**5. Coercivity of the composite nanomaterials**

*Mineralogy - Significance and Applications*

**4. Sol-gel synthesis**

of TEOS or TMOS, respectively).

listed requirements, hematite nanoparticles for application in biomedicine are coated by biocompatible materials (usually with silica, although different materials, such as dextran or citric acid, also could be applied [41–44]) and then further functionalized by attaching groups on the surface (different antibodies, oligonucleotides, or peptide ligands, depending on the desired application [45–47]) via various chemical methods [48–52]. An alternative way for production of suitable nanocomposites presents performing core-shell

In order to better examine the magnetic behavior of the hematite nanoparticles

that could be used as a starting material for different biomedical applications, nanocomposite materials based on hematite are often prepared by sol-gel method. From the point of view of correlation of the synthesis conditions with characteristics of the investigated nanomaterial, this synthesis method is of great significance. Advantages of this type of synthesis are low price of the chemicals, gelation process under ambient conditions, as well as possibility of the synthesis of very small nanoparticles (~1 nm) [53]. Basic compounds used in the sol-gel synthesis are iron ion precursor, silica ion precursor (tetraethyl orthosilicate, TEOS, or tetramethyl orthosilicate, TMOS), water, and the compound miscible with (mutually nonmiscible) alkoxide precursor and water (ethanol or methanol, depending on the usage

Mechanism of the sol-gel synthesis contains few stages. The first stage consisted nucleation of the Fe2O3 and SiO2 nanoparticles during the hydrolysis of TEOS. Reactions of condensation and polycondensation occurred during the mixing solution and resulted in the nanoparticle growth through the process of Ostwald ripening [53]. Mentioned processes occur at room temperature, conditioning the usage of catalyst, which initiates the changes in the structure and properties of the resulting material. The aging of the prepared sol enables its conversion into gel, which presents the second important stage, followed by drying of the gel (third stage), and subsequently annealing treatment (fourth stage). During the annealing process, Ostwald growth at higher temperatures initiated phase transformation of the iron oxide nanoparticles and finished with the formation of the most stable phase—α-Fe2O3 phase. The presence of the porous, nonmagnetic matrix enables minimization of the nanoparticle interaction and enables the control of the particles size [53]. Synthesis factors of importance for every stage of the sol-gel synthesis are the choice/ratio of the precursors and pH. The influence of the variation of the synthesis conditions onto the properties of the final synthesis product is still not sufficiently investigated. What is known from the literature is that the influence of pH is reflected in the defining of the pore size. Base-catalyzed sol-gel synthesis conditioned slow hydrolysis of the alkoxide precursor and fast condensation. Final matrix pores are determined by the sizes from 2 to 50 nm [54]. In contrary, acid-catalyzed sol-gel synthesis favors rapid hydrolysis, consequently bringing to the formation of a huge number of small SiO2 nuclei. The obtained gel consisted of the pores, with the size less than 2 nm [54]. This is explained by the influence of the hydrolysis and condensation rate on the formation of different polymers: base catalysis enabled formation of the longer, branched polymers, while acid catalysis resulted in the formation of linear polymers [55, 56]. Consequently, auto-, acid-, or base-catalyzed sol-gel syntheses could be used for the preparation of the α-Fe2O3/

SiO2 nanocomposite, significantly different in its properties.

strategy or encapsulation of the particles in a silica matrix.

**108**

If someone needs to get a better insight in the biomedical application of nanocomposite materials based on hematite, it is important to understand in detail synthesis, reaction mechanism, and correlation between synthesis conditions and properties of prepared samples containing nano-hematite particles. Some basic magnetic properties of nano-hematite phase are not well-established up until today, which complicate its biomedical application. From a fundamental point of view, the understanding of coercivity behavior is of great importance because coercivity presents magnetic property that significantly influences and determines application of the investigated nanomaterial.

An obstacle in a determination of the precise coercivity value of nanocomposite materials consisting hematite nanoparticles presents often occurrence of the intermediate iron oxide phase—epsilon phase (ε-Fe2O3) that is obtained during the synthesis of high-temperature hematite phase, by sol-gel method. ε-Fe2O3 polymorph is formed in the course of Fe3O4/γ-Fe2O3 → α-Fe2O3 structural transformation and often coexisted concomitantly with the α-Fe2O3 phase. The situation is additionally complicated by the inability to clearly separate a temperature range during which pure hematite or epsilon phase is formed. Pure hematite phase could be synthesized by various synthesis approaches at different temperatures up to 1100°C, while the epsilon phase is obtained only by sol-gel method and still is not prepared pure.

Having in mind that the behavior of α-Fe2O3 and ε-Fe2O3 phases is still not properly understood, the primary question regarding Hci value of nanocomposite materials containing hematite nanoparticles becomes: what is the difference between the mechanism of the coercivity field variations of the nano-hematite and nano-epsilon phase, since both of these phases could be characterized by coercivity field value ranging from zero to few thousand Oe?

To answer this question, more detailed scientific research should be performed. The correlation between Hci value and material microstructure is not sufficiently understood neither for bulk nor for nanomaterials [57]. Intrinsic coercivity field presents a reverse field required to reduce the magnetization (M) from the remnant magnetization (Mr) again to zero. The main problem in interpretation of the intrinsic coercivity field value is that the field measured by magnetic devices is not the coercivity field, but some critical field influenced by the magnetic interactions [57]:

$$\mathbf{H}\_{\rm crit} = \mathbf{H}\_{\rm ci} + \mathbf{H}\_{\rm int} \tag{2}$$

When we deal with attempts to understand the origin of the coercivity field variations in nano-sized samples, of big importance is the independent analysis of the Hcrit and Hci values, which is difficult, since consensus about the factors that influence Hint and Hci still is not achieved in the scientific community and presents a problem that should be overcome in the future.

The correlation between synthesis conditions and Hci value occurred through the competition of different parameters, which influence and contribute in the different measure of the final Hcrit and Hci values. The mathematical expression that would describe dependence of Hcrit on different parameters which influenced the coercivity field value has yet to be found, but roughly, it can be expressed as a function of different parameters (denoted according to the Greek alphabet) [Eq. (3)] [58]:

$$\mathbf{H}\_{\rm crit} = \mathbf{f}(\alpha, \beta, \gamma, \delta, e, \eta, \theta, \iota, \kappa) \tag{3}$$

The main influence on the coercivity value arises from:


In the next section, the impact of the parameters α and β on the measured magnetic field, that is in literature labeled as coercivity field, of the synthesized nanocomposite materials will be considered. X-ray diffraction and hysteretic measurements were performed in order to investigate the influence of the variation of synthesis parameters onto the formation of hematite phase as well as on the measured magnetic field value of the samples containing the pure hematite phase and hematite phase (as a dominant phase) in combination with the epsilon phase (appeared in traces). Few examples of the peculiar Hcrit behavior of the investigated samples will be represented.
