Now we investigate the effect of elongation. As discussed earlier, due to the contribution in Eq. (2.16), even a small elongation may have a strong effect on the energy barrier of the multispin particle, and in particular on the effective uniaxial constant , as will be seen below. Fig. 2.9 shows the energy potential of an ellipsoidal multispin particle with aspect ratio 2:3, cut from an fcc lattice. Unlike the energy potentials of spherical multispin particle, the result here shows that for large surface anisotropy the energy minimum corresponds to , see Fig. 2.9(d). Indeed, due to a large number of local easy axes on the surface pointing perpendicular to the core easy axis, the total effect is to change this point from a saddle for small to a minimum when has large values.


The effective uniaxial and cubic anisotropy constants are shown in Fig. 2.10, for nanoparticles cut from fcc and sc lattice. As expected, the effective uniaxial constant is linear in shows a strong variation and even changes sign at some value of , as opposed to the case of a spherical multispin particle. On the other hand, as for the latter case, the constant retains its behavior as a function of , i.e. is proportional . Again, in the case of an sc lattice and on an fcc lattice .
Rocio Yanes