**2.2. Size**

are occupied, Fe3+ ions are regularly distributed in only two-thirds of the sites and the rest of the sites remain vacant. After two sites filled with Fe3+ ions follows one vacant site [18, 19]. Maghemite is a metastable oxide, product of magnetite oxidation or a product resulting from

product of the transformation of other iron oxides, is a red powder when it is finely divided, very stable at room temperature and very widespread in rocks and soils (**Table 1**) [20, 21].

are materials with iron-magnetic properties under their Curie temperatures (858 K and 986 K) (**Table 1**). The ferro- and ferrimagnetic compounds in their raw state present a multidimensional magnetic structure, without a permanent magnetic moment. The magnetic properties of a material depend on following parameters: (i) temperature, (ii) pressure and (iii) applied magnetic field. The properties of iron oxide nanoparticles by their usual sizes are not similar to the properties of larger scale compounds, which explain their use and interest in nanomedicine [23]. In order to define the behavior of the magnetic field, the key lays in the size and distribution of nanoparticles morphology [24]. A spherical, small nanoparticle made of soft materials with a diameter below the domain size shows an expendable magnetic anisotropy, so that their magnetic moment is free to rotate relatively to the particle and is thus superparamagnetic, i.e., paramagnetic under the Curie temperature [25]. The direction of the magnetic moment of the nanoparticles is determined by thermal fluctuation and the magnetic anisot-

ropy, which tend to fixate on the crystalline structure or particle morphology [26].

The interaction between an external magnetic field and the magnetic field of a nanoparticle determines: (i) the orientation of the magnetic moment of the particle as to become parallel with the magnetic field applied to minimize energy and bipolar interaction and (ii) the transition of the particle in the direction of the gradient, as in magnetophoresis [26]. Many applications of the magnetic nanoparticles are based on their ability to be manipulated using magnetic fields. This capability depends on the effectiveness of the magnetophoretic force, determined by the time of the particle and the magnetic field gradient, to fasten or to move the particle [25]. The magnetophoretic force exercised over superparamagnetic nanoparticles with a single core is less effective due to their small diameter and magnetic moment, but in the case of multicore particles, the magnetic momentum induced in the field is strong enough to allow magnetic targeting to moderate values of the magnetic field intensity and field gradient. Therefore, in order to assess the applicability of magnetic particles or magnetic fixing, the magnetic momentum of the particles is more relevant than mass magnetization [25, 27].

O3

) has a corundum crystal structure with Fe3+ ions distributed

O4

)—an anti-ferromag-

O3

) and maghemite (γ-Fe<sup>2</sup>

, the final

O3 )

At temperatures over 300°C, magnetite is oxidized to hematite (α-Fe<sup>2</sup>

in octahedral sites and oxygen ions in hexagonal close-packed arrangement. α-Fe<sup>2</sup>

O3

Magnetic iron oxide nanoparticles (M-IONPs), magnetite (Fe3

**2. Properties of magnetic nanoparticles**

the heating of other iron oxides.

232 Iron Ores and Iron Oxide Materials

netic iron oxide. Hematite (α-Fe<sup>2</sup>

**2.1. Magnetic behavior**

The size and the size distribution of superparamagnetic iron oxide nanoparticles are important parameters for their biological application. Also, their magnetic properties are in close touch with their size. It has been demonstrated that the magnetic dipole-dipole interactions are significantly reduced in superparamagnetic iron oxide nanoparticles due to their scale of r6, r being the radius of the particle [28]. The advantages of using magnetic nanoparticles with sizes smaller than 100 nm are due to their surface efficiency to easily attach ligands and small settling velocities which give a high stability in suspension and improve tissue diffusion. Particles should be small enough to bypass the endothelial reticule system. They are supposed to remain in circulation after injection and be able to pass through the capital systems, organs and tissues, avoiding the embolus. Particle size is also important for getting an effect of improved permeability and retention. For example, particles larger than 10 nm may not penetrate the endothelium in physiological conditions, but can enter in pathological conditions, such as inflammations or tumors [28].

When magnetic nanoparticles loaded with medicinal substances are injected into the systemic circulation, size, morphology and surface charge are the three important parameters for their behavior in the bloodstream. Kupffer cells in the liver are very sensitive to both microorganisms and nanoparticles. Plasma proteins can easily adsorb onto their surface nanoparticles, depending on their size, surface charging and their morphology. Particles with sizes larger than 200 nm or below 10 nm are not suitable due to their absorption by the endoplasmic reticulum system [28].
