## 3 Results
### Epr  
In Figure 4a,b, the room-temperature EPR spectra for the slowly cooled and quenched samples were reported. All the measurements have been performed using the same experimental conditions; signal intensity always referred to the mass unit of the samples for the sake of comparison. All the spectra appeared to be composed of or dominated by a broad signal. The spectra of NMO, Cu02, and Fe02 (Figure 4a) were centered at the same g-value (1.991(1)). The undoped sample showed the narrowest line (about 800 G). An analysis of this spectrum disclosed the presence of two overwhelmed contributions: a main, narrower Lorentzian signal that was 750 G wide, and a secondary broader component with a Gaussian character and a linewidth of about 2500 G. Taking into account the data from Table 1 and according to the literature [26], the main signal is ascribable to the P2 phase, while the broader secondary component could be the result of contributions from other phases in the sample (P'2 and O phases). Indeed, after doping, line broadening was observed with respect to the NMO signal, correspondingly to the decrease in the P2 amount and to the overall increase in the other phases (see Table 1). Moreover, for Fe02, clear changes in the extreme parts of the line shape were also revealed. It should be noted that this sample contained the highest overall amount of P'2 and secondary different \(O\) phases, which then seem to play a role in affecting EPR line width and line shape. For this series of samples, the signal intensity (area) strongly decreased after Cu doping. This is quite unexpected because Cu/Mn substitution, as well as Fe/Mn substitution, should not lead to a decrease in the amount of Mn\({}^{4+}\) (see Table 2), Mn\({}^{4+}\) being the main component responsible for the EPR signal in these compounds. Moreover, the dopants themselves could make a contribution to the EPR spectrum, as both Cu\({}^{2+}\) and Fe\({}^{3+}\) are EPR-active ions. For the quenched samples (Figure 4b), a strongly different situation occurred, confirming the crucial role of annealing in affecting the structural and compositional features of these compounds. In this case, the spectra were centered at different g-values, with a line width that is much broader than that of the slowly cooled samples with the same composition. In particular, the broadest line pertains to NMO-Q (\(\Delta\)B \(\cong\) 2700G), which is  
Figure 4: Room-temperature EPR spectra for (**a**) slowly cooled and (**b**) quenched samples. also centered at the lowest detected g-value (1.919) among all the investigated samples. This value is then the farthest from g = 1.996, that is, the expected value for Mn\({}^{4+}\) when present as a unique magnetic ion in the sample [30]. Besides this, NMO-Q also showed the lowest signal intensity among all the investigated samples. According to the EPR results obtained for the slowly cooled samples, the EPR features achieved for NMO-Q perfectly agreed with the compositional data reported in Table 1 and in the literature [26]: both the _Cmcm_ orthorhombic phase and the P'2 and O phases are expected to give rise to different broad contributions to the spectra. Moreover, the markedly different g-value with respect to the one observed for all the other samples can be ascribed to _Cmcm_, which was present as the main phase in the sample, and to the high value of the Mn\({}^{3+}\)/Mn\({}^{4+}\) ratio, as also indicated by the EDS results (Table 2). Indeed, the broadest EPR signal observed in the literature for these compounds was related to the presence of Mn\({}^{3+}\)-Mn\({}^{4+}\) couples, which led to the development of strong antiferromagnetic interactions, giving rise to a progressive line broadening by lowering the temperature [26]. It is worth noting that among our samples, the highest signal intensity pertains to NMO (about 3 times higher than the one achieved by NMO-Q).