## 3 Results
### Epr
A high intensity of the EPR signal is consistent with the presence of a large number of Mn\({}^{4+}\) ions, in environments mainly containing Mn\({}^{4+}\) ions or Mn-vacancies (see [26] and the references therein). The Mn-vacancies formation in Mn-O layers was related to oxygen uptake, favored by the slow cooling of the samples, as indeed occurred for NMO. However, the g-value detected for NMO (1.991), which was still different from the expected 1.996 value, was matched with the presence of a certain amount of Mn\({}^{3+}\) ions coexisting with the Mn\({}^{4+}\) and Mn-vacancies. For the quenched series, the doping led to line-narrowing with respect to NMO-Q, consistently with the presence of the P2 polymorph as the main phase instead of the orthorhombic _Cmcm_ polymorph in NMO-Q. In particular, the narrower line was observed for Cu02-Q, for which a g-value very near to the expected 1.996 was also detected, together with an important increase in signal intensity with respect to NMO-Q. For Fe02-Q, the presence of a greater number and variety of the O phases again seems to play a role in affecting the features of the EPR spectrum. Besides this, in this case, the spectrum was centered at g \(\simeq\) 2, suggesting a non-negligible contribution to the spectrum by Fe\({}^{3+}\) ions. From the EPR results, it can be inferred that the doping seems to play a contrasting role against the annealing procedure, reducing the differences between samples with the same composition with respect to the undoped samples, looking, in particular, at the line width values. This was evident in Figure 5, where the line width values as a function of temperature were compared for all the samples. This picture also evidenced the wider line widths for the Q-sample with respect to its analogous slowly cooled sample. Besides this, the increase in \(\Delta\)B va  
Figure 5: EPR linewidth values as a function of temperature for all the samples. temperature confirmed that magnetic correlations occurred in these samples, which is in agreement with the findings reported in the literature [26]. The peculiar behavior of NMO-Q is clearly evidenced in Figure 6, where the temperature dependence of the g-factor for all the samples is shown. It can also be observed that the g-factor holds a nearly constant value in the whole investigated temperature range for all the samples but for Fe02-Q, for which an increase was detected by decreasing the temperature. This is possibly related to the low amount of Mn vacancies, as a consequence of quenching, with respect to the analogous slowly cooled sample (Fe02). Useful indications can be drawn from the temperature dependence of the signal intensity (area), as reported in Figure 7 for all the investigated samples: vertically, a comparison can be made between those samples obtained via the same annealing procedure and with different compositions, while horizontally, samples with the same composition but obtained after different annealing treatments can be compared. These trends suggested that the stronger correlations concern the Q-samples with respect to the slowly cooled ones, correspondingly to the presence of a greater number of Mn\({}^{3+}\)-Mn\({}^{4+}\) couples. Besides this, in each series of samples, Cu-doping seems  
Figure 6: The g-factor as a function of temperature for all the samples. Figure 7: The trend of signal intensity (area) for slowly cooled (left column) and quenched (right column) samples vs. temperature. Black, red, and blue symbols represent undoped, Cu-doped, and Fe-doped samples, respectively. to favor a paramagnetic-like behavior, while Fe-doping tends to support the onset of magnetic correlations.