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 D
0p 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.