Surface anisotropy, fine particles

The calculation of the relaxation time t of the magnetic moment m of a particle is very important because the t value states all the experimental results. Accurate formulas are needed but difficult to establish because an actual sample of fine particles does not correspond to any simple case, due to the various anisotropies, the interparticle interactions, the surface effects, and so forth. On the other hand, experiments do not always measure the same parameters and the formulas have to be adapted. 

It is well known that ferromagnetic resonance (FMR) is a sensitive tool to study the local fields. In fine-particle systems, as the different contributions to the total anisotropy (magnetocrystalline, magnetostatic, surface, shape, interparticle interactions) induce local fields, FMR experiments allow one to obtain information on the anisotropy energy and on the distribution of easy axes. 

In conclusion, FMR experiments represent a useful tool for studying the dynamical behavior of fine magnetic particles and for obtaining information on the anisotropy energy and on the orientational distribution of easy axes. The proposed theoretical models are able to fit qualitatively the experimental spectra. However some important aspects, strongly affecting the thermal fluctuations of particle moments, have not yet been accounted for, for example, the effect of dipolar interactions in dynamical conditions (the dipolar local fields fluctuate in time in a system of nonidentical particles) and the effect of surface layers. Moreover, some models are valid only if H and cannot be applied when H is small. 

---