also increasing the loss [22].

For practical application the quality factor is often used as a measure of performance. The RQF increases with an increase of frequency, showing a peak and then decreases with further increase in frequency as shown in Figure 9. The variation of RQF with frequency showed a similar trend for all the samples. It is observed that the sample sintered at 1250˚C has the highest RQF (5420) for Mn0.40Zn0.50Cu0.10Fe2O4. The highest RQF for Mn0.20Zn0.50Cu0.30Fe2O4 is 5420 sintered at 1250˚C. This is probably due to the growth of less imperfection and defects compared to those of other samples [9].

Figure 7. The variation of initial permeability and resonance frequency of Mn0.50-xZn0.50CuxFe2O4 at different Ts.


Figure 8. The initial permeability of Mn0.50-xZn0.50CuxFe2O4  with Cu2+ content at different frequencies sintered at (a) 1200˚C, (b) 1250˚C and (c) 1300˚C.

Figure 9. The RQF of Mn0.50-xZn0.50CuxFe2O4 with Cu2+ content sintered at 1200˚C, 1250˚C and 1300˚C.

3.3.2. Temperature Dependent Permeability

The as a function of temperature for various Mn0.50-xZn0.50CuxFe2O4 sintered at 1200˚C is shown in Figure 10(a).

It is observed that there is a sudden drop in at Néel temperature, TN, where the magnetic state of the ferrite changes from ferrimagnetic to paramagnetic state. This is because at TN, the thermal agitation is so high that it reduces the alignment of the magnetic moment along a given axis to zero [29]. Figure 10(b) shows the variation of TN with Cu2+ content. It is observed that TN increases


Figure 10. (a) The temperature dependent initial permeability and (b) Néel temperature of Mn0.50-xZn0.50CuxFe2O4 with Cu2+ content sintered at 1200˚C.

With increase in Cu2+ substitution for Mn2+ in Mn0.50-xZn0.50CuxFe2O4 due to strengthening of A-B interaction between the two sublattices. This could be attributed to the decrease in distance (hooping length) between the magnetic ions of Aand B-sites and is confirmed by the decrease in the lattice parameter with increase in Cu2+ content as shown in Figure 2. The shorter distance between magnetic cations leads to the increase in A-B interaction and consequently TN increases.

3.3.3. B-H Loops of Mn0.50-xZn0.50CuxFe2O4

Figure 11(a) shows the B-H loops of Mn0.50-xZn0.50CuxFe2O4 sintered at 1200˚C at constant frequency 1000 Hz and Figure 11(b) is the B-H loops for low field.

The magnetic induction, B of all samples increases linearly with increase in applied magnetic field up to 120 A/m (depending on compositions). Beyond 120 A/m, B increases slowly and then reaches to the saturation induction, Bs. Increasing B with applied low field H indicates that all compositions are in ferromagnetic state at room temperature. From the loops Bs has been calculated. It is observed that Bs increases with increasing Cu2+ content which is shown in Figure 12(a).

The variation in saturation magnetization with Cu2+ content could be explained by cation distribution and exchange interaction. It is well known that Zn2+ ion has strong A-site occupancy, Mn2+ ions prefer to go both sites (80% Aand 20% B-sites) [16]. Also Cu2+ and Fe3+ ions can occupy both Aand B-sites. Substitution of Cu2+ in place of Mn2+ results into migration of some of Fe3+ from Ato B-site. The change in Br (residual induction) and Bs with Cu2+ content are shown in Figure 12(b). The coercive field (Hc) for each sample has been measured from the B-H loops of Mn0.50-xZn0.50CuxFe2O4 which is shown in Figure 12(b). It is observed that Hc decreases


Figure 11. (a) B-H loops of Mn0.50-xZn0.50CuxFe2O4 (b) B-H loops at low field sintered at 1200˚C.


Figure 12. (a) The Bs and Br and (b) Coercive field of Mn0.50-xZn0.50CuxFe2O4 with Cu2+ content sintered at 1200˚C.

with increase in Cu2+ content, whereas increases. Mazen and Abu-Elsaad reported that Hc is directed to gradually decreasing with D [29]. Our experimental result is in agreement with the reported value. Therefore, samples having larger grains are expected to have lower Hc.

4. Conclusion

Substitution of Cu2+ causes appreciable change in the structural and magnetic properties of the Mn-Cu-Zn ferrites. The XRD patterns confirm single phase cubic spinel structure of Mn0.50-xZn0.50CuxFe2O4. The a0 decreases with the increase in Cu2+ content while, D, and TN increase with the increase in Cu2+. The Hc decreases with the increase in Cu2+ content. is greatly enhanced from 258 to 1061 (~400%). The highest RQF (5420) has been found for the sample Mn0.20Zn0.50Cu0.30Fe2O4 sintered at 1250˚C. These ferrites are important and suitable for technological applications because of their high permeability.


Conflicts of Interest

The authors declare no conflicts of interest.


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