4.5 Coercivity of Fe particles with surface anisotropy

Figure 4.13: Comparison with the experimentally measured values of the simulated data obtained for the ribbon width dependence of the coercive force in ribbons having different values of the surface anisotropy.

The contribution of surface anisotropy to the reversal process can not be ``a priori'' ruled out considering the simulated system dimensions. In Fig. 4.13 we present the results of our simulations corresponding to the consideration into the system energy of a surface magnetocrystalline anisotropy term. The local easy axes at the surfaces were considered to be parallel to the simulated system surfaces normals and replacing the anisotropy of the surface cubes by an effective anisotropy. Our results show that, for the considered surface anisotropy constants (taken from [Lorenz 96]) the coercivity of the thin ribbons is in reasonable agreement with the experiment but that this is not the case for the ribbons having the smaller aspect ratios. This difference can be related to the fact that the direction of the surface anisotropy easy axis favors the orientation of the moments perpendicular to the ribbon surface and consequently the reduction of the thickness of the Néel wall-like structures originated at the lateral surfaces by the minimization of the magnetostatic energy. Since the ribbon coercivities are linked to the creation of these structures (that process coincides with the rotation of the magnetization of the core of the ribbons towards the easy axis direction closest to the demagnetizing field direction) their reduction should be related to the easiest formation of the Néel wall-like structures in systems having significant surface anisotropy.