In the present section we aim at the modeling of nanoparticles experimentally prepared in Ref. [8,100,101,10,9]. In those articles it has been reported that the magnetic anisotropy of the Co cluster prepared by sequential sputtering of Co and (or Cu; or Au; or Ag) suffers an increment when the diameter of the cluster is reduced which is associated to the surface anisotropy. The same group suggested that such an increment of the effective anisotropy produces an increase of the effective energy barrier, stabilizing the magnetization against thermal fluctuations.
With the aim to study the effective energy landscape of such nanoparticles, we consider a Co nanoparticle with fcc internal structure, cubic magnetocrystalline anisotropy, varying the strength of the surface anisotropy. To compare with the experimental results in this subsection the anisotropy parameter was taken from the experimental value and is measured as the anisotropy density per volume, . The Co nanoparticles are "numerically prepared" in two forms: truncated octahedral, see Fig. 2.13(a) and elongated truncated octahedral, see Fig. 2.13(b). For truncated octahedra, we use a symmetrical construction which allowed the same surface atom arrangements in all rectangular facets, as it was described above. In the case of perfect truncated octahedral nanoparticle, the nanoparticle's diameter is ( ), is the total number of spin. For the elongated truncated octahedron the planes, perpendicular to zaxis, used to cut the polygon from the fcc lattice, were additionally separated to obtain the desired elongation, in our case that elongation has been chosen as , where and are dimensions in and directions, respectively (corresponding to experimentally prepared nanoparticles). The nanoparticle size in direction was considered (the total spin number is equal to ).

Some examples of the effective energy landscapes for several values of the Néel surface anisotropy constants are presented in Fig. 2.14.

In Figs. 2.15 we present 2D energy landscapes, corresponding to
(squares) and
(circles). Namely, the energy landscape cannot be fitted
to the cubic anisotropy with the first cubic anisotropy constant only.
We have found that these
landscapes could be fitted to the effective macrospin energy, slightly different
from that of Eq. (2.23):
For symmetric particles, (see previous sections) and we have observed that is relevant when has a large value, when , as we can see in Fig. 2.17 and Fig. 2.18.
In Fig. 2.16 we present the minimum and the saddle point spin configurations (on plane) for . We would like to mention that when the surface anisotropy is increased ( ) in comparison with the exchange parameter J, then the spin arrangement shows a high noncollinearity. Those spin noncollinearities are similar to the ones reported previously for strong surface anisotropy cases [74,90,102,103], and it is possible that the EOSP approximation is no more valid [73].

In Figs. 2.17 and 2.18 we present the values of the uniaxial and cubic macroscopic anisotropy constants extracted by fitting the 2D landscapes to expression (2.25). To make the comparison with experimental values easier we supply the values of the macroscopic anisotropy as energy density. The effective uniaxial anisotropy constant is zero in practice in the case of a perfect truncated octahedral particle. Nevertheless, for elongated particle, there is a nonzero uniaxial contribution coming from the surface effect, see Fig. 2.18(b). However its value is much smaller than the additional cubic anisotropy.
In both cases of octahedral and elongated particles the effective macroscopic first cubic anisotropy constant changes the sign, see Figs. 2.17 and 2.18(a), and therefore the easy axis of the system changes its position, as have been shown clearly in Figs. 2.15 and 2.14.
Rocio Yanes