  We investigate the effect of surface anisotropy in a spherical many-spin
magnetic nanoparticle. By computing minor loops, two-dimensional (2D) and 3D
energyscape, and by investigating the behavior of the net magnetization, we
show that in the case of not too strong surface anisotropy the behavior of the
many-spin particle may be modeled by that of a macrospin with an effective
energy containing uniaxial and cubic anisotropy terms. This holds for both the
transverse and N\'eel's surface anisotropy models.
