In this section we directly model the effective energy landscapes of multi-spin Co nanoparticles, with typical experimental parameters, varying the strength of the local surface anisotropy value. The obtained effective anisotropy, defined as , where is the energy barrier and V is the particle volume, is then compared with those experimentally determined in . Consequently, we get rid of formula (3.6) and obtain the local on-site surface anisotropy from a "direct" comparison of the experimental and numerical effective anisotropy constants.
In Figs. 3.10 we present the effective anisotropy constant evaluated for truncated octahedral nanoparticles with two diameters nm. and nm., and for an elongated nanoparticle with e=1.228 and the smaller dimension nm (experimental parameters). One can see that for the energy barriers of multispin particles increase with surface anisotropy increment, confirming multiple experimental results. The horizontal lines in these figures indicate the experimental results for the energy barriers obtained in Refs. [10,8]. From this comparison we have estimated the corresponding local surface anisotropy values. The uncapped Co nanoparticle would have the same value of the effective anisotropy as bulk Co, provided that ; this value is only slightly higher than that estimated from the calculations ( ) of Daalderop et al . The results for capped nanoparticles are presented in Tab. 3.2. The estimated surface anisotropy values are 20-40 times higher than those obtained via formula (3.6) and are almost of the order of the exchange parameter . These values look higher than those normally expected. On the other hand they are in agreement with estimations for the surface anisotropy based on first principles in thin films .