The first method is based on the phenomenological expression [132] that allows to determine the effective surface anisotropy from the total effective anisotropy varying the size of the system (see section 3.1.1).We assume that this formula is valid for all temperatures:
where is the temperaturedependent macroscopic volume anisotropy constant, is the temperaturedependent effective surface anisotropy constant, is the number of moments that belongs to the surface and is the number of total moments in the magnetic system. Then if we know the fraction of moments that belongs to the surface, and the value of the total anisotropy, as a function of the size of the system, we will be able to determine .

The modeling at different temperatures of a set of thin films (see Fig. 4.12) provide the effective bulk and surface anisotropy constants. The data are perfectly scaled with the ratio . The fitting of to the expression (4.23) allows to extract the effective surface and volume anisotropies as a function of the temperature, see Fig. 4.13.
Rocio Yanes