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Influence of Mg and Cr substitution on structural and magnetic properties of polycrystalline Ni0.50Zn0.50−x−yMgxCryFe2O4
Materials Chemistry and Physics 113 (2009) 172–178
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Influence of Mg and Cr substitution on structural and magnetic properties of polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 A.K.M. Akther Hossain a,∗ , T.S. Biswas a , S.T. Mahmud a , Takeshi Yanagida b , Hidekazu Tanaka b , Tomoji Kawai b a b
Department of Physics, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 11 June 2008 Accepted 8 July 2008 Keywords: Magnetic materials Annealing Magnetometer Microstructure Magnetic properties
a b s t r a c t Structural, surface morphological and magnetic properties of polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 (for x = 0.0, 0.1, 0.2 and y = 0.0, 0.1, 0.1) were thoroughly investigated. The lattice parameters of the samples investigated are found to decrease with increasing Mg and Cr contents. The bulk density and the average grain diameter are found to decrease with increasing Mg and Cr substitution in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 for any particular sintering temperature. However, they are found to increase with increasing sintering temperatures for any individual composition. In the present investigation, the maximum bulk density and the largest average grain diameter are found for Ni0.50 Zn0.50 Fe2 O4 sintered at 1350 ◦ C. The observed variation of lattice parameters and bulk densities of the samples investigated are explained with the help of ionic radii and atomic masses of the substituted cations. The real part of initial permeability, i , and the saturation magnetization, Ms , are found to decrease with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 , whereas the Neel temperature, TN , increases. The highest i , the value of which is 432, is obtained for Ni0.50 Zn0.50 Fe2 O4 (x = 0, y = 0) composition sintered at 1350 ◦ C with corresponding resonance frequency, fr of 2 MHz. On the other hand, the highest fr (45 MHz) is obtained for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 sintered at 1250 ◦ C with corresponding i = 88. The relative quality factor, Q, decreases with increasing Mg and Cr contents. The highest Q values for Ni0.50 Zn0.50 Fe2 O4 and Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 are observed for the samples sintered at 1250 ◦ C. On the other hand, the highest Q value for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 is obtained for the sample sintered at 1350 ◦ C. The observed i values and Ms are related to the chemical composition, morphological nature of the grain boundaries and average grain diameters. It is also observed that increase of i is accompanied by the decrease of fr , which confirms the Snoek relation for polycrystalline ferrites. © 2008 Elsevier B.V. All rights reserved.
1. Introduction There is an intense demand for high performance and miniaturization of many electronic devices, which exclusively needs soft magnetic materials with high permeability. Most modern soft ferrites have a spinel structure. It has tetrahedral A-sites and octahedral B-sites in AB2 O4 crystal structure. It shows various magnetic properties depending on their compositions and cation distribution. Various cations can be placed in A site and B site to tune its magnetic properties. Depending on A site and B site cations it can exhibit ferrimagnetic, antiferromagnetic, spin (cluster) glass, and paramagnetic behaviour [1]. The general composition of such fer-
∗ Corresponding author. Tel.: +88 2 966 5613; fax: +88 2 861 3046. E-mail addresses: [email protected], [email protected] (A.K.M.A. Hossain). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.045
rites is MeFe2 O4 , where Me represents one or several of the divalent metals. These types of ferrites have been extensively used in many electronic devices because of its high permeability in the radio frequency region, high electrical resistivity, mechanical hardness, and chemical stability [2–4]. Among them Ni–Zn ferrites are most popular and versatile which was investigated by many researchers [2,5–8]. It is found that structural and magnetic properties of Ni–Zn ferrites are effectively changed by nonmagnetic Mg2+ [9–12]. In this paper, the effect of structural and magnetic properties due to Mg2+ and Cr3+ substitution in Ni0.50 Zn0.50 Fe2 O4 are discussed thoroughly. 2. Experimental The Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 (for x = 0.0, 0.1, 0.2 and y = 0.0, 0.1, 0.1) compositions were prepared by the standard solid-state reaction technique. Powder of NiO (99.9%), ZnO (99.9%), MgO (99.9%), Cr2 O3 (99.9%) and Fe2 O3 (99.9%) were used as raw materials. Appropriate amounts of required powders were mixed thoroughly and then calcined at 950 ◦ C for 5 h. The calcined powders were then pressed into
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173
