ABSTRACT

Co_0.7Me_0.3Fe₂O₄ (Me = Zn, Ni and Mn) were synthesized through co-precipitation method. Cationic distribution for these ferrites was proposed on the basis of magnetization measurements and available occupancy of the substituent ions into the spinel lattice. Theoretical lattice constant calculations confirm the proposed cationic distributions were the correct ones.

1. Introduction

Recently cobalt and cobalt containing ferrites have gained significant impact as they possess high coercivity, modern magnetization, very high positive magnetocrystalline anisotropy and high negative magnetostriction. Partial replacement of Fe by Mn or Co by Mn in CoFe₂O₄ is shown to affect the magnetostrictive response of the material. Since the structural, electrical and magnetic properties of ferrites depend on magnetic interaction and cation distribution, much attention is needed to understand the distribution of ions in the two sublattices of spinel in various synthesis routes.

Petitt et al. [1] have studied Co_1−xZn_xFe₂O₄ ferrite system and found an increase in saturation magnetization with the increase in zinc concentration up to about x = 0.4 and beyond which a decrease in saturation magnetization. Mossbauer study in Co_1−xZn_xFe₂O₄ system revealed that Co²⁺ ions enter A-sites up to 10% for Co-ferrite and the concentration of Co²⁺ ions at A-site gets reduced with the increasing concentration of Zn²⁺ ions at A-site. From the Neutron diffraction study, MK Fayek et al. [2] reported that the distribution in Zn_0.85Co_0.15Fe₂O₄ as. Partial replacement of Co by Zn in CoFe₂O₄ helps to understand the exact cation distributions from Mossbauer and Neutron diffraction studies. It is of importance to determine their degree of inversion. The inversion is not complete in cobalt ferrite and the degree of inversion depends on annealing temperature and method of preparation. Krieble et al., have noticed that Mn substitution in Co-ferrite in place of Fe³⁺ ions, not only replaces Fe³⁺ ions at B-site, but also has the effect of displacing the Co²⁺ ions to the A-site [3]. Sonal Singhal et al. observed increase in saturation magnetization with the increase of annealing temperature in nickel substituted cobalt ferrites.

The aim of the present work is 1) processing of fine particles of cobalt containing ferrite samples Co_0.7Mn_0.3Fe₂O₄, Co_0.7Ni_0.3Fe₂O₄ and Co_0.7Zn_0.3Fe₂O₄ by co-precipitation method followed by annealing, 2) study of crystallite size from X-ray diffraction and Curie temperature of the ferrites, 3) determination of cation distribution from saturation magnetization measurements, 4) calculation of the theoretical lattice parameter from the proposed cation distribution and comparison of theoretically estimated and experimental obtained lattice parameters.

2. Experimental Details

For the preparation of Co_0.7Me_0.3Fe₂O₄ (Me = Zn, Ni and Mn) through co-precipitation, high pure cobalt, zinc, nickel, manganese and iron chlorides were taken in stioichiometric proportions and made dissolved separately in minimum amount of deionized water according to the chemical formula.

(0.7)CoCl₂·6H₂O + (0.3)MeCl₂·6H₂O + 2FeCl₃ + 8NaOH → Co_0.7Me_0.3Fe₂O₄ + 8NaCl +8H₂O Thus prepared cationic solution were mixed intimately so as to obtain the required compositions and stirred intensely for 1 hr to improve homogeneity. The resulting solutions, precursors, subjected to constant heating at 60˚C under continuous stirring. At this stage the co-precipitating base, aqueous NaOH in required proportion was added slowly drop by drop to the precursor solutions till a massy precipitate was formed at the bottom of the reaction vessel. Washing of the precipitate was done several times with deionized water and finally with acetone to remove the traces of water any. Pellets were prepared using 15% polyvinyl alcohol under a uniaxial pressure of 10⁵ psi. X-ray diffraction patterns of all the samples were recorded using Rigaku Miniflex X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å). Saturation magnetization was measured using Ponderometer method [4]. Curie temperature was determined with Soohoo method [5].

3. Results and Discussion

X-ray diffraction patterns of the Co_0.7Me_0.3Fe₂O₄ (Me = Zn, Ni and Mn) powders confirmed the single phase spinel structure [Figure 1(a)]. The average crystallite size of all the samples has been estimated using FWHM of each diffraction peak and Debye-Scherrer formula [6,7]. In order to obtain FWHM, a non-linear least square fit has been constructed assuming experimentally obtained diffraction peaks obey Voigt function [Figure 1(b)].

The value of average crystallite size is also estimated by constructing Williamson-Hall plots (versus) using the values of FWHM and incident angle θ corresponding to each diffraction peak. Extrapolation of these graphs provides the value of corresponding to zero strain, the reciprocal of which determines the crystallite size [Figure 2(a)]. Average crystallite size calculated from both the methods is found to be in good agreement and increasing with increasing ionic radii of substituted metal cation which might be attributed to entry of each ion into the lattice and variations in the rates of reaction for each specific ion [Figure 2(b)]. Similar tendency has been noticed in lattice constant, obtained from Nelson-Riley function [8].

