**2. Properties of magnetic nanoparticles**

#### **2.1. Magnetic behavior**

Magnetic iron oxide nanoparticles (M-IONPs), magnetite (Fe3 O4 ) and maghemite (γ-Fe<sup>2</sup> O3 ) 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 anisotropy, 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].
