Figure 1 shows the typical XRD analysis of the prepared samples which were calcinated at 600˚C. In general the XRD pattern of a pure cobalt ferrite (CoFe₂O₄) represents eight reflection peaks in which the highest intensity reflection peak (311) exists nearly at 2θ = 35˚ and the obtained XRD pattern exactly matched. However, the complete XRD pattern confirms the formation of single phase cubic spinal structure. And for the composition x = 0.00, the intensity peaks are indexed with the corresponding miller indices (hkl) of pure CoFe₂O₄ and there are no impurity peaks appeared. It is also observed that the spinal structure of cobalt ferrite does not alter with small amount of bismuth ion substitution [20] . The existence of additional peaks in the XRD pattern for the compositions x = 0.05 to 0.25 are indexed with the “♦” sign in terms of bismuth as a dopent ion. And the mean crystallite size of prepared samples was evaluated for the strongest peak (311) with Debye-scherrer’s Formula (1) as follows.

where D is the mean crystallite size; λ is the wavelength of incident X-ray beam; β is the full width at half of the maxima (FWHM) in radians and θ is the Bragg’s diffraction angle.

With analysis of these results the lattice parameters such as crystallite size (D), lattice constant (a) were increasing with the increase of doping concentration (x). This is due to the smaller ionic radiuses of Fe³⁺ (0.64Å) ions are partially replaced by the greater ionic radius of Bi³⁺ (1.31Å) ions at octahedral position (B-site). The calculated crystallite size was found to be as 21 (±5) nm and these results were well agreement with literature [21] [22] . The lattice constant of all these ferrites were evaluated by using Equation (2). Similar work was

observed by different authors with various dopent metal ions into the spinal ferrite like Cr³⁺, Zn³⁺, Mn³⁺, etc. [23] .

Figures 2(a)-(c) show the surface morphology and microstructure of the composition (x = 0.20) with Transmission Electron microscope (TEM). From the obtained TEM images, it is observed that the particles were found to be in cubic shape with less agglomeration. From Figure 2(a) and Figure 2(b), the lattice parameters such as crystallite size (D) and inter planar distance (d) are observed at different magnifications. Figure 2(c) shows the Selected Area of Electron Diffraction (SAED) patterns of the sample, which represent the most appeared intensity peaks in terms of miller indices (hkl) of different crystals in the composition. With the analysis of these images it is confirmed that, the synthesised particles are exhibiting well defined crystalline nature.

Figure 3 shows a typical hysteresis loop of all the prepared compositions which are sintered at 600˚C. By using a vibrating sample magnetometer, the measurements of magnetisation for all the compositions were carried out at room temperature under the applied magnetic field of range ± 10k Oe. It is observed that, the magnetic properties such as saturation magnetisation (Ms), remnant magnetisation (Mr) and coercivity (Hc) values are gradually decreasing with the increase of bismuth ion concentration and the values are listed in Table 1. The saturation magnetisation of CoFe₂O₄ sintered around 600˚C is nearly 64 emu/g, which is in good agreement with the literature values [24] . It is observed from Table 1 as the doping concentration increases in the spinal CoFe₂O₄, the saturation magnetisation decreased from 63.52 - 48.73 emu/g. This may be attributed to the larger magnetic moment of Fe³⁺ ions (5 µB) are partially replaced by the smaller magnetic moment of Bi³⁺ ions (3 µB) in the octahedral site [25] [26] . Therefore the ratio of Fe³⁺ (B)/Fe³⁺ (A) ionic concentration was gradually decreased in the ferrite composition [27] [28] . This is due to the cation distribution in between A and B sub lattices which were proposed by Neel’s two sub lattice model [29] [30] . According to this, in any ferrite the magnetic order of tetrahedral clusters (A-site) and octahedral clusters (B-site) was found to be anti parallel to each other. In this the A-A and B-B super exchange interaction was predominated by A-B super exchange interaction. So

the net magnetic moment of the composition is given as M = M_B − M_A, where M_B and M_A are the magnetic moment of B and A sub lattices respectively. It is also observed from Table 1 the coercivity values are gradually decreasing with increase of bismuth content, due to the decrease in magneto crystalline anisotropy (K). As compared with greater magneto crystalline anisotropy of iron with bismuth is having negative magneto crystalline anisotropy [29] , therefore the values of anisotropy and coercivity were decreasing with the increase of bismuth content. The correlation of saturation magnetisation and coercivity is clearly shown in Figure 4. A similar work was observed in the literature [27] [29] . The anisotropy of the composition can be evaluated by the formula K = Hc x Ms/0.98 and the values are tabulated in Table 1. From Figure 5 it is clear that the anisotropy is decreasing with the increasing of Bi³⁺ ion concentration and which leads the decrease in magneto static energy [31] [32] .

Sqareness ratio or remnant ratio (Mr/Ms) of a material is one of the important characteristic which depends on its anisotropy. The values of squreness ratio represent the random arrangement of uniaxial particles along with the cubic magneto crystalline anisotropy [33] [34] .

In the present work the squreness ratio of pure CoFe₂O₄ is 0.54 at room temperature and which was decreased to 0.47 with the increase of bismuth ion concentration. And it has been observed from literature that the squreness ratio is less than 0.6, which indicate the presence of non interacting single domain particles with cubic anisotropy in the respective compositions [35] . The values of Bohr Magneton (µ_B) of these samples were also evaluated by using the following Equation (3).

where: M_w is molecular weight of the sample; M_s is saturation magnetisation.

5585 = β × N [β is Conversion factor (9.27 × 10⁻²¹); N is Avogadro’s number].

It is observed from Table 1 that Bohr Magneton values are gradually decreased from 2.668 to 2.381. With the research interest one can modified the compositions of ferrite materials in accordance with sqareness ratio (S), for the development of new electromagnetic materials [29] . In the present work it is observed a clear variation on magnetic properties of bismuth doped cobalt ferrite nano particles.