Effect of Cation Proportion on the Structural and Magnetic Properties of Ni-Zn Ferrites Nano-Size Particles Prepared By Co-Precipitation Technique

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1 CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 21, NUMBER 4 AUGUST 27, 2008 ARTICLE Effect of Cation Proportion on the Structural and Magnetic Properties of Ni-Zn Ferrites Nano-Size Particles Prepared By Co-Precipitation Technique Santosh S. Jadhav a, Sagar E. Shirsath c, B. G. Toksha c, S. J. Shukla b, K. M. Jadhav c a. DSM College of Arts Commerce and Science, Jintur, Dist. Parbhani , India b. Postgraduate and Research Centre of Physics, Department of Physics, Deogiri College, Aurangabad, India c. Department of Physics, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad , India (Dated: Received on February 7, 2008; Accepted on June 12, 2008) Ferrites having general formula Ni 1 x Zn xfe 2 O 4 with x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 were prepared by wet chemical co-precipitation method. The structural and magnetic properties were studied by means of X-ray diffraction, magnetization, and AC susceptibility measurements. The X-ray analysis confirmed the single-phase formation of the samples. The lattice parameter obtained from XRD data was found to increase with Zn content x. The cation distribution was studied by X-ray intensity ratio calculations. Magnetization results exhibit collinear ferrimagnetic structure for x 0.4, and which changes to non-collinear for x>0.4. Curie temperature T C obtained from AC susceptibility data decreases with increasing x. Key words: Ferrites, Magnetization, Yafet-Kittel angle, Curie temperature I. INTRODUCTION II. EXPERIMENTS For the last few years, the soft ferrites having fine particle size have been extensively studied due to their wide range of technological applications. The structural as well as magnetic properties of the ferrites have been found to depend upon the particle size, which depends entirely on the method of synthesis [1]. The chemical methods of synthesis of ferrites require low temperature and consequently produce powders with fine particle size having high homogeneity. The wet chemical co-precipitation [2], sol-gel [3], hydrothermal synthesis [4], citrate precursor [5], and combustion synthesis [6] are the few chemical methods of synthesis which can yield the ferrite powder with fine particle size of few nanometers. Among the soft ferrites, Ni-Zn ferrites are of commercial importance due to their high frequency applications in different devices such as radio frequency coils, transformer cores, high quality filters, rod antennas, and read-write heads for high speed digital tape and operating devices [7]. Such a large number of potential applications of Ni-Zn ferrite is possible due to their high resistivity, low dielectric loss, high Curie temperature, high permeability, etc. [8]. Owing to impact of fine particles on properties of Ni-Zn ferrites and considering its possible consequent effects on the technological applications, it was decided to study the structural and magnetic properties of Ni-Zn ferrites prepared by wet chemical co-precipitation technique. Author to whom correspondence should be addressed. drkmjadhav@yahoo.com The samples of Zn substituted nickel ferrite (Ni 1 x Zn x Fe 2 O 4 with x= ) were prepared by air oxidation of an aqueous suspension containing N 2+, Zn 2+, and Fe 3+ cations in proper proportions. The starting solutions were prepared by mixing appropriate amounts of FeSO 4 7H 2 O, NiSO 4 7H 2 O, and ZnSO 4 7H 2 O (all 99.9% pure supplied by s.d. fine, India). A two molar NaOH solution is used as precipitating agent and H 2 O 2 was used as an oxidant, which helps to convert Fe 2+ to Fe 3+ by the following reaction [9], Fe 2+ + H 2 O 2 Fe 3+ +OH +OH. The details of method of preparation were reported in our previous work [10]. The samples were filtered, and washed several times by distilled water. The wet samples of Ni-Zn system were annealed at 800 C for 12 h. The X-ray powder diffraction patterns were recorded on a Philips X-ray diffractometer (Model Joel-DX-8030) at room temperature in a 2θ scanning range from 20 to 80. The magnetic data for these samples were obtained with the help of high field hysteresis loop technique [11]. The low field AC susceptibility measurements on powdered samples were carried out in the temperature range of K using a double coil setup [12] operating at a frequency of 263 Hz and in an r.m.s. field of 7 Oe. III. RESULTS AND DISCUSSION Figure 1 depicts the X-ray diffraction patterns of the typical samples for x=0.0 and 0.7 of the series Ni 1 x Zn x Fe 2 O 4. All the peaks of the XRD patterns were indexed using Bragg s law. The analysis of X-ray 381

