# Unexpected Ferromagnetism--A Review
## 1 Conventional vs. \(d^{0}\) Magnetism
There are only three elements in the periodic table that exhibit ferromagnetism at room temperature, namely iron, cobalt, and nickel. \(d^{0}\) magnetism is a term that has been used to describe materials that display room temperature ferromagnetism when they should not be ferromagnetic at all [1; 2]. Some examples of this behavior have been found in hexaborides, non-magnetic oxides, and carbon structures, further discussed in the following sections, as well as exemplified in Table 1.  
As resumed in [3], the appearance of ferromagnetism in various semiconductors or in some insulators doped with 3d ions occurs even if the concentration of these ions is much lower than the percolation required for their nearby coupling. In these materials, the mediated moment per cation is near or even greater than the moment induced by uncoupled spins in conditions where dopant concentrations were low, and this moment decreases as the concentration approaches towards those of percolation [3]. It was also highlighted that the magnetic properties show great variations for materials with the same apparent compositions synthesized by different researcher groups or by various methods [3]. Known for about three decades, non-oxidic diluted magnetic semiconductors (DMSs) with p-type electronic conduction are based on combinations between elements from main group III and V (such as GaAs, InAs, GaP, GaSb, GaN) and even from a single element from group IV such as Ge, all doped with Mn\({}^{2+}\) ions [4]. For example, the materials from Ga\({}_{1-x}\)Mn\({}_{x}\) as a system having x = 0.03 to 0.07 have a ferromagnetic (FM) behavior with a Curie temperature Tc \(\sim\) 100 K [5]. For this class of materials, the authors from [5] concluded that the apparition of a ferromagnetic phase when the temperature was sufficiently decreased was caused by the interaction between magnetic manganese ions induced by the charge carriers (in these cases being holes) in the host matrix. Thus, the exchange interaction of localized holes with magnetic dopants leads to the formation of so-called bound magnetic polarons (BMPs), which are composed of one localized hole (in the p valence band) and a considerable number of magnetic ions around it [5]. Even if the nearby spins of the magnetic impurities tend to align in the same direction and are in opposition with the hole spin, thus having a direct antiferromagnetic exchange interaction inside BMPs, there is a possibility of FM interaction between BMPs at a high concentration of magnetic dopants [5]. Moreover, when the temperature is sufficiently low, the BMPs tend to overlap while mutually interacting via nearby magnetic dopants. Consequently, the spins of polarons will align and thus, when interacting polarons clusters grow to the dimensions of all the material, it can be considered that a ferromagnetic transition occurred [5]. Itmust be pointed out that, for this kind of DMSs (especially for the insulating ones), their observed saturation magnetization is much lower than a similar system with the ideal full ordering of magnetic dopants, which can be explained if a great fraction of magnetic dopants have a zero contribution the sample ferromagnetism [4]. Additionally, in the range of concentrations where DMS FM is observable, the magnetic ions are intercalated between non-magnetic atoms, so they are separated, thus any antiferromagnetic interactions between these ions can be neglected due to their rapid diminution with the increasing distance [4]. The BMPs approach was also used to explain FM in diluted magnetic oxides having an n-type conduction type [3].