3. RESULTS AND DISCUSSION
Figure 1 shows the X-ray diffractograms of the a) PBM1; b) PBM2 samples and indicating the diffraction angles in 2-Theta of the hematite (Fe2O3) and of the lead hexaferrite (M-Pb) phases. In both cases it is possible to see that hematite is not present in these samples, only the M-type hexaferrite. This indicates that lead hexaferrite was obtained as single phase through these two methods of preparation under the conditions described above for each one. Figure 2 shows a thermogravimetric analysis of the powders prepared by coprecipitation, here is indicated that the Pb-hexaferrite phase crystallizes around of 700 °C, without loss of lead. This result is in agreement with the X-rays analysis. The structural characterization was obtained through the refinement of the crystal parameters by the Rietveld method. The results of the refinement of the structure of the analyzed samples are present in Table 1.
From the results of the crystal structure refinement we can see a variation in the density of the samples that is related with a change of the parameters a, and c of the unit cell. The cell parameters tend to be smaller and consequently, the density is bigger in the sample prepared via the solid state reaction method. This behavior is only attributed to the different sintering temperatures used. In PBM1 sample the sintering temperature was 950 °C, in contrast with the PBM2 sample, (700°C) which has not been reported before. The samples prepared by the coprecipitation technique did not present loss of lead due the temperature used to sintering the powder is lower than the temperature at which the lead evaporates (~900 °C) [16]. Another remarkable result is the diminishing in the crystallite size of the PBM2 sample prepared by the coprecipitation method, in this case the obtained crystallite size is only of 53 nm compared with the crystallite size obtained in the PBM1 sample with size of 161 nm. The reduction in the crystallite size is attributed to the small size of the precipitated particles that is possible to obtain by coprecipitation, also the reduced sintering temperature prevents the growth of crystallites [17].
Table 1. Rietveld refinement results of the samples PBM1 and PBM2.
| Sample | Sintering temperature (°C) | Cell parameter (Å) | Crystallite size (nm) | Density (g/cm3) |
|---|---|---|---|---|
| PBM1 | 950 | a = 5.8939 (2) c = 23.1290 (5) | 161 (1) | 5.77 |
| PBM2 | 700 | a = 5.8955 (4) c = 23.2299 (9) | 53 (1) | 5.61 |
χin = Happ/M (1)
where Happ is the applied magnetic field and M is the magnetization. The initial susceptibility is obtained in the linear region at low values of applied field.
The hysteresis loops of the PBM1 and PBM2 samples are shown in Figure 3. The PBM1 sample has higher value of remanence and saturation, however, the PBM2 sample has higher value of the coercive field. In Table 2 the results of the magnetic characterization of both samples are indicated. The magnetic saturation is determined using the law of approximation to the saturation [18], as consequence of the high magnetocrystalline anisotropy of powders randomly oriented. Thus, the crystallites oriented perpendicularly to the magnetic field reaches the saturation at a very high magnetic field.
Table 2. Results of the magnetic characterization.
Sample σs (emu/g) σr (emu/g) Hc (KOe) χin
PBM1 72.0 39.0 1.7 0.0129 PBM2 38.3 20.0 3.5 0.0031
Figure 3. Hysteresis loops of the PBM1 and PBM2 samples.
Differences in the magnetic behavior observed from the hysteresis loops of the PBM1 and PBM2 samples are attributed to the interparticle interaction and to the way in which this interactions take place, in most of the cases is very complicated and related to the form, size and distribution of the particles which in turn depend on the way of the hexaferrite is prepared.
]]> The initial susceptibility of the PBM2 sample indicates that the magnetization reversal is mainly controlled by the pinning of the wall motion produced by structural defects which act as anchoring centers, when the applied field increases above a critical (Hp), the defects which oppose to the wall motion are exceeded and the initial curve reaches the saturation in an easy way. For the PBM2 sample the Hp is about 4.0 KOe. In contrast, in the PBM1 sample the pinning are negligible and the reversal magnetization is controlled mainly by nucleation of inverse domains. Then for the same compound we can see two different behaviors in reversal magnetization, these behavior and the differences in magnetic properties can be explained in terms of the microstructure acquired from the use of each method of preparation.The differences in the magnetic parameters can be explained in terms of the microstructural characteristics of the samples. The morphology and grain size were studied using scanning electronic microscopy (SEM). For the PBM1 sample, it was found that the morphology of the particles was hexagonal platelets with an average particle size of
2.0 µm, see Figure 4A. In the case of the PBM2 sample, the particles are shown as agglomerates of small wires which tend to form big particles by effect of the diffusion of the atoms with the increase of the sintering temperature. In Figure 4B the micrograph of the PBM2 sample is presented.
could be large, then the magnetization of the system tend to diminish. Some investigations are carried out to understand the effect of the pores in different magnetic systems [19, 20]; is well known that small grain sizes are related with high values of coercivity [20, 21]. Then, the small grain sizes of the particles obtained in the PBM2 generated an increase in the coercivity field, and when the particles size increases the coercivity decreased, as in the PBM1 sample.