As we have mentioned previously, the magnetic anisotropy can be increased when the size of the system is reduced. It is usually explained by the existence of the surface anisotropy different to the bulk one.
Néel in 1954 was the first to suggest the existence of this kind of anisotropy. After that many experimental evidences have corroborated that claim , and there exist multiple works on calculation of the surface anisotropy from the first principles [44,45,46]. An important point that we should take into account is that for small particles or clusters or ultrathin films where the number of spin at surface is large, the surface anisotropy can easily dominate the bulk one, especially in cubic materials.