1. Introduction
M-type Barium hexaferrites are magnetically hard materials that possess versatile properties like high saturation magnetization, high coercivity, large uniaxial anisotropy and excellent chemical and magnetic stability. These properties make them well suited materials for numerous applications. They have been extensively used in permanent magnets, high density recording media and magneto optical recording technologies [1-4]. In addition, they are suitable microwave absorbing materials due to a significant permeability value (>1) [5].
BaFe12O19 has a magnetoplumbite structure and its unit cell is a combination of two structural blocks aligned in the direction of hexagonal c-axis: RSR*S*. In R block, the lattice is made of O2– ions, forming a hexagonal closed packed structure, with iron ions occupying the tetrahedral, octahedral and bipyramidal sites. However, in S block, O2– ions form a cubic closed packed lattice and iron ions occupy the tetrahedral and octahedral sites [6]. It is well known that their structural and magnetic properties are closely connected to the distribution of Fe ions among various interstitial sites, and it has been found that these properties can be changed by doping of Fe3+ and Ba2+ with different types of cations and cation combinations. Due to this, researchers have a lot of interest in modifying structural, magnetic and electrical properties of nanoferrites, according to the requirement for various practical applications by different cation substitutions and by using different synthetic routes under varying conditions [7-11]. Sandaranarayanan et al. [7] studied the effect of annealing temperature on Barium ferrites and reported that their crystallization begins around 550℃ and fully crystalline phase is obtained in the range of 700℃ - 900℃. Mendoza-Suarez et al. [8] synthesized Zn-Sn doped Ba ferrites, BaFe12-2xZnxSnxO19, using ball milling method and found that the coercivity decreases due to reduction of the magnetocrystalline anisotropy. The saturation magnetization has also been reported to decrease with increase in Zn-Sn concentration. Kresisel et al. [9] reported that with increasing concentration of Co-Ti dopants in barium ferrites, axial anisotropy reduced and further changed to nearly planar magnetic anisotropy. Teh et al. [10] synthesized Co2+ and Co3+ substituted Ba ferrites via sol-gel method and found that Co2+ doping decreases the value of coercivity and saturation magnetisation significantly. However, Co3+ doping shows less change in magnetic properties. Ghasemi et al. [11] prepared Mn-Co-Zr substituted Ba ferrites and studied that substitution of these cations is very effective in reduction of coercivity at low level of substitution.
The present work deals with the formation of nanoferrites of cobalt substituted M-type barium ferrites via citrate sol-gel method. Their characterisation has been done by Fourrier Transform Infrared (FT-IR) spectroscopy, powder X-Ray Diffraction (XRD) studies and Vibrating Sample Magnetometry (VSM). The effect of increasing Co2+ concentration on the magnetic properties has also been studied.
2. Experimental
2.1. Synthesis
BaCoxFe12-xO19 (x = 0.2, 0.4, 0.6, 0.8 & 1.0) were synthesized using the citrate sol-gel method [12,13]. AR grade Ba(NO3)2, Fe (NO3)3·9H2O, Co(NO3)2·6H2O and citric acid were weighed in appropriate stoichiometric proportions and dissolved in minimum amount of distilled water at 80℃. Ethylene glycol was added as gelling agent. Then, all the dissolved solutions were mixed with continued stirring and a homogenous mixture was obtained. The resulting solution was then heated at 10℃ and stirred using a hot plate magnetic stirrer that resulted in gel formation. Thermal decomposition was carried out and the ferrite powder was obtained, which was annealed at 1000℃ for 2 hours in muffle furnace.
2.2. Physical measurements
Fourier Transform infra red (FT-IR) spectra have been recorded using Perkin Elmer RX-1 FT-IR spectrophotometer with KBr pellets in the range 4000 - 400 cm–1. Powder X-ray diffraction (XRD) studies have been carried out using a Bruker AXS, D8 Advance spectrophotometer with Cu-Kα radiation Hitachi (H-7500). The magnetic properties have been measured at room temperature by a Vibrating Sample Magnetometer (VSM) (155, PAR) up to a magnetic field of ±10 kOe.
