Magnetic anisotropies, interparticle interactions

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. 

Other parameters such as the particle shape and the spatial organization of the particles will have an effect on the heating capacity of the magnetic materials through their dependence on the magnetic anisotropy and interparticle magnetic interactions, respectively. 

We have pointed out the complexities of the phenomena, which can now be experimentally detected because of the progress in measurement techniques and in materials. These complexities lead to difficulties for an accurate analysis of the experimental results. In particular, we have pointed out that the use of simple expressions, imprecisely stated, for the anisotropy energies and the interparticle magnetic interactions can lead to wrong conclusions, due to the close dependence of the relaxation time on the anisotropic part of the energies. Experimental process too is of fundamental importance for stating the measuring properties. In the same way, comparison without precaution of the results obtained by experiments that do not experience exactly the same properties or do not strictly use the same process can lead to erroneous inferences. 

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