disk-shaped and toroid-shaped samples. The samples were sintered at various temperatures 1250, 1300 and 1350 ◦ C in air for 5 h. The temperature ramp for sintering was 5 ◦ C min−1 for heating, and 10 ◦ C min−1 for cooling. The samples were polished and then thermal etching was carried out on them. Morphological properties were studied by a high-resolution optical microscope (Olympus DP 70) on polished pellets. Structural characterization was carried out with an X-ray diffractometer. The lattice parameter for each composition was calculated using Nelson Riley function [13]. The theoretical density, th , was calculated using the relation, th = 8 MA /NA a3 , where NA is the Avogadro’s number, MA is the molecular weight of the composition, and ‘a’ is the lattice constant. The porosity, P, was calculated using the formula P (%) = th − B /th
× 100, where B is the bulk density. Average grain sizes of the
samples were determined from the optical micrographs by linear-intercept technique [14]. The initial permeability spectra of the samples were measured using an Impedance Analyzer. The complex permeability investigations on toroid-shaped samples were carried out at room temperature in the frequency range 1 kHz to 100 MHz. The real (i ) and imaginary part (i ) of the complex permeability were calculated using the following relations: i = Ls /Lo and i = i tan ı, where Ls is the self-inductance of the sample core and Lo = o N 2 S/d¯ is derived from geometrical relations. Here Lo is the inductance of the winding coil without the sample core, N is the number of turns of the coil (N = 5), S is the area of cross-section and d¯ is the mean diameter of the sample. The temperature-dependent permeability was measured at a fixed frequency (100 kHz). The Neel temperature, TN , was calculated from the temperature-dependent permeability measurement. The DC magnetization measurements were made on pieces of the samples (approximate dimensions 2 mm × 1 mm × 1 mm) using the SQUID magnetometer (MPMS-5S; Quantum design Co. Ltd.).
Fig. 1. The X-ray diffraction patterns for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 .
3. Results and discussion 3.1. Lattice parameters, density and porosity of the Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 The X-ray diffraction (XRD) patterns for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 compositions are shown in Fig. 1. The XRD patterns clearly show their single phase and formation of spinel structure. Analyzing the XRD patterns, it is noticed that the positions of the peaks comply with the reported values for the spinel structure [6]. The lattice parameter, density, porosity and average grain size for different samples sintered at different temperatures are presented in Table 1. It is noticed that lattice parameter decreases with increasing Mg and Cr substitution in the polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 as shown in Fig. 2. The decrease in the lattice parameter with increasing Mg and Cr substitution can be explained in terms of ionic radii. The radius of the Zn2+ (0.88 Å) is greater than both of the Mg2+ (0.86 Å) and Cr3+ (0.755 Å) [15]. There is no noticeable variation of lattice parameters with sintering temperatures for a particular composition. Fig. 3 shows bulk density of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 decreases with increase of Mg and Cr substitution. This is expected as atomic weight of Zn (65.39 amu) is greater than that of both Mg (24.31 amu) and Cr (52.0 amu) [15]. Bulk density increases as the sintering temperature increases. On the other hand, porosity decreases with increasing sintering temperatures. Porosity values of various samples are presented in Table 1. It is known that
Fig. 2. The variation of ‘a’ with ‘x’ and ‘y’ for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 .
the porosity of the ceramic samples results from two sources, intragranular porosity and intergranular porosity. Thus, the total porosity could be written as P = Pintra + Pinter . The intergranular porosity mostly depends on the grain size [2]. At higher sintering temperatures grains are formed uniformly and voids are reduced, as a result density increases.
Table 1 The lattice parameter, density, porosity, average grain size, natural resonance frequency, Neel temperature, i at resonance frequency, and i at 100 kHz for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at different temperatures with fixed dwell time 5 h x
y
0
0
0.1
0.1
0.2
0.1
Grain size (m)
i (at fr )
i (at 100 kHz)
4 2.5 2.0
8.2 10.6 12.5
415 415 423
405 417 432
333
20 10 6.5
4.3 5.9 8.2
160 199 210
137 181 206
365
45 20 15
3.5 4.1 5.8
88 111 137
77 100 129
a (Å)
Ts (◦ C)
th (g cm−3 )
B (g cm−3 )
8.4130
1250 1300 1350
4.93 4.95 4.98
7.1 6.7 6.2
241
5.31
8.3804
1250 1300 1350
5.24
4.54 4.80 4.97
13.4 8.4 5.3
8.3781
1250 1300 1350
5.16
4.51 4.73 4.91
12.7 8.3 4.8
P (%)
TN (◦ C)
fr (MHz)
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ing temperature. On the other hand, elevated sintering temperature enhances grain growth. 3.3. Complex permeability of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4
Fig. 3. The variation of bulk density with x and y for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 .
