**6. Conclusions**

It can be established that systematic and significant differences occur in the resonance signal of ferrite NPs, associated with the magnetic structure. Paramagnetic phases lead to a high resonance field (typically in the ~330 mT range for the X band) with a small line width and a tendency to a symmetric signal (*R* ~ 1) (see signal at 448 K in Figure 15), in contrast with the ferromagnetic phase, which exhibits a lower resonance field, broad line width and asymmetric shape (signal at 103 K, Figure 15). Between these two different behaviors, there is an SPM phase showing a resonance field closer to the one for the paramagnetic phase, a progressive decrease in the broadening of the minimum section of the line (absorption at *H* > *H*res), and therefore an increase in the *R* parameter toward 1.0. The exact transition temperatures are not easy to establish since evolution from one phase to the next is quite continuous.

As a general conclusion, these reviews confirm that electron spin resonance is a very sensitive experimental method that can be used to characterize magnetic phases in the complex field of magnetic nanoparticles.
