*2.1. Magnetic Nanomedicines*

Nanomedicines, nanotechnologically generated drugs, cover a broad range of sizes of a few up to several hundred nanometers [56]. Depending on the nature of the material, they can be classified into two major groups, organic or inorganic. Owing to their small size, nanoparticles can easily be dispersed in aqueous solutions, which is crucial for intravenous administration [57,58]. The nanoscale exhibits a high surface to volume ratio, meaning that the comparatively high surface can be functionalized with ligands. These ligands can change the pharmacokinetics of the particles, i.e., polyethylene glycol (PEG) can increase the circulation time of nanoparticles [58–60]. Specific ligands for cellular receptors can enable specific binding to specific cell types. Moreover, this valuable surface can be conjugated via drugs for their delivery and on-demand release [59,61], or with fluorescence marker and other molecules for multimodal imaging or therapy [62,63]. The prefix "magnetic" means that MNPs are sensitive to the magnetic field. This field brings numerous options that are attractive for biomedical applications since this field can easily penetrate the body and interact with MNPs, for example, for their detection, visualization, manipulation, or heating.

The magnetite Fe3O4 and magnetite γ-Fe2O3 are the most applicable magnetic materials in the biomedical field. Bulk magnetite has a cubic inverted spinel structure and exhibits a ferrimagnetic behavior at room temperature. The nano-sized magnetite oxidizes rapidly into maghemite, which also has similar ferrimagnetic properties and spinel structures with some valences. The iron oxide-based MNPs of a diameter (d) below 30 nm act in the SPM regime at room temperature and are abbreviated as SPIONs (superparamagnetic iron oxide nanoparticles) [53]. Particles sized less than 10 nm are usually attributed to ultra-small SPIONs (USPIONs [53]) and are characterized by reduced magnetization and increased anisotropy because of the influence of non-collinear spins at the surface [64,65]. The SPIONs and USPIONs are in the frame of interest for biomedical applications, especially for magnetic resonance imaging (MRI) where they are applied as contrast agents for both T1 and T2 relaxation times [66–68]. For specific applications, for instance, magnetic hyperthermia, the adjustable anisotropy of MNPs, is needed [69,70]. For this reason, magnetic anisotropy can be tuned, for instance, by variation of the chemical composition of ferrites (Me2<sup>+</sup>Fe2O4) by doping the spinel structure of ferrite with the ions of transition metals (Me2<sup>+</sup> = Co, Mn, Zn, and others [71].
