  In this work we present a new method to calculate the classical properties of
magnetic nanoparticles. Based on the Bethe-Peierls (pair) approximation, we
developed a simple system of equations for the classical magnetization of spins
at any position within the nanoparticle. The nearest neightbor pair
correlations are treated exactly for Ising spins, and the method can be
generalized for various lattice symmetries. The master equation is solved for
the Glauber dynamics (single-spin-flip) in order to obtain the time evolution
of the magnetization. The capabilities of the model are demonstrated through
the calculation of hysteresis loops as well as field cooling (FC) and zero
field cooling (ZFC) magnetization curves of heterogeneous nanoparticles. The
present method can be an alternative to the usually complex and time consuming
methods employed in micromagnetism.
