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American Journal of Applied Sciences
Year: 2009  |  Volume: 6  |  Issue: 1  |  Page No.: 43 - 47

Effect of Partial Orientation in [100] Direction on the Magnetic Properties of Co-Ferrite Prepared from Nano Particles

H.M. El-Sayed    

Abstract: Chemical co-precipitation method was used for the preparation of Co-ferrite nano particles. The particle size was about 18 nm. A magnetic anisotropy of Co-ferrite could be increased by applying an external magnetic field during the pressing of the nano particles before the final sintering. This anisotropy enhanced the squareness and the coercivity of investigated samples.

P = 100(1 - d/dx)%.

The magnetization (M), at room temperature and the hysteresis parameters were measured using the vibrating sample technique. The magnetizing field ranged from 0.0 up to 12 K Oe.

The particle size (Dhkl) of the powder sample was measured from the x-ray chart according to Debye-Sherrer formula[7] which is given by,

Dhkl = 0.9λ/βcosθ

where, λ is the wave length of the used x-ray (λ = 0.154 nm) , β is the half width and θ is the half diffraction angle. The particle size was determined by taking the average of the strongest peaks D220,D311, D400, D511 and D440. Also, the Scanning electron microscope (SEM) was used for measuring the grain size of the investigated samples.

RESULTS AND DISCUSSION

X-ray analysis: X-ray diffraction of the powder sample is shown in Fig. 1a. It is obvious that, the prepared sample from the chemical co-precipitation method has one cubic spinel phase. The lattice parameter was found to be about 8.383 Ao which is in good agreement with that reported by Armulmurugan et al. and Pandya et al.,[8,9]. Also, from x-ray pattern, the average particle size of the powder sample is found to be about 18 nm. This result confirms that, the prepared sample has nano particle size. Figure 1b shows the image of SEM of the powder sample. It is noticed that, the powder sample has nano size particles in the range of 12 nm which is in a good agreement with x-ray measurements.

The x-ray diffraction patters of the bulk samples are shown in Fig. 2. It is clear that, all patterns have single spinel phase. It was found that, the lattice parameter of all samples is in the same order of the powder sample. Furthermore, it is valuable to note that, the relative intensities of (100) and (400) planes increase with increasing the applied magnetic field during pressing the samples (Hext) while, at the same time, the relative intensities of the (220) and (440) decrease. This may be discussed as follows:

As the average particle size of the powder sample is much less than the magnetic domain size, one may conclude that, each particle has one domain i.e., each particle could be considered as a permanent magnet. Moreover, It is well known that, the easy magnetization axis of Co-ferrite is [100] where, this direction has the highest magnetic anisotropy [10,11]. Thus the domain direction of each particle prefers to be in [100] direction. Thus, by applying the external field, the nano particles will rotate in the direction of the applied field i.e., the crystallographic direction of the powder will have partial orientation in [100] direction, Fig. 3. By increasing the applied field (Hext), the degree of orientation in [100] direction will increase. This orientation causes the increase of the relative intensity of (100) plane. As the plane (400) is parallel to (100) then, the relative intensity of this plane also increases on the account of the two parallel planes (220) and (440).

Fig. 1: (a) X-ray diffraction pattern and (b) SEM of Co-Ferrite nano-particles.

Fig. 2: X-ray Diffraction Patterns of Co-Ferrite subjected to external applied magnetic field (Hext.).

Fig. 3: An illustration figure shows the rotation of the magnetic nano-particles due to the application of magnetic field during pressing the samples.

Fig. 4: SEM images for Co-ferrite at different Hext.

Figure 4 shows the SEM images of the investigated samples. The average grain sizes of the samples at different Hext are shown in Table 1. It is noticed that, the average grain size increases with increasing Hext.. This increase of the grain size with the magnetic field (Hext.) could be attributed to the orientation effects of the nano particles with the applied magnetic field (Hext.). From Table(1), it is also observed that, the porosity has an inverse relation with the applied field (Hext.) which may be attributed to the increase of the grain size.

Magnetization measurements: In fact, the enhancement of the degree of orientation of the Co-ferrite samples with the applied field (Hext.) in [100] direction is expected to affect the magnetic properties of the investigated samples. Figure 5 shows the magnetization curve in the perpendicular and parallel directions to the surface of the sample at Hext.= 20KA/m. It is clear that, the perpendicular direction is easy to be magnetized than the parallel one. This means that, there is magnetic anisotropy in the sample.

Fig. 5: Magnetization curve for the sample with Hext. = 20 KA/m in Parallel and perpendicular directions to the surface of the sample

This is attributed to the degree of orientation of the samples in [100] direction. The magnetization curves of the four samples in perpendicular direction to the surface of the sample are shown in Fig. 6. It is obvious that, the sample which is pressed under higher magnetic field is easier to be magnetized i.e. the anisotropy becomes higher.

Hysteresis parameters: The hysteresis loops of the four investigated samples are shown in Fig. 7. The hysteresis parameters, the saturation magnetization (Ms), remanance (Mr), ratio (Mr/Ms) ,which represents the squareness, and the coercivity (Hc) for the all samples are shown in Table 1.

Fig. 6: Magnetization Curves for all samples

Fig. 7: Hysteresis loops of Co-ferrite samples at different Hext

It is obvious that, the ratio (Mr/Ms) increases with increasing the applied field during the pressing. This means that there is an enhancement of the squareness of the samples. On the other hand, there is an increase of the coercivity with increasing Hext. Since according to Brown`s relation[12]

Hc ≥ (2K1oMs)

where, K1 is the anisotropy constant.

Table 1: Grain size, porosity and the hysteresis parameters of Co-Ferrite

One can see that, the coercivity has a direct relation with the anisotropy constants K1. Therefore, the increase of HC with the applied field, during the pressing of samples, proves the increase of the magnetic anisotropy. This means that, the application of a magnetic field during the pressing of Co-ferrite nano particles before sintering could enhance the magnetic anisotropy without the need of substituting any other ions.

CONCLUSIONS

Co-ferrite nano particles of size 18 nm could be obtained by chemical co-precipitation method at low temperature. The application of an external magnetic field on the sample during pressing causes great effect on the magnetic properties of Co-ferrite. During this work an enhancement of the squareness of Co-ferrite was obtained. Also, the increase of the magnetic anisotropy due to the application of the magnetic field during the pressing causes an increase in the coercivity of the Co-ferrite.

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