2 382 Chin. J. Chem. Phys., Vol. 21, No. 4 Santosh S. Jadhav et al. TABLE I Lattice constant (a), X-ray density (d x), and particle size (t) of Ni 1 xzn xfe 2O 4. x a/å d x/(gm/cm 3 ) t/nm Ceramic particle size/µm Wet chemical Standard ceramic [13-15] XRD SEM [27,28] FIG. 1 XRD patterns for the samples x=0.0 and x=0.7 of the series Ni 1 xzn xfe 2O 4. diffraction identified that all the samples have singlephase cubic spinel structure. The values of lattice constant were obtained from XRD data with an accuracy of ±0.002 Å and are given in Table I. The variation of lattice constant a with Zn content x is shown in Fig.2. The linear increase in lattice constant with Zn content obeys Vegard s law [13] and the increase in lattice constant is due to the replacement of smaller radius ions Ni 2+ (0.74 Å) by larger radius ions Zn 2+ (0.84 Å). Few researchers have reported similar behavior for Ni-Zn ferrite prepared by the standard ceramic method [14-16]. A comparison of the values of lattice constant obtained by the standard ceramic method (Table I) with those obtained in the present investigation shows considerable change. The values of X-ray density of ferrite samples were calculated from the molecular weight and the volume of the unit cell are given in Table I. It is observed from X-ray density values that X-ray density decreases with an increase in Zn content, which may be due to the increase in lattice constant. The particle size of all the samples were obtained from broadening of XRD peaks by using the Scherrer equation [13], and are given in Table I. It is evident from FIG. 2 Variation of lattice constant a with Zn content x of the series Ni 1 xzn xfe 2O 4. Table I that the particle size is of the order of nanometer dimension. The particle size was also confirmed by SEM data (Fig.3). There are very few reports available on particle size measurements of ceramically prepared Ni-Zn samples. The comparison of particle size of the samples prepared by co-precipitation technique in the present work and that of ceramically prepared samples from literature is given in Table I. The bond length R A (the shortest distance between A-site cations and oxygen ion) and R B (the shortest distance between B-site cations and oxygen ion) were calculated using the following relation [17], R A = a 3(δ 1 8 ) (1) ( R B = a 3δ 2 δ ) 1/2 (2) 16 where, a is lattice constant, δ is oxygen position parameter. The values of bond length R A and R B are given in Table II. It is evident from Table II that the bond length R A and R B increases with Zn content. The increase in bond length can be attributed to the increase in lattice constant with Zn content. According to Levine [18], there exists an inverse relationship between the covalent character of the spinel and bond lengths. In the present

3 Chin. J. Chem. Phys., Vol. 21, No. 4 2 µm Structural and Magnetic Properties of Ni-Zn Ferrites 2 (a) 2 µm 383 µm (c) (b) FIG. 3 Scanning electron micrograph for the samples x=0.2 (a), 0.4 (b), and 0.6 (c) of the series Ni1 x Znx Fe2 x O4. TABLE II Hopping length (LA ), (LB ), and bond lengths (RA ), (RB ) of Ni1 x Znx Fe2 O4. Lengths are all in A. x FIG. 4 Variation of hopping lengths in octahedral (LA ) and tetrahedral (LB ) sites with the Zn content x of the series Ni1 x Znx Fe2 O4. study, the bond lengths RA and RB increase with Zn content, therefore, there is a decrease of iono-covalent character of the spinel with Zn content. Similar results were reported in the case of Li-Cu [19], Li-Cd [20], and Cu-Zn [21]. The distance between magnetic ions, the hopping lengths in tetrahedral sites (LA ) and in octahedral sites (LB ) were also calculated using the relation reported in Ref.