Figure 3(a) shows the variations in saturation magnetization with ionic radius of the dopant. The saturation magnetization of cobalt ferrite decreases with the partial replacement of Co²⁺ ions by Ni²⁺ ions because the magnetic moment associated with nickel ion is 2, less than that of Co²⁺ ion (3). Moreover, nickel ions are known to occupy octahedral sites and cause decrease in the B-site magnetic moment. As the net magnetic moment of the sample is equal to the difference in B site and A-site magnetic moments, M_B-M_A, a decrease in overall magnetic moment of the sample is expected. This kind of explanation is valid provided the strongest A-B interaction persists among various cations existing on A and B sublattices.

It is well known [9] that Zn²⁺ ions occupy tetrahedral sites, Ni²⁺ ions occupy octahedral sites and Mn²⁺ ions occupy tetrahedral sites. Co²⁺ ions are reported to occupy A-sites up to a maximum of 10% in cobalt ferrite [10,11] depending on the annealing temperature. From Mossbauer study of cobalt-zinc ferrite [1] it has been observed that the amount of cobalt at A-site decreases from 10% with the increase of zinc content.

In case of Co-Zn ferrite system, non-magnetic Zn²⁺ ions are known to occupy tetrahedral (A) sites due to their strong preference and decrease the magnetic moment of A-sublattice and hence an increase in the total magnetic moment of the sample is predicted. Similar

behaviour has been reported [12] in Co_1−xZn_xFe₂O₄ system up to x = 0.4. The highest value of saturation magnetization for the composition Co_0.9Zn_0.1Fe₂O₄ [13] was noticed and explained the result on the basis of cationic distribution and their occupancy in specific sites. The substitution of Mn²⁺ for Co²⁺ produces changes in the value of saturation magnetization depending on the heat treatment. Mn³⁺ ions will have strong preference for the octahedral sites when Fe³⁺in cobalt ferrite is replaced by Mn³⁺ [9]. On the other hand, Mn²⁺ ions will have stronger preference for the tetrahedral sites [10] if Co²⁺ is replaced by Mn²⁺. In the present study the experimentally observed value of magnetization in Co-Mn ferrite is in accordance with the replacement of Co²⁺ ions by Mn²⁺.

Figure 3(b) shows the variations in Curie temperature. As is clear from the table, except Co-Mn ferrite Table 1 the other two ferrite systems show less value of Curie temperatures than that of the cobalt ferrite. The observed variations in Curie temperature are explained on the basis of the super exchange interactions among the metallic cations distributed between the two sublattices. As described in saturation magnetization, the substitution of Ni²⁺ in place of Co²⁺ should decrease the magnetic interactions since the ion having less magnetic moment (Ni²⁺) is replacing an ion having high magnetic moment (Co²⁺). In case of Co-Mn ferrite, as Mn²⁺ ion magnetic moment (5) more than magnetic moment of Co²⁺ ion (3), the rise in magnetic interactions is generally expected which increase the Curie temperature. In case of Co-Zn ferrite system, the substitution of non-magnetic zinc in place of ferromagnetic cobalt leads to a decrease in Curie temperature owing to diminishing A-B super exchange interaction.

Cation distribution for these ferrites has been proposed on the basis of saturation magnetization measurements. In the present study, the occupancy amount of cobalt ions at tetrahedral sites has been chosen to be about 5% to provide the following better possible cationic distributions agreeable to experimentally observed number of Bohrmagnetons per molecule.

Besides providing the experimentally observed lattice constant, theoretical lattice constants have also been estimated using the following formulae by proposing cati-

onic distribution of metal ions in the spinel lattice.

where and are radii of tetrahedral and octahedral sites respectively. is the radius of oxygen ion. u is oxygen positional parameter. For face centered cubic structure the value of oxygen parameter is 0.375. In the Co-Zn ferrite system the radii of Zn²⁺ and Co²⁺ ions are greater than the space occupied by the oxygen ions. This will cause distortion in the cubic lattice and therefore the oxygen parameter may differ from its usual value 3/8 [14].

Theoretical lattice constants corresponding to these ferrites have been calculated using the proposed cationic distributions. The calculated lattice constants are found to be in agreement with the experimentally observed and reported lattice constant values (Table 2).

4. Conclusions

X-ray diffraction measurements confirmed the single phase spinel structure for all the ferrites and lattice constant has been measured by using Nelson-Riley function. Cation distribution for the compositions has been proposed on the basis of saturation magnetization values and the preferred site occupancy of the ions in the ferrite lattice. Confirmation of cation distribution has been done through theoretical lattice constant calculations. A good agreement between theoretical and experimental lattice constants has been found for all the compositions.

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