En las fronteras de la Materia Condensada

Since 1977, when it was possible to extend the temperature range studied of the a phase of Fe using the Ion Beam Sputtering (IBS) technique, it is known that a-Fe self-diffusion does not follow the Arrhenius behaviour. The paramagnetic to ferromagnetic transformation produces a hardening of the crystal that is reflected by a change in the slope of the Arrhenius line at the Curie temperature and a curvature obtained in the ferromagnetic zone, where the diffusion coefficients are lower than predicted if a straight line would have been obtained in the a phase.
The study of the hetero-diffusion of other elements in a-Fe is presented in this work. Measurements were made in extended temperature ranges combining several techniques like Rutherford Backscattering Spectrometry (RBS), Heavy Ion RBS (HIRBS) and serial sectioning. The Arrhenius plots show the same variation as in the self-diffusion case. The curvature observed is modeled by adding to the activation energy in the paramagnetic region, a term depending on spontaneous magnetization:

where D₀^p and Q^p are the frequency factor and the activation energy in the paramagnetic state, s is the ratio of the spontaneous magnetization at temperature t to that at 0 K (reduced magnetization) and a depends on the increment of the formation and migration energy of a vacancy induced by the magnetic ordering.
All the diffusers studies are well fitted by this model. A correlation between the required a parameters to fit the Arrhenius plot and the identity and the magnetic or no- magnetic character of the diffusing impurity is discussed.