[22] and are given in Table II. Figure 4 shows the relation between the hopping lengths in octahedral and tetrahedral sites as a function of Zn content. The distance between the magnetic ions increases as the Zn content increases. This may be explained on the basis of difference in ionic radii of the constituent ions. In the present series Ni1 x Znx Fe2 O4, the distribution of cations over the available tetrahedral (A) and octahedral [B] site was obtained by using X-ray intensity ratio calculation. The calculated intensity ratios were compared with the observed intensity ratios to determine the cation distribution. The planes (220), (400), (422) and (440) are chosen to calculate intensity ratio, because it is observed that these planes are sensitive to cation distribution. The temperature and absorption factors are not taken into account in our calculations LA LB RA RB because they do not affect the intensity ratio calculations at room temperature [23]. Considering the site preference energy of the cations, Zn2+ ions have strong preference towards tetrahedral (A) site whereas Ni2+ and Fe3+ have no definite site preference. Taking this into account and giving various values of concentration of Ni2+, Zn2+ and Fe3+ at tetrahedral (A) site and octahedral [B] site, intensities for various planes were calculated and their ratios taken. The calculated intensity ratios were then compared with the observed intensity ratios. The results indicate that an observed and calculated intensity ratio does not match perfectly to any combination of cations. The combination of cations for which observed and calculated intensity ratios agree most closely is taken as the most correct cation distribution and is given in Table III. Table III indicates that Zn2+ occupies tetrahedral (A) site, and Ni2+ occupies octahedral [B] site, and Fe3+ occupies both tetrahedral and octahedral sites. These results of cation distribution are in agreement with the earlier reports [24]. The magnetic properties like saturation magnetization (σs ) and magneton number (nb ) were studied using the high field hysteresis loop technique [11]. The magnetization data were recorded at room temperature. The values of saturation magnetization (σs ) and magneton number (nb ) (the saturation magnetization per c 2008 Chinese Physical Society

4 384 Chin. J. Chem. Phys., Vol. 21, No. 4 Santosh S. Jadhav et al. TABLE III Cation distribution and X-ray intensity ratios of Ni 1 xzn xfe 2O 4. x A-site B-site I (400) /I (422) I (220) /I (400) Obs. Cal. Obs. Cal. 0.0 Zn 0.0Fe 1.0 Ni 1.0Fe Zn 0.1Fe 0.9 Ni 0.9Fe Zn 0.2Fe 0.8 Ni 0.8Fe Zn 0.3Fe 0.7 Ni 0.7Fe Zn 0.4Fe 0.6 Ni 0.6Fe Zn 0.5Fe 0.5 Ni 0.5Fe Zn 0.6Fe 0.4 Ni 0.4Fe Zn 0.7Fe 0.3 Ni 0.3Fe formula unit in µ B ) obtained from the hysteresis loop technique are given in Table IV. It is clear from Table IV that magneton number increases up to a certain value i.e. x=0.2 and then decreases with increase of Zn-concentration. The variation of magneton number (n B ) with Zn content is shown in Fig.5. In the present series of Ni 1 x Zn x Fe 2 O 4 magnetic Ni 2+ ions are replaced by non-magnetic Zn 2+ and magnetic Fe 3+ ions are replaced by non-magnetic Zn 2+. Thus, A-B interaction decreases in the system. However, for x 0.4 the difference of magnetic moment of A and B site ions increases and therefore the magneton number (n B ) increases. For x>0.4, n B decreases with the increasing Ni-Zn content. Chukalkin et al. reported the variation of n B with Zn content for Ni-Zn ferrites prepared by standard ceramic method [15]. There was a linear increase in n B for x<0.45, for x=0.45 there is a small deviation of n B from linear growth as predicted by Neel s theory, while for x>0.6 there is decrease in n B which reaches zero at x=0.8. Thus, in both the cases it is confirmed that the variation of magnetic moment with Zn content is not governed by a two-sublattice model. According to Neel s two-sublattice model of ferrimagnetism, Neel s magnetic moment per formula unit in µ B, n N B is expressed as n N B = M B (x) M A (x) (3) where M B and M A are the B and A sublattice magnetic moments in µ B. The variation of calculated values of n B obtained by using the cation distribution (Table IV) and Neel s Eq.(6), as a function of Zn content is shown in Fig.5. Figure 5 shows the discrepancy in the observed and calculated values of n B indicating that significant canting exists at octahedral B sites, suggesting the magnetic structure is non-collinear for x>0.4. The variation of n B with x for x>0.4 can be explained on the basis of Yafet-Kittel model [25]. A similar type of spin canting was reported by Kakatkar et al. for Ni- Zn ferrite prepared by standard ceramic method [14], where for x>0.3 the magnetic structure was found to be non-collinear. The Yafet-Kittel (Y-K) angles were FIG. 5 Variation of n B with Zn content x of the series Ni 1 xzn xfe 2O 4. (a) observed and (b) calculated. FIG. 6 Variation of AC susceptibility χ T/χ RT with