# Unexpected Ferromagnetism--A Review
## 1 Conventional vs. \(d^{0}\) Magnetism
Thus, the Fermi wavevector noted KF has a small value on the base of the band, translated in the fact that, at low electron densities, the RKKKY exchange interactions will be of FM type. However, at higher electron density, the system will behave like a spin glass due to the fact that the exchange of positive and negative bonds of the equal number will cancel each other [3]. One main type of defect which gives rise to the donors is considered to be the oxygen vacancies in n-type oxides. Whereas in the two-electron occupied vacancy (F0 centers) the 1s2 state is usually in a singlet state and is responsible only for weak antiferromagnetic exchange, the one-electron occupied vacancy (F+ centers) can behave as described above. However, when F0 centers can overlap with the 4s band, or in the case when a partial compensation would eliminate some electrons from the impurity band, the FM will be favored [3]. Other types/variants of defects were also suspected of playing a role in unusual FM, such as defects introduced at the narrow region of the film-substrate interface, and also defects having two electrons in a triplet ground state or even in excite state formed by a cation vacancy together with a nearby monoelectronic vacancy. Thus, the molecular orbitals in the triplet state may also form an impurity band, replacing the previously discussed BMPs. This band can suffer a spontaneous split (with the condition to be narrow enough) or to interact with the nearby 3d magnetic cations and undergo polarization by exchange with them, events that can occur at quite low concentrations of dopant ions (less or equal with 1%) [3]. Regarding the values of Curie temperatures measured in diluted magnetic oxides and nitrides, some examples were presented by [PERSON] et al. [3]. Thus, in the cases of cation-doped nitrides: GaN-Mn(9%): 940 K, whereas for GaN-Cr: \(>\)400 K and for AlN-Cr(7%): \(>\)600 K. For titania doped with Co(1-2%): \(>\)300 K, with Co(7%): 650-700 K, with V(5%): \(>\)400 K, with Fe(2%): \(>\)300 K. For tin dioxide doped with Fe(5%): 610 K, with Co(5%): 650 K. For zinc oxide doped with: with: V(15%): \(>\) 350 K, Mn(2.2%): \(>\)300 K., with: Co(10%): 280-300 K and Ni(0.9%) \(>\)300 K. For cuprous oxide co-doped with Co(5%) + Al(0.5%): \(>\)300 K. For ITO In1.8 SnO.2O3 doped with Mn(5%): \(>\)300 K. All these selected examples show quite high Curie transition temperatures values, many being well above room temperature [3]. Another mechanism for the emergence of FM in diluted magnetic oxide nanoparticles was proposed in [6], and was associated with surface or interior positioned defects in these nanostructures. The authors considered that the FM is manifested in a limited fraction of the nanosized material, which contains zones rich in defects that can also be on the surface, zones being characterized by a narrow (\"sharply peaked\") local DOS (density of states) in the conditions that the Fermi level (positioned either below or above the mobility edge) will usually not coincide with a local DOS. The authors introduce the concept of a \"local charge reservoir\" (which contains, for example, dopant cations existing in two different ionized states or is formed by some type of surface charge-transfer complex), which will make possible the increase of Fermi level to a peak in local DOS by electron transfer, named \"Stoner splitting\" of local DOS. It is highlighted that this spontaneous Stoner FM can occur especially in surface zones that are rich in defects (percolating zones) [6]. Alternative to the bound magnetic polaron BMP model, another mechanism was presented in [7; 8] and shortly after described in [9]. Thus, for a certain concentration of holes, the hole \"localization length\" has a size that greatly surpasses those between acceptors as average. As a result, the holes can now be seen as being delocalized at a length scale that can be compatible with those required for coupling of magnetic ions.