**Author details**

**Figure 21.** Interpretation of the line broadening in ferrites with cubic anisotropy. On a [110] crystal plane, containing two easy axis of the <111> family, which have an influence on the magnetization dynamics, leading to an increase in

These results were obtained for the case of uniaxial materials; in the case of most ferrites with cubic anisotropy axes, it would be expected a larger broadening. In a very cartoon-like model, it is possible to imagine that the magnetization is first oriented along the applied field, which has a value closer to the anisotropy field. Once close to the saturation and once satisfied the resonance conditions, the magnetization is driven by the microwave radiation to precess uniformly, that is in phase. However, magnetocrystaline anisotropy is strong at these temper‐ atures, and it is possible to consider that the magnetization vector can still feel its influence and precesses with a tendency toward the cubic axes. This dynamics leads to a broadening of the line, as far as the main result will be the combination of precession along several anisotropy axes, and has to be averaged over a large NPs population. Figure 21 illustrates this interpre‐

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.

the precession amplitude.

234 Advanced Electromagnetic Waves

tation.

**6. Conclusions**

Rebeca Díaz-Pardo and Raúl Valenzuela\*

\*Address all correspondence to: monjaras@unam.mx

Institute for Materials Research, National Autonomous University of Mexico, Circuito Exterior, Ciudad Universitaria, Coyoacán, México D.F., México