the absolute temperature (T ) of the series Ni 1 xzn xfe 2O 4. calculated by using the following formula and given in Table IV. n B = M B cosα YK M A (4) The Y-K angles of Ni-Zn series prepared from standard ceramic method [13] are given in Table IV for the sake of comparison. It can be observed that Y-K angles show an increasing trend with Zn content in both the cases, but the values in present system are greater than that of ceramically prepared samples. This behavior may be attributed to the single domain nature of particles. Preparation method, chemical composition, microstructure, and grain size play a crucial role in deciding the magnetic susceptibility. The variation of AC susceptibility as a function of annealing temperature and composition is studied. Thermal variation of AC susceptibility of the typical samples x=0.1, 0.2, and 0.3 is shown in Fig.6. All the samples exhibit ferimagnetic behavior, which decreases as Zn-content increases. The plots of χ T /χ RT can be used to determine the Curie temperature and the values are given in Table IV. It is clear from Table IV that Curie temperature goes on decreasing with the addition of non-magnetic Zn content. This is attributed to the decrease in A-B inter-

5 Chin. J. Chem. Phys., Vol. 21, No. 4 Structural and Magnetic Properties of Ni-Zn Ferrites 385 TABLE IV Saturation magnetization (σ s), magneton number (n B), Curie temperature (T C) and Yafet Kittle angle (θ YK) of Ni 1 xzn xfe 2O 4. x σ s/(emu/gm) n B/µ B θ YK/( ) T C/K Obs. Cal. Wet chemical Standard ceramic [13] ACS a DCR b TEP c [15] CP [27,28] a AC susceptibility. b DC resistivity. c Thermoelectric power. action resulting from the replacement of magnetic Fe 3+ by non-magnetic Zn 2+. The decrease in T C is uniform and linear. The comparison of Curie temperature of the samples prepared by co-precipitation technique in the present work and ceramically prepared samples from literature is given in Table IV. The variation of Curie temperature with Zn content is represented in Fig.7. According to Neel s model A-B interaction is the most dominant in ferrites, therefore, the Curie temperatures of the ferrites are determined from the overall strength of A-B interaction. The strength of A-B interaction is a function of the number of FeA + -O 2 -FeB 3+ linkages, which, in turn, depends upon the number of Fe 3+ ions in the formula unit and their distribution amongst tetrahedral (A) and octahedral [B] sites. In the present system Ni 2+ (2 µ B ) ions are replaced by Zn 2+ (0 µ B ). This results in decreasing the A-B interaction, which leads to a decrease in Curie temperature (T C ). The Curie temperatures are nearly the same as observed from DC resistivity plots [26]. The values of Curie temperatures of the samples prepared by wet chemical co-precipitation method are higher than those prepared by ceramic method. Thus the ferrite samples having high Curie temperatures may be prepared by wet chemical co-precipitation method which is attributed to nano particle nature. IV. CONCLUSION Samples of Ni-Zn spinel ferrite with x= were successfully prepared by wet chemical co-precipitation technique with relatively low cost. The fine particle nature results from the co-precipitation technique. The XRD peaks are broader as compared to ceramic samples, which indicates nanoparticle nature. A change in the structural parameter is observed compared with the ceramic samples. The lattice constant increases with Zn content. The observed and calculated magneton num- FIG. 7 Variation of Curie temperature T C with Zn content x of the series Ni 1 xzn xfe 2O 4. ber shows discrepancy in their values for x>0.4. The Curie temperature T C decreases with increase of Zn content. Having finer particles led in turn to enhanced magnetic properties of the ferrite samples. The system with incorporation of Zn ions in Ni matrix exhibits interesting magnetic behavior. The substitution of Zn 2+ in nickel ferrite reduces the magnetic properties as Zn is non-magnetic and occupies tetrahedral A-sites. The reduction in the particle size leads us to conclude that the properties of the present samples are superior to that of their bulk counter part. The A-B interaction is found to weaken with increase in non-magnetic Zn content which is echoed through the decrease in Curie temperature. [1] P. P. Hankare, P. D. Kamble, M. R. Kadam, K. S. Rane, and P. N. Vasambekar, Mater. Lett. 61, 2769 (2007).

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