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Effect of Cd on the structural, magnetic and electrical properties of nanostructured Mn–Zn ferrite
Journal of Magnetism and Magnetic Materials 323 (2011) 1817–1822
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of Cd on the structural, magnetic and electrical properties of nanostructured Mn–Zn ferrite C. Venkataraju a,n, G. Sathishkumar b, K. Sivakumar c a
Department of Physics, A.M.A .College Of Engineering, Vadamavandal, Kancheepuram 604 401, India Department of Physics, Sri Sairam Engineering College, West Tambaram, 600 044 Chennai, India c Department of Physics, Anna University Chennai, Chennai 600 025, India b
a r t i c l e i n f o
abstract
Article history: Received 8 September 2010 Received in revised form 3 February 2011 Available online 17 February 2011
Nanoparticles of Mn0.5Zn0.5 xCdxFe2O4 (x ¼ 0.0, 0.1, 0.2 and 0.3) have been synthesized by a chemical co-precipitation method. The lattice constant increases with increasing Cd content. X-ray calculations indicate that there is deviation in the cation distribution in the nanostructured spinel ferrite. The dielectric constant and dielectric loss decrease for the samples with Cd content up to x ¼ 0.2. However the dielectric constant rises for x ¼ 0.3. This is due to an increase in the hopping process at the octahedral (B sites). The dielectric constant increases with increase in temperature, indicating a thermally activated hopping process. The DC resistivity increases with Cd content up to x ¼0.2 and decreases for Cd content x ¼0.3. The maximum magnetization of all the samples decreases with increase in Cd content. & 2011 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials Chemical synthesis X-ray diffraction Dielectric response Magnetic property
1. Introduction Manganese zinc ferrites are technologically important materials because of their high permeability and low loss. The structural, electrical and magnetic properties of the spinel ferrite depend on the distribution of cations among the two sublattices, tetrahedral (A site) and octahedral (B site). The cation distribution depends on the chemical composition and the method of preparation [1–3]. By altering the chemical composition of these ferrites, the physical properties can be changed to suit a particular application. Ferrites have proved to be good in microwave applications because of their low cost, high resistivity and low eddy current loss. Kharabe et al. [4] have reported the dielectric properties of mixed Li–Ni–Cd ferrites. They have reported an inverse trend between dielectric property and resistivity for all compositions. Rajesh Iyer et al. [5] have synthesized nanosized Mn(1 x)CdxFe2O4 by a co-precipitation technique. They have reported the effect of cadmium substitution on saturation magnetization value. Ladgaonkar and Vaingankar [6] have reported the X-ray diffraction investigation of cation distribution in CdxCu(1 x)Fe2O4. They have reported a new method wherein a graph of theoretical intensity is plotted against the inversion parameter for the (2 2 0) plane. The experimentally observed intensity of the same plane is then used to get an exact inversion parameter with the help of this plot. In the present investigation the
n
Corresponding author. E-mail address: [email protected] (C. Venkataraju).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.017
structural, electrical and magnetic properties of cadmium substituted Mn–Zn ferrite (Mn0.5Zn0.5 xCdxFe2O4 with x¼0.0, 0.1, 0.2 and 0.3) is reported.
2. Experimental details Nanoparticles of Mn0.5Zn(0.5 x)CdxFe2O4 with x varying from 0.0 to 0.3 were prepared by a co-precipitation method. Aqueous solutions of MnCl2, ZnSO4, CdCl2 and FeCl3 in their respective stoichiometry (100 ml of solution containing 0.5 M MnCl2, (x) M CdCl2, (0.5 x) M ZnSO4 and 100 ml of 2 M FeCl3) were mixed thoroughly at 80 1C and this mixture was added to a boiling solution of NaOH (0.55 M dissolved in 1600 ml of distilled water) within 10 s under constant stirring and a pH of 12 was maintained throughout the reaction. Conversion of metal salts into hydroxides and subsequent transformation of metal hydroxide to nanoferrites takes place on heating to 100 1C and maintaining it for 60 min. The nanoparticles thus formed were isolated by centrifugation and washed several times with deionized water followed by acetone and then dried at room temperature. The dried powder was ground thoroughly in a clean agate mortar. The ground powder was then pelletized using a hydraulic press and fired at 500 1C for 5 h. The data collection was performed on a PAN analytical X’PERT-PRO X-ray diffractometer using CuKa radiation. Data were collected every 0.021 in the angle range 20–701 of 2y. The dielectric measurements were carried out in the frequency range 5 Hz–5 MHz at room temperature using a HIOKI
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3532-50 LCR HiTester. Resistivity as a function of temperature was measured using the two-probe method. The magnetization measurements were done at room temperature up to a maximum field of 20 kOe using a vibration sample magnetometer (Lakeshore Model 7000).