3.2. Microstructures of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 The optical micrographs of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C are shown in Fig. 4. It is revealed that chemical compositions and sintering temperature have the great influence on grain size. The average grain size (Table 1) decreases with increasing Mg2+ and Cr3+ content in the samples for a fixed sinter-
Fig. 5 shows the real and imaginary permeability spectra for various polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1350 ◦ C. It is found that initial permeability decreases with Mg and Cr substitution in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . However, the natural resonance frequency, fr , increases as result of Mg and Cr substitution. The i ’s are found to be independent of frequency up to resonance frequency. It is also noticed that there are a sharp decrease in i and increase in i above the resonance frequency. The resonance frequencies of all samples are listed in Table 1. It is observed that the fr and i are inversely proportional, which really confirm with Snoek’s relation i fr = constant [16]. Fig. 6 shows the real and imaginary permeability spectra for various polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C. We observed that there is a variation of i values with sintering temperatures for all compositions. The i value increases with increasing sintering temperature. Although i increases, the corresponding fr value decreases with sintering temperature. As the sintering temperature increases from 1250 to 1350 ◦ C, the fr shifted from 4 to 2 MHz for Ni0.50 Zn0.50 Fe2 O4 , from 20 to 6.5 MHz for Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 , and from 45 to 15 MHz for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 . For all chemical compositions, higher i values are observed for samples sintered at 1350 ◦ C. The values of i at fr and at 100 kHz for all compositions are tabulated in
Fig. 4. The micrographs of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C. (a) x = 0.0, y = 0.0; (b) x = 0.1, y = 0.1; (c) x = 0.2, y = 0.1.
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Fig. 5. (a) The real and (b) imaginary permeability spectra for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1350 ◦ C in air.
Fig. 6. (a) The real and (b) imaginary permeability spectra for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 samples sintered at 1250 ◦ C in air.
Table 1. The highest i is observed for Ni0.50 Zn0.50 Fe2 O4 composition sintered at 1350 ◦ C. On the other hand, the highest fr (45 MHz) is observed for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 composition sintered at 1250 ◦ C with corresponding i = 88. The variation of i with frequency for Ni0.50 Zn0.50 Fe2 O4 sintered at 1250, 1300 and 1350 ◦ C are shown in Fig. 7. It is obvious from this figure that when i is higher, fr is lower. Fig. 8 shows the variation of i and B with sintering temperature, Ts , for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . It is observed that both i and B increases with sintering temperature. This is because pores and voids are reduced with increasing sintering tem-
perature. The permeability of polycrystalline ferrite is related to two different magnetizing mechanisms: spin rotation and domain wall motion [17–20], which can be described as, i = 1 + w + spin , where w is the domain wall susceptibility, spin is intrinsic rotational susceptibility. The w and spin may be written as w = (3Ms2 D/4) and spin = (2Ms2 /Ku ) with Ms the saturation magnetization, Ku the total anisotropy, D the average grain diameter, and the domain wall energy. Since i is a function of grain size and magnetization, therefore, i decreases with increase of Mg2+ and Cr3+ content in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O for a fixed sintering temperature. Our microstructure study shows that the average grain
Fig. 7. (a) The real and (b) imaginary initial permeability spectra for Ni0.50 Zn0.50 Fe2 O4 sintered at 1250, 1300 and 1350 ◦ C in air.
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Fig. 8. The variation of i and with Ts for (a) Ni0.50 Zn0.50 Fe2 O4 , (b) Ni0.50 Zn0.30 Mg0.1 Cr0.1 Fe2 O4 and (c) Ni0.50 Zn0.20 Mg0.2 Cr0.1 Fe2 O4 .
diameter increases with increasing sintering temperature (Fig. 4). Therefore, an increase of i with increasing sintering temperature is expected. The variation of relative quality factors, Q, (Q = (i /loss factor)) for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at various temperatures are shown in Fig. 9. It is noticed that Q factor decreases with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . It is also noticed that highest Q values for Ni0.50 Zn0.50 Fe2 O4
and Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 are observed for the sample sintered at 1250 ◦ C. On the other hand, highest Q value for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 is obtained for the sample sintered at 1350 ◦ C. This is probably due to the growth of less imperfections and defects in the samples sintered at lower sintering temperature (e.g. 1250 ◦ C). For some composition, there is an abnormal grain growth at higher sintering temperature with trapped pores inside the grain, which causes relatively higher loss.
Fig. 9. The variation of quality factor with frequency for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at: (a) 1250 ◦ C, (b) 1300 ◦ C and (c) 1350 ◦ C.
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Fig. 10. The magnetization as a function of applied magnetic field, M(H) curves for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C for 5 h in air. Fig. 11. The variation of i as a function Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C.