3. Results and Discussion
3.1. FT-IR Measurements
The FT-IR spectra of all the ferrite compositions have been recorded in the range of 4000 - 400 cm–1, using KBr plates. In the low frequency range, the spectra show two main peaks corresponding to the vibrational modes of metal oxide of ferrites that are interpreted in the light of literature study of absorption region of ferrite [14]. The peak observed in the range of 580 cm–1 is attributed to the stretching vibration of tetrahedral group. However, the peak in the range of 460 cm–1 is due to stretching vibration of octahedral group.
3.2. X-Ray Diffraction Studies
The powder X-Ray diffractographs for all the as obtained as well as annealed samples have been recorded and are shown in Figure 1. The XRD patterns of the ferrite samples show characteristic diffraction peaks corresponding to the M-type barium ferrite structure, having point group P63/mmc, indicating that the crystal structure does not transform and remains hexagonal magnetoplumbite after substitution with cobalt ions. The average crystallite size for each composition has been calculated from the line broadening of the most intense peak corresponding to (1,1,4) plane of magnetoplumbite structure according to Scherrer equation [15] given below
D = kλ/βcosθ
where D is the average size of the crystallites, k is the scherrer constant, λ is the wavelength of radiation (1.5405 Å), β is the peak width at half height or full width half maximum. The values of crystallite size are listed in Table 1 and the average crystallite size is found to be ~55 nm.
The lattice parameters, a and c, have been calculated using Powley as well as Le-Bail refinement methods (built in TOPAS V2.1 of BRUKER AXS), and are listed in Table 1. It is observed that the lattice parameters remain constant on increasing Co2+ concentration.
3.3. Magnetic Properties
The room temperature magnetic hysteresis loops for all the as obtained as well as annealed samples have been recorded. Typical loops for all the annealed ferrite samples are shown in Figure 2. From these plots, saturation magnetization (Ms), coercivity (Hc) and squareness ratio (Sq) have been calculated and are given in Table 1. From Table 1, it is seen that the saturation magnetization decreases as the Co2+ concentration is increased. This is
Figure 1. X-Ray diffractographs of (a) BaCo0.2Fe11.8O19; (b) BaCo0.4Fe11.6O19; (c) BaCo0.6Fe11.4O19; (d) BaCo0.8Fe11.2O19; (e) BaCo1.0Fe11O19 annealed at 1000℃.
Table 1. Crystallite size, D (nm); Lattice parameter (a); Saturation Magnetisation (Ms), Coercivity (Hc) and Squareness Ratio (Sq) of the ferrites annealed at 1000℃.
Figure 2. Hysteresis loops of BaCoxFe12-xO19 (x = 0.0, 0.2, 0.4, 0.6, 1.0) annealed at 1000℃.
attributed to the lesser magnetic moment of Co2+ ions (3 µB) as compared to Fe3+ ions(5 µB) and their substitution in 12k and 2a sites of the lattice, in preference to the 4f2 sites. The coercivity value decreases from 5200 to 1750 Oe with increasing cobalt concentration (x = 0.0 to 1.0) due to reduction in magnetocrystalline anisotropy [16,17].
The value of squareness ratio has been calculated by formula
Sq = Mr/Ms
where Mr is the remnance and Ms is the saturation magnetisation. From Table 1, it is observed that the squareness ratio increases with increase in cobalt ion concentration.
4. Conclusions
Cobalt substituted barium ferrites BaCoxFe12-xO19 (0.2, 0.4, 0.6 & 1.0) were synthesized using citrate sol-gel method. The formation of M-type ferrites has been confirmed by FT-IR and XRD characterization. The crystallite size is found to be ~55 nm. The values of saturation magnetization and coercivity decrease with increasing cobalt substitution due to less magnetic moment of Co2+ ions as compare to Fe3+ ions and reduced magnetocrystalline anisotropy respectively.
5. Acknowledgements
Grateful thanks to the UGC for the financial support for this research work under the scheme of UGC-major project.