3. Result and discussion 3.1. XRD analysis The X-ray diffraction for Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0, 0.1, 0.2 and 0.3) is shown in Fig. 1.These diffraction lines provide a clear evidence for the formation of ferrite phase in all the samples. The broad XRD line indicates that the ferrite particles are of nanosize. The average particle size for each composition was calculated from the XRD line width of (3 1 1) peak using the Scherrer formula [7]. The values of the particle size and lattice constant as deduced from the X-ray data are given in Table 1. The lattice constant increases with increasing cadmium concentration, which can be explained based on the relative ionic radius. ˚ of Cd2 + ions is larger than the ionic The ionic radius (0.78 A) 2+ ˚ radius (0.60 A) of Zn ions. Replacement of larger Cd2 + ion for smaller Zn2 + ions in the manganese zinc ferrite causes an increase in lattice constant. It is observed that there is a systematic decrease in the particle size as the Cd2 + ion concentration increases. The average particle size of Mn0.5Zn0.5Fe2O4 is 15 nm, which decreases with increasing Cd2 + ion concentration and becomes 11 nm in the case of
Mn0.5Zn0.2Cd0.3Fe2O4. It is understood from the above observation that the presence of Cd2 + ions obstructs the crystal growth in the spinel ferrite. The average cation distribution in tetrahedral (A site) and octahedral (B site) can be calculated on the basis of proposed cationic distribution parameter by using the relation suggested by Mazen et al. [8]: i pffiffiffi 8 h ath ¼ pffiffiffi ðrA þ Ro Þ þ 3ðrB þRo Þ ð1Þ 3 3 ˚ rA and rB are the where Ro is the radius of oxygen ion (1.32 A); ionic radii of tetrahedral (A Site) and octahedral (B Site), respectively. The ionic radius for each site was calculated according to h i 2þ þ CA Fe r Fe3 þ rA ¼ CA Mn r Mn2 þ þ CA Zn r Zn2 þ þCA Cd r Cd ð2Þ h
i 2þ þCB Fe r Fe3 þ rB ¼ CB Mn r Mn2 þ þ CB Zn r Zn2 þ þ CB Cd r Cd ð3Þ 2+
2+
2+
3+
where r (Mn ), r (Zn ), r (Cd ) and r (Fe ) are the cationic radius of Mn, Zn, Cd and Fe, respectively. Computerized calculation of theoretical lattice constant ath is performed for various values of cationic distribution parameter. The value of cationic distribution parameter for which the theoretical lattice constant ath and the experimental lattice constant aexp nearly match each other is taken as the correct one. It is observed from Table 1, that there is a deviation in cation preferences in nanosize particles, leading to a metastable state.
Fig. 1. XRD pattern for the system Mn0.5Zn(0.5 x)CdxFe2O4 with (x ¼0.0, 0.1, 0.2 and 0.3).
Table 1 Structural parameters of Mn0.5Zn0.5 xCdxFe2O4 (x ¼0.0, 0.1, 0.2, 0.3). Sl.no.
Composition x
A site
B site
Lattice constant ath
Lattice constant aexp
Particle size (nm)
1. 2. 3. 4.
0.0 0.1 0.2 0.3
+ Mn0.25Zn0.295Fe30.455 Mn0.26Zn0.2Cd0.07Fe3 + 0.47 Mn0.262Zn0.135Cd0.13Fe3 + 0.473 Mn0.263Zn0.1Cd0.17Fe3 + 0.467
+ Mn0.25Zn0.205 Fe3 + 1.435Fe20.11 Mn0.24Zn0.2Cd0.03Fe3 + 1.53 Mn0.238Zn0.165Cd0.07Fe3 + 1.527 Mn0.237Zn0.1Cd0.13Fe3 + 1.533
8.374 8.402 8.433 8.454
8.373 8.391 8.428 8.446
13 13 12 11
C. Venkataraju et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1817–1822
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Fig. 2. (a) and (b) SEM micrograph for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0 and 0.2).
Cd2 + ions have chemical affinity towards tetrahedral A sites. In the present investigation it is observed that a fraction of Cd2 + ions occupy octahedral B site. This forces some of the Fe3 + ions to migrate from B site to A site. However for the sample x ¼0.3 there is a slight increase in the Cd2 + ions on A site, causing migration of Fe2 + ions from A site to B site. 3.2. Scanning electron microscope analysis The surface morphology, grain size and elemental composition of the samples play an important role on the properties of ferrites. Hence to investigate the surface morphology and grain size variation of the system Mn0.5Zn(0.5 x)CdxFe2O4 with respect to cadmium content gives more information of the grain nature of the samples. Typical micrographs for the samples x¼0.0 and 0.3 are shown in Fig. 2(a) and (b). SEM analysis shows that all the samples are highly homogeneous and the grains are spherical in shape. The clear SEM images of all the samples also reveal that there are no secondary phases. This is supported by the absence of additional peaks in the XRD patterns. The micrograph of all the samples shows the presence of more number of smaller grains having a large number of interfaces, which have a direct effect on the properties of these ferrites. 3.3. Dielectric studies 3.3.1. Compositional variation of dielectric constant The variation of dielectric constant e with Cd content for different frequencies is shown in Fig. 3. The dielectric constant decreases with increase in Cd content up to x ¼0.2. This is due to migration of Fe3 + ions from B site to A site. This decreases the hopping between Fe3 + –Fe2 + ions in the octahedral B site. Therefore polarization decreases, resulting in lower dielectric constant. Further, the presence of Zn2 + ions at the octahedral B sites blocks the Verweys hopping mechanism, resulting in a decrease in dielectric constant [9]. The dielectric constant increases for Cd content x¼0.3. As the concentration is increased to x¼ 0.3, the Cd2 + ions substitute for the Zn2 + ions at A site and B sites. Hence the concentration of Zn2 + ions at A site and B site decreases. Therefore the hopping between Fe3 + –Fe2 + ions increases. As a result, polarization and hence the dielectric constant increase. 3.3.2. Frequency variation of dielectric constant It is observed from Fig. 3 that for each sample the dielectric constant decreases with increase of frequency and becomes a constant at higher frequency. The values of the dielectric constant
Fig. 3. Variation of dielectric constant with frequency for the Mn0.5Zn(0.5 x)CdxFe2O4 (x¼ 0.0, 0.1, 0.2 and 0.3).