3.4. DC magnetization of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 The magnetization as a function of applied magnetic field, M(H), for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 compositions at room temperature (300 K) is shown in Fig. 10. The magnetization of all samples increases linearly with increasing the applied magnetic field up to 0.1 T. Beyond 0.1 T applied field, magnetization increases slowly and then saturation occurs. Therefore, it is clear that at room temperature all samples are in ferrimagnetic state. The saturation magnetizing field, 0 H, saturation magnetization, Ms , number of Bohr magnetons calculated from Ms are tabulated in Table 2. The number of Bohr magneton, n, is calculated using the following formula n = (MA × Ms /NA × 9.27 × 10−21 ). The M(H) value shows that Ms decreases with increasing Mg2+ and Cr3+ contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . It is well known that Zn2+ prefer A site, and Ni2+ prefer B site in the spinel structure. The net magnetization of spinel ferrite is given by M(T) = MB (T) − MA (T). When Zn2+ is substituted by nonmagnetic Mg2+ and magnetic Cr3+ , the magnetic moment of A site increases as a result the net magnetic moment decreases. 3.5. Temperature-dependent permeability and Neel temperature The i as a function of temperatures for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 compositions are shown in Fig. 11. The i was measured at a constant frequency (100 kHz) of a sinusoidal wave. The i value falls sharply, when the magnetic state changes from ferrimagnetic to paramagnetic. The sharp change of i with temperature shows the degree of homogeneity of measured samples. The Neel temperature, TN, is determined from the minima of d i /dT , and the values of TN are tabulated in Table 1. It is found that TN increases with increase of Mg2+ and Cr3+ contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . The increase of TN with Mg2+ and Cr3+ substitution is probably due to the strengthening of the A–B interaction. This could be attributed to the decrease in distance between the A and B sites, which is confirmed by the
of
temperature
for
decrease in the lattice parameter with increasing Mg2+ and Cr3+ contents (Fig. 2). 4. Conclusions The lattice parameter of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 decreases with increasing Mg and Cr contents. There is a decrease of bulk density and average grain diameter with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 for a fixed sintering temperature. However, there is an increase of these values with increasing sintering temperature for a particular composition. The maximum bulk density and the largest average grain diameter are found for Ni0.50 Zn0.50 Fe2 O4 sintered at 1350 ◦ C. There is a decrease of i and Ms values with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . These results are explained with the help of average grain diameter and cations distribution in tetrahedral and octahedral sites, respectively. The increase of TN , can be explained with the help of lattice compression as a result of Mg and Cr substitution. The maximum i is found for Ni0.50 Zn0.50 Fe2 O4 sintered at 1350 ◦ C with corresponding fr = 2 MHz. On the other hand, highest fr (45 MHz) is obtained for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 sintered at 1250 ◦ C with corresponding i = 88. The Q value decreases with increasing Mg and Cr contents. The maximum Q values for Ni0.50 Zn0.50 Fe2 O4 and Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 are observed for the sample sintered at 1250 ◦ C. On the other hand, highest Q value for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 is obtained for the sample sintered at 1350 ◦ C. These results are helpful for practical applications of Mg and Cr substituted Ni–Zn ferrites. Acknowledgment The present study was supported by CASR, Bangladesh University of Engineering & Technology (BUET). References
Table 2 The saturation magnetizing field, saturation magnetization, and number of Bohr magneton of the Ni0.5 Zn0.5−x−y Mgx Cry Fe2 O4 samples sintered at 1250 ◦ C x
y
The field at which saturation occurs, o Hs (T)
Ms (emu g−1 )
0.00 0.10 0.20
0.00 0.10 0.10
0.4 0.3 0.15
83 62 59
Number of B (n)
3.54 2.58 2.41
[1] R. Peelamedu, C. Grimes, D. Agrawal, R. Roy, J. Mater. Res. 18 (2003) 2292. [2] A.K.M. Akther Hossain, M. Seki, T. Kawai, H. Tabata, J. Appl. Phys. 96 (2) (2004) 1273. [3] A. Goldman, Handbook of Modern Ferromagnetic Materials, Kulwer Academic Publishers, Boston, USA, 1999. [4] R. Valenzuela, Magnetic Ceramics, Cambridge University Press, Cambridge, 1994. [5] C. Upadhyay, H.C. Verma, S. Annand, J. Appl. Phys. 95 (10) (2004) 5746. [6] S.T. Mahmud, A.K.M. Akther Hossain, A.K.M. Abdul Hakim, M. Seki, T. Kawai, H. Tabatta, J. Magn. Magn. Mater. 305 (2006) 269. [7] O.F. Caltun, L. Spinu, A. Sraneu, IEEE Trans. Magn. 37 (4) (2001) 2353.
