**6. Effect of particle size**

#### **6.1 Magnetic separation of the nanoparticles**

The force exerted on a magnetic particle placed in a static magnetic field is proportional to its volume (FM~Vp) [58]. So, according to their sizes, magnetic nanoparticles dispersed in a liquid can be efficiently separated by static magnetic field. In the experiment, the LSr900-15 h powder is dispersed in distilled water with mechanical stirring and then subjected to a magnetic field applied by a permanent magnet. The powder retained after 5 minutes is designated by LSr900b (bigger), and the powder retained from the supernatant after 30 minutes is designated by LSr900s (smaller).

#### **6.2 Structural characterization sand ERS measurement**

**Figure 6** shows X-ray diffraction patterns of powders LSr900b and LSr900s with LSr900-15 h. The diffraction peaks are characteristic of the perovskite phase. The broadening of the (0 2 4) reflection at 2θ = 46° is in harmony with that of the original compound LSr900-15 h. Thus, the crystallite's size for all the compounds is 55 nm.

The ESR spectra obtained for the three samples have the same general shape (**Figure 7**). They are characteristic of a multi-domain ferromagnetic compound. In coherence with the results of Valenzuela et al., the LFMA signal becomes more intense with increased particle sizes [24]. The linewidth ESR depends on particle sizes, shows a gradual decrease with decreasing size, and is explained by external causes resulting from a distribution of the anisotropy axes [60]. In fact, when a ferromagnetic particle is exposed to a magnetic field, the crystallites tend to orient in the direction of easy magnetization. The assembly of these crystallites in agglomerates prevents some of them from being oriented in the preferred direction, which translates to the spectrum level by the widening of the ESR lines. These observations confirm that the absorption line envelops ensembles of narrower and indistinguishable lines, each of them coming from the resonance of a set of spins called by Portis "spin packets" [61].

**29**

**Figure 7.**

*ESR spectra of the samples LSr900-15h, LSr900a and LSr900b.*

**Figure 6.**

*(0 2 4) reflection.*

*Synthesis and ESR Study of Transition from Ferromagnetism to Superparamagnetism…*

*X-ray diffraction patterns of the samples LSr900-15h, LSr900b and LSr900s. Inset shows the details of the* 

*DOI: http://dx.doi.org/10.5772/intechopen.89951*

*Synthesis and ESR Study of Transition from Ferromagnetism to Superparamagnetism… DOI: http://dx.doi.org/10.5772/intechopen.89951*

**Figure 6.**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

confirms multi-domain state of samples higher size 32 nm.

separation from the sample LSr900- 15 h (55 nm).

**6.1 Magnetic separation of the nanoparticles**

**6.2 Structural characterization sand ERS measurement**

**6. Effect of particle size**

LSr900s (smaller).

is 55 nm.

In the multi-domain state, application in the measurement of weak magnetic

These results are comparable to the overall results obtained with samples of analogous compositions [35, 57]. In particular, Sujoy R. et al. reported an exponential increase of the magnetic core between 8 and 22 nm [18], favoring the transition

In addition to the intrinsic causes of line broadening due to the change of magnetic state, extrinsic causes related to the size and shape of the magnetic particles are to be considered. Nanocrystalline powders are formed by several crystallites bonded together to form a wide variety of sizes and shapes that have different magnetic properties [58, 59]. "Crystallite size" is not synonymous with "particle size";

To study the effect of particle sizes on the line shape, ESR measurements have been performed for two populations with diverse particle size selected by magnetic

The force exerted on a magnetic particle placed in a static magnetic field is proportional to its volume (FM~Vp) [58]. So, according to their sizes, magnetic nanoparticles dispersed in a liquid can be efficiently separated by static magnetic field. In the experiment, the LSr900-15 h powder is dispersed in distilled water with mechanical stirring and then subjected to a magnetic field applied by a permanent magnet. The powder retained after 5 minutes is designated by LSr900b (bigger), and the powder retained from the supernatant after 30 minutes is designated by

**Figure 6** shows X-ray diffraction patterns of powders LSr900b and LSr900s with LSr900-15 h. The diffraction peaks are characteristic of the perovskite phase. The broadening of the (0 2 4) reflection at 2θ = 46° is in harmony with that of the original compound LSr900-15 h. Thus, the crystallite's size for all the compounds

The ESR spectra obtained for the three samples have the same general shape (**Figure 7**). They are characteristic of a multi-domain ferromagnetic compound. In coherence with the results of Valenzuela et al., the LFMA signal becomes more intense with increased particle sizes [24]. The linewidth ESR depends on particle sizes, shows a gradual decrease with decreasing size, and is explained by external causes resulting from a distribution of the anisotropy axes [60]. In fact, when a ferromagnetic particle is exposed to a magnetic field, the crystallites tend to orient in the direction of easy magnetization. The assembly of these crystallites in agglomerates prevents some of them from being oriented in the preferred direction, which translates to the spectrum level by the widening of the ESR lines. These observations confirm that the absorption line envelops ensembles of narrower and indistinguishable lines, each of them coming from the resonance of a set of spins called by

field can easily move the magnetic domain walls. The magnetization vectors approach the direction of the applied field, thus leading to reduce ΔHpp. Thus, the decrease of ΔHpp in addition to the appearance of the absorption at low field

from superparamagnetic state to the single-domain ferromagnetic state.

X-ray diffraction is sensitive to the size of crystallite inside the particles.

**28**

Portis "spin packets" [61].

*X-ray diffraction patterns of the samples LSr900-15h, LSr900b and LSr900s. Inset shows the details of the (0 2 4) reflection.*

**Figure 7.** *ESR spectra of the samples LSr900-15h, LSr900a and LSr900b.*

However, the resonance frequency remains constant, equal to Hres = 2920 G for the three samples. This can be explained in terms of the same internal field, which is directly related to the crystallite size without exchange interaction between adjacent crystallite. Autocombustion synthesis gives voluminous powders of high specific surfaces, with a structure of "sponge" form [62, 63]. These structures weaken the exchange interactions between the crystallites. In addition to this, the formation of a magnetically dead layer, which increases in thickness with decreasing crystallite size, prevents crystallite-crystallite interactions [34, 62, 64]. Thus, the resonance field is controlled principally by crystallite size.