system
for Mn0.5Zn0.5Fe2O4 are quite low and are found to be lower by several orders than those values obtained for samples prepared by ceramic method [10]. This low value of dielectric constant is attributed to homogeneity, better symmetry, uniformity and smaller grains [11]. Smaller grains contain large surface boundaries and are regions of high resistance. This reduces the interfacial polarization and hence the dielectric constant is found to be lower than that reported for bulk material. It is also observed that the dispersion of dielectric constant is maximum for Mn0.5Zn0.5Fe2O4. This can be explained on the basis of space charge polarization model of Wagner [12] and Maxwell [13] and is also in agreement with Koops [14] phonological theory. 3.3.3. Dielectric loss It is observed from Fig. 4, that the position of the dielectric loss maxima shifts towards the lower frequency with increase in cadmium concentration. This shifting of the peak towards lower frequency suggests that the dipole–dipole interaction becomes stronger at lower frequency, causing hindrance to the rotation of the dipoles. The value of the dielectric loss obtained in the present work is found to be lower than those obtained by other methods. This low dielectric loss obtained in the present work is attributed to better symmetry and homogeneity of the ferrites prepared by the co-precipitation method.
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3.3.4. Variation of dielectric constant with temperature The variation of dielectric constant as a function of temperature at different frequencies is shown in the Fig. 5a–c. The increase in the dielectric constant with increase in temperature is higher at lower frequency (1 kHz), while at higher frequencies
the increase is very small. For the sample with x ¼0.0, a slight decrease in the dielectric constant is observed above 423 K. A similar variation has been reported earlier [15–17]. In general the dielectric constant of any material is due to dipolar, electronic, ionic and interfacial polarizations. At low frequencies dipolar and interfacial polarizations are the main contributors. Both of these polarizations are strongly temperature dependent. Interfacial polarization increases with temperature, whereas dipolar polarization decreases with increase in temperature. This rapid increase in the dielectric constant with increase in temperature at lower frequency suggests that the effect of temperature is more pronounced on the interfacial than on dipolar polarization. At higher frequency electronic and ionic polarizations are the main contributors which are temperature independent. Fig. 6a–c shows the temperature dependence of tan d at some selected frequencies. The dielectric loss increases with temperature at a higher rate for 1 kHz than for higher frequencies. This suggests an thermally activated hopping process with increasing temperature at low frequencies.
3.4. DC resistivity
Fig. 4. Variation of dielectric loss with frequency for the system Mn0.5Zn(0.5 x) CdxFe2O4 (x¼0.0, 0.1, 0.2 and 0.3).
Compositional variation of DC resistivity at room temperature is shown in Fig. 7. It is observed that resistivity increases for increase in Cd content up to x ¼0.2. However, resistivity decreases for the sample with Cd content x¼0.3. It is well known that cadmium and zinc ions occupy tetrahedral (A site) positions,
Fig. 5. (a), (b) and (c). Variation of dielectric constant with temperature for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x ¼0.0, 0.1 and 0.3).
C. Venkataraju et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1817–1822
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Fig. 6. (a), (b) and (c) Variation of dielectric loss with temperature for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x ¼ 0.0, 0.1 and 0.3).
evident from Table 1. As the concentration is increased to x ¼0.3, the Cd2 + ions largely occupy tetrahedral sites (A site) and also substitute for Zn2 + ion at octahedral sites (B site). This decrease in the concentration of Zn2 + ions from octahedral site increases the hopping mechanism. Therefore the resistance decreases. The DC resistivity obtained in the present investigation is higher than the reported value obtained by other methods. Singh et al. [18] have reported values of DC resistivity between 1.4 105 and 7.3 107 O cm for NixMn0.4 xZn0.6Fe2O4 (with x ¼0.1–0.4) synthesized by the citrate precursor method. In the present investigation the measured value of DC resistivity for all samples lies in the range 3.2 107–1.2 1010 O cm. This higher value of resistivity is attributed to small grain size. SEM analysis reveals the presence of a large number of small grains. Small grains contain a large number of grain boundaries, which act as scattering centers for the flow of electrons and hence the resistivity increases.