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[8] T. Tsutaoka, J. Appl. Phys. 93 (5) (2003) 2789. [9] S.J. Shukla, K.M. Jadhav, G.K. Bichile, J. Magn. Magn. Mater. 195 (1999) 692. [10] E. Rezlescu, L. Sachelarie, P.D. Popa, N. Rezlescu, IEEE Trans. Magn. 36 (6) (2000) 3962. [11] M. Singh, J. Magn. Magn. Mater. 299 (2006) 397. [12] M.A. Amer, M. El Hiti, J. Magn. Magn. Mater. 234 (2001) 118. [13] A.K.M. Akther Hossain, S.T. Mahmud, M. Seki, T. Kawai, H. Tabata, J. Magn. Magn. Mater. 312 (2007) 210. [14] M.I. Mendelson, J. Am. Ceram. Soc. 52 (8) (1967) 443.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Influence of Mg and Cr substitution on structural and magnetic properties of polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 A.K.M. Akther Hossain a,∗ , T.S. Biswas a , S.T. Mahmud a , Takeshi Yanagida b , Hidekazu Tanaka b , Tomoji Kawai b a b
Department of Physics, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 11 June 2008 Accepted 8 July 2008 Keywords: Magnetic materials Annealing Magnetometer Microstructure Magnetic properties
a b s t r a c t Structural, surface morphological and magnetic properties of polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 (for x = 0.0, 0.1, 0.2 and y = 0.0, 0.1, 0.1) were thoroughly investigated. The lattice parameters of the samples investigated are found to decrease with increasing Mg and Cr contents. The bulk density and the average grain diameter are found to decrease with increasing Mg and Cr substitution in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 for any particular sintering temperature. However, they are found to increase with increasing sintering temperatures for any individual composition. In the present investigation, the maximum bulk density and the largest average grain diameter are found for Ni0.50 Zn0.50 Fe2 O4 sintered at 1350 ◦ C. The observed variation of lattice parameters and bulk densities of the samples investigated are explained with the help of ionic radii and atomic masses of the substituted cations. The real part of initial permeability, i , and the saturation magnetization, Ms , are found to decrease with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 , whereas the Neel temperature, TN , increases. The highest i , the value of which is 432, is obtained for Ni0.50 Zn0.50 Fe2 O4 (x = 0, y = 0) composition sintered at 1350 ◦ C with corresponding resonance frequency, fr of 2 MHz. On the other hand, the highest fr (45 MHz) is obtained for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 sintered at 1250 ◦ C with corresponding i = 88. The relative quality factor, Q, decreases with increasing Mg and Cr contents. The highest Q values for Ni0.50 Zn0.50 Fe2 O4 and Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 are observed for the samples sintered at 1250 ◦ C. On the other hand, the highest Q value for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 is obtained for the sample sintered at 1350 ◦ C. The observed i values and Ms are related to the chemical composition, morphological nature of the grain boundaries and average grain diameters. It is also observed that increase of i is accompanied by the decrease of fr , which confirms the Snoek relation for polycrystalline ferrites. © 2008 Elsevier B.V. All rights reserved.
1. Introduction There is an intense demand for high performance and miniaturization of many electronic devices, which exclusively needs soft magnetic materials with high permeability. Most modern soft ferrites have a spinel structure. It has tetrahedral A-sites and octahedral B-sites in AB2 O4 crystal structure. It shows various magnetic properties depending on their compositions and cation distribution. Various cations can be placed in A site and B site to tune its magnetic properties. Depending on A site and B site cations it can exhibit ferrimagnetic, antiferromagnetic, spin (cluster) glass, and paramagnetic behaviour [1]. The general composition of such fer-
∗ Corresponding author. Tel.: +88 2 966 5613; fax: +88 2 861 3046. E-mail addresses: [email protected], [email protected] (A.K.M.A. Hossain). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.045
rites is MeFe2 O4 , where Me represents one or several of the divalent metals. These types of ferrites have been extensively used in many electronic devices because of its high permeability in the radio frequency region, high electrical resistivity, mechanical hardness, and chemical stability [2–4]. Among them Ni–Zn ferrites are most popular and versatile which was investigated by many researchers [2,5–8]. It is found that structural and magnetic properties of Ni–Zn ferrites are effectively changed by nonmagnetic Mg2+ [9–12]. In this paper, the effect of structural and magnetic properties due to Mg2+ and Cr3+ substitution in Ni0.50 Zn0.50 Fe2 O4 are discussed thoroughly. 2. Experimental The Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 (for x = 0.0, 0.1, 0.2 and y = 0.0, 0.1, 0.1) compositions were prepared by the standard solid-state reaction technique. Powder of NiO (99.9%), ZnO (99.9%), MgO (99.9%), Cr2 O3 (99.9%) and Fe2 O3 (99.9%) were used as raw materials. Appropriate amounts of required powders were mixed thoroughly and then calcined at 950 ◦ C for 5 h. The calcined powders were then pressed into
