3 Analytical background
In Refs. [74,87,97,73]
analytical as well as numerical calculations showed that a
multispin particle, cut from a sc lattice, and when its surface
anisotropy is small with respect to the exchange coupling,
may be modeled by an effective onespin particle (EOSP), i.e., a
single macroscopic magnetic moment
representing the net
magnetic moment of the multispin particle. The energy of this EOSP
(normalized to ) may be written as

(23) 
The
is the firstorder anisotropy energy with surface contribution,
is the secondorder anisotropy energy with surface contribution,
is a mixed contribution to the energy that is second order in surface anisotropy and first order in core anisotropy and
is the core anisotropy energy (per spin).
The
energy has the following form:

(24) 
The other three contributions (
,
and
) stem from the surface, which we discuss now.
Subsections
Rocio Yanes