Fig. 7. DC resistivity for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0, 0.1, 0.2 and 0.3).
while iron and manganese occupy both A sites and B sites. However in nanodimensions zinc and cadmium occupies octahedral sites. When Cd2 + ions are introduced at the cost of Zn2 + ions, some of the iron ions migrate from B sites to A sites, decreasing thereby the Fe3 + ion concentration at the octahedral (B sites) as is
3.5. Compositional variation of maximum magnetization The compositional variation of maximum magnetization is shown in Fig. 8. It is observed that the maximum magnetization decreases with increase in substitution of non-magnetic Cd ion in the Mn–Zn ferrite spinel lattice. Cd2 + ions have preference for A site, but in nanodimension Cd ion occupies both A site and B site. Due to this Fe3 + ions migrate from B site to A site.
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magnetization. The existence of random canting of particle surface spins, surface effects and the occurrence of a glassy state were reported to be playing an active role in the decline of magnetization value [21,22].
4. Conclusion Nanosized Mn–Zn–Cd ferrites were prepared by a co-precipitation method. The dielectric constant decreases with increase in Cd content except for the sample x ¼0.3, where it shows a rise. The dielectric constant increases with temperature, indicating a thermally activated hopping process. The sample with Cd content x¼0.2 shows a higher resistivity as compared to other samples. The reduced magnetization values and variation of maximum magnetization with Cd content provide the evidence for the deviation in cation distribution and non-colinear magnetic spin ordering in nanoparticles. References Fig. 8. Maximum magnetization for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0, 0.1, 0.2 and 0.3).
According to Neel’s two sublattice model of ferrimagnetism [19] ,the magnetic moment per formula unit, ZN B with known cation distribution at A site and B site is given by the equation
ZNB ¼ MB ðxÞMA ðxÞ
ð4Þ
where MB and MA are the B sublattice and A sublattice magnetic moments, respectively, in ZB. The theoretical value of magnetic moment per formula unit in Bohr’s magneton is estimated using cation distribution. The magnetic moments of Fe3 + ion and Mn2 + ion are 5mB and 4.5mB, respectively. Due to migration of Fe3 + ions from B site to A site, the magnetic moment of B sublattice decreases and the magnetic moment of A sublattice increases. Therefore the net magnetic moment decreases with increase in Cd content. However the maximum magnetization in the nanodimension is found to be lower than the corresponding bulk counterpart [20]. The respective reduction in magnetization can be attributed to the change in cation distribution between A site and B site. In addition to the change in the cation distribution, many other factors reported in literature may contribute to the reduction in maximum
[1] G. Herzer, M. Vaznez, M. Knobel, A. Zhokov, T. Reininger, H.A. Davies, R. Grossinger, Li J L Sanchez, J. Magn. Magn. Mater. 294 (2005) 252. [2] N.W. Grimes, Phys. Technol. (1975). [3] R.E. Vandenberghe, R. Vanleeberghe, E. Degrave, G. Robbrechi, J. Magn. Magn. Mater. 117 (1980) 15. [4] R.G. Kharabe, R.S. Devan, C.M. Kanamadi, B.K. Chougule, Smart Mater. Struct. 15 (2006) N36–N39. [5] Rajesh Iyer, Rucha Desai, R.V. Upadhyay, Indian J. Pure Appl. Phys. 47 (2009) 180–185 March. [6] B.P. Ladgaonkar, A.S. Vaingankar, Mater. Chem. Phys. 56 (1998) 280–283. [7] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Co. Inc., 1959. [8] S.A. Mazen, M.H. Abdallah, B.A. Sabrah, H.A.M. Hashem, Phys. Status Solidi A 134 (1992) 263–271. [9] J.W. Verwey, J.H. de Boer, Recl. Trav. Chim. Pays–Bas. 55 (1936) 531. [10] K. Latha, K. Sathya Mohan, D. Ravinder, Phys. Status Solidi (a) 142 (1994) K103. [11] P. Mathur, A. Thakur, M. Singh, Phys. Scr. 77 (2008). [12] K.W. Wagner, Am. Phys 40 (1973) 817. [13] J.C. Maxwell, Electricity and Magnetisim, vol.1, Oxford University Press, New York, 1973, p. 828. [14] C.G. Koops, Phys. Rev. 83 (1951) 121. [15] D. Ravinder, K.Vijaya Kumar, Bull. Mater. Sci. 24 (2001) 505–509. [16] A.K. Singh, T.C. Goel, R.G. Mendiratta, J. Appl. Phys. 91 (2002) 6626–6629. [17] S.A. Olofa, J. Magn. Magn. Mater. 131 (1994) 103. [18] A.K. Singh, T.C. Goel, R.G. Mendiratta, J. Magn. Magn. Mater. 281 (2004) 276–280. [19] L. Neel, C. R. Acad. Sci. 230 (1950) 375. [20] J.M.D. Coe1, Phys. Rev. Lett. 27 (1971) 140.y. [21] M. Garcia del Muro, X. Battle, A. Labarta, Phys. Rev. B 59 (1999) 13584.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of Cd on the structural, magnetic and electrical properties of nanostructured Mn–Zn ferrite C. Venkataraju a,n, G. Sathishkumar b, K. Sivakumar c a