A.K.M.A. Hossain et al. / Materials Chemistry and Physics 113 (2009) 172–178
173
disk-shaped and toroid-shaped samples. The samples were sintered at various temperatures 1250, 1300 and 1350 ◦ C in air for 5 h. The temperature ramp for sintering was 5 ◦ C min−1 for heating, and 10 ◦ C min−1 for cooling. The samples were polished and then thermal etching was carried out on them. Morphological properties were studied by a high-resolution optical microscope (Olympus DP 70) on polished pellets. Structural characterization was carried out with an X-ray diffractometer. The lattice parameter for each composition was calculated using Nelson Riley function [13]. The theoretical density, th , was calculated using the relation, th = 8 MA /NA a3 , where NA is the Avogadro’s number, MA is the molecular weight of the composition, and ‘a’ is the lattice constant. The porosity, P, was calculated using the formula P (%) = th − B /th
× 100, where B is the bulk density. Average grain sizes of the
samples were determined from the optical micrographs by linear-intercept technique [14]. The initial permeability spectra of the samples were measured using an Impedance Analyzer. The complex permeability investigations on toroid-shaped samples were carried out at room temperature in the frequency range 1 kHz to 100 MHz. The real (i ) and imaginary part (i ) of the complex permeability were calculated using the following relations: i = Ls /Lo and i = i tan ı, where Ls is the self-inductance of the sample core and Lo = o N 2 S/d¯ is derived from geometrical relations. Here Lo is the inductance of the winding coil without the sample core, N is the number of turns of the coil (N = 5), S is the area of cross-section and d¯ is the mean diameter of the sample. The temperature-dependent permeability was measured at a fixed frequency (100 kHz). The Neel temperature, TN , was calculated from the temperature-dependent permeability measurement. The DC magnetization measurements were made on pieces of the samples (approximate dimensions 2 mm × 1 mm × 1 mm) using the SQUID magnetometer (MPMS-5S; Quantum design Co. Ltd.).
Fig. 1. The X-ray diffraction patterns for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 .
3. Results and discussion 3.1. Lattice parameters, density and porosity of the Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 The X-ray diffraction (XRD) patterns for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 compositions are shown in Fig. 1. The XRD patterns clearly show their single phase and formation of spinel structure. Analyzing the XRD patterns, it is noticed that the positions of the peaks comply with the reported values for the spinel structure [6]. The lattice parameter, density, porosity and average grain size for different samples sintered at different temperatures are presented in Table 1. It is noticed that lattice parameter decreases with increasing Mg and Cr substitution in the polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 as shown in Fig. 2. The decrease in the lattice parameter with increasing Mg and Cr substitution can be explained in terms of ionic radii. The radius of the Zn2+ (0.88 Å) is greater than both of the Mg2+ (0.86 Å) and Cr3+ (0.755 Å) [15]. There is no noticeable variation of lattice parameters with sintering temperatures for a particular composition. Fig. 3 shows bulk density of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 decreases with increase of Mg and Cr substitution. This is expected as atomic weight of Zn (65.39 amu) is greater than that of both Mg (24.31 amu) and Cr (52.0 amu) [15]. Bulk density increases as the sintering temperature increases. On the other hand, porosity decreases with increasing sintering temperatures. Porosity values of various samples are presented in Table 1. It is known that
Fig. 2. The variation of ‘a’ with ‘x’ and ‘y’ for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 .
the porosity of the ceramic samples results from two sources, intragranular porosity and intergranular porosity. Thus, the total porosity could be written as P = Pintra + Pinter . The intergranular porosity mostly depends on the grain size [2]. At higher sintering temperatures grains are formed uniformly and voids are reduced, as a result density increases.
Table 1 The lattice parameter, density, porosity, average grain size, natural resonance frequency, Neel temperature, i at resonance frequency, and i at 100 kHz for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at different temperatures with fixed dwell time 5 h x
y
0
0
0.1
0.1
0.2
0.1
Grain size (m)
i (at fr )
i (at 100 kHz)
4 2.5 2.0
8.2 10.6 12.5
415 415 423
405 417 432
333
20 10 6.5
4.3 5.9 8.2
160 199 210
137 181 206
365
45 20 15
3.5 4.1 5.8
88 111 137
77 100 129
a (Å)
Ts (◦ C)
th (g cm−3 )
B (g cm−3 )
8.4130
1250 1300 1350
4.93 4.95 4.98
7.1 6.7 6.2
241
5.31
8.3804
1250 1300 1350
5.24
4.54 4.80 4.97
13.4 8.4 5.3
8.3781
1250 1300 1350
5.16
4.51 4.73 4.91
12.7 8.3 4.8
P (%)
TN (◦ C)
fr (MHz)
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ing temperature. On the other hand, elevated sintering temperature enhances grain growth. 3.3. Complex permeability of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4
Fig. 3. The variation of bulk density with x and y for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 .
