**7. Conclusion**

*Mineralogy - Significance and Applications*

sis routes, are shown in **Figure 9(a)** and **(b)**.

nano-hematite particles, TEM micrographs of the chosen nanocomposite samples annealed at 1050°C, obtained by auto-catalyzed and base-catalyzed sol-gel synthe-

Detailed TEM analysis is given in Ref. [59, 63]. Quantitative description of morphological properties of the investigated particles is performed by measuring ellipticity. The results of the analysis showed that the shape of observed Fe2O3 nanoparticles (ε-Fe2O3 and α-Fe2O3) varies from ellipticity to circularity. In other words, **Figure 9** confirms the presence of nonideally spherical particles, whose shape deviates from circularity in a different measure [59]. Fe2O3 particle sizes, presented in **Figure 9(a)**, are ranging between 10 and 20 nm, while the sample presented in **Figure 9(b)** showed a wider particle size distribution, from 4 to 50 nm, and the same variations from ellipticity to circularity. Wide size distribution leads to the presence of different particle shapes during the annealing treatment, elliptic/spherical (**Figure 9(a)**). This feature appeared as a consequence of the fact that sol-gel method consisted of coprecipitation of the particles within the SiO2 pores (coprecipitated samples are characterized by wide particle size distribution) [53]. The best way to overcome the mentioned problem is the coating of the nanoparticles within the SiO2 pores [69]. Notwithstanding, a significant difference between micrographs is the presence of nanorod particles within the sample synthesized by base-catalyzed sol-gel synthesis (**Figure 9(b)**). Rod-like morphology appeared as a consequence of the participation of group II element, Sr2+, in the synthesis procedure (**Figure 9(b)**). The addition of Sr2+ ions accelerated the growth of the ε-Fe2O3 particles in one crystallographic axis [63], inducing more pronounced shape variations and formation of rod-like nanoparticles. If we recall the fact that TEM image shown in **Figure 9(b)** revealed the presence of the only γ-Fe2O3 and ε-Fe2O3 nanoparticles in the investigated sample (discussed in more detail in the Ref. [63]), as well as having in mind that α-Fe2O3 formation occurs as a consequence of phase transformations ε-Fe2O3 → α-Fe2O3, it can be assumed that

the sample presented in **Figure 8** contained rod-like α-Fe2O3 nanoparticles.

It is important to note that the origin of dependence of Hmeas behavior on the synthesis conditions of the samples investigated in this chapter is found in quantum

Briefly, the quantity that strongly affects the shape of hysteresis loop is magnetic anisotropy [parameter ι, in Eq. (3)]. For the most simplest case, in crystal systems whose symmetry is determined by a single axis of high symmetry (uniaxial sym-

Ea~KVsin<sup>2</sup> θ (4)

where K is the anisotropy constant, V is the volume, and θ is the angle between two spins with respect to each other [17]. The overall magnetic anisotropy energy is dependent on the symmetry of the investigated systems and defined by various contributions, such as magnetocrystalline anisotropy, shape anisotropy, surface

Anisotropy energy appeared as a consequence of spin-orbit interaction and the partial quenching of the angular momentum [17]. From the aspect of nanomaterial preparation and dependence of samples of magnetic properties on synthesis conditions (annealing temperature and time), it is important to emphasize that the anisotropy constant is strongly temperature dependent [17]. Independent of the presence of the same or different iron oxide polymorph phases within the sample, differences in the structure and morphology characteristics of each individual

Noteworthy, alteration of the SiO2 matrix during the annealing treatment impacts magnetic properties of the samples [59]. Gas diffusion in the SiO2 matrix,

**118**

mechanics.

metry), anisotropy energy is defined as:

anisotropy, strain anisotropy and stress anisotropy.

nanoparticle resulted in the changes in a magnetic anisotropy.

The main message of this chapter was to emphasize the importance of the investigation of the influence of the synthesis parameter variations onto the magnetic properties of the composite materials containing nano-hematite particles that could be used as a starting material for preparation of multifunctional nanoparticles, used in different areas of biomedicine. Since coercivity field presents a parameter of importance for application of this type of materials, alterations of measured measured Hcrit value, initiated by changing the synthesis parameters, are discussed. To get a better insight into relation between synthesis conditions and magnetic properties of composites containing α-Fe2O3 nanoparticles, sol-gel synthesis is recognized as a suitable preparation method. Alterations of measured Hcrit value of the samples are driven by the variation of the pH of the performed sol-gel synthesis (auto-, acid-, or base-catalyzed), initial Fe**<sup>3</sup>**<sup>+</sup> and Si4+ precursor ratio, amount of the iron precursor, and annealing conditions (T and t) and by performing postannealing treatment. The author expected that this chapter will facilitate a current and objective evaluation of the knowledge regarding the search for the exact mathematical expression of the measured intrinsic coercivity field value of the composite nanomaterials containing nano-hematite phase, which is of significance for improvement of the preparation of a high-quality nano-α-Fe2O3 particles for biomedical application.