Department of Physics, A.M.A .College Of Engineering, Vadamavandal, Kancheepuram 604 401, India Department of Physics, Sri Sairam Engineering College, West Tambaram, 600 044 Chennai, India c Department of Physics, Anna University Chennai, Chennai 600 025, India b
a r t i c l e i n f o
abstract
Article history: Received 8 September 2010 Received in revised form 3 February 2011 Available online 17 February 2011
Nanoparticles of Mn0.5Zn0.5 xCdxFe2O4 (x ¼ 0.0, 0.1, 0.2 and 0.3) have been synthesized by a chemical co-precipitation method. The lattice constant increases with increasing Cd content. X-ray calculations indicate that there is deviation in the cation distribution in the nanostructured spinel ferrite. The dielectric constant and dielectric loss decrease for the samples with Cd content up to x ¼ 0.2. However the dielectric constant rises for x ¼ 0.3. This is due to an increase in the hopping process at the octahedral (B sites). The dielectric constant increases with increase in temperature, indicating a thermally activated hopping process. The DC resistivity increases with Cd content up to x ¼0.2 and decreases for Cd content x ¼0.3. The maximum magnetization of all the samples decreases with increase in Cd content. & 2011 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials Chemical synthesis X-ray diffraction Dielectric response Magnetic property
1. Introduction Manganese zinc ferrites are technologically important materials because of their high permeability and low loss. The structural, electrical and magnetic properties of the spinel ferrite depend on the distribution of cations among the two sublattices, tetrahedral (A site) and octahedral (B site). The cation distribution depends on the chemical composition and the method of preparation [1–3]. By altering the chemical composition of these ferrites, the physical properties can be changed to suit a particular application. Ferrites have proved to be good in microwave applications because of their low cost, high resistivity and low eddy current loss. Kharabe et al. [4] have reported the dielectric properties of mixed Li–Ni–Cd ferrites. They have reported an inverse trend between dielectric property and resistivity for all compositions. Rajesh Iyer et al. [5] have synthesized nanosized Mn(1 x)CdxFe2O4 by a co-precipitation technique. They have reported the effect of cadmium substitution on saturation magnetization value. Ladgaonkar and Vaingankar [6] have reported the X-ray diffraction investigation of cation distribution in CdxCu(1 x)Fe2O4. They have reported a new method wherein a graph of theoretical intensity is plotted against the inversion parameter for the (2 2 0) plane. The experimentally observed intensity of the same plane is then used to get an exact inversion parameter with the help of this plot. In the present investigation the
n
Corresponding author. E-mail address: [email protected] (C. Venkataraju).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.017
structural, electrical and magnetic properties of cadmium substituted Mn–Zn ferrite (Mn0.5Zn0.5 xCdxFe2O4 with x¼0.0, 0.1, 0.2 and 0.3) is reported.
2. Experimental details Nanoparticles of Mn0.5Zn(0.5 x)CdxFe2O4 with x varying from 0.0 to 0.3 were prepared by a co-precipitation method. Aqueous solutions of MnCl2, ZnSO4, CdCl2 and FeCl3 in their respective stoichiometry (100 ml of solution containing 0.5 M MnCl2, (x) M CdCl2, (0.5 x) M ZnSO4 and 100 ml of 2 M FeCl3) were mixed thoroughly at 80 1C and this mixture was added to a boiling solution of NaOH (0.55 M dissolved in 1600 ml of distilled water) within 10 s under constant stirring and a pH of 12 was maintained throughout the reaction. Conversion of metal salts into hydroxides and subsequent transformation of metal hydroxide to nanoferrites takes place on heating to 100 1C and maintaining it for 60 min. The nanoparticles thus formed were isolated by centrifugation and washed several times with deionized water followed by acetone and then dried at room temperature. The dried powder was ground thoroughly in a clean agate mortar. The ground powder was then pelletized using a hydraulic press and fired at 500 1C for 5 h. The data collection was performed on a PAN analytical X’PERT-PRO X-ray diffractometer using CuKa radiation. Data were collected every 0.021 in the angle range 20–701 of 2y. The dielectric measurements were carried out in the frequency range 5 Hz–5 MHz at room temperature using a HIOKI
1818
C. Venkataraju et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1817–1822
3532-50 LCR HiTester. Resistivity as a function of temperature was measured using the two-probe method. The magnetization measurements were done at room temperature up to a maximum field of 20 kOe using a vibration sample magnetometer (Lakeshore Model 7000).