3.2. Microstructures of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 The optical micrographs of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C are shown in Fig. 4. It is revealed that chemical compositions and sintering temperature have the great influence on grain size. The average grain size (Table 1) decreases with increasing Mg2+ and Cr3+ content in the samples for a fixed sinter-
Fig. 5 shows the real and imaginary permeability spectra for various polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1350 ◦ C. It is found that initial permeability decreases with Mg and Cr substitution in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . However, the natural resonance frequency, fr , increases as result of Mg and Cr substitution. The i ’s are found to be independent of frequency up to resonance frequency. It is also noticed that there are a sharp decrease in i and increase in i above the resonance frequency. The resonance frequencies of all samples are listed in Table 1. It is observed that the fr and i are inversely proportional, which really confirm with Snoek’s relation i fr = constant [16]. Fig. 6 shows the real and imaginary permeability spectra for various polycrystalline Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C. We observed that there is a variation of i values with sintering temperatures for all compositions. The i value increases with increasing sintering temperature. Although i increases, the corresponding fr value decreases with sintering temperature. As the sintering temperature increases from 1250 to 1350 ◦ C, the fr shifted from 4 to 2 MHz for Ni0.50 Zn0.50 Fe2 O4 , from 20 to 6.5 MHz for Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 , and from 45 to 15 MHz for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 . For all chemical compositions, higher i values are observed for samples sintered at 1350 ◦ C. The values of i at fr and at 100 kHz for all compositions are tabulated in
Fig. 4. The micrographs of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C. (a) x = 0.0, y = 0.0; (b) x = 0.1, y = 0.1; (c) x = 0.2, y = 0.1.
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175
Fig. 5. (a) The real and (b) imaginary permeability spectra for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1350 ◦ C in air.
Fig. 6. (a) The real and (b) imaginary permeability spectra for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 samples sintered at 1250 ◦ C in air.
Table 1. The highest i is observed for Ni0.50 Zn0.50 Fe2 O4 composition sintered at 1350 ◦ C. On the other hand, the highest fr (45 MHz) is observed for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 composition sintered at 1250 ◦ C with corresponding i = 88. The variation of i with frequency for Ni0.50 Zn0.50 Fe2 O4 sintered at 1250, 1300 and 1350 ◦ C are shown in Fig. 7. It is obvious from this figure that when i is higher, fr is lower. Fig. 8 shows the variation of i and B with sintering temperature, Ts , for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . It is observed that both i and B increases with sintering temperature. This is because pores and voids are reduced with increasing sintering tem-
perature. The permeability of polycrystalline ferrite is related to two different magnetizing mechanisms: spin rotation and domain wall motion [17–20], which can be described as, i = 1 + w + spin , where w is the domain wall susceptibility, spin is intrinsic rotational susceptibility. The w and spin may be written as w = (3Ms2 D/4) and spin = (2Ms2 /Ku ) with Ms the saturation magnetization, Ku the total anisotropy, D the average grain diameter, and the domain wall energy. Since i is a function of grain size and magnetization, therefore, i decreases with increase of Mg2+ and Cr3+ content in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O for a fixed sintering temperature. Our microstructure study shows that the average grain
Fig. 7. (a) The real and (b) imaginary initial permeability spectra for Ni0.50 Zn0.50 Fe2 O4 sintered at 1250, 1300 and 1350 ◦ C in air.
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Fig. 8. The variation of i and with Ts for (a) Ni0.50 Zn0.50 Fe2 O4 , (b) Ni0.50 Zn0.30 Mg0.1 Cr0.1 Fe2 O4 and (c) Ni0.50 Zn0.20 Mg0.2 Cr0.1 Fe2 O4 .
diameter increases with increasing sintering temperature (Fig. 4). Therefore, an increase of i with increasing sintering temperature is expected. The variation of relative quality factors, Q, (Q = (i /loss factor)) for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at various temperatures are shown in Fig. 9. It is noticed that Q factor decreases with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . It is also noticed that highest Q values for Ni0.50 Zn0.50 Fe2 O4
and Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 are observed for the sample sintered at 1250 ◦ C. On the other hand, highest Q value for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 is obtained for the sample sintered at 1350 ◦ C. This is probably due to the growth of less imperfections and defects in the samples sintered at lower sintering temperature (e.g. 1250 ◦ C). For some composition, there is an abnormal grain growth at higher sintering temperature with trapped pores inside the grain, which causes relatively higher loss.
