# Unexpected Ferromagnetism--A Review
## 3 Drawbacks of Unexpected Ferromagnetism  
Even though there are many decades of studies on the presence of weak ferromagnetism in systems such as diluted magnetic semiconductors and oxides, and also in many other exotic systems, there is still much controversy about the origin of this phenomenon, despite the different theoretical models. For example, donor impurity band exchange, implying bound magnetic polarons; or charge-transfer ferromagnetism for doped oxide nanoparticles [8] developed and employed to explain their behavior. The study on this class of materials is also hampered by the high dependence of the magnetic properties on the synthesis method, their morphology (bulk, thin films, nanoparticles, etc. ), and the presence of surface defects [109]. Moreover, there is also the problem of reproducibility and contamination of samples with unwanted magnetic impurities. For example, single-phase iron-doped zinc oxide ceramics obtained by solid-state reaction, having Zn\({}_{1\times}\) Fe\({}_{\text{x}}\)O (0 \(\leq\) x \(\leq\) 0.07 mol) composition, showed a weak superparamagnetic behavior without a clear saturation of hysteresis magnetization curve even at magnetic fields as high as 18 kOe, while the corresponding magnetization values increased from 0.02 emu/g at 1% Fe up to 0.18 emu/g at 7% Fe [110]. Additionally, ZnO-Fe-doped nanoparticles synthesized by sol-gel auto-combustion method [111], having x = 0.02, 0.04, 0.06 and 0.08, presented a similar tendency with magnetization values approaching 0.85 emu/g at 2000 G for the highest dopant concentration. Whereas pure ZnO presented a much lower magnetization (4 \(\times\) 10\({}^{-5}\) emu/g) at the same external field value. However, studies performed on ZnO single crystals implanted with Fe ions and analyzed by Mossbauer spectroscopy and X-ray magnetic circular dichroism (XMCD) revealed that, at a very low doping regime (10\({}^{-5}\) at.%), only isolated paramagnetic trivalent iron ions had \"a spin-lattice type of relaxation\", whereas at higher Fe\({}^{3+}\) doping regimes (0.02-0.2 at%) the relaxation between nearby iron ions becomes spin-spin type, and at even higher concentrations, up to 2.2 at.%, the researchers did not find any signature of magnetic ordering [112]. Thus, they concluded that, oppositely to other observations of dilute magnetism in zinc oxide systems doped with 3d ions, there was \"no evidence of any long-range magnetic ordering between isolated Fe atoms incorporated in the ZnO lattice\" [112]. Thus, it seems that, without an efficient mechanism for mediating long-range interactions, there is no magnetic ordering at room temperature in this wide band gap and \"truly diluted\" oxides/semiconductors, the authors implying that the cause for this could be the conditions of the absence of precipitation or 3d-aggregates or surface contaminants [112]. In their article revising the topic of d0 ferromagnetism in undoped zinc oxide nanostructures [113], the authors summarized the possible origin of erroneous magnetic signals foreign to measured samples, which can be applied to other diluted magnetic materials. Thus, small influences that are not very important for materials with strong magnetization start to compete with the true magnetic signals when one deals with very weakly magnetic materials, such as those from the undoped zinc oxide class whose magnetization is around or even smaller than \(\sim\)10\({}^{-4}\) emu [114].