3. Result and discussion 3.1. XRD analysis The X-ray diffraction for Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0, 0.1, 0.2 and 0.3) is shown in Fig. 1.These diffraction lines provide a clear evidence for the formation of ferrite phase in all the samples. The broad XRD line indicates that the ferrite particles are of nanosize. The average particle size for each composition was calculated from the XRD line width of (3 1 1) peak using the Scherrer formula [7]. The values of the particle size and lattice constant as deduced from the X-ray data are given in Table 1. The lattice constant increases with increasing cadmium concentration, which can be explained based on the relative ionic radius. ˚ of Cd2 + ions is larger than the ionic The ionic radius (0.78 A) 2+ ˚ radius (0.60 A) of Zn ions. Replacement of larger Cd2 + ion for smaller Zn2 + ions in the manganese zinc ferrite causes an increase in lattice constant. It is observed that there is a systematic decrease in the particle size as the Cd2 + ion concentration increases. The average particle size of Mn0.5Zn0.5Fe2O4 is 15 nm, which decreases with increasing Cd2 + ion concentration and becomes 11 nm in the case of
Mn0.5Zn0.2Cd0.3Fe2O4. It is understood from the above observation that the presence of Cd2 + ions obstructs the crystal growth in the spinel ferrite. The average cation distribution in tetrahedral (A site) and octahedral (B site) can be calculated on the basis of proposed cationic distribution parameter by using the relation suggested by Mazen et al. [8]: i pffiffiffi 8 h ath ¼ pffiffiffi ðrA þ Ro Þ þ 3ðrB þRo Þ ð1Þ 3 3 ˚ rA and rB are the where Ro is the radius of oxygen ion (1.32 A); ionic radii of tetrahedral (A Site) and octahedral (B Site), respectively. The ionic radius for each site was calculated according to h i 2þ þ CA Fe r Fe3 þ rA ¼ CA Mn r Mn2 þ þ CA Zn r Zn2 þ þCA Cd r Cd ð2Þ h
i 2þ þCB Fe r Fe3 þ rB ¼ CB Mn r Mn2 þ þ CB Zn r Zn2 þ þ CB Cd r Cd ð3Þ 2+
2+
2+
3+
where r (Mn ), r (Zn ), r (Cd ) and r (Fe ) are the cationic radius of Mn, Zn, Cd and Fe, respectively. Computerized calculation of theoretical lattice constant ath is performed for various values of cationic distribution parameter. The value of cationic distribution parameter for which the theoretical lattice constant ath and the experimental lattice constant aexp nearly match each other is taken as the correct one. It is observed from Table 1, that there is a deviation in cation preferences in nanosize particles, leading to a metastable state.
Fig. 1. XRD pattern for the system Mn0.5Zn(0.5 x)CdxFe2O4 with (x ¼0.0, 0.1, 0.2 and 0.3).
Table 1 Structural parameters of Mn0.5Zn0.5 xCdxFe2O4 (x ¼0.0, 0.1, 0.2, 0.3). Sl.no.
Composition x
A site
B site
Lattice constant ath
Lattice constant aexp
Particle size (nm)
1. 2. 3. 4.
0.0 0.1 0.2 0.3
+ Mn0.25Zn0.295Fe30.455 Mn0.26Zn0.2Cd0.07Fe3 + 0.47 Mn0.262Zn0.135Cd0.13Fe3 + 0.473 Mn0.263Zn0.1Cd0.17Fe3 + 0.467
+ Mn0.25Zn0.205 Fe3 + 1.435Fe20.11 Mn0.24Zn0.2Cd0.03Fe3 + 1.53 Mn0.238Zn0.165Cd0.07Fe3 + 1.527 Mn0.237Zn0.1Cd0.13Fe3 + 1.533
8.374 8.402 8.433 8.454
8.373 8.391 8.428 8.446
13 13 12 11
C. Venkataraju et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1817–1822
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Fig. 2. (a) and (b) SEM micrograph for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0 and 0.2).
Cd2 + ions have chemical affinity towards tetrahedral A sites. In the present investigation it is observed that a fraction of Cd2 + ions occupy octahedral B site. This forces some of the Fe3 + ions to migrate from B site to A site. However for the sample x ¼0.3 there is a slight increase in the Cd2 + ions on A site, causing migration of Fe2 + ions from A site to B site. 3.2. Scanning electron microscope analysis The surface morphology, grain size and elemental composition of the samples play an important role on the properties of ferrites. Hence to investigate the surface morphology and grain size variation of the system Mn0.5Zn(0.5 x)CdxFe2O4 with respect to cadmium content gives more information of the grain nature of the samples. Typical micrographs for the samples x¼0.0 and 0.3 are shown in Fig. 2(a) and (b). SEM analysis shows that all the samples are highly homogeneous and the grains are spherical in shape. The clear SEM images of all the samples also reveal that there are no secondary phases. This is supported by the absence of additional peaks in the XRD patterns. The micrograph of all the samples shows the presence of more number of smaller grains having a large number of interfaces, which have a direct effect on the properties of these ferrites. 3.3. Dielectric studies 3.3.1. Compositional variation of dielectric constant The variation of dielectric constant e with Cd content for different frequencies is shown in Fig. 3. The dielectric constant decreases with increase in Cd content up to x ¼0.2. This is due to migration of Fe3 + ions from B site to A site. This decreases the hopping between Fe3 + –Fe2 + ions in the octahedral B site. Therefore polarization decreases, resulting in lower dielectric constant. Further, the presence of Zn2 + ions at the octahedral B sites blocks the Verweys hopping mechanism, resulting in a decrease in dielectric constant [9]. The dielectric constant increases for Cd content x¼0.3. As the concentration is increased to x¼ 0.3, the Cd2 + ions substitute for the Zn2 + ions at A site and B sites. Hence the concentration of Zn2 + ions at A site and B site decreases. Therefore the hopping between Fe3 + –Fe2 + ions increases. As a result, polarization and hence the dielectric constant increase. 3.3.2. Frequency variation of dielectric constant It is observed from Fig. 3 that for each sample the dielectric constant decreases with increase of frequency and becomes a constant at higher frequency. The values of the dielectric constant
Fig. 3. Variation of dielectric constant with frequency for the Mn0.5Zn(0.5 x)CdxFe2O4 (x¼ 0.0, 0.1, 0.2 and 0.3).