Fig. 9. The variation of quality factor with frequency for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at: (a) 1250 ◦ C, (b) 1300 ◦ C and (c) 1350 ◦ C.
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177
Fig. 10. The magnetization as a function of applied magnetic field, M(H) curves for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C for 5 h in air. Fig. 11. The variation of i as a function Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 sintered at 1250 ◦ C.
3.4. DC magnetization of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 The magnetization as a function of applied magnetic field, M(H), for various Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 compositions at room temperature (300 K) is shown in Fig. 10. The magnetization of all samples increases linearly with increasing the applied magnetic field up to 0.1 T. Beyond 0.1 T applied field, magnetization increases slowly and then saturation occurs. Therefore, it is clear that at room temperature all samples are in ferrimagnetic state. The saturation magnetizing field, 0 H, saturation magnetization, Ms , number of Bohr magnetons calculated from Ms are tabulated in Table 2. The number of Bohr magneton, n, is calculated using the following formula n = (MA × Ms /NA × 9.27 × 10−21 ). The M(H) value shows that Ms decreases with increasing Mg2+ and Cr3+ contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . It is well known that Zn2+ prefer A site, and Ni2+ prefer B site in the spinel structure. The net magnetization of spinel ferrite is given by M(T) = MB (T) − MA (T). When Zn2+ is substituted by nonmagnetic Mg2+ and magnetic Cr3+ , the magnetic moment of A site increases as a result the net magnetic moment decreases. 3.5. Temperature-dependent permeability and Neel temperature The i as a function of temperatures for Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 compositions are shown in Fig. 11. The i was measured at a constant frequency (100 kHz) of a sinusoidal wave. The i value falls sharply, when the magnetic state changes from ferrimagnetic to paramagnetic. The sharp change of i with temperature shows the degree of homogeneity of measured samples. The Neel temperature, TN, is determined from the minima of d i /dT , and the values of TN are tabulated in Table 1. It is found that TN increases with increase of Mg2+ and Cr3+ contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . The increase of TN with Mg2+ and Cr3+ substitution is probably due to the strengthening of the A–B interaction. This could be attributed to the decrease in distance between the A and B sites, which is confirmed by the
of
temperature
for
decrease in the lattice parameter with increasing Mg2+ and Cr3+ contents (Fig. 2). 4. Conclusions The lattice parameter of Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 decreases with increasing Mg and Cr contents. There is a decrease of bulk density and average grain diameter with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 for a fixed sintering temperature. However, there is an increase of these values with increasing sintering temperature for a particular composition. The maximum bulk density and the largest average grain diameter are found for Ni0.50 Zn0.50 Fe2 O4 sintered at 1350 ◦ C. There is a decrease of i and Ms values with increasing Mg and Cr contents in Ni0.50 Zn0.50−x−y Mgx Cry Fe2 O4 . These results are explained with the help of average grain diameter and cations distribution in tetrahedral and octahedral sites, respectively. The increase of TN , can be explained with the help of lattice compression as a result of Mg and Cr substitution. The maximum i is found for Ni0.50 Zn0.50 Fe2 O4 sintered at 1350 ◦ C with corresponding fr = 2 MHz. On the other hand, highest fr (45 MHz) is obtained for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 sintered at 1250 ◦ C with corresponding i = 88. The Q value decreases with increasing Mg and Cr contents. The maximum Q values for Ni0.50 Zn0.50 Fe2 O4 and Ni0.50 Zn0.3 Mg0.1 Cr0.1 Fe2 O4 are observed for the sample sintered at 1250 ◦ C. On the other hand, highest Q value for Ni0.50 Zn0.2 Mg0.2 Cr0.1 Fe2 O4 is obtained for the sample sintered at 1350 ◦ C. These results are helpful for practical applications of Mg and Cr substituted Ni–Zn ferrites. Acknowledgment The present study was supported by CASR, Bangladesh University of Engineering & Technology (BUET). References
Table 2 The saturation magnetizing field, saturation magnetization, and number of Bohr magneton of the Ni0.5 Zn0.5−x−y Mgx Cry Fe2 O4 samples sintered at 1250 ◦ C x
y
The field at which saturation occurs, o Hs (T)
Ms (emu g−1 )
0.00 0.10 0.20
0.00 0.10 0.10
0.4 0.3 0.15
83 62 59
Number of B (n)
3.54 2.58 2.41
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