**Author details**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

perovskite structure of similar composition [18, 35, 58].

**7.2 Superparamagnetic crystallite with multicore**

manetic multicore crystallite.

of magnetism in these compounds.

**8. Conclusion**

the LFMA.

The increase in the magnetic volume due to a sharp decrease in the thickness of the dead layer between 24.5 and 32 nm breaks the superparamagnetism. The system evolves toward the single-domain state, which results in a significant increase of the g-factor and ∆Hpp. Beyond 32 nm, nanoparticles change into a multi-domain state. This passage is confirmed by the asymmetrical EMR signal and the appearance of

These results are similar to those of the literature for manganite with the

consisting of different spatial regions with different magnetic orders.

Phase separation leads to the formation of a nanometer-sized ferromagnetic droplet cloud ranging from tens of nanometers to several hundred nanometers [71, 72]. In particular, La1−xSrxMnO3 manganites tend to form mixed phases near the paramagnetic-ferromagnetic phase transition [65, 73]. In this case, transition from the single-domain to the superparamagnetic state is not due to increasing the thickness of the dead layer but to the subdivision of the magnetic volume in small volumes. Thus, the superparamagnetic particles are formed by a paramagnetic volume containing several distinct magnetic cores that we have called superparag-

The multicore superparamagnetic state has a comparable magnetic structure with multicore magnetic particles (MCP)that have been shown to be promising for a wide range of biomedical applications, in particular in magnetic hyperthermia treatment and magnetic resonance imaging [74, 75]. So, we can conclude that control of crystallite size in the synthesis of manganites allows a controlled change

In recent years, a remarkable progress has been made in understanding the magnetic phenomenon in nanomanganites with perovskite structure. This advance is mainly made possible through experimental measurements and theoretical approaches. In this work, ESR spectroscopic measurements were adopted to study crystallite size-dependent magnetic properties of La0.8Sr0.2MnO3 nanomanganite. Samples with different crystallite sizes ranging from 9 to 57 nm were prepared by autocombustion method with a two-step synthesis process. Significant differences in the ESR spectrum parameters, namely, resonant field (Hres), line shape, low-field microwave absorption, and linewidth (ΔHpp), are used to determine the critical sizes of magnetic state changes. The findings from the ESR measurements

In **Figure 8**, we notice that in the powders with crystallite size less than 24.5 nm (superparamagnetic region), the value of the factor g is practically constant. This suggests that the magnetic core size is invariant regardless of crystallite size. To rely on the core-shell model with a single magnetic core, in order that the magnetic core keeps the same volume, the thickness of the dead layer must increase with increasing crystallite size. This hypothesis is in contradiction with the literature which reveals that the thickness of the shell decreases [18, 34, 48]. To explain these experimental data, we resorted to the phenomena of magnetic phase separation, which is a usual phenomenon observed in manganites [69, 70]. The competition between several interactions in perovskite manganites means that there are only small energy differences between the different possible phases of the system. As a result, the perovskite manganite oxide is magnetically inhomogeneous,

**32**

Mondher Yahya1 \*, Faouzi Hosni<sup>2</sup> and Ahmed Hichem Hamzaoui1

1 Laboratory of Useful Materials Valuation, National Center for Research in Materials Sciences, Soliman, Tunisia

2 Faculty of Sciences, University of Bisha, Bisha, Saudi Arabia

\*Address all correspondence to: mondher.yahya@cnrsm.rnrt.tn

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