system
for Mn0.5Zn0.5Fe2O4 are quite low and are found to be lower by several orders than those values obtained for samples prepared by ceramic method [10]. This low value of dielectric constant is attributed to homogeneity, better symmetry, uniformity and smaller grains [11]. Smaller grains contain large surface boundaries and are regions of high resistance. This reduces the interfacial polarization and hence the dielectric constant is found to be lower than that reported for bulk material. It is also observed that the dispersion of dielectric constant is maximum for Mn0.5Zn0.5Fe2O4. This can be explained on the basis of space charge polarization model of Wagner [12] and Maxwell [13] and is also in agreement with Koops [14] phonological theory. 3.3.3. Dielectric loss It is observed from Fig. 4, that the position of the dielectric loss maxima shifts towards the lower frequency with increase in cadmium concentration. This shifting of the peak towards lower frequency suggests that the dipole–dipole interaction becomes stronger at lower frequency, causing hindrance to the rotation of the dipoles. The value of the dielectric loss obtained in the present work is found to be lower than those obtained by other methods. This low dielectric loss obtained in the present work is attributed to better symmetry and homogeneity of the ferrites prepared by the co-precipitation method.
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3.3.4. Variation of dielectric constant with temperature The variation of dielectric constant as a function of temperature at different frequencies is shown in the Fig. 5a–c. The increase in the dielectric constant with increase in temperature is higher at lower frequency (1 kHz), while at higher frequencies
the increase is very small. For the sample with x ¼0.0, a slight decrease in the dielectric constant is observed above 423 K. A similar variation has been reported earlier [15–17]. In general the dielectric constant of any material is due to dipolar, electronic, ionic and interfacial polarizations. At low frequencies dipolar and interfacial polarizations are the main contributors. Both of these polarizations are strongly temperature dependent. Interfacial polarization increases with temperature, whereas dipolar polarization decreases with increase in temperature. This rapid increase in the dielectric constant with increase in temperature at lower frequency suggests that the effect of temperature is more pronounced on the interfacial than on dipolar polarization. At higher frequency electronic and ionic polarizations are the main contributors which are temperature independent. Fig. 6a–c shows the temperature dependence of tan d at some selected frequencies. The dielectric loss increases with temperature at a higher rate for 1 kHz than for higher frequencies. This suggests an thermally activated hopping process with increasing temperature at low frequencies.
3.4. DC resistivity
Fig. 4. Variation of dielectric loss with frequency for the system Mn0.5Zn(0.5 x) CdxFe2O4 (x¼0.0, 0.1, 0.2 and 0.3).
Compositional variation of DC resistivity at room temperature is shown in Fig. 7. It is observed that resistivity increases for increase in Cd content up to x ¼0.2. However, resistivity decreases for the sample with Cd content x¼0.3. It is well known that cadmium and zinc ions occupy tetrahedral (A site) positions,
Fig. 5. (a), (b) and (c). Variation of dielectric constant with temperature for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x ¼0.0, 0.1 and 0.3).
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Fig. 6. (a), (b) and (c) Variation of dielectric loss with temperature for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x ¼ 0.0, 0.1 and 0.3).
evident from Table 1. As the concentration is increased to x ¼0.3, the Cd2 + ions largely occupy tetrahedral sites (A site) and also substitute for Zn2 + ion at octahedral sites (B site). This decrease in the concentration of Zn2 + ions from octahedral site increases the hopping mechanism. Therefore the resistance decreases. The DC resistivity obtained in the present investigation is higher than the reported value obtained by other methods. Singh et al. [18] have reported values of DC resistivity between 1.4 105 and 7.3 107 O cm for NixMn0.4 xZn0.6Fe2O4 (with x ¼0.1–0.4) synthesized by the citrate precursor method. In the present investigation the measured value of DC resistivity for all samples lies in the range 3.2 107–1.2 1010 O cm. This higher value of resistivity is attributed to small grain size. SEM analysis reveals the presence of a large number of small grains. Small grains contain a large number of grain boundaries, which act as scattering centers for the flow of electrons and hence the resistivity increases.
Fig. 7. DC resistivity for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0, 0.1, 0.2 and 0.3).
while iron and manganese occupy both A sites and B sites. However in nanodimensions zinc and cadmium occupies octahedral sites. When Cd2 + ions are introduced at the cost of Zn2 + ions, some of the iron ions migrate from B sites to A sites, decreasing thereby the Fe3 + ion concentration at the octahedral (B sites) as is
3.5. Compositional variation of maximum magnetization The compositional variation of maximum magnetization is shown in Fig. 8. It is observed that the maximum magnetization decreases with increase in substitution of non-magnetic Cd ion in the Mn–Zn ferrite spinel lattice. Cd2 + ions have preference for A site, but in nanodimension Cd ion occupies both A site and B site. Due to this Fe3 + ions migrate from B site to A site.
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magnetization. The existence of random canting of particle surface spins, surface effects and the occurrence of a glassy state were reported to be playing an active role in the decline of magnetization value [21,22].
4. Conclusion Nanosized Mn–Zn–Cd ferrites were prepared by a co-precipitation method. The dielectric constant decreases with increase in Cd content except for the sample x ¼0.3, where it shows a rise. The dielectric constant increases with temperature, indicating a thermally activated hopping process. The sample with Cd content x¼0.2 shows a higher resistivity as compared to other samples. The reduced magnetization values and variation of maximum magnetization with Cd content provide the evidence for the deviation in cation distribution and non-colinear magnetic spin ordering in nanoparticles. References Fig. 8. Maximum magnetization for the system Mn0.5Zn(0.5 x)CdxFe2O4 (with x¼ 0.0, 0.1, 0.2 and 0.3).
According to Neel’s two sublattice model of ferrimagnetism [19] ,the magnetic moment per formula unit, ZN B with known cation distribution at A site and B site is given by the equation
ZNB ¼ MB ðxÞMA ðxÞ
ð4Þ
where MB and MA are the B sublattice and A sublattice magnetic moments, respectively, in ZB. The theoretical value of magnetic moment per formula unit in Bohr’s magneton is estimated using cation distribution. The magnetic moments of Fe3 + ion and Mn2 + ion are 5mB and 4.5mB, respectively. Due to migration of Fe3 + ions from B site to A site, the magnetic moment of B sublattice decreases and the magnetic moment of A sublattice increases. Therefore the net magnetic moment decreases with increase in Cd content. However the maximum magnetization in the nanodimension is found to be lower than the corresponding bulk counterpart [20]. The respective reduction in magnetization can be attributed to the change in cation distribution between A site and B site. In addition to the change in the cation distribution, many other factors reported in literature may contribute to the reduction in maximum
[1] G. Herzer, M. Vaznez, M. Knobel, A. Zhokov, T. Reininger, H.A. Davies, R. Grossinger, Li J L Sanchez, J. Magn. Magn. Mater. 294 (2005) 252. [2] N.W. Grimes, Phys. Technol. (1975). [3] R.E. Vandenberghe, R. Vanleeberghe, E. Degrave, G. Robbrechi, J. Magn. Magn. Mater. 117 (1980) 15. [4] R.G. Kharabe, R.S. Devan, C.M. Kanamadi, B.K. Chougule, Smart Mater. Struct. 15 (2006) N36–N39. [5] Rajesh Iyer, Rucha Desai, R.V. Upadhyay, Indian J. Pure Appl. Phys. 47 (2009) 180–185 March. [6] B.P. Ladgaonkar, A.S. Vaingankar, Mater. Chem. Phys. 56 (1998) 280–283. [7] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Co. Inc., 1959. [8] S.A. Mazen, M.H. Abdallah, B.A. Sabrah, H.A.M. Hashem, Phys. Status Solidi A 134 (1992) 263–271. [9] J.W. Verwey, J.H. de Boer, Recl. Trav. Chim. Pays–Bas. 55 (1936) 531. [10] K. Latha, K. Sathya Mohan, D. Ravinder, Phys. Status Solidi (a) 142 (1994) K103. [11] P. Mathur, A. Thakur, M. Singh, Phys. Scr. 77 (2008). [12] K.W. Wagner, Am. Phys 40 (1973) 817. [13] J.C. Maxwell, Electricity and Magnetisim, vol.1, Oxford University Press, New York, 1973, p. 828. [14] C.G. Koops, Phys. Rev. 83 (1951) 121. [15] D. Ravinder, K.Vijaya Kumar, Bull. Mater. Sci. 24 (2001) 505–509. [16] A.K. Singh, T.C. Goel, R.G. Mendiratta, J. Appl. Phys. 91 (2002) 6626–6629. [17] S.A. Olofa, J. Magn. Magn. Mater. 131 (1994) 103. [18] A.K. Singh, T.C. Goel, R.G. Mendiratta, J. Magn. Magn. Mater. 281 (2004) 276–280. [19] L. Neel, C. R. Acad. Sci. 230 (1950) 375. [20] J.M.D. Coe1, Phys. Rev. Lett. 27 (1971) 140.y. [21] M. Garcia del Muro, X. Battle, A. Labarta, Phys. Rev. B 59 (1999) 13584.